Abstract:
A quantum communication system including an emitter and a receiver, the emitter including an encoder and at least one photon source and being configured to pass a signal pulse and a reference pulse, which are separated in time, through the encoder and output the signal pulse and the reference pulse. The reference pulse has a higher probability of containing more than one photon than the signal pulse. The receiver includes a decoder and at least one detector for measuring the signal pulse and the reference pulse.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a division of and claims the benefit of priority under 35 U.S.C. §120 from U.S. Ser. No. 10/890,286 filed Jul. 14, 2004, the entire contents of which are incorporated herein by reference. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The present invention is concerned with the field of quantum communication systems and emitters and receivers which may be used in such systems. Specifically, the present invention is concerned with the use of a reference pulse in a quantum communication system in order to provide active stabilisation of the system. 
         [0004]    2. Discussion of Background 
         [0005]    In quantum communication systems, information is transmitted between a sender and a receiver by encoded single quanta, such as single photons. Each photon carries one bit of information encoded upon a property of the photon, such as its polarisation, phase or energy/time. The photon may even carry more than one bit of information, for example, by using properties such as angular momentum. 
         [0006]    Quantum key distribution which is a technique for forming a shared cryptographic key between two parties; a sender, often referred to as “Alice”, and a receiver often referred to as “Bob”. The attraction of this technique is that it provides a test of whether any part of the key can be known to an unauthorised eavesdropper (Eve). In many forms of quantum key distribution, Alice and Bob use two or more non-orthogonal bases in which to encode the bit values. The laws of quantum mechanics dictate that measurement of the photons by Eve without prior knowledge of the encoding basis of each causes an unavoidable change to the state of some of the photons. These changes to the states of the photons will cause errors in the bit values sent between Alice and Bob. By comparing a part of their common bit string, Alice and Bob can thus determine if Eve has gained information. 
         [0007]    Examples of quantum communication systems are described in GB 2 368 502 from the current applicant. 
         [0008]    When the photons are encoded using phase, typically, a Mach-Zender interferometer is provided in both Alice&#39;s sending equipments and Bob&#39;s receiving equipment. Each interferometer has a long path and a short path. Details of how the photons are encoded using this arrangement will be described later. However, it is required that photons that contribute to the key or the encoded information through the short arm of one interferometer and the long arm of the other interferometer. Thus, the photons may follow one of two paths: Path 1, the short arm of Alice&#39;s interferometer and the long arm of Bob&#39;s interferometer; and Path 2, the long arm of Alice&#39;s interferometer and the short arm of Bob&#39;s interferometer. 
         [0009]    Both interferometers will contain a phase modulator which can be used to either randomly vary the phase of photons passing through the interferometer either randomly or under the control of either Alice or Bob. 
         [0010]    However, it is necessary that any other phase delay between Path 1 and Path 2 is constant throughout transmission as any other phase delay can increase the quantum bit error rate and can even make the system unusable if it exceeds a certain level. Thus, in practice, one has to calibrate the phase delay every several tens of seconds or several minutes depending on the stability of the system. This introduces a dead time to the system. Also, during key distribution, no information concerning the phase drift can be obtained. This causes extra difficulties in identifying the presence of an eavesdropper as Alice and Bob cannot identify the source of the quantum bit error rate. It can arise from either an eavesdropper or a variation in the phase drift. 
         [0011]    It is also required that the polarisation of photons be stabilised. However, this presents difficulties as photons will generally be sent from Alice to Bob along a single mode fibre link and the polarisation of photons passing through this link will vary due to birefringence regions commonly existing in the single mode fibre. For example, variations in the temperature can cause the polarisation to vary randomly. It is necessary to be able to correct any rotation of the polarisation which has occurred in the fibre link because some of the components of Bob&#39;s equipment are polarisation sensitive and variations in the polarisation will again result in a higher error rate. Also, the bit rate of the system may decrease in equipment where polarisation beam splitters are used to ensure that photons pass through the short arm of one interferometer and the long arm of the other. 
       SUMMARY OF THE INVENTION 
       [0012]    The present invention attempts to address the above problems and, in a first aspect provides a quantum communication system comprising an emitter and a receiver, said emitter comprising encoding means and at least one photon source, said emitter being configured to pass a signal pulse and a reference pulse, which are separated in time, through said encoding means and output the signal pulse and the reference pulse, said reference pulse having a higher probability of containing more than one photon than said signal pulse, said receiver comprising decoding means and at least one detector for measuring said signal pulse and said reference pulse. 
         [0013]    By outputting a reference pulse which passes through the encoding means as well as the signal pulse, the reference pulse may be measured in order to determine variations in the system, for example, phase variations and polarisation variations. 
         [0014]    By outputting a reference pulse and a signal pulse through the same encoding means it is possible to output a reference pulse for each signal pulse. This allows any variations in phase or polarisation of the system to be monitored during transmission of a key. Thus, Alice and Bob can determine if an increase in the bit error rate is due to an eavesdropper or due to phase or polarisation drift. 
         [0015]    Further, preferably, the receiver comprises feedback means for altering a component of the receiver on the basis of the measured reference signal. For example, the component may be a means to alter the polarisation or phase of photons, specifically, the component may be a polarisation rotation, delay line or phase modulator. Thus, the system may be actively balanced or aligned during transmission of the key. 
         [0016]    Typically, the reference pulse will be 10 to 10,000 times stronger than the signal pulse. 
         [0017]    In a preferred embodiment, the encoding means are phase encoding means comprising an encoding interferometer with an entrance member connected to a long arm and a short arm, said long arm and said short arm being joined at their other ends by an exit member, one of the said arms having a phase modulator which allows the phase of a photon passing through that arm to be set to one of at least two values. 
         [0018]    Where the encoding means comprises an interferometer, the decoding means should also comprise a decoding interferometer, said decoding interferometer comprising an entrance member connected to a long arm and a short arm, said long arm and said short arm being joined at their other ends by an exit member, one of said arms having a phase modulator which allows the phase of a photon passing through that arm to be set to at least one of two values. 
         [0019]    Typically, the phase modulators will be provided in the short arms. However, the phase modulators may also be provided in the long arms of both interferometers. Only photons which have passed through the long arm of one interferometer and the short arm of the other are of use in communicating the key. In the four-state protocol, which is sometimes referred to as BB84, Alice modulates her phase modulator to one of four different values, corresponding to phase shifts of 0°, 90°, 80° and 270°. Phase 0° and 180° are associated with bits  0  and  1  in a first encoding basis, while 90° and 270° are associated with bits zero and one in a second encoding basis. The second encoding basis is chosen to be non-orthogonal to the first. 
         [0020]    Information may alternatively be sent using the B92 protocol where Alice applies phase shifts of 0° and 90° on her phase modulator randomly. Alice associates 0° with bit=0and 90° delay with bit=1. Bob applies 180° or 270° to his phase modulator randomly and associates 180° with bit=1 and 270° with bit=0. 
         [0021]    In order to increase the bit rate, it is preferable for the system to comprise polarisation directing means for directing photons which have passed through the long arm of the encoding interferometer through the short arm of the decoding interferometer and for directing photons which have passed through the short arm of the encoding interferometer through the long arm of the decoding interferometer. 
         [0022]    These means may be achieved by configuring the encoding interferometer to ensure that photons which have passed through the long arm of the interferometer exit the interferometer with a first polarisation and photons which have passed through the short arm of the interferometer exit the interferometer with a second polarisation. The first polarisation being orthogonal to the second polarisation direction. A polarisation beam splitter may then be provided as the entrance member to the decoding interferometer to direct photons with the first polarisation along the short arm of the decoding interferometer and photons with the second polarisation through the long arm of the decoding interferometer. 
         [0023]    The reference pulse and the signal pulse pass through the encoding means in the same manner. However, to avoid an eavesdropper obtaining information about the signal pulse from the reference pulse, the reference pulse is either not encoded as it passes through the interferometer, for example, the phase modulator is switched to a fixed encoding position for the reference pulse or the reference pulse is encoded in a different manner to that of the signal pulse. The coding of the reference pulse may be decided between Alice and Bob before transmission begins so that Bob can correctly measure the reference pulse. 
         [0024]    The receiver comprises at least one detector for measuring said signal pulse and the reference pulse. As the reference pulse and the signal pulse arrive at the receiver at different times, it is possible to use a single detector to monitor both the signal and reference pulses. However, this is not advantageous because typically, avalanche photo diodes are used as the detectors. When one of these detectors detects a photon, a large number of charge carriers are generated within the diode forming an easily detectable current. Some of these carriers may be localised at hetero-junctions or at trap states within the semiconductor. Carriers confined in these traps can have a lifetime of several microseconds. Therefore, the diode can only be used once the trapped carriers have decayed and thus the detector has a fixed sampling rate which is usually the limiting factor in the information bit rate of the system. Thus, although it is possible, it is not desirable to have the same detector detecting both the reference and signal pulses. 
         [0025]    Previously, a system has been described comprising polarisation directing means which ensures that photons which pass along the long path of one interferometer pass through the short path of the other interferometer. In such a system, where there is no variation in the polarisation due to the passage of photons through the fibres, a reference detector provided at an output of the exit member of the decoding interferometer would expect to just detect a single reference peak due to photons flowing along the long path of one interferometer and the short path of the other. However, if the polarisation of the photons is varied during their passage though the emitter and receiver or though the fibre link connecting the emitter and receiver, some photons will flow along both long paths through the interferometers and some photons will pass through both short paths of the two interferometers. Thus, an early satellite peak is seen due to photons which pass through the two short arms and a late satellite peak is seen due to photons which pass through the two long arms. Thus, the reference detector may be configured to monitor either the late or early satellite peak. The presence of either of these peaks indicates that the polarisation of the photons is being rotated as it passes through the fibres of the system. 
         [0026]    The reference pulse will be outputted from one of two ports of the exit member of the decoding interferometer. Typically, the exit member will be a fibre coupler. The phase of the encoding phase modulator and the phase of the decoding phase modulator may be set to ensure that the reference pulse is directed to the port which outputs to the reference detector. 
         [0027]    Preferably, the receiver comprises a polarisation rotator positioned in the photon path prior to the decoding means. The reference detector may be connected to a feedback circuit which controls the polarisation rotator in order to correct the polarisation directions prior to the decoding means. 
         [0028]    It is also desirable to correct for any rotations in the polarisation direction in systems which do not use polarisation in order to direct photons down the desired arms of the interferometers. One reason for this is that phase modulators are sensitive to the polarisation direction, variations in the polarisation may still increase the bit error rate. In systems which do not use polarisation directing means, photons in the emitter are generally outputted with just a single polarisation direction. 
         [0029]    This polarisation direction may be monitored by inserting a polarisation beam splitter before the decoding means in the receiver. The polarisation beam splitter is configured to pass photons with the desired polarisation and reflect photons with an orthogonal polarisation into a reference detector. Preferably, the reference detector is connected to a feedback circuit which is in turn connected to a polarisation controller provided in the photon path before the polarisation beam splitter. Thus, the polarisation controller may be used to correct the rotation of the polarisation to minimise the signal at the reference detector. 
         [0030]    The reference pulse may also be used to stabilise and monitor the phase of the system. The reference detector will be connected to one of the outputs of the decoding interferometer&#39;s exit member. The exit member will typically be a fibre coupler. If the phase of the system remains stable (except for the phase changes introduced by the phase modulators of the interferometers), then a constant count rate is expected at the reference detector. Any variation in the phase drift of the system will be manifested by a varying count rate. Thus, by monitoring the arrival time of the reference peaks, any variations in the count rate may be established. Preferably, integration means are provided so that the count rate may be integrated over a period of time in order to average statistical fluctuations in the arrival rate of the reference pulse. The integration time may typically be a fraction of a second, for example, 0.1 seconds. 
         [0031]    Preferably, the reference detector is connected to a feedback circuit which is in turn connected to a phase correcting means provided in one of the arms of the decoding interferometer. The phase correcting means may be provided by a variable fibre stretcher or a variable air gap, etc. Alternatively, the phase correcting means may be provided by an adjustment means for the phase modulator of the receiver to allow the phase to be balanced. In other words, feedback is used to re-calibrate the zero points of the phase modulators. Thus, the interferometer phase may be balanced using the results from the reference detector. 
         [0032]    The phase reference detector may be used to monitor the central reference pulse to monitor variations in the phase alone. However, if polarisation direction control means are provided in the system, then the reference monitoring may be used to monitor either the early or late satellite peaks in order to calibrate both the polarisation and the phase at the same time. 
         [0033]    In the B92 communication protocol, it is only necessary to use one detector for the signal peak. Therefore, the system may be arranged with a signal pulse detector connected to one output of the exit member and reference pulse detector connected to the other output of the exit member. 
         [0034]    In the BB84 protocol, two signal pulse detectors are required, one connected to one output of the exit member and the other connected to the other output of the exit member. In this arrangement, both the reference detector and a signal pulse detector may be connected to the same output of the exit member. A fibre couple may be provided to direct photons from the port of the exit member into either the reference detector or the signal pulse detector. 
