Abstract:
At a sender site of a secure communication network, a first coherent light pulse sequence is phase modulated with a random bit sequence by a phase modulator, and a second coherent light pulse sequence synchronised to the first coherent light pulse sequence is transformed by an optical transducer to a superposition of coherent states. The outputs of the modulator and the transducer are multiplexed and transmitted over an optical communication link. At a receiver site, a homodyne detector receives the transmitted light pulse sequence and detects a random bit sequence and a superposition of quantum states. The homodyne detector may include a local light oscillator, phase control circuitry for controlling the local light source so that the local light oscillator produces first and second local light oscillations having a phase difference of 90 degrees therebetween, and a beamsplitter for receiving light from the optical communication link and mixing the first coherent light pulse sequence with the first local light oscillations and mixing the second coherent light pulse sequence with the second local light oscillation.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates generally to secure communication systems and more specifically to distributing key information using quantum cryptography which is unconditionally secure against eavesdropping. 
     2. Description of the Related Art 
     Quantum cryptography is known as the powerful technique for secure communication, because it provides unconditional security for distribution of secret key information between remote users. Quantum cryptographic key distribution consists of two parts: quantum information transmissions between legitimate users over a quantum channel and classical information transmission between the legitimate users over a public channel. Any activities of eavesdroppers are detected from the measured results of the two kinds of transmissions, which is ensured from the principles of quantum mechanics such as Heisenberg&#39;s uncertainty principle and violation of the Bell theorem. The protocol describes a process whereby the legitimate users determine a secret key while confirming that no eavesdropping is taking place. The security of the secret key is guaranteed by the uncertainty principle whereby disturbance is introduced in the quantum information by any eavesdropping attempt, and hence unconditional security against any wiretapping is achieved. By combining quantum cryptography With a one-time-pad scheme, an unconditional secure communication can be implemented. 
     A variety of protocols have been proposed so far, for example, the four-state scheme, the two-photon interferometric scheme, the nonorthogonal two-state scheme and the delayed interferometric transmission scheme. One measure of the performance of a protocol is the sensitivity to eavesdropping (specifically, it represents the precision of the amount of information leakage to an eavesdropper determined from the data bit error). Another measure is the data transmission rate which is determined by the reduction of data being discarded or sacrificed for detecting eavesdropping during the protocol. It has been found from the current study that the four-state quantum scheme and the two-photon interferometric scheme are better because of their high sensitivity to eavesdropping and high transmission rate. 
     The four-state scheme is the first one of the protocols invented. As described in Proceedings of the IEEE International Conference on Computers, Systems and Signal Processing, Bangalore, India (IEEE, New York, 1984), C. H. Bennet and G. Brassard, pages 175-179 (Reference  1 ), the four-state scheme (currently known as the BB84 protocol) uses a single-photon source  10  (see FIG. 1) to produce a pulsed photon carrier  11  for carrying one bit of information, a light modulator  12 , an optical channel  13  for conveying the modulated photon carrier  11 , and a public channel  16  (for which an eavesdropper can access, but cannot alter transmitted messages) for exchanging classical messages between two legitimate users at the sender and receiver sites to test the correlation of the data sent and those actually received. Light modulator  12  modulates the photon carrier  11  and encodes random bit sequence consisting of a bit “0” and a bit “1” produced from a controller  15  onto the photon carrier  11  so that bits “0” and “1” are encoded by two orthogonal polarisation states of a photon. Two nonorthogonal polarisation bases (oil is linear polarisations of 0° and 90° rectilinear basis, and the other is linear polarisations of 45° and 135°; diagonal basis) are used to encode the “0” and “1”. Logical “0” and “1” are encoded with the 0° and 90° polarisations respectively (for rectilinear basis) and the 45° and 135° polarisations respectively (for diagonal basis). Circular polarisations (clockwise and counterclockwise) may be used, instead of one of these two polarisation bases (rectilinear basis or diagonal basis). 
