Abstract:
A quantum key distribution (QKD) system is provided that makes use of a quantum signal of polarized photons and comprises a QKD device and complimentary QKD apparatus. The QKD device has a QKD subsystem comprising one of a QKD transmitter and receiver for inter-working with a complimentary QKD receiver or transmitter of said apparatus. The device also has an alignment subsystem arranged to wirelessly interact with the QKD apparatus to enable generation of user feedback and/or adjustment signals for use in aligning the QKD transmitter and receiver such that the QKD transmitter is pointing at the QKD receiver and the polarization axes of the QKD transmitter and receiver are aligned.

Description:
FIELD OF THE INVENTION 
     The present invention relates to quantum key distribution apparatus and methods. 
     BACKGROUND TO THE INVENTION 
     With advances in computing, and in particular with the possibility of quantum computing platforms becoming available, the once secure public key infrastructure based on RSA encryption is coming under question. As part of the desire to address possible security shortcomings work is currently underway to develop a quantum key based infrastructure. 
     Preferred embodiments of the present invention aim to provide apparatus usable in such an infrastructure. 
     SUMMARY OF THE INVENTION 
     According to one aspect of the present invention, there is provided a device for quantum key distribution, herein QKD, using a quantum signal of polarized photons, the QKD device being intended for use with complimentary QKD apparatus, the device comprising:
         a QKD subsystem comprising one of a QKD transmitter and receiver for inter-working with a complimentary QKD receiver or transmitter of said apparatus; and   an alignment subsystem arranged to wirelessly interact with said apparatus to enable generation of adjustment signals for use in aligning the QKD transmitter and receiver such that the QKD transmitter is pointing at the QKD receiver and the polarization axes of the QKD transmitter and receiver are aligned.       

     According to another aspect of the present invention, there is provided apparatus for quantum key distribution, herein QKD, using a quantum signal of polarized photons, the QKD apparatus being intended for use with complimentary QKD device, the apparatus comprising:
         a QKD subsystem comprising one of a QKD transmitter and receiver for inter-working with a complimentary QKD receiver or transmitter of said device; and   an alignment subsystem arranged to wirelessly interact with said device to generate adjustment signals for use in aligning the QKD transmitter and receiver such that the QKD transmitter is pointing at the QKD receiver and the polarization axes of the QKD transmitter and receiver are aligned.       

    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention will now be described, by way of example only, with reference to the accompanying diagrammatic drawings of example embodiments, in which: 
         FIG. 1  is a schematic illustration of a quantum key distribution system embodying the present invention; 
         FIG. 2  is a schematic illustration of an embodiment of a quantum key distribution transmitting apparatus usable in the  FIG. 1  system; 
         FIG. 3  is a schematic plan view of an array of light emitting diodes used in the transmitting apparatus of  FIG. 2 ; 
         FIG. 4  is a schematic illustration of a shaped shutter used to generate a shaped light beam in the transmitting apparatus of  FIG. 2 ; 
         FIG. 5  is a schematic illustration of an embodiment of a quantum key distribution receiving apparatus usable in the  FIG. 1  system; 
         FIGS. 6A and 6B  together form a functional flow diagram illustrating an example method of operation of the system shown in  FIGS. 1-5 ; 
         FIGS. 7A and 7B  is a schematic illustration of an arrangement for aligning the optical axes of the transmitting apparatus of  FIG. 2  and of the receiving apparatus of  FIG. 5 ; and 
         FIG. 8  is a schematic illustration of a further embodiment of a quantum key distribution receiving apparatus usable in the  FIG. 1  system. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to  FIG. 1  of the accompanying drawings there is shown an quantum key distribution (QKD) system comprising a quantum-key-distribution transmitting apparatus  2  and a quantum-key-distribution receiving apparatus  4 . The transmitting apparatus  2  can be any mobile device such as a mobile phone, Personal Digital Assistant (PDA), laptop, digital camera etc; preferably, the transmitting apparatus is hand-portable and adapted for hand-held operation. The receiving apparatus  4  is a base station and can be included in any convenient location or equipment such as an ATM, information point, cashier&#39;s terminal, etc. For the purpose of illustrating this embodiment of the present invention, the transmitting apparatus  2  is a mobile phone and the receiving apparatus  4  is a bank&#39;s ATM. 