         [0035]    The reference pulse will contain more photons than the signal pulse. Therefore, the detector coupler is typically a coupling ratio of 95/5 is used with the 95% port attached to the signal pulse detector and the 5% port attached to the reference pulse detector. This is chosen to ensure that the reference pulse detector does not reduce the photon count rate of the signal pulses significantly at the signal pulse detector. As the reference pulse will contain many photons, even with a high coupling ratio, the reference detector should still receive some photons of the reference pulse. As the reference pulse and the signal pulse arrive at the detectors at different times, the signal pulse detector can be switched off when the reference pulse arrives and is thus not affected by the reference pulse photons. 
         [0036]    In order to create both the signal pulse and reference pulse, the emitter may comprise a separator for dividing photon pulses emitted from a photon source into a signal pulse and a reference pulse. 
         [0037]    The separator may comprise an entrance member, for example, a fibre coupler, connected to a long arm and short arm, said long arm and said short arm being connected at the other ends to an exit member. Typically, the exit member will also be a fibre coupler. 
         [0038]    As the reference pulse needs to be larger than the signal pulse, the coupler may be an asymmetric coupler and may allow 90 to 99.99% of the input along one arm to form the reference pulse. The reference pulse may either just proceed the signal pulse or may be delayed after the signal pulse. 
         [0039]    The exit member of the separator is also preferably a coupler. The coupler will have two output ports and the photons will exit through just one of the output ports into the encoding interferometer or other encoding means. In an alternative arrangement, the entrance member of the encoding interferometer is provided by the exit member of the separator. 
         [0040]    Typically, the separator is configured to introduce a time delay of about 10 ns. 
         [0041]    Alternately, the reference pulse and the signal pulse may be generated by separate sources. For example, a laser diode may be used to generate the reference pulse and a dedicated single photon source may be used to generate the signal pulse. Delay means will be provided in order to delay the reference pulse with respect to the signal pulse. 
         [0042]    As previously explained, the detectors for both the reference pulse and the signal pulse may be avalanche photo diodes. Preferably, the receiver comprises a gating means in order to keep the detectors in an on mode only for the time when they expect to receive a signal pulse or a reference pulse. 
         [0043]    In a second aspect, the present invention provides an emitter for a quantum communication system, said emitter comprising encoding means and at least one photon source, said emitter being configured to pass a signal pulse and a reference pulse, which are separated in time, through said encoding means and output the signal pulse and the reference pulse, said reference pulse having a higher probability of containing more than one photon than said signal pulse. 
         [0044]    In a third aspect, the present invention provides a receiver for a quantum communication system, said receiver comprising decoding means and at least one detector for measuring a signal pulse and a reference pulse, said signal pulse and said reference pulse being separated in time and said reference pulse having a higher probability of containing more than one photon than said signal pulse. 
         [0045]    In a fourth aspect, the present invention provides a method of communicating photon pulses from an emitter to a receiver, comprising generating a signal pulse and a reference pulse separated in time in an emitter; passing both the signal pulse and the reference pulse through the same encoding means in said emitter and sending said pulses to a receiver; and measuring both the signal pulse and the reference pulse in said receiver. 
         [0046]    In a fifth aspect, the present invention provides a method of outputting photons from an emitter, the method comprising generating a signal pulse and a reference pulse separated in time in an emitter; passing both the signal pulse and the reference pulse through the same encoding means in said emitter and outputting said pulses. 
         [0047]    The present invention will now be described with reference to the following non-limiting embodiments in which: 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0048]      FIG. 1   a  is a known quantum communication system and  FIG. 1   b  is a plot of the probability of a photon arriving at either of the detectors of the system of  FIG. 1   a  against time; 
           [0049]      FIG. 2   a  is a plot of clock signal against time for the system of  FIG. 1   a,    FIG. 2   b  is a plot of the output of the signal laser against time for the system of  FIG. 1   a,    FIG. 2   c  is a plot of the probability of the photon arriving at either of Bob&#39;s detectors in the system of  FIG. 1   a,    FIG. 2   d  is a plot of the modulator voltage against time for the systems of  FIG. 1   a  and  FIG. 2   e  is a plot of detector gating bias against time for the system of  FIG. 1   a;    
           [0050]      FIG. 3   a  is a communication system in accordance with the preferred embodiment of the present invention and  FIG. 3   b  is a plot of the probability of a photon arriving at either of the detectors in the system of  FIG. 3   a  against time; 
           [0051]      FIG. 4   a  is a plot of the clock signal against time for the system of  FIG. 3   a ,  FIG. 4   b  is a plot of the signal laser output against time for the system of  FIG. 3   a ,  FIG. 4   c  is a plot of the probability of a photon arriving at either the reference detector or the signal detector of  FIG. 3   a ,  FIG. 4   d  is a plot of the modulator bias against time applied to the interferometers of  FIG. 3 ,  FIG. 4   e  is a plot of the gating voltage for the signal detector against time and  FIG. 4   f  is a plot of the gating voltage applied to the reference detector of the system of  FIG. 3   a;    
           [0052]      FIG. 5   a  is a quantum communication system in accordance with a preferred embodiment of the present invention where the exit member of the separator provides the entrance member for the interferometer and  FIG. 5   b  is a plot of the probability of a photon arriving at either of the detectors of the system of  FIG. 5   a  against time; 
           [0053]      FIG. 6   a  is a quantum communication system in accordance with a preferred embodiment of the present invention optimised for use with the BB84 protocol and  FIG. 6   b  is plot of the probability of a photon arriving at any of the three detectors of the system of  FIG. 6   a  against time; 
           [0054]      FIG. 7   a  is quantum communication system in accordance with a preferred embodiment of the present invention having a separate signal pulse source and reference pulse source and  FIG. 7   b  is a plot of the probability of a photon arriving at either of the detectors of the system of  FIG. 7   a  against time; 
           [0055]      FIG. 8   a  is a quantum communication system in accordance with a preferred embodiment of the present invention showing a variation on the source arrangement of the system of  FIG. 7   a  and  FIG. 8   b  is a plot of the probability of a photon arriving at either of the detectors of the system of  FIG. 8   a  against time; and 
           [0056]      FIG. 9   a  is a quantum communication system in accordance with a preferred embodiment of the present invention where photons transmitted between the emitter and receiver have the same polarisation and  FIG. 9   b  is a plot of the probability of a photon arriving at either of the two detectors connected to the interferometer of the receiver of  FIG. 9   a  against time. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0057]      FIG. 1   a  shows a prior art quantum cryptography system based upon phase encoding using a polarisation sensitive fibre interferometer. 
         [0058]    The sender “Alice”  101  sends encoded photons to receiver “Bob” over optical fibre  105 . 
         [0059]    Alice&#39;s equipment  101  comprises a signal laser diode  107 , a polarisation rotator  108  configured to rotate the polarisation of pulses from signal laser diode  107 , an imbalanced fibre Mach-Zender interferometer  133  connected to the output of polarisation rotator  108 , an attenuator  137  connected to the output of the interferometer  133 , a bright clock laser  102 , a wavelength division multiplexing (WDM) coupler  139  coupling the output from attenuator  137  and clock laser  102  and bias electronics  109  connected to said signal laser diode  107  and clock laser  102 . 
         [0060]    The interferometer  133  comprises an entrance coupler  130 , one exit arm of entrance coupler  130  is joined to long arm  132 , long arm  132  comprises a loop of fibre  135  designed to cause an optical delay, the other exit arm of entrance coupler  130  is joined to a short arm  131 , short arm  131  comprises phase modulator  134  an exit polarising beam combiner  136  is connected to the other ends of long arm  132  and short arm  131 . All components used in Alice&#39;s interferometer  133  are polarisation maintaining. 
         [0061]    During each clock signal, the signal diode laser  107  outputs one optical pulse. The signal diode laser  107  is connected to biasing electronics  109  which instruct the signal diode laser  107  to output the optical pulse. The biasing electronics are also connected to clock laser  102 . 
         [0062]    The linear polarisation of the signal pulses outputted by diode laser  107  is rotated by a polarisation rotator  108  so that the polarisation of the pulse is aligned to be parallel to a particular axis of the polarisation maintaining fibre (usually the slow axis) of the entrance coupler  130  of the interferometer  133 . Alternatively the polarisation rotator  108  may be omitted by rotating the signal laser diode  107  with respect to the axes of the entrance polarisation maintaining fibre coupler  130 . 
         [0063]    After passing through the polarisation from rotator (if present) the signal pulses are then fed into the imbalanced Mach-Zender interferometer  133  through a polarisation maintaining fibre coupler  130 . Signal pulses are coupled into the same axis (usually the slow axis) of the polarisation maintaining fibre, of both output arms of the polarisation maintaining fibre coupler  130 . One output arm of the fibre coupler  130  is connected to the long arm  132  of the interferometer while the other output arm of the coupler  130  is connected to the short arm  131  of the interferometer  133 . 
         [0064]    The long arm  132  of the interferometer  133  contains an optical fibre delay loop  135 , while the short arm  131  contains a fibre optic phase modulator  134 . The fibre optic phase modulator  134  is connected to biasing electronics  109  which will be described in more detail later. The length difference of the two arms  131  and  132  corresponds to an optical propagation delay of t delay . Typically the length of the delay loop  135  may be chosen to produce a delay t delay ˜5 ns. Thus, a photon travelling through the long arm  132  will lag that travelling through the short arm  131  by a time of t delay  at the exit  136  of the interferometer  133 . 
         [0065]    The two arms  131 ,  132  are combined together with a polarisation beam combiner  136  into a single mode fibre  138 . The fibre inputs of the polarisation beam combiner  136  are aligned in such a way that only photons propagating along particular axes of the polarisation maintaining fibre, are output from the combiner  136 . Typically, photons which propagate along the slow axis or the fast axis are output by combiner  136  into fibre  138 . 
         [0066]    The polarising beam combiner  136  has two input ports, an in-line input port and a 90° input port. One of the input ports is connected to the long arm  132  of the interferometer  133  and the other input port is connected to the short arm  131  of the interferometer  133 . 
         [0067]    In this example, only photons polarised along the slow axis of the in-line input fibre of the in-line input port are transmitted by the polarising beam combiner  136  and pass into the fibre  138 . Photons polarised along the fast axis of the in-line input fibre of the input port are reflected and lost. 
         [0068]    Meanwhile, at the 90° input port of the beam coupler  136 , only photons polarised along the slow axis of the 90° input fibre are reflected by the beam combiner  136  and pass into the output port, while those polarised along the fast axis will be transmitted out of the beam combiner  136  and lost. 
         [0069]    This means that the slow axis of one of the two input fibres is rotated by 90° relative to the output port. Alternatively the polarisation may be rotated using a polarisation rotator (not shown) before one of the input ports of the polarising beam combiner ( 136 ). 
         [0070]    Thus, photon pulses which passed through the long  132  and short arms  131  will have orthogonal polarisations. 
         [0071]    The signal pulses are then strongly attenuated by the attenuator  137  so that the average number of photons per signal pulse μ&lt;&lt;1. 
         [0072]    The signal pulses which are outputted by the combiner  136  into single mode fibre  138  are then multiplexed with a bright laser clock source  102  at a different wavelength using a WDM coupler  139 . The multiplexed signal is then transmitted to the receiver Bob  103  along an optical fibre link  105 . The biasing electronics  109  synchronises the output of the clock source  102  with the signal pulse. 
         [0073]    Bob&#39;s equipment  103  comprises WDM coupler  141 , a clock recovery unit  142  connected to an output of coupler  141 , a polarisation controller  144  connected to the other output of WDM coupler  141 , an imbalanced Mach-Zender interferometer  156  connected to the output of polarisation controller  144 , two single photon detectors A  161 , B  163  connected to the output arms of interferometer  156  and biasing electronics  143  connected to the detectors  161 ,  163 . Bob&#39;s interferometer  156  contains an entrance polarising beam splitter  151  connected to both: a long arm  153  containing a delay loop  154  and a variable delay line  157 ; and a short arm  152  containing a phase modulator  155 . The long arm  153  and short arm  152  are connected to an exit polarisation maintaining 50/50 fibre coupler  158 . All components in Bob&#39;s interferometer  156  are polarisation maintaining. 
         [0074]    Bob first de-multiplexes the transmitted signal received from Alice  101  via fibre  105  using the WDM coupler  141 . The bright clock laser  102  signal is routed to an optical receiver  142  to recover the clock signal for Bob  103  to synchronise with Alice  101 . 
         [0075]    The signal pulses which are separated from the clock pulses by WDM coupler  141  are fed into a polarisation controller  144  to restore the original polarisation of the signal pulses. This is done so that signal pulses which travelled the short arm  131  in Alice&#39;s interferometer  133 , will pass the long arm  153  in Bob&#39;s interferometer  156 . Similarly, signal pulses which travelled through the long arm  132  of Alice&#39;s interferometer  133  will travel through the short arm  152  of Bob&#39;s interferometer. 