     Since the 0° polarisations state and 90° polarisation state are orthogonal, photons with such polarisations can be reliably distinguished. A single measurement device  14  at the receiver site that has the ability to distinguish such polarisations is called a rectilinear measurement device. Likewise, photons with 45°-135° linear polarisation can he reliably distinguished by another single measurement device  14  that is called a diagonal measurement device. Quantum mechanical operator, having the eigenstates of rectilinear polarisation states and those having the eigenstate of diagonal basis are non-commuting. Thus, the rectilinear measurement device cannot distinguish the state of the photons which are in the eigenstate of diagonal basis and the diagonal measurement device cannot distinguish the state of the photons which are in the eigenstate of rectilinear basis (they will produce an error with a probability of ½). In particular, when a light pulse contains only one photon, these measurement devices cannot distinguish the state of the photons which are in the eigenstate of rectilinear basis and the state of the photons which are in the eigenstate diagonal basis at the same time (that is the uncertainty principle). The output of the measurement device  14  is supplied to a controller  17 . 
     The basis (rectilinear basis or diagonal basis) are chosen at random at the sender site when encoding the bit onto the photon carrier. At the receiver site, the basis are also chosen at random independently of the sender site when decoding the modulated carrier. After transmissions of quantum information encoded in the photon carriers over the quantum channel  13 , messages are exchanged over the public channel  16  between the controllers  15  and  17  to test whether both users used the same linear polarisation basis to transmit and receive the data. They discard the data that the legitimate users used a different basis to encode and decode the bit data. The bit value of the remaining data should agree for both legitimate users and are used to obtain the shared key data. An eavesdropper, having no means at all to match his/her polarisation basis to those chosen at the sender and receiver, inevitably produces an error in the shared bit sequence of the legitimate users when he/she attempts to measure the photons to eavesdrop the data. Several bits are then extracted from the shared bit sequence at each site and tested whether they agree by exchanging information over the public channel to determine if eavesdropping is taking place. If the extracted data agreed then the legitimate users find that there is no eavesdropping, and they produce a sequence of common random bits from the remaining data that were not used for this test and use these common random bits as a secret key. 
     The BB84 protocol is based on the uncertainty principle that in a single quantum system two sets of mutually nonorthogonal bases cannot he measured with certainty at the same time. A given orthogonal basis (e.g., the diagonal basis) can be always represented by a superposition of another basis nonorthogonal to it (e.g., the rectilinear basis). A measurement that can reliably distinguish a given basis would inevitably destroy the superposition state of a given basis (that is, nonorthogonal basis) and cause it to collapse to a given basis. More generally, a measurement that can partially distinguish a given basis would partially destroy the superposition state of given basis and the state after measurement approaches statistical mixture of a given basis. 
     It is shown in Physical Review Vol. A 56, No. 2, August 1997, Christopher A. Fuchs at al., pages 1163 to 1172 (hereinafter Reference  2 ) that the BB84 protocol is equivalent to a procedure in which the presence of an eavesdropper is detected through the collapse of quantum mechanical superposition. Reference  2  shows that the two-photon interferometric scheme is as strong as the four-state quantum cryptography. This two-photon interferometric scheme, known as the E91 protocol, uses the so-called Einstein-Podolsky-Rosen correlation, that is, non-local correlation in the non-separable quantum state of composite system, see Physical Review Letters Vol. 67, No. 6, August 1991, Artur K. Eckert, pages 661 to 663, (Reference  3 ), and Physical Review Letters Vol. 69, No. 9, August 1992, Artur K. Eckert, pages 1293 to 1295 (Reference  4 ). In addition, Physical Review Letters Vol. 81, No. 14, October 1998, Dagmar Bruss, pages 3018 to 3021, Reference  5 , indicates that the security of quantum cryptography can be further increased by using a set of three different pairs of two orthogonal Basis states (i.e., a total of six states) for encoding the data. 
     A s It has hitherto been believed that it is required that the measured system must be comprised by single quanta for a measurement with wrong basis to cause disturbance to a quantum mechanical superposition state. However, it is not a true requirement, but quantum mechanics allows the system to contain more than single quanta (photon) to be affected by the uncertainty principle. As will be described later, the present invention is based on the utilization of mesoscopic quantum mechanical states where the measured system, i.e. carriers, comprises multiple quanta or photons. 