     The transmitting apparatus  2  includes emitters for three channels between the transmitting apparatus  2  and the receiving apparatus  4 . The first channel  6  is a classical communication channel (that is, one not relying on quantum technology) such as an Infrared Data (IrDA), BLUETOOTH (Trade Mark) or the normal wireless communication channel of the mobile phone. The second channel  8  is a quantum channel provided by the sending of a quantum signal. The third channel  10  is an alignment channel for facilitating directional and angular alignment of the transmitting apparatus  2  and receiving apparatus  4 ; in some embodiments, the alignment channel is made up of multiple sub-channels. 
     A quantum signal, in the present context, is a signal capable of conveying sufficient data to enable a quantum cryptographic transaction with another entity. Thus, for example, in one embodiment, a source and transmitter are required which are capable of preparing and transmitting the quantum state which it is desired to send to a requisite degree of accuracy. 
     A requirement for the successful transmission of the quantum signal in the quantum channel  8  is that the quantum signal is correctly aligned with a quantum signal detector of the receiving apparatus  4 , both directionally and such that the polarization directions of the transmitting an receiving apparatus  2 ,  4  have the same orientation. This is achieved using the alignment channel  10 . If it is considered that the transmitting apparatus emits along a z-direction, with the x, y and z-directions all being mutually orthogonal (see  FIG. 1 ), aiming correction refers to relative adjustment of the transmitting and receiving apparatus (or of components of one or both apparatus) so that the z axis of the transmitting apparatus substantially intersects the centre of the lens  54 , and orientation correction refers to relative rotation of the transmitting and receiving apparatus (or of components of one or both apparatus) about the z axis to align the directions of polarization of polarizers of the transmitting and receiving apparatus  2 ,  4 . 
     Referring to  FIG. 2  of the accompanying drawings, the transmitting apparatus  2  is shown to comprise a first (classical) channel transceiver  12 , a second (quantum) channel emitter  14  and a third (alignment) channel emitter  16 . 
     The first (classical) channel emitter  12  is a IrDA transmitter. This provides a data-communication channel for wireless communication between the transmitting apparatus  2  and receiving apparatus  4 . 
     Referring to  FIGS. 2 and 3  of the accompanying drawings, the second (quantum) channel emitter  14  comprises an array  18  of light emitting diodes (LEDs)  20 ,  22 ,  24  and  26 . Referring now specifically to  FIG. 2 , in front of each LED  20 ,  22 ,  24  and  26  is a respective polarising filter  28 ,  30 ,  32 ,  34 . Filter  28  polarises the photons emitted from LED  20  vertically, filter  30  polarises the photons emitted from LED  22  horizontally, filter  32  polarises the photons emitted from LED  24  diagonally and filter  34  polarises the photons emitted from LED  26  anti-diagonally (the directions of polarisation are stated relative to an intended orientation of the apparatus  2  when in use). Thus, after passing through the filters  28 ,  30 ,  32 ,  34 , the photons are polarised in four directions, each at 45° to another thus providing two pairs of orthogonal polarisations. The LEDs  20 ,  22 ,  24 ,  26  are narrow frequency emitters such as those available from Agilent Technologies, Inc. of 395 Page Mill Rd, Palo Alto, Calif. 94306, United States e.g., one of the Sunpower series, emitting at 590 nm or 615 nm. 
     A fibre optic light guide  36  is provided to convey the polarised photons to an attenuation filter  37  and narrow band pass frequency filter  38 . The purpose of the attenuation filter  37  is to reduce the number of photons emitted and the frequency filter  38  is to restrict the emitted photons to a narrow frequency range (typically plus or minus 1 nm). Without the attenuation filter  37  in place the number of photons emitted per LED pulse would be of the order of one million. With the filter in place, the average emission rate is 1 photon per 100 pulses. Importantly this means that more than one photon is rarely emitted per pulse. The attenuation filter  37  and frequency filter  38  can be combined in a single device if preferred. A spatial filter is provided to limit light leakage outside the channel. 