         [0076]    The signal then passes through Bob&#39;s interferometer  156 . An entrance polarising beam splitter  151  divides the incident pulses with orthogonal linear polarisations. The two outputs of the entrance polarisation beam splitter  151  are aligned such that the two output polarisations are both coupled into a particular axis, usually the slow axis, of the polarisation maintaining fibre. This ensures that signal pulses taking either arm will have the same polarisation at the exit 50/50 polarisation maintaining coupler  158 . The long arm  153  of Bob&#39;s interferometer  156  contains an optical fibre delay loop  154  and a variable fibre delay line  157 , and the short arm  152  contains a phase modulator  155 . The two arms  152 ,  153  are connected to a 50/50 polarisation maintaining fibre coupler  158  with a single photon detector A  161 , B  163  attached to each output arm. Due to the use of polarising components, there are, in ideal cases, only two routes for a signal pulse travelling from the entrance of Alice&#39;s interferometer to the exit of Bob&#39;s interferometer:
   i. Alice&#39;s Long Arm  132 −Bob&#39;s Short Arm  152  (L-S) and   ii. Alice&#39;s Short Arm  131 −Bob&#39;s Long Arm  153  (S-L).   
 
         [0079]    The variable delay line  157  at Bob&#39;s interferometer  156  is adjusted to make the propagation time along routes (i) and (ii) almost equal, within the signal laser coherence time which is typically a few picoseconds for a semiconductor distributed feed back (DFB) laser diode, and thereby ensure interference of the two paths. Bob achieves this by adjusting the variable fibre delay line  157  prior to key transfer. 
         [0080]      FIG. 1   b  is a plot of probability of a photon arriving at either of detectors A  161 , B  163  against time. The early satellite signal peak shown in  FIG. 1   b  is due to photons travelling through Alice&#39;s short arm  131  to Bob&#39;s short arm  152 , and the late satellite signal peak is due to those travelling through Alice&#39;s long arm  132  to Bob&#39;s long arm  153 . Photons taking these non-ideal routes are due to incomplete polarisation control by the polarisation controller  144 , and they reduce quantum key distribution rate. So, Bob has to adjust the polarisation controller  144  prior to key distribution to minimise the strength of the satellite signal pulses in  FIG. 1   b.    
         [0081]    By controlling the voltages applied to their phase modulators  134 ,  155 , Alice and Bob determine in tandem whether paths (i) and (ii) undergo constructive or destructive interference at detectors A  161  and B  163 . The phase modulators  134 ,  155  are connected to respective biasing means  109  and  143  to ensure synchronisation. 
         [0082]    The variable delay line  157  can be set such that there is constructive interference at detector A  161  (and thus destructive interference at detector B  163 ) for zero phase difference between Alice and Bob&#39;s phase modulators. Thus for zero phase difference between Alice&#39;s and Bob&#39;s modulators and for a perfect interferometer with 100% visibility, there will be a negligible count rate at detector B  163  and a finite count rate at A  161 . 
         [0083]    If, on the other hand, the phase difference between Alice and Bob&#39;s modulators  134 ,  155  is 180°, there should be destructive interference at detector A  161  (and thus negligible count rate) and constructive at detector B  163 . For any other phase difference between their two modulators, there will be a finite probability that a photon may output at detector A  161  or detector B  163 . 
         [0084]    In the four-state protocol, which is sometimes referred to as BB84, Alice sets the voltage on her phase modulator to one of four different values, corresponding to phase shifts of 0°, 90°, 180°, and 270°. Phase 0° and 180° are associated with bits  0  and  1  in a first encoding basis, while 90° and 270° are associated with 0 and 1 in a second encoding basis. The second encoding basis is chosen to be non-orthogonal to the first. The phase shift is chosen at random for each signal pulse and Alice records the phase shift applied for each clock cycle. 
         [0085]    Meanwhile Bob randomly varies the voltage applied to his phase modulator between two values corresponding to 0° and 90°. This amounts to selecting between the first and second measurement bases, respectively. Bob records the phase shift applied and the measurement result (i.e photon at detector A  161 , photon at detector B  163 , photon at detector A  161  and detector B  163 , or no photon detected) for each clock cycle. 
         [0086]    In the BB84 protocol, Alice and Bob can form a shared key by communicating on a classical channel after Bob&#39;s measurements have taken place. Bob tells Alice in which clock cycles he measured a photon and which measurement basis he used, but not the result of the measurement. Alice then tells Bob the clock cycles in which she used the same encoding basis and they agree to keep only those results, as in this case Bob will have made deterministic measurements upon the encoded photons. This is followed by error correction, to remove any errors in their shared key, and privacy amplification to exclude any information known to an eavesdropper. 
         [0087]    The system in  FIG. 1   a  is also suitable for implementing the two-state protocol known as B92. In this case only one detector is needed on one output arm of Bob&#39;s interferometer  156 . The arm lengths are calibrated so that for zero phase delay the photon rate into the detector is maximum (constructive interference). For the B92 protocol Alice applies phase shifts of 0 and 90° on her phase modulator randomly. Alice associates 0 phase delay with bit=0, and 90° phase delay with bit=1. Bob applies 180″ or 270° to his phase modulator randomly, and associates 180° with bit=1 and 270° with bit=0. After Bob&#39;s detections, he tells Alice in which clock cycle he detected a photon and they keep these bits to form a shared sifted key. They then perform error correction and privacy amplification upon the sifted key. 
         [0088]    Paragraph moved to introduction  FIG. 2  shows plots of the timing schemes which may be used for the prior art quantum cryptographic system of  FIG. 1   a.    
         [0089]      FIG. 2   a  shows the clock signal produced by the biasing electronics  109  as a function of time. The clock has a repetition period T clock . The rising edge of the clock signal is used to synchronise Alice&#39;s signal laser  107 , Alice&#39;s phase modulator  134 , Bob&#39;s phase modulator  155  and Bob&#39;s detectors A  161  and B  163 . 
         [0090]    The output of the signal laser  107  is shown in  FIG. 2   b . For each clock period, the signal laser  107  is triggered to produce one pulse of width d laser . 
         [0091]      FIG. 2   c  plots the probability of a photon arriving at Bob&#39;s detectors A  161  and B  163  (i.e. sum of the probabilities of a photon arriving at detector A or detector B) as a function of time. Each signal pulse now has a width of d bob , which may be greater than d laser  due to dispersion in the fibres of the system. Three arrival windows can be seen for each clock cycle. In order of arrival time, these correspond to photons taking the short-short, long-short or short-long and long-long paths through Alice&#39;s-Bob&#39;s interferometer as described with reference to  FIG. 1   b.  Thus the first and second, as well as the second and third pulses are separated by a time delay t delay . The short-short and long-long paths are due to imperfect polarisation beam splitting at the entrance  151  of Bob&#39;s interferometer  156 . 
         [0092]    Only photons arriving in the central window of each clock cycle undergo interference and are thus of interest. The single photon detectors A  161  and B  163  are gated to be on only when the central pulse arrives in each clock cycle, as shown in  FIG. 2   e . This is achieved by biasing the detector with a voltage V det2  for which it is in an active state for a short duration d det  during each clock cycle when the central pulse arrives. The bias voltage duration d det  is typically chosen to be longer than d bob  and is typically a few nanoseconds. At other times the detector is held at a voltage V det1  for which it is inactive. 
         [0093]    For a single photon detector based upon an avalanche photodiode, time gating can be achieved by choosing V det2  to be greater than the avalanche breakdown voltage of the diode and V det1  to be less than the breakdown voltage. An avalanche can only be triggered when the diode bias exceeds the breakdown threshold. 
         [0094]    The avalanche process generates a large number of charge carriers within the diode forming an easily detectable current. Some of these carriers may be localised at hetero-junctions or at trap states within the semiconductor. Carriers confined in such traps can have a lifetime of several microseconds. If the diode is biased above the avalanche breakdown threshold, before the trapped carriers have decayed, there is a possibility that a trapped carrier could be released and then trigger another avalanche. The resultant spurious signal is called an ‘afterpulse’. 
         [0095]    To minimise the rate of afterpulse counts, the APD is biased inactive for a sufficiently long time to allow most of the trapped charge to decay. Thus in a conventional quantum cryptography system, afterpulsing limits the minimum period between APD detection gates and thus the minimum clock period T clock . Typically T clock ˜1 μs. 
         [0096]    Alice&#39;s and Bob&#39;s phase modulators  134  and  155  are driven by separate voltage pulse generators. The voltage pulse generators are also synchronised with the clock signal (of  FIG. 2   a ), as shown in  FIG. 2   d.    
         [0097]    During the pass of each signal pulse through the phase modulator, the pulse generator outputs one of a number of voltage levels, V mod1 , V mod2  etc. For the BB84 protocol, for instance, Alice applied one of four different voltage levels, corresponding to phase shifts of 0°, 90°, 180°, and 270°. Meanwhile Bob applies two voltage levels to his modulator corresponding to phase shifts of 0° and 90°. Alice and Bob vary the applied phase shifts for each signal pulse randomly and independently of one-another.  FIG. 3   a  shows an apparatus for quantum cryptography with active stabilisation. 
         [0098]    Alice and Bob&#39;s equipment is similar to that described with reference to  FIG. 1   a.  As described with reference to  FIG. 1   a,  Alice  201  sends photons to Bob  203  along fibre  205 . 
         [0099]    Alice&#39;s equipment  201  comprises a signal laser diode  207 , a polarisation rotator  208  connected to the output of said signal laser diode  207 , a signal/reference pulse separator  224  connected to the output of said polarisation rotator  208 , an imbalanced fibre Mach-Zender interferometer  233  for encoding photons connected to the output of the signal/reference pulse separator  224 , an attenuator  237  connected to the output of the interferometer  233 , a bright clock laser  202 , a wavelength division multiplexing (WDM) coupler  239  connected to both the output of the attenuator  237  and the bright clock laser  202  and bias electronics  209 . The biasing electronics  209  are connected to both the clock laser  202  and the signal laser  207 . 
         [0100]    The signal/reference pulse separator  224  comprises an entrance fibre optic coupler  220  with a first output connected to a long arm  222  with a loop of fibre  223  designed to cause an optical delay and short arm  221 . The separator  224  further comprises an exit fibre optic coupler  225  combining two arms  221  and  222 . All fibre in separator  224  is polarisation maintaining. 
         [0101]    The encoding interferometer  233  is identical to that described in  FIG. 1   a  and comprises an entrance coupler  230 , a long arm  232  with a loop of fibre  235  designed to cause an optical delay, a short arm  231  with a phase modulator  234 , and an exit polarising beam combiner  236 . All components used in Alice&#39;s interferometer  233  are polarisation maintaining. 
         [0102]    During each clock signal, the signal laser diode laser  207  outputs one optical pulse under the control of biasing electronics  209 . 
         [0103]    The polarisation of the signal pulses is rotated by a polarisation rotator  208  so that the polarisation is aligned to be parallel to a particular axis of the polarisation maintaining fibre, usually the slow axis, of the entrance coupler  220  of separator  224 . Alternatively the polarisation rotator  208  may be omitted by rotating the signal laser diode  207  with respect to the axes of the entrance coupler  220  of separator  224 . 
         [0104]    The signal pulses are then fed into the signal/reference pulse separator  224  through polarisation maintaining fibre coupler  220 . Signal pulses are coupled into the same axis, usually the slow axis of the polarisation maintaining fibre, from both output arms of the polarisation maintaining fibre coupler  220 . 
         [0105]    The long arm  222  of the signal/reference pulse separator  224  contains an optical fibre delay loop  223 . The length difference of the two arms  221  and  222  corresponds to an optical propagation delay of t reference . Typically the length of the delay loop  223  may be chosen to produce a delay t reference ˜10 ns. A photon travelling through the long arm  222  will lag that travelling through the short arm  221  by a time of t reference  at the exit coupler  225  of the splitter  224 . 
         [0106]    The two arms  221  and  222  are combined together with an exit polarisation maintaining fibre optic coupler  225 . One output is connected into one input of the encoding Mach-Zender interferometer  233 . 
         [0107]    Coupling ratio of two couplers  220  and  225  can be either fixed or variable. The ratios are chosen so that the reference and signal pulses have unequal intensities. Typically, before entering the encoding interferometer  233 , the later reference pulse is 10-10000 times stronger than the earlier signal pulse. For example, the entrance coupler  220  may be asymmetric so as to allow 90% to 99.99% of the input into arm  221  and the exit coupler  225  may be a 50/50 coupler. Alternatively, both the entrance  220  and exit couplers  225  may be 50/50 couplers and an appropriate attenuator placed in arm  221 . 
         [0108]    Properties of the signal and reference pulses are exactly the same, for example polarisation, wavelength etc, except of course for their intensity and time of injection into the interferometer  233 . 
         [0109]    The signal and reference pulses are then fed into the imbalanced Mach-Zender interferometer  233  through a polarisation maintaining fibre coupler  230 . Signal and reference pulses are coupled into the same axis, usually the slow axis of the polarisation maintaining fibre, from both output arms of the polarisation maintaining fibre coupler  230 . 