     The two-state scheme, known as the B92 protocol, is described in Physical Review Letters Volume 68, Number 21, May 1992, Charier. H. Bennett, pages 3121 to 3124 (hereinafter Reference  6 ) and Physical Review Volume 30, Number 2, August 1994, A. K. Eckert, B. Huttner, G. M. Palma and A. Peres, pages 1047 to 1056 (hereinafter Reference  7 ). As shown in simplified form in FIG. 2, Reference  6  discloses an interferometric quantum key distribution scheme in which the sender site uses beam-splitter  22  to split a low-intensity coherent light pulse  21  into light pulses  23  and  24 . The light pulse  23  is modulated by a phase modulator  25  so that information bits “0” and “1” are encoded into 0° and 180° phase shift, respectively. The modulated light pulse  23  is launched into one arm (quantum channel)  26  and the non-modulated light pulse  24  is launched into the other arm (quantum channel)  27  of a Mach-Zehnder interferometer. At the receiver site, the light pulses  23  and  24  are combined by beam-splitter  28  to cause interference. The phase difference between light pulses  23  and  24  is controlled by a phase modulator  29  so that the “0” bit pulses are delivered to a photodetector  30  and the “1” bit pulses are delivered to a photodetector  31 . In order that the probability of light pulse  23  having two or more photons is as small as possible, the average number of photons contained in the low-intensity coherent light pulse  21  must be much smaller than 1 (0.1, for example). In this way, a prospect eavesdropper is prevented from copying a light pulse and the nonorthogonality (overlap) of the 0° and 180° phase shifted states of the light pulse  21  increases. Since the intensity of light pulse  21  is sufficiently dim to realize the two nonorthogonal quantum states, the contribution of vacuum state in the light pulse  21  necessarily increases. 
     Because of the large contribution of the vacuum state, it can be conclusively determine whether the light pulses incident on the photodetectors  30  and  31  are bits “0” and “1”, respectively, although most of the time no photons are detected. 
     The B92 protocol relies on conclusive measurement of two nonorthogonal quantum states of this kind. According to the uncertainty principle, there exist no measurement that can unambiguously distinguish two nonorthogonal quantum states. Two nonorthogonal states can only be distinguished with a certain error probability. However, consider a measurement that allows three different outcomes to be gained from two nonorthogonal quantum states. If such a measurement is allowed, there exists a so-called unambiguous (conclusive) measurement that can give a unambiguous conclusion about some outcomes. For example, an measurement of two nonorthogonal quantum states A and B, three conclusions can be drawn in such a measurement: (i) state A cannot be true, (ii) state B cannot be true, and (iii) neither of these can be determined as true or false. If a given quantum state is none other than states A and B, these results are equivalent to the conclusions that (i) the state is unambiguously B, (ii) the state is unambiguously A, and (iii) neither of these can be determined. If conclusion (i) or (ii) is designated as “conclusive results” and conclusion (iii) as “inconclusive results”, it is only necessary for the receiver to tell the sender the fact that the results are conclusive or inconclusive in order to share information about the state unambiguously. The contents of the conclusions (i) and (ii) are not transmitted, but shared by the sender and the receiver. However, there is no correlation between what data are conclusive results and what data are inconclusive results between the legitimate receiver and the eavesdropper. Thus it is impossible for the eavesdropper to share the same information with the legitimate sender and receiver. Therefore, an eavesdropper cannot tap a quantum channel without causing errors in the shared bit stream. The sender and the receiver extract test bits from the shared bit stream using a public channel to check for errors and determine if unauthorised interception has occurred. If it is ascertained that no eavesdropping has occurred, a secret key is determined from the remaining, untested bits. Since this protocol requires low-intensity coherent light, the receiver suffers from frequent instances of inconclusive results of measurement of quantum states, resulting in a low transmission speed. 
