JP2007184700A - Quantum cryptography communication system and method, polarization/phase modulation converter, and phase/polarization modulation converter - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a quantum cryptography communication system capable of directly transmitting a meaningful message. <P>SOLUTION: A quantum cryptography communication apparatus A includes: a polarization EPR pair generating source 1 for generating an EPR pair of a polarization state; a polarization phase modulation converter 2 for a polarization state of one of photons of the photon pair into a phase state; a fidelity degree detection means 3A for measuring a quantum state of the other photon of the photon pair to decide the fidelity of the EPR pair; a polarization modulator 5 for modulating the photon generated from a single photon source 4 into the polarization state according to data desirably to be transmitted; and a Bell state measurement means 6 for measuring a Bell state of the photons accepted by the fidelity degree detection means 3A and the modulated photons, and a quantum cryptography receiver B includes: a phase/polarization modulation converter 7 for converting the EPR pair into the polarization state; a fidelity degree detection means 3B for measuring the quantum of the photons to decide the fidelity of the EPR pair; and a quantum state measurement means 8 for measuring the quantum state of the photons accepted by the fidelity degree detection means 3B and exclusively ORing the quantum state and the Bell state measurement result to decode the transmission data. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention uses streamlined quantum teleportation, and at this time, qubits on the transmission path use qubits that are phase-encoded so that they can be transmitted over long distances, and quantum cryptography communication that directly transmits meaningful messages. The present invention relates to a system and method, a polarization / phase modulation converter, and a phase / polarization modulation converter.

  Conventional quantum cryptography provides secret sharing of random information used as an encryption key with security guaranteed by quantum mechanics (see, for example, Non-Patent Document 1). It is also conceivable to realize cryptographic communication by using quantum teleportation technology that can transmit an arbitrary quantum state to a remote place by transmitting only classical information (for example, see Non-Patent Document 2). ).

C. H. Bennett and G. Brassard “Quantum cryptography: Public key distribution and coin tossing” in Proc. Of IEEE Int. Conf. Computers, Systems and Signal Processing, Bangalore, India, pp.175-179, 1984 D. Bouwmeester, J.-W. Pan, K. Mattle, M. Eibl, H. Weinfurter, and A. Zeilinger “Experimental quantum teleportation” Nature 390, pp. 575-579, 1997

  Conventional quantum cryptography provides secret sharing of random information used as an encryption key, and there is a problem that classical cryptographic communication must be performed separately using a classical communication channel for the confidential transmission of meaningful messages. there were.

  In addition, in the conventional method for realizing cryptographic communication using the quantum teleportation technique, a complete realization method has not yet been proposed in two-dimensional quantum teleportation for transmitting one qubit of quantum information corresponding to one bit of classical information. The failure probability is about 50% and the transmission efficiency is poor. Furthermore, transmission of classical information of 2 bits per qubit is required, and the communication cost is high. Further, the quantum teleportation receiving side has a problem that it is necessary to restore the quantum state using the quantum unitary transformation using the received classical information. Furthermore, in quantum teleportation, a special two-photon state called EPR (acronym for three names of Einstein et al.) Pair is used, but a specific implementation method is presented only for the polarization-modulated quantum state. On the other hand, the polarization-modulated quantum state has a problem in that long-distance transmission is impossible because the transmission distance is at most about 10 km.

  The present invention has been made to solve the above-described problems, and an object of the present invention is to provide a quantum cryptography communication system and method capable of directly transmitting a meaningful message, a polarization / phase modulation converter, and a phase. / A polarization modulation converter is obtained.

  This quantum cryptography communication system applies quantum teleportation technology in its realization, but it guarantees a 100% success probability even with the imperfect scheme that can be realized at present, and is half as classical transmission information. It is sufficient to measure the quantum state on the receiving side, and an operation for converting the quantum state is unnecessary. Also, the polarization-modulated quantum state is modulated and converted to a phase-modulated quantum state, and the phase-modulated quantum state is converted to a polarization-modulated modulation, so that the phase-modulated quantum state is used for quantum state transmission. Long distance transmission is possible.

  The quantum cryptography communication system according to the present invention is a quantum cryptography communication system provided with a quantum cryptography transmission device and a quantum cryptography reception device connected to the quantum cryptography transmission device by a quantum communication channel and a classical communication channel, The quantum cryptography transmitter is encoded with a polarization state EPR pair generation source that generates a photon pair called a polarization state EPR pair, and a polarization state of one photon of the photon pair generated by the polarization state EPR pair generation source. Measuring the quantum state of the other photon of the photon pair generated by the polarization state EPR pair source, and a polarization / phase modulation converter that converts the quantum state into a quantum state encoded in the phase state, First fidelity test means for determining the fidelity of an EPR pair by classical communication with the same means on the quantum cryptography receiver side, a single photon source that generates a single photon, and the single photon source A polarization modulator that modulates a generated photon into a polarization state according to data to be transmitted, a photon that has passed the first fidelity test means, and a two-photon state called a Bell state of a photon modulated by the polarization modulator. A degenerate Bell state measuring unit that measures and classifies and identifies four Bell states into two groups, and the quantum cryptography receiving device transmits the polarization communication from the polarization / phase modulation converter through the quantum channel. A phase / polarization modulation converter that converts the phase-encoded photons of the EPR pair into a polarization state; and a quantum state of a photon output from the phase / polarization modulation converter; A second fidelity test means for determining the fidelity of the EPR pair by performing classical communication with the fidelity test means and the classical channel, and the amount of photons that have passed the second fidelity test means State was measured, and has a quantum state measuring means for decoding the transmission data by taking an exclusive OR of the Bell state measurements degenerate transmitted from the measurement result and the Bell state measurement means.