     The third (alignment) channel emitter  16  comprises a bright visible light laser emitter  40  and a shaped shutter  42  with an aperture in the form of an elongate cross  44  with the centre filled in as illustrated in  FIG. 4  of the accompanying drawings. This emits a visible beam of light in the shape of the elongate cross. Preferably, a central portion the cross-shaped beam is blocked out (see dashed circle  45  in  FIG. 4 ) and the quantum channel emitter  14  is arranged to transmit its signal along the resulting hole in the alignment beam. The cross shape enables the orientation of the transmitting apparatus about the z axis to be determined; other beam cross-sectional shapes can be used provided the shape has an asymmetry allowing orientation of the transmitting apparatus to be determined. 
     The frequency of the LEDs used for the quantum channel is different to that of the laser emitter  40  used for the alignment channel so as to avoid cross-talk and overload of the quantum channel detectors. 
     Additionally, the transmitting apparatus  2  comprises a control processor  46 , a user interface  47 , and a memory  48  for storing both data and control programs for controlling operation of the control processor  46  to operate the transmitting apparatus  2  as described below. 
     The receiving apparatus  4  is further explained with reference to  FIG. 5  of the accompanying drawings. The receiving apparatus  4  comprises a classical-channel transceiver  50  for receiving signals transmitted in the classical channel from transmitting apparatus  2 , a quantum signal receiver  52  for receiving the second channel  30  output from transmitting apparatus  2  and a detector  53  for detecting the light signal output from the third (alignment) emitter  16 . 
     The quantum signal receiver  53  comprises a lens  54 , a yoke  55  for controlling the positioning of the lens  54 , a quad-detector arrangement  85 , and a fibre optic light guide for conveying photons received through the lens to the quad-detector arrangement  85 . The end of the light guide  57  nearest the lens  54  is fixed on the optical axis of the lens  55  and is arranged to move with the lens  54  when the latter is moved by the yoke  55 . The quad-detector arrangement  85  comprises a beam splitter  56 , a first paired-detector unit  80 , and a second paired-detector unit  81 . The first paired-detector unit  80  comprises a beam splitter  82 , polarizers  58 ,  59 , and detectors  60 ,  61 . The second paired-detector unit  81  comprises a beam splitter  83 , polarizers  62 ,  63 , and detectors  64 ,  65 . The polarizers  58 ,  59  of the first paired-detector unit  80  have their directions of polarization orthogonal to each other; similarly, the polarizers  58 ,  59  of the second paired-detector unit  81  also have their directions of polarization orthogonal to each other. The polarization directions of the polarizers of the first paired-detector unit  80  are at 45° to the polarization directions of the polarizers of the second paired-detector unit  81 . The beam splitters  56 ,  82  and  83  are depicted in  FIG. 5  as half-silvered mirrors but can be of other forms such as diffraction gratings. 
     The detectors  60 ,  61 ,  64 ,  65  are avalanche photo-diodes, such as those available from Perkin Elmer Optoelectronics of 22001 Dumberry Road, Vaudreuil, Quebec, Canada, J7V 8P7 types C30902E, C30902S, C30921E and C30921S. 
     Dotted line  86  depicts the paths of photons passing through the lens  54  to the detectors  60 ,  61 ,  64  and  65  of the quad-detector arrangement  85 . 
     The yoke  55  is a mounting for the lens  54  enabling electromechanical control of the lens position, using a positioning unit  90  (for example, similar to that used for conventional Compact Disc players). The yoke  55  can adjust the position of the lens  54  in a plane generally perpendicular to the optical path through the lens  54 . The quad-detector arrangement  85  is rotatable about the optical axis of the lens  54  by an orientation unit  91 . 