     Although the four-state scheme (the BB84 protocol) and the low-photon interferometric scheme (the E91 protocol) are highly secure and have high transmission rate, they need to use single-photon transmission in which each pulse contains only a single photon to ensure secure communication. This requires devices that can be precisely controlled to generate a single-photon sequence. However, no practical single-photon source is implemented with the current technology. In this regard, Physical Review Volume 51, Number 3, March 1995, B. Huttner, N. Gisin and T. Mor, pages 1863 to 1869 (Reference  8 ) and Optics Communications 123, 1996, Yi Mu et al., pages 344 to 352 (Reference  9 ) discuss a practical four-state quantum cryptographic key distribution system using a combination of two nonorthogonal quantum states to artificially create a four-states. However, it is also necessary to reduce the average number of photons sufficiently to ensure high security for these systems. This is achieved only at the cost of transmission rate. 
     SUMMARY OF THE INVENTION 
     It is therefore an object of the present invention to provide a key distribution system using four-state coherent light pulses comprised of multiple photons that is improved over prior art systems in terms of security and data transmission rate. 
     In general terms, the present invention provides a secure communication network comprising a sender node for randomly selecting, a first coherent light pulse sequence encoded with a random bit sequence and a second coherent light pulse sequence containing a superposition of two coherent states and transmitting the randomly selected first and second sequences over an optical communication link. A receiver node is connected to the optical communication link for receiving the randomly selected fist and second sequences. The receiver node determines whether or not the received second light pulse sequence is destroyed with the aid of exchanging the classical messages after quantum transmission that specifies which are the second sequences among total transmitted sequences, and produces a key from the received random bit sequence from the first light pulse sequence if the second light pulse sequence is found not destroyed by an unauthorised interception. It is the key point that the random bit sequence is encoded as a pair of orthogonal quantum states and detection of eavesdropping is carried by a superposition of these orthogonal quantum states. The first light pulse sequence in which the random hit is encoded may be two high-intensity nearly orthogonal coherent states, and the second light pulse sequence may the a superposition of two coherent states. This superposition of coherent states collapsed to one of coherent state by a measurement that can decode the random bit encoded in the first light pulse sequence. It is also collapsed even if a measurement is made at a single quantum level. 
     In further specific terms, the secure communication network of the present invention comprise, a first light source for producing a first coherent light pulse sequence, a phase modulator for modulating the first coherent light pulse sequence with a random bit sequence, a second light source synchronised in phase to the first light source for producing a second coherent light pulse sequence, an optical transducer for converting quantum states of the second coherent light pulse sequence to superposition of coherent states, an optical switch for switching outputs of the phase modulator and the optical transducer into a temporally mixed light pulse sequence and transmitting the mixed light pulse sequence over an optical communication link, and a homodyne detector for receiving the transmitted light pulse sequence via the optical communication link and detecting a random bit sequence and a superposition state. 
     The homodyne detector may include a local light oscillator, phase control circuitry for controlling the phase of the local light oscillator so that it produces first and second local light having a phase difference of 90 degrees therebetween, and a beam-splitter for receiving light from the optical communication link and mixing the first coherent light pulse sequence with the first local light and mixing the second coherent light pulse sequence with the second local light. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention will be described in further detail with reference to the accompanying drawings, in which: 
     FIG. 1 is a block diagram of a prior art four-state quantum cryptographic communication network; 
     FIG. 2 is a block diagram of a prior art two-state quantum cryptographic communication network; 
     FIG. 3A is a block diagram of a sender site of a quantum cryptographic communication network according to the present invention; 
     FIG. 3B is a block diagram of a receiver site of the quantum cryptographic communication network of the present invention according to a first mode of operation; 
     FIG. 3C is a block diagram of a receiver site of the quantum cryptographic communication network of the invention according to a second mode of operation; and 
     FIGS. 4A,  4 B and  4 C are graphic illustrations of probability distributions of signal detected by a legitimate receiver of the present invention. 
    
    
     DETAILED DESCRIPTION 
     The present invention is based on the uncertainty principle which states that, in a single quantum system, an attempt to distinguish given orthogonal states, though imperfectly, at least partially destroys a superposition of a given orthogonal states (that is nonorthogonal to given states) and alters them into one of given orthogonal states (statistically mixed states). The key distribution system of the present invention can detect the presence of an eavesdropper by identifying whether the superposition state is collapsed or not. 