         [0110]    The long arm  232  of the interferometer  233  contains an optical fibre delay loop  235 , while the short arm  231  contains a fibre optic phase modulator  234 . The length difference of the two arms  231  and  232  corresponds to an optical propagation delay of t delay . Typically the length of the delay loop  235  may be chosen to produce a delay t delay ˜5 ns. A photon travelling through the long arm  232  will lag that travelling through the short arm  231  by a time of t delay  at the exit  236  of the interferometer  233 . 
         [0111]    The two arms  231 ,  232  are combined together with a polarisation beam combiner  236  into a single mode fibre  238 . The fibre inputs of the polarisation beam combiner  236  are aligned in such a way that only photons propagating along particular axes of the polarisation maintaining fibre are output from the combiner  236 . Typically, photons which propagate along the slow axis or the fast axis are output by combiner  236  into single mode fibre  238 . 
         [0112]    The polarising beam combiner  236  has two input ports, an in-line input port and a 90° input port. One of the input ports is connected to the long arm  232  of the interferometer  233  and the other input port is connected to the short arm  231  of the interferometer  233 . 
         [0113]    Only photons polarised along the slow axis of the in-line input fibre of the in-line input port are transmitted by the polarising beam combiner  236  and pass into the fibre  238 . Photons polarised along the fast axis of the in-line input fibre of the input port are reflected and lost. 
         [0114]    Meanwhile, at the 90° input port of the beam coupler  236 , only photons polarised along the slow axis of the 90° input fibre are reflected by the beam combiner  236  and pass into the output port, while those polarised along the fast axis will be transmitted out of the beam combiner  236  and lost. 
         [0115]    This means that the slow axis of one of the two input fibres is rotated by 90° relative to the output port. Alternatively the polarisation may be rotated using a polarisation rotator before one of the input ports of the polarising beam combiner. 
         [0116]    Thus, photon pulses which passed through the long  232  and short arms  231  will have orthogonal polarisations. 
         [0117]    Both the signal and reference pulses are then strongly attenuated by the attenuator  237  so that the average number of photons per pulse μ&lt;&lt;1 for the signal pulses. The reference pulses are typically 10-1000 stronger than the signal pulses, and do not have to be attenuated to single photon level as information is only encoded upon signal pulses. 
         [0118]    The attenuated pulses are then multiplexed with a bright laser clock source  202  at a different wavelength using a WDM coupler  239 . The multiplexed signal is then transmitted to the receiver Bob  203  along an optical fibre link  205 . 
         [0119]    The clock may also be delivered in other ways. For example Alice may multiplex the signal pulses with a bright clock laser pulse at the same or different wavelength which is delayed relative to the start of the clock cycle and which does not coincide with the signal pulses. Alternatively the clock signal may be sent on a separate optical fibre. Alternatively, synchronisation may be achieved by using a timing reference. 
         [0120]    Bob&#39;s equipment  203  comprises WDM coupler  241 , a clock recovery unit  242  connected to one output of said WDM coupler  241 , a polarisation controller  244  connected to the other output of said WDM coupler  241 , an imbalanced Mach-Zender interferometer  256  connected to the output of polarisation controller  244 , two single photon detectors R  261 , B  263  connected to the two outputs of interferometer  256  and biasing electronics  243 . 
         [0121]    Bob&#39;s interferometer  256  contains an entrance polarising beam splitter  251 , a long arm  253  containing a delay loop  254  and a variable delay line  257  is connected to an output of beam splitter  251 , a short arm  252  containing a phase modulator  255  is connected to the other output of said beam splitter  251 , and an exit polarisation maintaining 50/50 fibre coupler  258  coupling the output from the long  253  and short  252  arms. All components in Bob&#39;s interferometer  256  are polarisation maintaining. 
         [0122]    Bob first de-multiplexes the transmitted signal received from fibre  205  using the WDM coupler  241 . The bright clock laser  202  signal is routed to an optical receiver  242  to recover the clock signal for Bob to synchronise with Alice. 
         [0123]    If Alice delivers the clock using an alternative method, Bob will recover the clock accordingly. If Alice sends the clock signal as a single bright pulse within each clock cycle which is delayed relative to signal pulses then Bob may use an imbalanced coupler, such as 90/10, to extract a fraction of the combined signal which is measured with a photo-diode. A clock pulse is then recovered if the incident intensity is above an appropriately set threshold. Alternatively Bob may detect the clock on a separate fibre or using a timing reference. 
         [0124]    The signal pulses which are separated from the clock pulses by WDM coupler  241  are fed into a polarisation controller  244  to restore the original polarisation of the signal pulses. This is done so that signal pulses which travelled the short arm  231  in Alice&#39;s interferometer  233 , will pass the long arm  253  in Bob&#39;s interferometer  256 . Similarly, signal pulses which travelled through the long arm  232  of Alice&#39;s interferometer  233  will travel through the short arm  252  of Bob&#39;s interferometer  256 . 
         [0125]    The signal/reference pulses from signal laser  207  then pass through Bob&#39;s interferometer  256 . An entrance polarising beam splitter  251  divides the incident pulses with orthogonal linear polarisations. The two outputs of the entrance polarisation beam splitter  251  are aligned such that the two output polarisations are both coupled into a particular axis, usually the slow axis, of the polarisation maintaining fibre. This ensures that signal pulses taking either arm will have the same polarisation at the exit 50/50 polarisation maintaining coupler  258 . The long arm  253  of Bob&#39;s interferometer  256  contains an optical fibre delay loop  254  and a variable fibre delay line  257 , and the short arm  252  contains a phase modulator  255 . The two arms  252 ,  253  are connected to a 50/50 polarisation maintaining fibre coupler  258  with a single photon detector R  261 , B  263  attached to each output arm. 
         [0126]    Due to the use of polarising components, there are, in ideal cases, only two routes for a signal pulse travelling from the entrance of Alice&#39;s encoding interferometer  233  to the exit of Bob&#39;s interferometer  256 :
   i. Alice&#39;s Long Arm  232 −Bob&#39;s Short Arm  252  (L-S) and   ii. Alice&#39;s Short Arm  231 −Bob&#39;s Long Arm  253  (S-L).   
 
         [0129]    The variable delay line  257  at Bob&#39;s interferometer  256  is adjusted to make the propagation time along routes (i) and (ii) almost equal, within the signal laser coherence time which is typically a few picoseconds for a semiconductor distributed feed back (DFB) laser diode, and thereby ensure interference of the two paths. 
         [0130]    The variable fiber delay line  257  can either be an airgap, or a fibre stretcher, driven by a piezo-electric actuator. Alternatively, the two delays can be balanced by carefully controlling the length of fibre in Alice&#39;s  233  and Bob&#39;s  256  interferometers. Fine adjustment of the length of the two optical paths can be achieved through the calibration of zero phase delay in the two modulators  234 ,  255 . 
         [0131]    It is important that the central arrival time window of the signal pulses at single photon detectors do not overlap temporally with any arrival windows of the reference pulses. Otherwise, interference visibility will decrease. This can be guaranteed by carefully choosing the lengths of the delay loops  223 ,  235  to ensure t delay &lt;t reference . 
         [0132]    The references pulses are used to actively monitor and stabilise the phase drift of Alice-Bob&#39;s encoding interferometer. The detector R can be a single photon detector. It is gated to be on only upon the central arrival time of the reference peak and measure the count rate. If the system were perfectly stable, the counting rate is constant. Any variation in phase drift will be manifested by a varying counting rate. Bob uses any variation in the count rate measured by the reference detector R  261  as a feedback signal to the variable delay line  257 . Thus the optical delay is adjusted to stabilise the counting rate in the reference detector, and compensate any phase drifts of Alice or Bob&#39;s interferometers. 
         [0133]    Bob can avoid using the delay line  257 . The count rate measured by the reference detector R 261  can be used a feedback signal to the phase modulator. The DC-bias applied to the phase modulator is then varied to stabilising the counting rate, and compensate any phase drifts of Alice or Bob&#39;s interferometers. 
         [0134]    It is most convenient to maintain the reference detector with a minimum count rate. In this case, destructive interference is maintained at the reference detector R  261 . 
         [0135]    The reference detector R  261  and associated electronics should integrate the count rate over a certain period of time in order to average statistical fluctuation in the arrival rate of the reference photons. The integration time may typically be a fraction of a second, for example, 0.1 second. Such feedback times are sufficient since the phase drift of the Alice and Bob&#39;s interferometers occurs over much longer time scales. For highly unstable environment, much shorter feedback times, for example, 0.1 ms, may be employed. Alternatively, the feedback signal may be used to recalibrate the zero point of both phase modulators as described above. This may be done by varying the DC bias applied to modulators  255  and  234 . 
         [0136]    The feedback electronics may also condition system for sudden shocks to the system, such as a sudden change in temperature. If a sudden change in count rate is detected in the reference detector R  261 , the results in the signal detector B  263  can be ignored until the system regains stability. 
         [0137]    The references pulses are also used to actively monitor and stabilise the polarisation drift of photons. The two satellites peaks of the reference peak in  FIG. 3   b  are due to imperfect polarisation control by the controller  244  and therefore imperfect polarisation beam splitting of the entrance polarisation beam splitter  251  of Bob&#39;s interferometer  256 . The early satellite peak arises from the short arm  231  of Alice&#39;s encoding interferometer  233  to Bob&#39;s Short Arm  252 , and the late satellite peak arises from the long arm  232  of Alice&#39;s encoding interferometer  233  to Bob&#39;s long arm  253 . By gating the reference detector R  261  to detect during the arrival of one of the satellite peaks and measure the photon counting rate, Bob can monitor the drift in the polarisation of the photons and actively stabilise it by feeding the measurement result back into the polarisation controller  244 . The polarisation controller  244  rotates the polarisation of photons so as to minimise the count rate of the satellite peak of the reference pulse in the reference detector R  261 . 
         [0138]    The reference detector R  261  should integrate photon counts over a certain period of time in order to reduce statistical fluctuation. The integration time can again be as short as a fraction of a second, for example, 0.1 second. This is typically much faster than the time scale over which the polarisation drifts. Much shorter integration time can be chosen for system operates in unstable conditions. 
         [0139]    The system in  FIG. 3   a  is suitable for implementing the two-state protocol known as B92. In this case only one detector is needed on one output arm of Bob&#39;s interferometer  256  for detecting encoded single photons. As the arm lengths are stabilised so that for zero phase delay the photon rate into the detector R  261  is minimum, and the photon rate into the detector B  263  is maximum. 
         [0140]    For the B92 protocol Alice applies phase shifts of either 0 or 90° on her phase modulator  234  to the signal pulses. Alice associates 0 phase delay with bit=0, and 90° phase delay with bit=1. Bob applies either 180° or 270° to his phase modulator  255 , and associates 180° with bit=1 and 270° with bit=0. After Bob&#39;s detections, he tells Alice in which clock cycle he detected a photon and they keep these bits to form a shared sifted key. They then perform error correction and privacy amplification upon the sifted key. 
         [0141]    It is most important that Alice and Bob apply the modulation to the signal pulses only and not the reference pulses during the time the reference pulses passes their phase modulators, should be set to 0° or some other fixed value. This is to ensure that the reference pulses do not carry any encoded information and therefore an eavesdropper cannot gain any information from measuring the reference pulses. At the same time, interferences of these pulses are not affected by Alice and Bob&#39;s information encoding processes. 
         [0142]      FIG. 4  shows plots of the timing schemes which may be used for the quantum cryptographic system of  FIG. 3   a.    
         [0143]      FIG. 4   a  shows the clock signal produced by the clock laser  202  as a function of time. The clock has a repetition period T clock . The rising edge of the clock signal is used to synchronise Alice&#39;s signal laser  207 , Alice&#39;s phase modulator  234 , Bob&#39;s phase modulator  255  and Bob&#39;s detectors R  261  and B  263 . 
         [0144]    The output of the signal laser  207  is shown in  FIG. 4   b . For each clock period, the signal laser  207  is triggered to produce one pulse of width d laser . 
         [0145]      FIG. 4   c  plots the probability of a photon arriving at Bob&#39;s detectors R  261  and A  263  (i.e. sum of the probabilities of a photon arriving at detector R  261  or detector B  263 ) as a function of time. Each signal/reference pulse now has a width of d bob , which may be greater than d laser  due to dispersion in the fibre. Six arrival windows can be seen for each clock cycle. These correspond to signal or reference pulses taking the short-short, long-short or short-long and long-long paths through Alice&#39;s-Bob&#39;s interferometer. The first and second, as well as the second and third signal pulses are separated by a time delay t delay , and the first and second, as well as the second and third reference (strong) signal pulses are also separated by a time delay t delay . The central signal peak and central reference peak are separated by a time delay of t reference . The short-short and long-long paths are observed due to imperfect polarisation beam splitting at the entrance  251  of Bob&#39;s interferometer  256 . 
         [0146]    Only photons arriving in the central window of the signal pulses of each clock cycle contribute to quantum key distribution. The single photon detector B  263  is gated to be on only when the central pulse arrives in each clock cycle, as shown in  FIG. 4   e . This is achieved by biasing the detector with a voltage V det2  for which it is in an active state for a short duration d det  during each clock cycle when the central pulse arrives. The bias voltage duration d det  is typically chosen to be longer than d bob  and is typically a few nanoseconds. At other times the detector is held at a voltage V det1  for which it is inactive. 