     The receiving apparatus  4  further comprises a user display  66 , a control processor  68  and associated memory  70 , configured to operate the receiving apparatus  4  as described below. 
     The alignment beam detector  53  comprises a rectangular array  72  of light detection elements arranged to detect light of the wavelength emitted by the laser emitter  40  used in the third emitter  16 . This array  72  lies substantially in the plane of the lens  54  and surrounds the latter with the central zone  73  of the array  72  being left empty for the lens  54 . The output of the array  72  is fed to an alignment control functional block  92  which is arranged to analyze this output to determine where and at what orientation the alignment beam is incident on the array  72  whereby to provide user feedback via display  66  and control of the positioning unit  90  and orientation unit  91 . The alignment control block can be implemented by a program executed by the processor  68  and/or by dedicated circuitry. 
     The array  72 , positioning unit  90 , orientation unit  91 , alignment control block  92 , and display  66  (to the extent it is used to provide user feedback) together form an alignment subsystem of the receiving apparatus  4  intended to work in cooperation with the complementary alignment subsystem of the transmitting apparatus (the emitter  16  of the present embodiment) in order to correctly align the transmitting and receiving apparatus  2 ,  4 . 
     As will be more fully described below, coarse aiming correction is effected by user feedback and fine aiming correction by operation of the positioning unit  90 ; orientation correction is effected by the orientation unit  91 . 
     A method of operation according to a preferred embodiment of the present invention of the apparatus described above will now be described with reference to  FIGS. 6  A and B of the accompanying drawings. 
     The convention is followed that the transmitting side for the quantum signal is referred to as Alice and the receiving side as Bob. In  FIGS. 6A and 6B , the appearance of the name of Alice and/or Bob in block capitals in relation to a particular step indicates the active involvement of Alice and/or Bob, as the case may be, in that step. 
     When a user activates the transmitting apparatus  2  in step  100  ( FIG. 6A ) via the user interface  47 , Alice will initiate a dialog with Bob using the IrDA channel. Alice tells Bob who she is and Bob responds by telling Alice who he is. According to the present embodiment, this is done using a cache of shared secrets possessed by Alice and Bob and either generated by previous interactions between them or downloaded from a trusted source. Typically, the shared secrets will be of the order of 100 kbits to 10 Mbits long. The shared secrets=a∥b∥c∥rest_of_secrets where a, b and c are, for example, each 64 bits (the symbol ∥ representing string concatenation). In step  102 , Alice transmits (a) XOR (b) to Bob where XOR is the exclusive function. In step  104 , Bob searches through his set of shared secrets looking for a match. Once the match is found, in step  106  Bob transmits (a) XOR (c) back to Alice. In step  108 , Alice checks that this is the correct response. Both Alice and Bob then, in step  110 , delete a, b and c from their set of shared secrets. i.e. shared secrets=rest_of_secrets. 
     In step  112 , a user activates the alignment channel, typically by depressing an appropriately marked key on the relevant device. This causes the third emitter  16  to emit a bright visible beam of light through shutter  42  in the manner of a torch. Thus the user sees an elongate cross when the emitted beam strikes a suitable surface. 
     In step  114 , the user uses the cross as a directional guide to aim the output towards the receiving apparatus  4 . 
     As the user nears the target receiver (i.e. the lens  54 ), the alignment beam illuminates the array  72  enabling a determination to be made as to which way the beam should be moved to centre it on the array and thus on the lens  54 . In step  116 , the display  66  of the receiving apparatus  4  provides a visual indication of the direction in which the user should move the transmitting apparatus  2 . This may be in the form of a directional arrow, a colour showing where the current aim of the transmitting apparatus  2  lies or any other indicia. The display additionally provides in step  118  an “on target” signal when the transmitting apparatus  2  is correctly aimed at the receiving apparatus  4 , this signal being, for example, in the form of a displayed word/phrase, a circle around the centre of the target or any other suitable indicia. An audible signal can additionally/alternatively be provided. 