     According to Journal of Modern Optics, 1993, Vol. 40, No. 1. K. J. Blow et al., pages 33 to 36 (Reference  10 ), Physical Review Vol. 48, No. 1, July 1993, S. M. Barnett et al., pages R5 to R8 (Reference  11 ), Journal of Modern Optics, 1993, Vol 40, No 12, S. M. Barnett et al., pages 2501 to 2513 (Reference  12 ), Physical Review Letters, Vol. 80, No. 14, April 1998, T. Mor, pages 3137 to 3140 (Reference  13 ) and Japanese Laid-Open Patent Application 7-202880 (Reference  14 ), a quantum cryptographic protocol can be constructed from two orthogonal quantum states and a third quantum state that is non-orthogonal to the two orthogonal states. The two orthogonal states are used to transmit a random bit sequence to be shared by sender and receiver and the third quantum state is used to detect an eavesdropper. 
     According to the present invention, a sender alternately transmits to a receiver a high-intensity light pulse sequence containing nearly orthogonal, first and second coherent states and a light pulse sequence containing a superposition of first and second coherent states, as a third state. The receiver is synchronised to the sender to supply local light oscillation to a homodyne detector to detect the transmitted quantum states. 
     The conditions required for the receiver to detect the presence of an eavesdropper are: 
     a) A measurement made by an eavesdropper using a wrong basis is such that the superposition of quantum states is destroyed and evolves into one of the measured basis; and 
     b) A superposition of quantum states must be destroyed by a measurement even at a single quantum level. 
     Potential eavesdroppers will devise, at all costs, a sophisticated strategy that attempts to leave no disturbance on tapped quantum States. In a situation where a given state of carrier involves multiple photons, and the encoded two states are known to the public by a protocol, it is theoretically possible for an eavesdropper to distinguish the given state by stealing at least one photon from a carrier pulse using a beamsplitter and leave no significant disturbance in the given state. The condition (b) is usually satisfied if the carrier pulse contains only one photon. In most cases, it has been considered that the state of a carrier pulse that satisfies the conditions (a) and (b) is the single photon state. However, quantum mechanics allows multiple photons to meet the conditions (a) and (b). 
     The present invention is characterised by the use of a quantum superposition of coherent states that satisfy the conditions (a) and (b) as a third nonorthogonal state to detect an eavesdropping. The quantum state of a coherent light pulse is expressed as |α&gt; according to the convention used in quantum mechanics, where α represents the complex amplitude. If the light pulse were subject to phase modulation of 0°, 90°, 180° and 270°, the modulated light pulse would alter its quantum state to |α&gt;, |iα&gt;, |−α&gt; and |−iα&gt;, respectively. 
     FIGS. 3A and 3B show a three-state quantum key distribution system of the present invention. The key distribution system of this invention comprises a sender site  40  (FIG. 3A) and a receiver site  41  (FIG. 3B) connected by an optical link  42  and a message (public) channel  43  which is used as a public communication shout the phase setting to be used for proper homodyne detection of the transmitted states as well as control for timing. 
     The sender site  40  includes a first coherent light source (oscillator)  50  for key distribution. Light source  50  is implemented with a semiconductor laser for producing a light pulse  51 , which is incident on a phase modulator  52 . Light pulse  51  is modulated (encoded) by modulator  52  in accordance with a random bit sequence supplied from a random number generator  53  via a control circuit  61 . To produce a random bit sequence for key distribution, the average number of photons produced by light source  50  is nearly equal to 1. Phase modulator  52  modulates the light pulse  51  so that a bit “0” is encoded with phase delay 0° (= state |α&gt;) and a bit “1” is encoded with phase delay 180° (= state |−α&gt;). The output of the phase modulator  52  is coupled to an optical switch  54 . 