         [0147]    For a single photon detector based upon an avalanche photodiode, time gating can be achieved by choosing V det2  to be greater than the avalanche breakdown voltage of the diode and V det1  to be less than the breakdown voltage. An avalanche can only be triggered when the diode bias exceeds the breakdown threshold. 
         [0148]    The avalanche process generates a large number of charge carriers within the diode forming an easily detectable current. Some of these carriers may be localised at hetero-junctions or at trap states within the semiconductor. Carriers confined in such traps can have a lifetime of several microseconds. If the diode is biased above the avalanche breakdown threshold, before the trapped carriers have decayed, there is a possibility that a trapped carrier could be released and then trigger another avalanche. The resultant spurious signal is called an ‘afterpulse’. 
         [0149]    To minimise the rate of afterpulse counts, the APD has to be biased inactive for a sufficiently long time to allow most of the trapped charge to decay. Thus afterpulsing often limits the minimum period between APD detection gates and thus the minimum clock period T clock . Typically T clock ˜1 μs. 
         [0150]    Alice&#39;s and Bob&#39;s phase modulators  234  and  255  are driven by separate voltage pulse generators. The voltage pulse generators are also synchronised with the clock signal (of  FIG. 4   a ), as shown in  FIG. 4   d.    
         [0151]    During the pass of each signal pulse through the phase modulator, the pulse generator outputs one of a number of voltage levels, V mod1 , V mod2  etc. For the B92 protocol, for instance, Alice applied one of two different voltage levels, corresponding to phase shifts of 0° and 90°, to her phase modulator  234 . Meanwhile Bob applies two voltage levels to his modulator  255  corresponding to phase shifts of 180° and 270°. Alice and Bob vary the applied phase shifts for each signal pulse randomly and independently of one-another. 
         [0152]    It is important that Alice and Bob only modulate only the signal pulses, but not the reference pulses. The phase modulator should be set to zero or some other fixed value during the time that the reference pulse passes. 
         [0153]    If the modulators are also used to compensate for phase drift, the DC bias applied to the modulators shown in  FIG. 4   d  may be slightly above or below the levels shown in  FIG. 4   d . The variation from the DC bias illustrated in  FIG. 4   d  will be controlled by the feedback from the measurements of the reference pulse. 
         [0154]      FIG. 4   f  shows the bias scheme for the reference detector R  261 . To monitor and stabilise the phase drift, the detector is gated to be on only upon the central arrival window of the reference signal pulses to detect photons taking Long-Short or Short-Long route through Alice-Bob&#39;s encoding interferometer. 
         [0155]    This is achieved by biasing the detector with a voltage V det2  for which it is in an active state for a short duration d det1  during each clock cycle when the central reference pulse arrives, as shown by the solid line in  FIG. 4   f . The bias voltage duration d det  is typically chosen to be longer than d bob  and is typically a few nanoseconds. At other times the detector is held at a voltage V det1  for which it is inactive. 
         [0156]    To monitor and stabilise the polarisation, the reference detector R is gated to be on only upon one of the satellite arrival window of the reference signal pulses to detect photons. This is shown by the dash-dotted line in  FIG. 4   f . The reference detector may alternate between measurement of the central and the satellite peak. Measurements of the central reference peaks are averaged and feedback to stabilise the phase of the interferometer as described above. Measurements of the satellite peak are averaged and feedback to stabilise the polarisation input to Bob&#39;s interferometer as described above. 
         [0157]      FIG. 4   f  shows the case where one reference measurement is made per clock cycle. However, it is also possible to sample the reference pulse less frequently or to supply more than one reference pulse per clock cycle. 
         [0158]      FIG. 5   a  shows an apparatus for quantum cryptography with active monitoring and stabilisation on phase and polarisation drifts. 
         [0159]    Alice and Bob&#39;s equipment is similar to that described with reference to  FIG. 3   a . Alice  301  sends photons to Bob  303  along fibre  305 . 
         [0160]    Alice&#39;s equipment  301  comprises a signal laser diode  307 , a polarisation rotator  308  receiving output of laser diode  307 , a signal/reference pulse separator  324  receiving the output of polarisation rotator  308 , an imbalanced fibre Mach-Zender interferometer  333  for encoding photons receiving the output from separator  324 , an attenuator  337  connected to the output of interferometer  333 , a bright clock laser  302 , a wavelength division multiplexing (WDM) coupler  339  and bias electronics  309 . 
         [0161]    The signal/reference pulse separator  324  consists of an entrance fibre optic coupler  320 , a long arm  322  with a loop of fibre  323  designed to cause an optical delay connected to one output of entrance coupler  320 , a short arm  321  connected to the other output of entrance coupler  320 , and an exit fibre optic coupler  325  combining two arms  321 ,  322 . All fibres in the separator  324  are polarisation maintaining. 
         [0162]    The interferometer  333  shares its entrance coupler  325  with the signal/reference pulse separator  324 , and consists of a long arm  332  with a loop of fibre  335  designed to cause an optical delay, a short arm  331  with a phase modulator  334 , and an exit polarising beam combiner  336 . All components used in Alice&#39;s interferometer  333  are polarisation maintaining. 
         [0163]    During each clock signal, the signal laser diode laser  307  outputs one optical pulse. 
         [0164]    The polarisation of the signal pulses is rotated by a polarisation rotator  308  so that the polarisation is aligned to be parallel to a particular axis of the polarisation maintaining fibre, usually the slow axis, of the entrance coupler of the signal/reference pulse separator  324 . Alternatively the polarisation rotator  308  may be omitted by rotating the signal laser diode  307  with respect to the axes of the entrance polarisation maintaining fibre coupler  320 . 
         [0165]    The signal pulses are then fed into the signal/reference pulse separator  324  through a polarisation maintaining fibre coupler  320 . Signal pulses are coupled into the same axis, usually the slow axis of the polarisation maintaining fibre, from both output arms of the polarisation maintaining fibre coupler  320 . 
         [0166]    The long arm  322  of the signal/reference pulse separator  324  contains an optical fibre delay loop  323 . The length difference of the two arms  321  and  322  corresponds to an optical propagation delay of t reference . Typically the length of the delay loop  323  may be chosen to produce a delay t reference ˜10 ns. A photon travelling through the long arm  322  will lag that travelling through the short arm  321  by a time of t reference  at the exit coupler  325  of the signal/reference pulse separator  324 . 
         [0167]    The two arms  321  and  322  are combined with an exit polarisation maintaining fibre optic coupler  325  which also serves as an entrance coupler of the encoding interferometer  333 . 
         [0168]    Coupling ratio of two couplers  320  and  325  can be either fixed or variable. The ratios are chosen so that the reference and signal pulses have unequal intensities. Typically, before entering the encoding interferometer  333 , the later reference pulse is 10-10000 times stronger than the earlier signal pulse. For example, the entrance coupler  320  may be asymmetric so as to allow 90% to 99.99% of the input into arm  321  and the exit coupler  325  may be a 50/50 coupler. Alternatively, both the entrance  320  and exit couplers  325  may be 50/50 couplers and an appropriate attenuator placed in arm  321 . 
         [0169]    Properties of the signal and reference pulses are exactly the same, for example polarisation, wavelength etc, except of course for their intensity and time and port of injection into the interferometer  333 . 
         [0170]    The signal and reference pulses are then fed into the imbalanced Mach-Zender interferometer  333  through a polarisation maintaining fibre coupler  325 . Signal and reference pulses are coupled into the same axis, usually the slow axis of the polarisation maintaining fibre, from both output arms of the polarisation maintaining fibre coupler  325 . 
         [0171]    The long arm  332  of the interferometer  333  contains an optical fibre delay loop  335 , while the short arm  331  contains a fibre optic phase modulator  334 . The length difference of the two arms  331  and  332  corresponds to an optical propagation delay of t delay . Typically the length of the delay loop  335  may be chosen to produce a delay t delay ˜5 ns. A photon travelling through the long arm  332  will lag that travelling through the short arm  331  by a time of t delay  at the exit  336  of the interferometer  333 . 
         [0172]    The two arms  331 ,  332  are combined together with a polarisation beam combiner  336  into a single mode fibre  338 . The fibre inputs of the polarisation beam combiner  336  are aligned in such a way that only photons propagating along particular axes of the polarisation maintaining fibre are output from the combiner  336 . Typically, photons which propagate along the slow axis and the fast axis are output by combiner  336  into fibre  338 . 
         [0173]    The polarising beam combiner  336  has two input ports, an in-line input port and a 90° input port. One of the input ports is connected to the long arm  332  of the interferometer  333  and the other input port is connected to the short arm  331  of the interferometer  333 . 
         [0174]    Only photons polarised along the slow axis of the in-line input fibre of the in-line input port are transmitted by the polarising beam coupler  336  and pass into the fibre  338 . Photons polarised along the fast axis of the in-line input fibre of the input port are reflected and lost. 
         [0175]    Meanwhile, at the 90° input port of the beam coupler  336 , only photons polarised along the slow axis of the 90° input fibre are reflected by the beam coupler  336  and pass into the output port, while those polarised along the fast axis will be transmitted out of the beam coupler  336  and lost. 
         [0176]    This means that the slow axis of one of the two input fibres is rotated by 90° relative to the output port. Alternatively the polarisation may be rotated using a polarisation rotator before one of the input ports of the polarising beam combiner  336 . 
         [0177]    Thus, photon pulses which passed through the long  332  and short arms  331  will have orthogonal polarisations. 
         [0178]    Both the signal and reference pulses are then strongly attenuated by the attenuator  337  so that the average number of photons per pulse μ&lt;&lt;1 for the signal pulses. The reference pulses are typically 10-1000 stronger than the signal pulses, and do not have to be attenuated to single photon level as information is only encoded upon signal pulses. 
         [0179]    The attenuated pulses are then multiplexed with a bright laser clock source  302  at a different wavelength using a WDM coupler  339 . The multiplexed signal is then transmitted to the receiver Bob  303  along an optical fibre link  305 . 
         [0180]    The clock may also be delivered in other ways. For example Alice may multiplex the signal pulses with a bright clock laser pulse at the same or different wavelength which is delayed relative to the start of the clock cycle and which does not coincide with the signal pulses. Alternatively the clock signal may be sent on a separate optical fibre. Alternatively, synchronisation may be achieved by using a timing reference. 
         [0181]    Bob&#39;s equipment  303  comprises WDM coupler  341 , a clock recovery unit  342  connected to one output of the WDM coupler  341 , a polarisation controller  344  connected to the other output of the WDM controller  341 , an imbalanced Mach-Zender interferometer  356  connected to the output of the polarisation controller  344 , two single photon detectors R  361 , B  363  connected to either outputs of interferometer  356  and biasing electronics  343 . 
         [0182]    Bob&#39;s interferometer  356  contains an entrance polarising beam splitter  351 , a long arm  353  containing a delay loop  354  and a variable delay line  357  connected to one output of beam splitter  351 , a short arm  352  containing a phase modulator  355  connected to the other output of beam splitter  351 , and an exit polarisation maintaining 50/50 fibre coupler  358 . All components in Bob&#39;s interferometer  356  are polarisation maintaining. 
         [0183]    Bob first de-multiplexes the transmitted signal received from fibre  305  using the WDM coupler  341 . The bright clock laser  302  signal is routed to an optical receiver  342  to recover the clock signal for Bob to synchronise with Alice. 
         [0184]    If Alice delivers the clock using an alternative method, Bob will recover the clock accordingly. If Alice sends the clock signal as a single bright pulse within each clock cycle which is delayed relative to signal pulses then Bob may use an imbalanced coupler, such as 90/10, to extract a fraction of the combined signal which is measured with a photo-diode. A clock pulse is then recovered if the incident intensity is above an appropriately set threshold. Alternatively Bob may detect the clock on a separate fibre or using a timing reference. 
         [0185]    The signal/reference pulses which are separated from the clock pulses by WDM coupler  341  are fed into a polarisation controller  344  to restore the original polarisation of the signal pulses. This is done so that signal pulses which travelled the short arm  331  in Alice&#39;s interferometer  333 , will pass the long arm  353  in Bob&#39;s interferometer  356 . Similarly, signal pulses which travelled through the long arm  332  of Alice&#39;s interferometer  333  will travel through the short arm  352  of Bob&#39;s interferometer. 
         [0186]    The signal/reference pulses then pass through Bob&#39;s interferometer  356 . An entrance polarising beam splitter  351  divides the incident pulses with orthogonal linear polarisations. The two outputs of the entrance polarisation beam splitter  351  are aligned such that the two output polarisations are both coupled into a particular axis, usually the slow axis, of the polarisation maintaining fibre. This ensures that signal pulses taking either arm will have the same polarisation at the exit 50/50 polarisation maintaining coupler  358 . The long arm  353  of Bob&#39;s interferometer  356  contains an optical fibre delay loop  354  and a variable fibre delay line  357 , and the short arm  352  contains a phase modulator  355 . The two arms  352 ,  353  are connected to a 50/50 polarisation maintaining fibre coupler  358  with a single photon detector R  361 , B  363  attached to each output arm. 