     The beam detector  53  in step  120  uses the asymmetricc shape (elongate cross shape) of the beam emitted as the orientation signal from laser emitter  40  to determine the orientation of the transmitting apparatus  2  and cause the orientation unit  91  to rotate the quad-detector arrangement  85  so as to adjust the orientation of the polarising filters  58 ,  59 ,  62 ,  63  such that vertical/horizontal and diagonal/anti-diagonal quantum signals are received appropriately. To minimise the degree of rotation required of the quad-detector arrangement  85 , either of the paired-detector units  80 ,  81  can be used as the vertical/horizontal detector whilst the other unit is used as the diagonal/anti-diagonal detector. 
     Slight errors in the directional alignment of the transmitting apparatus  2  relative to the receiving apparatus  4 , such as those caused by minor hand movements, can be accommodated by the positioning unit  90  adjusting the position of the lens  54  in step  122 . Thus, when the beam detector  53  determines that the alignment signal is off-centre, the positioning unit  90  is used to adjust the lens position in the plane of the lens  54  to correct the alignment of the quantum communication signal. 
     When alignment is achieved, the quantum signal emitted by the emitter  14  will pass through the lens  54  and be guided by optical fibre  57  to the quad-detector arrangement  85 , and the polarization directions of the signal will align with those of the quad-detection arrangement  85 . 
     Once the quantum channel has been established, a quantum key transfer can be made. The transfer of information based on quantum cryptography is carried out using a variant of the BB84 quantum coding scheme. The specific algorithm according to the preferred embodiment will now be described. 
     Alice and Bob have a predetermined agreement as to the length of a time slot in which a unit of data will be emitted. To achieve initial synchronisation, Alice in step  124  (see  FIG. 6B ) overdrives the alignment emitter  40  to produce a “START” synchronisation signal. Alternatively, the quantum signal channel can be used for synchronisation. 
     In step  126 , Alice randomly generates a multiplicity of pairs of bits, typically of the order of 10 8  pairs. Each pair of bits consists of a message bit and a basis bit, the latter indicating the pair of polarization directions to be used for sending the message bit, be it vertical/horizontal or diagonal/anti-diagonal. A horizontally or diagonally polarised photon indicates a binary 1, while a vertically or anti-diagonally polarised photon indicates a binary 0. The message bit of each pair is thus sent over the quantum signal channel encoded according to the pair of polarization directions indicated by the basis bit of the same pair. Randomness in generating the pairs of bits can be achieved by a hardware random number generator such as a quantum-based arrangement in which a half-silvered mirror is used to pass/deflect photons to detectors to correspondingly generate a “0”/“1” with a 50:50 chance; an alternative form of random number generator can be constructed based around overdriving a resistor or diode to take advantage of the electron noise to trigger a random event. 
     When receiving the quantum signal from Alice, Bob randomly chooses which basis (pair of polarization directions) it will use to detect the quantum signal during each time slot and records the results. 
     The sending of the message bits of the randomly-generated pairs of bits is the only communication that need occur using the quantum channel. The remainder of the algorithm is carried out using the classical channel. 
     In step  128 , Bob informs Alice of the time slots in which a signal was received and the basis (i.e. pair of polarization directions) thereof. 
     In step  130 , Alice sends to Bob confirmation of which of those bases is correct. Alice and Bob then use the bits corresponding to the time slots where they used the same bases, as the initial new shared secret data. However, there may well be discrepancies (errors) between the versions of the new shared secret data held by Alice and Bob due, for example, to noise in the quad detector arrangement  85 . 
     In step  132 , error rate checking is carried out by Alice and Bob comparing their versions of a selected subset of the initial new shared secret data. The higher the error rate, the greater the probability is that the quantum signal has been intercepted. Error rates above about 12% are generally unacceptable and, preferably, an upper threshold of 8% is set since above this figure the number of bits available after error correction and privacy amplification is too low. 
     If the error rate is found to be greater than the 8% threshold, the session is abandoned and the new shared secret data is discarded (step  134 ). 