     Among this light pulse sequence from the coherent light source  51 , one-third contains zero photon, one-third contains a photon and one fifth contains two photons and one of every sixteen light pulses contains three photons in average. Homodyne (phase sensitive) detection is usually used to distinguish between the two coherent states |α&gt; and |−α&gt;. If the amplitude |α| were near 1, the standard quantum limit (SQL) of error rate of distinguishing these coherent states would he about 2%. Note that the error rate can be reduced to 0.400 if an optimum receiver as discussed in Physical Review Vol. 54, No. 4, October 1996, M. Sasaki et al., pages 2728 to 2735 (Reference  15 ) is implemented. While the legitimate users at the sender and receiver sites cannot share perfect error-free bits even if no eavesdropping occurs (since one bit out of fifty bits is in error because of SQL), this bit error may be corrected by a classical error correction procedure. Furthermore, it is possible to reduce the bit error rate of optical homodyne detection to a value smaller than 10 −9  by setting the average number of photons contained in the light pulse  51  to be greater than 10. Then, the sender and the receiver can share bits which substantially contain no errors. 
     For detecting eavesdropping, a quantum superposition of coherent states |α&gt;+|−α&gt; are used. In order to produce a light pulse of such quantum superposition state, the sender site  40  has a second coherent light source  55 . Light source  55  is implemented with a semiconductor laser which may be phase-synchronised to the light source  51  by light injection. A light pulse  56  from the light source  55  is incident on an attenuator  57  where the average number of incident light is adjusted to be same as that of the output from the light source  50 . Note that if the average number of photon is much larger than 1, the superposition state tends to become easily collapsed due to optical losses encountered during transmission, lowering the sensitivity of eavesdropping. 
     The low-intensity light pulse from the attenuator  57  is then incident on a non-linear crystal  58  (such as BBO, KTP, LBO, LiNO 3 ). Crystal  58  transforms the incident light so that the coherent states |α&gt; of the incident light are superposed into a state (|α&gt;+|−α&gt;), known as the Schrodinger&#39;s cat state. For further information, see Physical Review Letters Vol 57, No. 1, July 1986, B. Yurke et al, pages 13 to 16 (Reference  16 ), Physical Review Letters Vol. 58., No. 11, March 1987, A. Mecozzi et al., pages 1055 to 1058 (Reference  17 ), and Physical Review Letters Vol. 77, No. 24, December 1996, M. Brune et al., pages 4887 to 4890 (Reference  18 ). 
     As described in Reference  16 , the quantum superposition of coherent states has the following properties: 
     1) It can be determined by optical homodyne detection whether or not states of superposition are maintained. 
     2) Measurement setting on a first homodyne detector for distinguishing between coherent states |α&gt; and |−α&gt; are different from setting on a second homodyne detector for detecting superposition of coherent states |α&gt;+|−α&gt;. There is a phase difference of 90° between the local light oscillator of the first and second homodyne detectors. 
     3) If the first homodyne detector is used to detect a superposition of state |α&gt;+|−α&gt;, photon states evolve into state |α&gt; or state |−α&gt;. This implies that a quantum superposition of coherent states is destroyed by a measurement using an incorrect phase setting for the local oscillator. This satisfies the property (1). 
     4) A superposition of coherent states can be fragile to an optical loss. A loss of only a single photon is sufficient to destroy a superposition of coherent states, as indicated in Reference  16  as well as in Physical Review Vol. 31, No. 4, April 1985, D. F. Walls et. al., pages 2403 to 2408 (Reference  19 ) and Physical Review Vol. 31, No. 2, February 1985, A. O. Caldeira et al., pages 1059 to 1066 (Reference  20 ). This indicates that a single photon carries sufficient information to identity states |α&gt; and |−α&gt;. In principle, splitting even a single photon from a superposition of coherent state and detecting it make the state evolve into one of states |α&gt; and |−α&gt;. This satisfies the condition (b) mentioned previously. 
     Returning to FIG. 3A, a coherent light pulse  59  of superposed states is directed from the non-linear optical element  58  and reflected off a mirror  60  to the optical switch  54 , which is operated under control of the control circuit  61 . Optical switch  54  randomly selects one of the output of (the first and second quantum states) the modulator  52  and the output of the non-linear crystal  58  according to a control signal supplied from a random number generator  63  via the control circuit  61 , and forwards the selected optical signal to the quantum channel  42 . As a result, two kinds of pulsed light sequences, one for key distribution and the other for detecting eavesdropping, are randomly multiplexed into a single pulsed light sequence and transmitted over the quantum channel  42  to the receiver site  41 . The random bit sequence which has been used to encode the light pulse  51  is stored in a memory  62 . 