         [0187]    Due to the use of polarising components, there are, in ideal cases, only two routes for a signal pulse travelling from the entrance of Alice&#39;s encoding interferometer  333  to the exit of Bob&#39;s interferometer  356 :
   i. Alice&#39;s Long Arm  332 −Bob&#39;s Short Arm  352  (L-S) and   ii. Alice&#39;s Short Arm  331 −Bob&#39;s Long Arm  353  (S-L).   
 
         [0190]    The variable delay line  357  at Bob&#39;s interferometer  356  is adjusted to make the propagation time along routes (i) and (ii) almost equal, within the signal laser coherence time which is typically a few picoseconds for a semiconductor distributed feed back (DFB) laser diode, and thereby ensure interference of the two paths. 
         [0191]    The variable fibre delay line  357  can either be an airgap, or a fibre stretcher, driven by a piezo-electric actuator. Alternatively, the two delays can be balanced by carefully controlling the length of fibre in Alice&#39;s  333  and Bob&#39;s  356  interferometers. Fine adjustment of the length of the two optical paths can be achieved through the calibration of zero phase delay in the two modulators  334 ,  355 . 
         [0192]    It is important that the central arrival time window of the signal pulses at single photon detectors do not overlap temporally with any arrival windows of the reference pulses. Otherwise, interference visibility will decrease. This can be guaranteed by carefully choosing the lengths of the delay loops  323 ,  335  to ensure t delay &lt;t reference . 
         [0193]    The references pulses are used to actively monitor and stabilise the phase drift of Alice-Bob&#39;s encoding interferometer. The detector R  361  can be a single photon detector. It is gated to be on only upon the central arrival time window of the reference peak to measure the count rate. If the system were perfectly stable, the counting rate is constant. Any phase drift will be manifested by a varying counting rate. Bob uses any variation in the count rate measured by the reference detector R 361  as a feedback signal to the variable delay line  357 . Thus the optical delay is adjusted to stabilise the counting rate in the reference detector, and compensate any phase drifts of Alice or Bob&#39;s interferometers. 
         [0194]    It is most convenient to maintain that the reference detector with a minimum count rate. In this case, destructive interference is maintained at the reference detector R  361 . 
         [0195]    The reference detector R  361  and associated electronics should integrate the count rate over a certain period of time in order to average statistical fluctuation in the arrival rate of the reference photons. The integration time may typically be a fraction of a second, for example, 0.1 second. Such feedback times are sufficient since the phase drift of the Alice and Bob&#39;s interferometers occurs over much longer time scales. For highly unstable environment, much shorter feedback times, for example, 0.1 ms, may be employed. Alternatively, the feedback signal may be used to recalibrate the zero point of both phase modulator. 
         [0196]    The feedback electronics may also condition system for sudden shocks to the system, such as a sudden change in temperature. If a sudden change in count rate is detected in the reference detector R  361 , the results in the signal detector B  363  can be ignored until the system regains stability. 
         [0197]    The references pulses are also used to actively monitor and stabilise the polarisation drift of photons. The two satellites peaks of the reference peak in  FIG. 5   b  are due to imperfect polarisation control by the polarisation controller  344  and therefore imperfect polarisation beam splitting of the entrance polarisation beam splitter  351  of Bob&#39;s interferometer  356 . The early satellite peak arises from the short arm  331  of Alice&#39;s encoding interferometer  333  to Bob&#39;s short arm  352 , and the late satellite peak arises from the long arm  332  of Alice&#39;s encoding interferometer  333  to Bob&#39;s long arm  353 . By gating the reference detector R  361  to detect during the arrival time of one of the satellite peaks and measuring the photon counting rate, Bob can monitor the polarisation drift of photons and actively stabilise it by feeding the measurement result back into the polarisation controller  344 . The polarisation controller  344  rotates the polarisation of photons and minimise the count rate of the reference detector R  361 . 
         [0198]    The reference detector R  361  should integrate photon counts over a certain period of time in order to reduce statistical fluctuation. The integration time can again be as short as a fraction of a second, for example, 0.1 second. This is typically much faster than the time scale over which the polarisation drifts. Much shorter integration time can be chosen for system operates in unstable conditions. 
         [0199]    The system in  FIG. 5   a  is suitable for implementing the two-state protocol known as B92. In this case only one detector is needed on one output arm of Bob&#39;s interferometer for detecting encoded single photons. As the arm lengths are stabilised so that for zero phase delay the photon rate into the detector R is minimum, and the photon rate into the detector B  363  is minimum if the applied phase shift difference by two phase modulator is 0. 
         [0200]    For the B92 protocol Alice applies phase shifts of 0 and 90° on her phase modulator  334  to the signal pulses. Alice associates 0 phase delay with bit=0, and 90° phase delay with bit=1. Bob applies 0 or 90° to his phase modulator  355  to the signal pulses, and associates 0° with bit=1 and 90° with bit=0. Note that Bob now applies phase shifts for bits  0  and  1  which differ by 180° compared to scheme in  FIG. 3 . After Bob&#39;s detections, he tells Alice in which clock cycle he detected a photon and they keep these bits to form a shared sifted key. They then perform error correction and privacy amplification upon the sifted key. 
         [0201]    It is most important that Alice and Bob apply the modulation to the signal pulses only and not the reference pulses during the time the reference pulses passes their phase modulators, should be set to 0° or some other fixed value. This is to ensure that the reference pulses do not carry any encoded information and therefore an eavesdropper cannot gain any information from measuring the reference pulses. At the same time, interferences of these pulses are not affected by Alice and Bob&#39;s information encoding processes. 
         [0202]    The biasing scheme for the apparatus shown in  FIG. 5   a  is the same as shown in  FIG. 4   a - f.    
         [0203]      FIG. 6   a  shows an apparatus for quantum cryptography with active monitoring and stabilisation of polarisation and phase drift. It is suitable for BB84 protocol. 
         [0204]      FIG. 6   a  is similar to  FIG. 3   a . To avoid unnecessary repetition, like reference numerals will be used to denote like features. The only difference is that one of the outputs of Bob&#39;s interferometer is attached with two single photon detectors R  465 , A  461  through an asymmetric fibre optic coupler  464 . The coupling ratio is typically 95/5, with 95% port attached with single photon detector A  461  for quantum key distribution, and the 5% port attached with single photon detector R  465  for monitoring and stabilising phase and polarisation drifts. The coupling ratio is chosen so high in order that the coupler  464  does not reduce photon count rate of the signal pulses significantly at the detector  461 . Also, as the reference pulses can be set arbitrarily strong, 5% or even smaller coupling into the reference detector is enough for monitoring photon count rate of references pulses. 
         [0205]    The references pulses are used to actively monitor and stabilise the phase drift of Alice-Bob&#39;s encoding interferometer. The detector R  465  can be a single photon detector. It is gated to be on only upon the central arrival time of the reference peak and measure the count rate. If the system were perfectly stable, the counting rate is constant. Any variation in phase drift will be manifested by a varying counting rate. Bob uses any variation in the count rate measured by the reference detector R  465  as a feedback signal to the variable delay line  257 . Thus the optical delay is adjusted to stabilise the counting rate in the reference detector, and compensate any phase drifts of Alice or Bob&#39;s interferometers. 
         [0206]    It is most convenient to maintain the reference detector with a minimum count rate. In this case, destructive interference is maintained at the reference detector R  465  and the signal detector A  461 . 
         [0207]    The reference detector R  465  and associated electronics should integrate the count rate over a certain period of time in order to average statistical fluctuation in the arrival rate of the reference photons. The integration time may typically be a fraction of a second, for example, 0.1 second. Such feedback times are sufficient since the phase drift of the Alice and Bob&#39;s interferometers occurs over much longer time scales. For highly unstable environment, much shorter feedback times, for example, 0.1 ms, may be employed. Alternatively, the feedback signal may be used to recalibrate the zero point of both phase modulator. 
         [0208]    The feedback electronics may also condition system for sudden shocks to the system, such as a sudden change in temperature. If a sudden change in count rate is detected in the reference detector R  465 , the results in the signal detectors A  461  and B  463  can be ignored until the system regains stability. 
         [0209]    The references pulses are also used to actively monitor and stabilise the polarisation drift of photons. The two satellites peaks of the reference peak in  FIG. 6   b  are due to imperfect polarisation control by the controller  244  and therefore imperfect polarisation beam splitting of the entrance polarisation beam splitter  251  of Bob&#39;s interferometer  256 . The early satellite peak arises from the short arm  231  of Alice&#39;s encoding interferometer  233  to Bob&#39;s Short Arm  252 , and the late satellite peak arises from the long arm  232  of Alice&#39;s encoding interferometer  233  to Bob&#39;s long arm  253 . By gating the reference detector R  465  to detect during the arrival of one of the satellite peaks and measure the photon counting rate, Bob can monitor the drift in the polarisation of the photons and actively stabilise it by feeding the measurement result back into the polarisation controller  244 . The polarisation controller  244  rotates the polarisation of photons so as to minimise the count rate of the satellite peak of the reference pulse in the reference detector R  465 . 
         [0210]    The reference detector R  465  should integrate photon counts over a certain period of time in order to reduce statistical fluctuation. The integration time can again be as short as a fraction of a second, for example, 0.1 second. This is typically much faster than the time scale over which the polarisation drifts. Much shorter integration time can be chosen for system operates in unstable conditions. 
         [0211]    In the four-state protocol, which is sometimes referred to as BB84, Alice sets the voltage on her phase modulator to one of four different values, corresponding to phase shifts of 0°, 90°, 180°, and 270°. Phase 0° and 180° are associated with bits  0  and  1  in a first encoding basis, while 90° and 270° are associated with 0 and 1 in a second encoding basis. The second encoding basis is chosen to be non-orthogonal to the first. The phase shift is chosen at random for each signal pulse and Alice records the phase shift applied for each clock cycle. 
         [0212]    Meanwhile Bob randomly varies the voltage applied to his phase modulator between two values corresponding to 0° and 90°. This amounts to selecting between the first and second measurement bases, respectively. Bob records the phase shift applied and the measurement result (i.e photon at detector A  461 , photon at detector B  463 , photon at detector A  461  and detector B  463 , or no photon detected) for each clock cycle. 
         [0213]    In the BB84 protocol, Alice and Bob can form a shared key by communicating on a classical channel after Bob&#39;s measurements have taken place. Bob tells Alice in which clock cycles he measured a photon and which measurement basis he used, but not the result of the measurement. Alice then tells Bob the clock cycles in which she used the same encoding basis and they agree to keep only those results, as in this case Bob will have made deterministic measurements upon the encoded photons. Bob associates a click at the signal detector A  461  with bit=1 and a click at the signal detector B  463  with bit=0. This is followed by error correction, to remove any errors in their shared key, and privacy amplification to exclude any information known to an eavesdropper. 
         [0214]      FIG. 7   a  shows an apparatus for quantum cryptography with a single photon source and a laser diode as a reference. 
         [0215]      FIG. 7   a  is similar to  FIG. 3   a . The main difference is that the signal pulse is replaced with a truly single photon source. 
         [0216]    Alice and Bob&#39;s equipment is similar to that described with reference to  FIG. 3   a . Alice  501  sends photons to Bob  503  along fibre  505 . 
         [0217]    Alice&#39;s equipment  501  comprises a reference laser diode  507 , a polarised single photon source  506 , a polarisation rotator  508  receiving the output of said laser diode  507 , an attenuator  504  receiving the output of said polarisation rotator  508 , a delay loop  523  receiving the output of said attenuator  504 , a polarisation maintaining fibre optic coupler  510  coupling the output from said delay loop  523  and said single photon source  506 , an imbalanced fibre Mach-Zender interferometer  533  receiving the output from said coupler  510 , a narrow band pass filter  537  receiving the output from said interferometer  533 , a bright clock laser  502 , a wavelength division multiplexing (WDM) coupler  539  coupling the output from filter  537  and clock laser  502  and bias electronics  509 . 
         [0218]    The interferometer  533  consists of an entrance coupler  530  connected to both: a long arm  532  with a loop of fibre  535  designed to cause an optical delay; and a short arm  531  with a phase modulator  534 , and an exit polarising beam combiner  536 . All components used in Alice&#39;s interferometer  533  are polarisation maintaining. 
         [0219]    During each clock signal, the reference laser diode  507  outputs one reference pulse and the single photon source  506  emits a polarised single photon signal pulse. 
         [0220]    The polarisation of the reference pulse is rotated by a polarisation rotator  508  so that the polarisation is aligned to be parallel to a particular axis of the polarisation maintaining fibre, usually the slow axis, of the entrance port of the polarisation maintaining fibre coupler  510 . Alternatively the polarisation rotator  508  may be omitted by rotating the signal laser diode  507  with respect to the axes of the entrance of polarisation maintaining coupler  510 . 
         [0221]    The reference pulses are then attenuated by the attenuator  504  so that on average each reference pulse typically contains 10-10000 photons when leaving Alice&#39;s apparatus  501 . 