     If the error rate is below the 8% threshold, error correction is then carried out on the initial new shared secret data (after the latter have been reduced by discarding the subsets used for error rate determination). 
     Error correction is effected using a version of the CASCADE algorithm in which two basic steps  136 ,  138  are repeated until a stable condition is reached (typically after six or seven iterations); alternatively, and as indicated by step  140  in  FIG. 6B , the number of iterations can be fixed. The two basic steps are:
     (1) A preliminary step  136  in which Alice and Bob effect the same random permutation of their respective versions of the new shared secret data. This is done as follows. Alice and Bob use the same subset of bits (typically 64 bits) of their new shared secret data as a seed for a deterministic pseudo random number generator. This pseudo random number generator is used to permute the data. This way both Alice and Bob will permute their data in the same way. The shared secret is then reduced by the subset used as the seed for the random number generator. This permutation step is designed to do two things—it uniformly redistributes the bits in error and also make life difficult for external observers (who do not know how the bits are being redistributed).
       The remaining new shared secret data is then treated as if divided into blocks of a size chosen such that for the measured error rate each block has, on average, one error.   
       (2) An error elimination step  138  in which Alice and Bob process each block of their respective versions of the shared secret data as follows. Both Alice and Bob determine the parity of the block and Bob sends its parity value to Alice. If Alice finds that Bob&#39;s parity value is the same value as Alice has determined for her block, that block is accepted as error free (although it could have any even number of errors); if Alice finds that her parity value differs from Bob&#39;s, the block is assumed to have one error (though it could have any odd number of errors); in this case, a binary search process is followed to track down the error. This search process involves the steps of halving the block in error, and determining which half contains the error by Bob sending Alice the parity of one of the half blocks which Alice compares with her parity value for the corresponding half block in her possession; if the parity values differ, the errored half block is the one being processed whereas if the parity values are the same, the errored half block is the one not being processed. The foregoing steps are then repeated for the errored half block and so on until the errored bit is identified). The errored bit is then either discarded or Bob flips the value of his version of the bit.   

     The above-described error correction process will generally achieve an error level of 1:10 6  or better which is sufficient for present purposes. 
     However, it will be appreciated that the error correction process involves the exchange of considerable amounts of parity information between Bob and Alice which is potentially of use to an eavesdropper. It is also to be noted that although the error-rate-based intercept check carried out in step  132  will detect interception of any substantial portion of the quantum signal transmission, an eavesdropper may still be able to successfully intercept a small number of bits of the quantum signal as there will be a finite (though very small) probability that more than one photon is sent during a time slot over the quantum channel thereby leaving open the possibility that an eavesdropper with a beam splitter can capture one photon while allowing Bob to receive the other photon. Accordingly, a privacy amplification step  142  is next performed. In this step both Alice and Bob reduce the size of their respective versions of the new shared secret data using a deterministic randomizing permutation, the reduction in size being dependent on the amount of parity information exchanged and the level of security required. 
     A detailed discussion of privacy amplification can be found, for example, in the paper “Generalized Privacy Amplification”, C. H. Bennett, G. Brassard, C. Crepeau, and U. M. Maurer; IEEE transactions on Information Theory, IT-41 (6), p1915-1923. In general terms, it can be said that if the new shared secret x has a length of n bit after error correction, and the eavesdropper has at most k deterministic bits of information about the new shared secret, then if an appropriate class of hash function h( ) is applied to the secret random data:
 
{0,1} n →{0,1} n·k−s  
 
where s is a safety parameter 0&lt;s&lt;n−k, the eavesdroppers expected information on h(x) is no more than (2 −s /ln 2) bits. Thus varying the value of (n−k−s) gives different levels of security for the result of the hash of x; in particular, increasing s increases the level of security.
 
     After the error correction and privacy amplification, Alice and Bob are very likely to have the same result. However, in step  144  Alice and Bob seek to re-assure themselves that this is the case by exchanging a hash of their new shared secret data; to protect the transmitted hash, it is XORed with bits popped from the store of shared secrets. If the hashes differ (checked in step  145 ), the newly shared data is discarded (step  146 ) together with the bits used from the store of shared secrets. 