     According to a first mode of operation, the control circuit  61 , after quantum transmissions over the optical link  42 , sends a measurement setting to the receiver site over the public channel  43  to inform it of the phase setting (local oscillator phase delay of 0°) appropriate for detecting the first and second quantum states, i.e., |α&gt; and |α&gt; and the phase setting (local oscillator phase delay of 90°) appropriate for detecting the third quantum state, i.e., |α&gt;+|−α&gt;. 
     In FIG. 3B, the optical delay line  77  is connected to the quantum channel  42  to receive and hold the transmitted quantum states until the measurement phase setting is received over the public channel  43 . After passing through the delay line  77 , the delayed optical signal is incident on a beam-splitter  70  where it is mixed with light from a local light source or oscillator  71 . The phase delay of local light oscillator  71  is controlled by a phase shifter  72  to introduce a phase shift of 0° and 90° to the local oscillations in synchronism with the sender site so that the two components of the multiplexed optical signal are respectively mixed with local oscillations having a phase difference of 90° therebetween. To establish this synchronisation the receiving site  41  includes a control circuit  73  that receives a phase setting and timing signal supplied from the sender&#39;s control circuit  61  over the public channel  43  and controls the switch timing of the phase shifter  72 . 
     Upon mixing with a local light oscillation at the beam-splitter  70 , the quantum states of the multiplexed light pulse beam are determined. The mixed optical signal is incident on a photodiode detector  74  where the signal is converted to an electrical signal and applied to an analog-to-digital converter  75  where the magnitude of the signal is determined and converted to a binary signal. It is seen that a whole set of the beamsplitter  70 , the local light oscillator  71 , the phase shifter  72  and the photodiode detector  74  function as a homodyne detector. 
     The measurement setting message from the sender&#39;s control circuit  61  is received by the control circuit  73 . In order for the homodyne detector to distinguish between the transmitted quantum states, the control circuit  73  controls the phase shifter  72  to introduce a stepwise phase shift of 0° and 90° according to the measurement setting message. Since the receiver site has possession of knowledge of the transmitted quantum states, they are properly distinguished. If the receiver site has no knowledge of such relationships in advance, one half of the transmitted quantum states would have to be discarded as stated below. Thus, the first mode of operation is advantageous in that it can achieve high transmission efficiency. 
     According to a second mode of operation, the sender&#39;s control circuit  61  transmits the measurement setting message after the receiver site has performed homodyne detection without the knowledge of quantum states transmitted by the sender site. In this case, the optical delay line.  77  is not used. Instead, the receiver node  42  includes a random number generator  78  as shown in FIG.  3 C. Phase shifter  72  is controlled by the random number generator  79  via the control circuit  73  in order to randomly introduce a phase shifts of 0° and 90°. During quantum transmissions over the optical link  42 , the control circuit  73  stores the digital output values of A/D converter  75  and the information about the phase setting for homodyne detection that were determined by the random number generator  78  in the memory  76 . After the quantum transmissions, the receiver&#39;s control circuit  73  receives the measurement setting message over the public channel  43 . Control circuit  73  utilises this measurement setting information to analyse the data stored in the memory  76  to discard data whose measurement setting do not coincide with the measurement setting specified by the measurement setting message. Since the receiver site has no knowledge of the measurement setting in advance to the homodyne detection, one half of the transmitted quantum states would have to be discarded. 
     The following is a detailed description of the homodyne detection of the present invention. 
     If φ denotes the synchronised phase of the sender&#39;s coherent light sources  50  and  55  and θ denotes the phase of the receiver&#39;s local light oscillator  71 , the quantum states |α&gt; and |−α&gt; are distinguished with a high degree of certainty if cos (φ+θ)=1 and the superposition states is distinguished with a high degree of certainty if sin (φ÷θ)−1. There is a phase difference of 90° between the local light oscillators used for distinguishing the two kinds of optical signals. 