         [0222]    The polarisation of the single photon signal pulse is aligned to the same axis to the polarisation maintaining fibre of the polarisation maintaining coupler  510  as the attenuated reference pulses. 
         [0223]    The reference pulse passes a delay loop  523 , and then is combined with the single photon signal pulses by a polarisation maintaining fibre optic coupler  510 . The coupling ratio is typically 99.5/0.5. It is chosen so that the single photon source is hardly attenuated when passing this fibre coupler  510  before entering the imbalanced interferometer  533 . 
         [0224]    The delay loop  523  causes an optical propagation delay of t reference . Typically the length of the delay loop  523  may be chosen to produce a delay t reference ˜10 ns. Reference pulses lag single photon pulses by a time of t reference  at the exit port of the polarisation maintaining fibre coupler  510 . 
         [0225]    The output of the coupler with hardly attenuated single photon pulses is connected into an input of the imbalanced Mach-Zender interferometer  533 . 
         [0226]    The wavelength of the reference laser diode  507  has to be chosen nearly the same as that of the single photon source. 
         [0227]    The single photon and reference pulses are then fed into the imbalance Mach-Zender interferometer  533  through a polarisation maintaining fibre coupler  530 . Signal pulses are coupled into the same axis, usually the slow axis of the polarisation maintaining fibre, from both output arms of the polarisation maintaining fibre coupler  530 . 
         [0228]    The long arm  532  of the interferometer  533  contains an optical fibre delay loop  535 , while the short arm  531  contains a fibre optic phase modulator  534 . The length difference of the two arms  531  and  532  corresponds to an optical propagation delay of t delay . Typically the length of the delay loop  535  may be chosen to produce a delay t delay ˜5 ns. A photon travelling through the long arm  532  will lag that travelling through the short arm  531  by a time of t delay  at the exit  536  of the interferometer  533 . 
         [0229]    The two arms  531 ,  532  are combined together with a polarisation beam combiner  536  into a single mode fibre  538 . The fibre inputs of the polarisation beam combiner  536  are aligned in such a way that only photons propagating along particular axes of the polarisation maintaining fibre are outputted from the combiner  536 . Typically, photons which propagate along the slow axis and the fast axis are output by combiner  536  into fibre  538 . 
         [0230]    The polarising beam combiner  536  has two input ports, an in-line input port and a 90° input port. One of the input ports is connected to the long arm  532  of the interferometer  533  and the other input port is connected to the short arm  531  of the interferometer  533 . 
         [0231]    Only photons polarised along the slow axis of the in-line input fibre of the in-line input port are transmitted by the polarising beam coupler  536  and pass into the fibre  538 . Photons polarised along the fast axis of the in-line input fibre of the input port are reflected and lost. 
         [0232]    Meanwhile, at the 90° input port of the beam coupler  536 , only photons polarised along the slow axis of the 90° input fibre are reflected by the beam coupler  536  and pass into the output port, while those polarised along the fast axis will be transmitted out of the beam coupler  536  and lost. 
         [0233]    This means that the slow axis of one of the two input fibres is rotated by 90° relative to the output port. Alternatively the polarisation may be rotated using a polarisation rotator before one of the input ports of the polarising beam combiner  536 . 
         [0234]    Thus, photon pulses which passed through the long  532  and short arms  531  will have orthogonal polarisations. 
         [0235]    The single photon signal pulses and reference pulses then pass through a narrow band pass filter  537  whose transmission window is spectrally centred at the wavelength of the single photon signal source. The filtered reference pulses then have the exact same wavelength as the single photon source. 
         [0236]    The filtered pulses are then multiplexed with a bright laser clock source  502  at a different wavelength using a WDM coupler  539 . The multiplexed signal is then transmitted to the receiver Bob  503  along an optical fibre link  505 . 
         [0237]    The clock may also be delivered in other ways. For example Alice may multiplex the signal pulses with a bright clock laser pulse at the same or different wavelength which is delayed relative to the start of the clock cycle and which does not coincide with the signal pulses. Alternatively the clock signal may be sent on a separate optical fibre. Alternatively, synchronisation may be achieved by using a timing reference. 
         [0238]    Bob&#39;s equipment  503  comprises WDM coupler  541 , a clock recovery unit  542  connected to one output of the WDM coupler  541 , a polarisation controller  544  connected to the other output of WDM coupler  541 , an imbalanced Mach-Zender interferometer  556  connected to the output of polarisation controller  544 , two single photon detectors R  561 , B  563  connected to the two outputs of interferometer  556  and biasing electronics  543 . 
         [0239]    Bob&#39;s interferometer  556  comprises an entrance polarising beam splitter  551 , a long arm  553  containing a delay loop  554  and a variable delay line  557 , a short arm  552  containing a phase modulator  555 , and an exit polarisation maintaining 50/50 fibre coupler  558 . All components in Bob&#39;s interferometer  556  are polarisation maintaining. 
         [0240]    Bob first de-multiplexes the transmitted signal received from fibre  505  using the WDM coupler  541 . The bright clock laser  502  signal is routed to an optical receiver  542  to recover the clock signal for Bob to synchronise with Alice. 
         [0241]    If Alice delivers the clock using an alternative method, Bob will recover the clock accordingly. If Alice sends the clock signal as a single bright pulse within each clock cycle which is delayed relative to signal pulses then Bob may use an imbalanced coupler, such as 90/10, to extract a fraction of the combined signal which is measured with a photo-diode. A clock pulse is then recovered if the incident intensity is above an appropriately set threshold. Alternatively Bob may detect the clock on a separate fibre or using a timing reference. 
         [0242]    The single photon and references pulses which are separated from the clock pulses by WDM coupler  541  are fed into a polarisation controller  544  to restore the original polarisation of the signal pulses. This is done so that signal pulses which travelled the short arm  531  in Alice&#39;s interferometer  533 , will pass the long arm  553  in Bob&#39;s interferometer  556 . Similarly, signal pulses which travelled through the long arm  532  of Alice&#39;s interferometer  533  will travel through the short arm  552  of Bob&#39;s interferometer  556 . 
         [0243]    The single photon source and reference pulses then pass Bob&#39;s interferometer  556 . An entrance polarising beam splitter  551  divides the incident pulses with orthogonal linear polarisations. The two outputs of the entrance polarisation beam splitter  551  are aligned such that the two output polarisations are both coupled into a particular axis, usually the slow axis, of the polarisation maintaining fibre. This ensures that signal pulses taking either arm will have the same polarisation at the exit 50/50 polarisation maintaining coupler  558 . The long arm  553  of Bob&#39;s interferometer  556  contains an optical fibre delay loop  554  and a variable fibre delay line  557 , and the short arm  552  contains a phase modulator  555 . The two arms  552 ,  553  are connected to a 50/50 polarisation maintaining fibre coupler  558  with a single photon detector R  561 , B  563  attached to each output arm. 
         [0244]    Due to the use of polarising components, there are, in ideal cases, only two routes for a single photon or a reference pulse travelling from the entrance of Alice&#39;s encoding interferometer  533  to the exit of Bob&#39;s interferometer  556 :
   i. Alice&#39;s Long Arm  532 −Bob&#39;s Short Arm  552  (L-S) and   ii. Alice&#39;s Short Arm  531 −Bob&#39;s Long Arm  553  (S-L).   
 
         [0247]    The variable delay line  557  at Bob&#39;s interferometer  556  is adjusted to make the propagation time along routes (i) and (ii) almost equal, within the signal laser coherence time which is typically a few picoseconds for a semiconductor distributed feed back (DFB) laser diode, and thereby ensure interference of the two paths. 
         [0248]    The variable fibre delay line  557  can either be an airgap, or a fibre stretcher, driven by a piezo-electric actuator. Alternatively, the two delays can be balanced by carefully controlling the length of fibre in Alice&#39;s  533  and Bob&#39;s  556  interferometers. Fine adjustment of the length of the two optical paths can be achieved through the calibration of zero phase delay in the two modulators  534 ,  555 . 
         [0249]    It is important that the central arrival time window of the signal pulses at single photon detectors do not overlap temporally with any arrival windows of the reference pulses. Otherwise, interference visibility will decrease. This can be guaranteed by carefully choosing the lengths of the delay loops  523 ,  535  to ensure t delay &lt;t reference . 
         [0250]    The references pulses are used to actively monitor and stabilise the phase drift of Alice-Bob&#39;s encoding interferometer. The detector R  561  can be a single photon detector. It is gated to be on only upon the central arrival time of the reference peak and measure the count rate. If the system were perfectly stable, the counting rate is constant. Any variation in phase drift will be manifested by a varying counting rate. Bob uses any variation in the count rate measured by the reference detector R  561  as a feedback signal to the variable delay line  557 . Thus the optical delay is adjusted to stabilise the counting rate in the reference detector, and compensate any phase drifts of Alice or Bob&#39;s interferometers. 
         [0251]    It is most convenient to maintain the reference detector with a minimum count rate. In this case, destructive interference is maintained at the reference detector R  561 . 
         [0252]    The reference detector R  561  and associated electronics should integrate the count rate over a certain period of time in order to average statistical fluctuation in the arrival rate of the reference photons. The integration time may typically be a fraction of a second, for example, 0.1 second. Such feedback times are sufficient since the phase drift of the Alice and Bob&#39;s interferometers occurs over much longer time scales. For highly unstable environment, much shorter feedback times, for example, 0.1 ms, may be employed. Alternatively, the feedback signal may be used to recalibrate the zero point of both phase modulator. 
         [0253]    The feedback electronics may also condition system for sudden shocks to the system, such as a sudden change in temperature. If a sudden change in count rate is detected in the reference detector R  561 , the results in the signal detector B  563  can be ignored until the system regains stability. 
         [0254]    The references pulses are also used to actively monitor and stabilise the polarisation drift of photons. The two satellites peaks of the reference peak in  FIG. 7   b  are due to imperfect polarisation control by the controller  544  and therefore imperfect polarisation beam splitting of the entrance polarisation beam splitter  551  of Bob&#39;s interferometer  556 . The early satellite peak arises from the short arm  531  of Alice&#39;s encoding interferometer  533  to Bob&#39;s Short Arm  552 , and the late satellite peak arises from the long arm  532  of Alice&#39;s encoding interferometer  533  to Bob&#39;s long arm  553 . By gating the reference detector R  561  to detect during the arrival of one of the satellite peaks and measure the photon counting rate, Bob can monitor the drift in the polarisation of the photons and actively stabilise it by feeding the measurement result back into the polarisation controller  544 . The polarisation controller  544  rotates the polarisation of photons so as to minimise the count rate of the satellite peak of the reference pulse in the reference detector R  561 . 
         [0255]    The reference detector R  561  should integrate photon counts over a certain period of time in order to reduce statistical fluctuation. The integration time can again be as short as a fraction of a second, for example, 0.1 second. This is typically much faster than the time scale over which the polarisation drifts. Much shorter integration time can be chosen for system operates in unstable conditions. 
         [0256]    The system in  FIG. 7   a  is suitable for implementing the two-state protocol known as B92. In this case only one detector is needed on one output arm of Bob&#39;s interferometer for detecting encoded single photons. As the arm lengths are stabilised so that for zero phase delay the photon rate into the detector R  561  is minimum, and the photon rate into the detector B  563  is maximum. 
         [0257]    For the B92 protocol Alice applies phase shifts of 0 and 90° on her phase modulator  534  to the signal pulses. Alice associates 0 phase delay with bit=0, and 90° phase delay with bit=1. Bob applies 180° or 270° to his phase modulator  555  to the signal pulses, and associates 180° with bit=1 and 270° with bit=0. After Bob&#39;s detections, he tells Alice in which clock cycle he detected a photon and they keep these bits to form a shared sifted key. They then perform error correction and privacy amplification upon the sifted key. 
         [0258]    It is most important that Alice and Bob apply the modulation to the signal pulses only and not the reference pulses during the time the reference pulses passes their phase modulators, should be set to 0° or some other fixed value. This is to ensure that the reference pulses do not carry any encoded information and therefore an eavesdropper cannot gain any information from measuring the reference pulses. At the same time, interferences of these pulses are not affected by Alice and Bob&#39;s information encoding processes. 
         [0259]      FIG. 8   a  shows an apparatus for quantum cryptography with a single photon source and a reference laser diode for active monitor and stabilisation of phase and polarisation drifts. 
         [0260]    Alice and Bob&#39;s equipment is similar to that described with reference to  FIG. 7   a . Thus, to avoid unnecessary repetition, like reference numerals will be used to denote like features The only difference is that the single photon source and the reference pulses enter Alice&#39;s interferometer  533  through different ports. Specifically, Alice&#39;s equipment comprises a reference laser diode  607 , a polarised single photon source  606 , a polarisation rotator  608  receiving the output of said reference laser diode  607 , an attenuator  604  receiving the output from said polarisation rotator  608 , a delay loop  623  connected to the output of said polarisation rotator, an imbalanced fibre Mach-Zender interferometer  533  receiving the output from both delay loop  623  and single photon source  606 , a bright clock laser  502 , a wavelength division multiplexing (WDM) coupler  539  coupling the output from both filter  537  and clock laser  502 , and finally bias electronics  509 . 