     On the assumption that Alice and Bob have the same new data, they merge the new data in with the existing shared secret. This merging involves the use of a hash function to ensure that the external observer has no knowledge of the final shared secret. Data from this new shared secret is then used to generate a session key (for example, a 128 bit session key) for encrypting the ex change of application data between the transmitting apparatus and receiving apparatus over the classical channel, the data used for creating the session key being discarded from the shared secret. 
     The quantum signal element of the quantum key distribution need only take 0.5-1.0 seconds so the user is not required to keep the transmitting apparatus  2  on target for a long period. 
     It will be appreciated that many variations are possible to the above-described embodiment of the invention. 
     For example, provision can be made for ensuring that the plane of the lens  54  is adjusted to be at least nearly orthogonal to the z axis of the quantum signal emitter since although the quantum signal detector  52  described about will tolerate some misalignment between the z axis of the emitter and the optical axis of the lens  54 , if the misalignment is too great, photons passing through the lens may not be channelled to the quad-detector arrangement. To this end, an element of the array  72  is replaced with an opaque plate  87  formed with a small aperture  88  behind which is an array  89  of light detectors (shown dashed in  FIG. 5 ). When the alignment beam falls on the array  72 , the angle between the z axis of the transmitting apparatus  2  and the optical axis of the lens  54  will determine which of the detectors of the array  89  will be activated. This is illustrated in  FIG. 7A  in which:
         dotted line  150  illustrates the path of the alignment beam through the aperture in plate  87  when the plane of the lens is orthogonal to the z axis of the quantum signal emitter—in this case, the beam strikes the central detector  151  of the array  89 ; and   dotted line  155  illustrates an example path of the alignment beam through the aperture in plate  87  when the plane of the lens is not orthogonal to the z axis of the quantum signal emitter—in the illustrated case, the beam strikes a detector  155  of the array  89 .       

     Depending on which detector of the array  89  is illuminated by the alignment beam, the angle of the lens  54  is adjusted by rotating it about orthogonal axes lying in the plane of the lens  54  (the yoke  55  and unit  90  being adapted, for example, to perform this task in response to signals from the control unit  92 , this latter being fed with the output from the array  89 ). The angle of the plate  87  and detector array  89  are similarly adjusted (for example, by mechanical linkage with the yoke  55 ) whereby upon the plane of the lens  54  becoming orthogonal to the z axis of the quantum signal emitter, the alignment beam  155  will strike the central detector  151  of the array  89  (see  FIG. 7B ) causing angular adjustment of the lens  54 , plate  87  and array  89  to be discontinued. The adjustment of the angling of the lens  54  to make it orthogonal to the z axis of the quantum signal emitter, can be considered as part of the overall alignment process. 
     Although in the embodiment described above, a single lens  54  is used, a plurality of independent lenses can be provided either leading to a common quad-detector arrangement for all such lenses or to a respective quad-detector arrangement for each lens. In this manner, the operative target area is effectively increased and it no longer necessary to mount the lenses on a yoke to compensate for small alignment errors. 
     Indications of any suitable sort can be used to guide a user to centre the quantum signal on the receiving apparatus  4  using the alignment beam. For instance, an audible indication can be used with beeps of increasing frequency the nearer the receiving apparatus  4  the user aims with a continuous noise when the signal is centred. 
     Equally, though it is convenient for the alignment signal to be visual, it need not be. 
     As an alternative embodiment, the alignment signal can be emitted using polarised photons of predetermined polarisation, whereby the polarisation of the photons is used as the orientation signal by the receiving apparatus  4 . In this embodiment a polarising filter is utilised in front of the alignment signal emitter. The polarising filter may be rotated through 90° periodically to assist the receiving apparatus  4  in receiving the orientation signal. The receiving apparatus  4  is modified by having a corresponding polarising filter in front of the orientation signal detector, which detector and polarising filter is rotated until the orientation signal is received, thus determining an orientation of the transmitting apparatus  2  relative to the receiving apparatus  4 . 