     More specifically, the homodyne detector performs a measurement using a condition “cos (φ+θ)=1” to distinguish the two quantum states |α&gt; and |−α&gt; modulated by the phase modulator  52  at the sender site and the A/D converter  75  recovers the transmitted random bit sequence according to the usual zero-threshold decision strategy. This random bit sequence is supplied to the control circuit  73  and stored in a memory  76 . Homodyne detector performs a measurement using a condition “sin (φ+θ)=1” to rest whether the superposition of coherent states produced by the non-linear optical crystal  58  at the sender site is destroyed or not. The output A/D converter  75  which is derived from the superposition state are supplied to the control circuit  73  to check for eavesdropping. 
     FIGS. 4A to  4 C illustrate the probability distributions of two mutually orthogonal, normalised field quadrature amplitudes (a 1 , a 2 ) for the states |α&gt;, |−&gt; and superposition of states |α&gt;+|−α&gt;, respectively. 
     In FIG. 4A, the probability distribution of (a 1 , a 2 ) for state |α&gt; concentrate in a circle of radius ½ with its center located at point (a 1 =1, a 2 =0), and the observation with the condition cos (φ+θ)=1 is equivalent to observing the probability distribution of a 1  that is projected onto a plane a 2 =0. If the state |α&gt; is transmitted, the expected probability distribution of the receiver&#39;s output is a Gaussian distribution with its peak at a 1 =1. On the other hand, the observation with the condition sin (φ÷θ)=1 is equivalent to observing the probability distribution of a 2  that is projected onto a plane a 1 1=0, and the expected probability distribution for state |α&gt; is a Gaussian distribution with its peak at a 2 =0. 
     FIG. 4B, the probability distribution of (a 1 , a 2 ) for states |−α&gt; concentrates in a circle of radius ½ with its center located at point (a 1 =−1, a 2 =0) and the observation with the condition cos (φ+θ)−1 is equivalent to observing the probability distribution of a 1  that is projected onto a plane a 2 =0. If the state |−&gt; is transmitted, the expected probability distribution is a Gaussian distribution having its peak at a 1 =−1. The observation with the condition sin (φ+θ)=1 for detecting state |−α&gt; is equivalent to observing its probability distribution on a plane a 1 =0, and its expected probability distribution is a Gaussian distribution with its peak at a 2 =0. 
     The legitimate receiver can recover the transmitted random bit sequence according to the usual zero-threshold decision strategy where the receiver obtains the bit value “0” when the electrical signal proportional to a 1  are negative and the bit value “1” when the electrical signal proportional to a 1  are positive. 
     As illustrated in FIG. 4C, when the transmitted signal is a superposition of stares |α&gt;|−α&gt;, two Gaussian distributions with peaks at a 1 =1 and a 1 =−1 can be observed when cos (φ+0)=1 is met, and a single Gaussian distribution with a peak at a 1 =0 is observed when sin (φ÷θ)=1 is satisfied. In the latter case, the Gaussian distribution has an interference fringe within its distribution. 
     As long as the superposition state is preserved, an interference fringe pattern is observed as illustrated in FIG.  4 C. This interference fringe pattern disappears when the superposition state is destroyed by eavesdropping or optical loss, leaving a fringeless pattern of Gaussian distribution. Thus, the legitimate user at the receiving site can determine the presence of eavesdropping by observing the presence of an interference fringe and its visibility. 
     If eavesdropping has occurred during transmission, the superposition state are inevitably collapsed because the eavesdropper know no proper setting to measure each of the data, and the control circuit  73  detector an absence of an interference fringe pattern and alerts the sender&#39;s control circuit  61  to the fact that eavesdropping is taking place. 
     If eavesdropping is not detected, key distribution data stored in memory  76  is accepted. The legitimate sender and receiver are thus guaranteed to share a random bit sequence of the same bit pattern safely that is exclusively composed of orthogonal pairs of the first and second quantum states |α&gt; and |−α&gt; since this random bit sequence is only known to the legitimate users, it is determined as a common secret key.