         [0261]    The interferometer is the same as that described with reference to  FIG. 7   a  and has an entrance coupler  630  connected to long arm  532  and short arm  531 . 
         [0262]    During each clock signal, the reference laser diode  607  outputs one reference pulse and the single photon source  606  emits a polarised single photon pulse. 
         [0263]    The polarisation of the reference pulse is rotated by a polarisation rotator  608  so that the polarisation is aligned to be parallel to a particular axis of the polarisation maintaining fibre, usually the slow axis, of one entrance port of the polarisation maintaining fibre coupler  630  of interferometer  533 . Alternatively, the polarisation rotator  608  may be omitted by rotating the signal laser diode  607  with respect to the axes of the selected entrance port of entrance couple  630 . The reference pulses are then attenuated by attenuator  604  so that on average, each reference pulse typically contains 10-10,000 photons when leaving Alice&#39;s apparatus  501 . 
         [0264]    The output of the single photon source  606  is received by the other entrance port of entrance coupler  630 . Although the reference pulse and the signal pulse enter the entrance coupler  630  through different ports, they are both aligned to the same polarisation axis of the polarisation maintaining fibres of coupler  630 . The photons are then processed in the same manner as described with reference to  FIG. 7   a.    
         [0265]      FIG. 8   b  is a plot of probability of a photon arriving at either of detectors R  561  and B  563  against time. As explained with reference to  FIG. 3   a , a central reference peak with early and late satellites and a central signal peak with late and early satellites are observed. 
         [0266]      FIG. 9   a  shows an apparatus for quantum cryptography with active stabilisation. 
         [0267]    Alice and Bob&#39;s equipment is similar to that described with reference to  FIG. 3   a . The main difference is that polarisation division is not used in  FIG. 9   a.    
         [0268]    Alice&#39;s equipment  701  comprises a signal laser diode  707 , a polarisation rotator  708  receiving the output of said signal laser diode  707 , a signal/reference pulse separator  724  receiving the output of said polarisation rotator  708 , an imbalanced fibre Mach-Zender interferometer  733  for encoding photons receiving the output from said pulse separator  724 , an attenuator  737  attenuating the output of said interferometer, a bright clock laser  702 , a wavelength division multiplexing (WDM) coupler  739  coupling the output from said clock laser  702  and said attenuator  737  and bias electronics  709 . 
         [0269]    The signal/reference pulse separator  724  comprises an entrance fibre optic coupler  720  connected to: a long arm  722  with a loop of fibre  723  designed to cause an optical delay; and a short arm  721 , the separator further comprising an exit fibre optic coupler  725  combining two arms  721  and  722 . All fibre in the separator  724  is polarisation maintaining. 
         [0270]    The encoding interferometer  733  consists of an entrance coupler  730  connected to both: a long arm  732  with a loop of fibre  735  designed to cause an optical delay; and a short arm  731  with a phase modulator  734 , the interferometer  733  further comprising an exit polarising maintaining fibre coupler  736 . All components used in Alice&#39;s interferometer  733  are polarisation maintaining. 
         [0271]    During each clock signal, the signal laser diode laser  707  outputs one optical pulse. 
         [0272]    The polarisation of the pulses is rotated by a polarisation rotator  708  so that the polarisation is aligned to be parallel to a particular axis of the polarisation maintaining fibre, usually the slow axis, of the entrance coupler  720  of the signal/reference pulse separator  724 . Alternatively the polarisation rotator  708  may be omitted by rotating the signal laser diode  707  with respect to the axes of the entrance coupler  720 . 
         [0273]    The pulses are then fed into the signal/reference pulse separator  724  through a polarisation maintaining fibre coupler  720 . The pulses are coupled into the same axis, usually the slow axis of the polarisation maintaining fibre, from both output arms of the polarisation maintaining fibre coupler  720 . 
         [0274]    The long arm  722  of the separator  724  contains an optical fibre delay loop  723 . The length difference of the two arms  721  and  722  corresponds to an optical propagation delay of t reference . Typically the length of the delay loop  723  may be chosen to produce a delay t reference ˜10 ns. A photon travelling through the long arm  722  will lag that travelling through the short arm  721  by a time of t reference  at the exit coupler  725  of the separator  724 . 
         [0275]    The two arms  721  and  722  are combined together with an exit polarisation maintaining fibre optic coupler  725 . One output is connected into one input of the encoding Mach-Zender interferometer  733 . 
         [0276]    Coupling ratio of two couplers  720  and  725  can be either fixed or variable. The ratios are chosen so that the signal and reference pulses have unequal intensities. Typically, before entering the encoding interferometer  733 , the reference pulse is 10-10000 times stronger than the signal pulse. 
         [0277]    Properties of the signal and reference pulses are exactly the same, for example polarisation, wavelength etc, except of course for their intensity and time of injection into the interferometer  733 . 
         [0278]    The signal and reference pulses are then fed into the imbalanced Mach-Zender interferometer  733  through a polarisation maintaining fibre coupler  730 . Signal and reference pulses are coupled into the same axis, usually the slow axis of the polarisation maintaining fibre, from both output arms of the polarisation maintaining fibre coupler  730 . 
         [0279]    The long arm  732  of the interferometer  733  contains an optical fibre delay loop  735 , while the short arm  731  contains a fibre optic phase modulator  734 . The length difference of the two arms  731  and  732  corresponds to an optical propagation delay of t delay . Typically the length of the delay loop  735  may be chosen to produce a delay t delay ˜5 ns. A photon travelling through the long arm  732  will lag that travelling through the short arm  731  by a time of t delay  at the exit  736  of the interferometer  733 . 
         [0280]    The two arms  731 ,  732  are combined together with a polarisation maintaining fibre coupler  736  into a single mode fibre. 
         [0281]    Thus, photon pulses which passed through the long  732  and short arms  731  will have same polarisation. 
         [0282]    Both the signal and reference pulses are then strongly attenuated by the attenuator  737  so that the average number of photons per pulse μ&lt;&lt;1 for the signal pulses. The reference pulses are typically 10-1000 stronger than the signal pulses, and do not have to be attenuated to single photon level as information is only encoded upon signal pulses. 
         [0283]    The attenuated pulses are then multiplexed with a bright laser clock source  702  at a different wavelength using a WDM coupler  739 . The multiplexed signal is then transmitted to the receiver Bob  703  along an optical fibre link  705 . 
         [0284]    Bob&#39;s equipment  703  comprises WDM coupler  741 , a clock recovery unit  742  connected to one output of said WDM coupler  741 , a polarisation controller  744  connected to the other output of said WDM coupler  741 , a polarisation beam splitter  745  connected to the output of said polarisation controller, an imbalanced Mach-Zender interferometer  756  connected to a first output of said polarisation beam splitter  745 , three single photon detectors R  761 , B  763 , P  765 , two, R  761  and B  763 , connected to the two outputs of interferometer  756  and the other P  765  connected to a second output of said polarisation beam splitter  745 , and biasing electronics  743 . Bob&#39;s interferometer  756  contains an entrance polarising maintaining coupler  751 , a long arm  752  containing a delay loop  754  and a variable delay line  757 , a short arm  753  containing a phase modulator  755 , and an exit polarisation maintaining 50/50 fibre coupler  758 . All components in Bob&#39;s interferometer  756  are polarisation maintaining. 
         [0285]    Bob first de-multiplexes the transmitted signal received from fibre  705  using the WDM coupler  741 . The bright clock laser  702  signal is routed to an optical receiver  742  to recover the clock signal for Bob to synchronise with Alice. 
         [0286]    The signal and reference pulses which are separated from the clock pulses by WDM coupler  741  are fed into a polarisation controller  744  to restore the original polarisation of the signal pulses. This is done so that all photons can pass through the polarisation beam splitter  745 . 
         [0287]    The signal and reference pulses then pass Bob&#39;s interferometer  756 . An entrance polarising maintaining fibre coupler  751  splits the incident pulses into two arms with same polarisation. The two outputs of the entrance coupler  751  are aligned such that the two output polarisations are both coupled into a particular axis, usually the slow axis, of the polarisation maintaining fibre. This ensures that signal pulses taking either arm will have the same polarisation at the exit 50/50 polarisation maintaining coupler  758 . The long arm  753  of Bob&#39;s interferometer  756  contains an optical fibre delay loop  754  and a variable fibre delay line  757 , and the short arm  752  contains a phase modulator  755 . The two arms  752 ,  753  are connected to a 50/50 polarisation maintaining fibre coupler  758  with a single photon detector R  761 , B  763  attached to each output arm. 
         [0288]    There are four routes for a signal pulse travelling from the entrance of Alice&#39;s encoding interferometer  733  to the exit of Bob&#39;s interferometer  756 :
   i. Alice&#39;s Short Arm  731 −Bob&#39;s Short Arm  753  (S-S);   ii. Alice&#39;s Long Arm  732 −Bob&#39;s Short Arm  753  (L-S);   iii. Alice&#39;s Short Arm  731 −Bob&#39;s Long Arm  752  (S-L); and   iv. Alice&#39;s Long Arm  732 −Bob&#39;s Long Arm  752 .   
 
         [0293]    The variable delay line  757  at Bob&#39;s interferometer  756  is adjusted to make the propagation time along routes (ii) and (iii) almost equal, within the signal laser coherence time which is typically a few picoseconds for a semiconductor distributed feed back (DFB) laser diode, and thereby ensure interference of the two paths. 
         [0294]    The variable fiber delay line  757  can either be an airgap, or a fibre stretcher, driven by a piezo-electric actuator. Alternatively, the two delays can be balanced by carefully controlling the length of fibre in Alice&#39;s  733  and Bob&#39;s  756  interferometers. Fine adjustment of the length of the two optical paths can be achieved through the calibration of zero phase delay in the two modulators  734 ,  755 . 
         [0295]    Only photons arriving at the central windows at detectors R  761  and B  763  undergo interferences and are thus of interest. 
         [0296]    It is important that the central arrival time window of the signal pulses at single photon detectors do not overlap temporally with any arrival windows of the reference pulses. Otherwise, interference visibility will decrease. This can be guaranteed by carefully choosing the lengths of the delay loops  723 ,  735  to ensure t delay &lt;t reference . 
         [0297]    The references pulses are used to actively monitor and stabilise the phase drift of Alice-Bob&#39;s encoding interferometer. The detector R  761  can be a single photon detector. It is gated to be on only upon the central arrival time of the reference peak and measure the count rate. If the system were perfectly stable, the counting rate is constant. Any variation in phase drift will be manifested by a varying counting rate. Bob uses any variation in the count rate measured by the reference detector R  761  as a feedback signal to the variable delay line  757 . Thus the optical delay is adjusted to stabilise the counting rate in the reference detector, and compensate any phase drifts of Alice or Bob&#39;s interferometers. 
         [0298]    It is most convenient to maintain the reference detector with a minimum count rate. In this case, destructive interference is maintained at the reference detector R  761 . 
         [0299]    The reference detector R  761  and associated electronics should integrate the count rate over a certain period of time in order to average statistical fluctuation in the arrival rate of the reference photons. The integration time may typically be a fraction of a second, for example, 0.1 second. Such feedback times are sufficient since the phase drift of the Alice and Bob&#39;s interferometers occurs over much longer time scales. For highly unstable environment, much shorter feedback times, for example, 0.1 ms, may be employed. Alternatively, the feedback signal may be used to recalibrate the zero point of both phase modulator. 
         [0300]    The feedback electronics may also condition system for sudden shocks to the system, such as a sudden change in temperature. If a sudden change in count rate is detected in the reference detector R  761 , the results in the signal detector B  763  can be ignored until the system regains stability. 
         [0301]    The references pulses are also used to actively monitor and stabilise the polarisation of photons with the help of the single photon detector P  765 . 
         [0302]    The single photon detector P  765  is attached to reflecting port of the polarisation beam splitter  745 . If the polarisation controller fully recover the polarisation of the signal pulses, all photons will be transmitted and no photons will be reflected into the single photon detector P  765 . If there is a polarisation drift, part of the signal photons will be reflected into the single photon detector P  765 . 
         [0303]    There will be two time windows of photons arriving at the detector P  765  for each signal or reference pulse. The separation of two arriving window is t delay . The early window corresponds to photons passing through the short arm  731  of Alice&#39;s interferometer, and the late window corresponds to photons passing through the long arm  732  of Alice&#39;s interferometer. 
         [0304]    By gating the detector P  765  to detect during at least one of the arrival windows of the reference pulses and measure the photon counting rate, Bob can monitor the drift in the polarisation of the photons and actively stabilise it by feeding the measurement result into the polarisation controller  744 . The polarisation controller  744  adjusts according to the feed back and minimise the photon count rate, and thus maintain the polarisation of the photon pulses. 
         [0305]    The detector P  765  should integrate photon counts over a certain period of time in order to reduce statistical fluctuation. The integration time can again be as short as a fraction of a second, for example, 0.1 second. This is typically much faster than the time scale over which the polarisation drifts. Much shorter integration time can be chosen for system operates in unstable conditions.