     Another simple way of detecting polarization orientation errors is to provide the mobile device with tilt sensors, the outputs of these sensors being sent over the classical communications channel to the receiving apparatus to enable the latter to automatically adjust the orientation of the quad-detector arrangement. 
     In the illustrated embodiments of the invention a single laser beam emitted from the alignment channel emitter  16  is used for both aiming and orientation alignment. It will be appreciated that rather than relying on a single alignment channel signal for all aspects of alignment, separate alignment signal (forming respective alignment sub-channels) can be used for the different alignment adjustments needed. 
     In another variant, depicted in  FIG. 8 , the above-described yoke arrangement  55  is replaced by a tip/tilt mirror  200  the angle of which is set by a mirror drive unit  201  in dependence on the output of the alignment control  92  such as to compensate for alignment errors and ensure that the quantum signal passes via the now static lens  54  to the quad detector arrangement  85 . Rather than relying on the alignment detector  53  to determine the amount of adjustment to be applied using the tip/tilt mirror (as was the case for adjustment of the yoke), in another embodiment a further detector array  202  is positioned around the aperture of the quad detector  85  to detect quantum signal misalignment (as depicted in  FIG. 8  by the dotted rays  204  and  205 ). The detector array  202  is sensitive to a laser beam emitted by the QKD transmitting apparatus that is sent along the path of the quantum signal channel prior to the quantum signal being transmitted (it will be appreciated that the adjustment of the tip/tilt mirror therefore takes place before the adjustment of the orientation of the quad detector that is effected for the purpose of polarization alignment, the quantum signal replacing the laser beam before the polarization alignment step; it will also be appreciated that the quad detector may need temporary screening to protect its sensitive detectors from the laser beam). 
     Although in the described embodiments the quantum signal emitter has been placed in the mobile device and the quantum signal detector in the complementary base station apparatus, it would alternatively be possible to put the quantum signal emitter in the complementary apparatus and the quantum signal detector in the mobile device. For cost reasons, however, mechanical adjustment mechanisms for effecting aiming and orientation alignment are preferably kept in the complementary apparatus and appropriately modified. 
     It would also be possible for the alignment signals to be emitted by the complementary apparatus and detected at the mobile device, the latter then providing feedback over the classical communication channel to the complementary apparatus to enable it to take appropriate alignment correction action, including by way of visual/audible feedback to the user. Alternatively, visual/audible feedback to the user can be provided directly by the mobile device. 
     In the situation where the quantum signal detector is provided at the complementary apparatus and has a significant operative area (for example, due to the replication of detector elements), it may be possible to eliminate any fine alignment adjustment action (such as effected using the yoke arrangement  55  of the  FIG. 5  embodiment or the tip/tilt mirror  200  of the  FIG. 8  embodiment); however, it is generally preferred to at least provide the user with audible/visual feedback that they are on target. 
     Whilst it is preferred to automatically correct for polarization orientation discrepancies between the mobile device and the complementary apparatus, it is also possible to arrange for feedback to be provided to the user to get the user to appropriately rotate the mobile device. 
     In a further variant, where the mobile device is a camera phone, it is possible to electronically place aiming cross-hairs on the image seen through the camera functionality of the device, these cross hairs indicating both where the quantum signal emitter of the device is being pointed and its polarisation orientation. 
     Thus, preferred embodiments of the present invention provide an apparatus enabling a possibly unsteady user to correctly line up and orientate a QKD transmitter-receiver pair to enable a quantum key distribution to take place. The mobile device  2  is portable in that it can conveniently be carried by a user, and although only effective over a relatively short range (typically 3-5 meters), is usable for quantum key distribution in typical consumer environments such as a high street, shop, bank etc. In many expected applications, an optional range of less than 1 meter will suffice. By providing apparatus enabling freestanding (i.e. no tripods, clamps etc.) to be used in a quantum key distribution, the use of the technique can extend into everyday devices.