  The quantum cryptography communication system according to the present invention has an effect that a meaningful message can be directly transmitted. In that realization, quantum teleportation technology is applied, but 100% success probability is guaranteed even with the imperfect schemes that can be realized at present. Only the quantum state is measured on the side, and an operation for converting the quantum state is unnecessary. Also, the polarization-modulated quantum state is modulated and converted to a phase-modulated quantum state, and the phase-modulated quantum state is converted to a polarization-modulated modulation, so that the phase-modulated quantum state is used for quantum state transmission. Long distance transmission is possible.

Embodiment 1 FIG.
A quantum cryptography communication system according to Embodiment 1 of the present invention will be described with reference to FIGS. FIG. 1 is a block diagram showing a configuration of a quantum cryptography communication system according to Embodiment 1 of the present invention. In addition, in each figure, the same code | symbol shows the same or equivalent part.

  In FIG. 1, a quantum cryptography transmitter A includes a polarization state EPR pair generation source 1, a polarization / phase modulation converter 2, a fidelity test means (first fidelity test means) 3A, a single A photon source 4, a polarization modulator 5, and a Bell (bell: person name) state measuring means 6 are provided. The quantum cryptography receiver B includes a phase / polarization modulation converter 7, a fidelity test unit (second fidelity test unit) 3 B, a quantum state measurement unit 8, and an exclusive OR gate 9. Is provided.

  The double line represents a classical communication path 32 such as a communication cable, and the other lines represent optical fibers 31 and 33. That is, the polarization / phase modulation converter 2 and the phase / polarization modulation converter 7 are connected by a quantum communication path 31 such as an optical fiber.

The polarization state EPR pair generation source 1 in the quantum cryptography transmission device A constantly generates an entangled two-photon state called an EPR pair. In particular, it generates a two-photon state that is entangled with respect to the polarization state. For example, it is well known that such a two-photon state is generated by irradiating a nonlinear optical crystal such as BBO (β-BaB ₂ O ₄ : Beta-Barium Borate) with a laser beam.

  The polarization / phase modulation converter 2 converts information held by a photon as a polarization state into phase information. The configuration and operation of the polarization / phase modulation converter 2 will be described later with reference to FIG. The fidelity test means 3A measures the state of the captured photon, exchanges classical information with the fidelity test means 3B in the quantum cryptography receiver B, and evaluates the quality of the EPR pair state. The single photon source 4 emits photons having a specific polarization state (horizontal polarization). For example, it can be realized using parametric down conversion using a nonlinear optical crystal or quantum dots. The polarization modulator 5 converts the polarization state of a single photon for each bit using classical information to be transmitted called data.

  The Bell state measurement means 6 takes in two photons having a polarization state, measures the Bell state, and notifies the quantum cryptography receiver of the measurement result through the classical communication path 32. In the complete Bell measurement, one of the four two-photon states is projected. In this degenerate Bell measurement, the four Bell states are divided into two groups, which group belongs to? It is a measurement that only needs to be determined. It can therefore be determined with a well-known Bell measuring instrument consisting of one beam splitter, two polarizing beam splitters and four photon detectors with a 100% success probability (K. Mattle, et. Al., Physical Review Letter 76 , pp.4656-4659, 1996).

  The phase / polarization modulation converter 7 in the quantum cryptography receiving device B converts information held by a photon as a phase state into polarization information. The configuration and operation of the phase / polarization modulation converter 7 will be described later with reference to FIG. The fidelity test means 3B measures the state of the captured photon, exchanges classical information with the fidelity test means 3A in the quantum cryptography transmitter A, and evaluates the quality of the EPR pair state. The quantum state measuring means 8 measures the polarization state of the captured photon and demodulates classical information called data from the degenerated Bell state measurement result notified from the quantum cryptography transmitter A.

  Next, the operation of the quantum cryptography communication system according to the first embodiment will be described with reference to the drawings. FIG. 4 is a flowchart showing the operation of the quantum cryptography communication system according to Embodiment 1 of the present invention.

  In step 101 (polarization EPR pair generation step), the polarization state EPR pair generation source 1 continuously generates EPR pairs. This two-photon state can be expressed by the following equation.

  Here, H and V in the formula indicate that there are photons in the horizontal polarization state and the vertical polarization state, respectively. The subscripts 1 and 2 indicate the state of photon 1 and photon 2. The photon 1 is input to the fidelity test means 3A, and the photon 2 is input to the polarization / phase modulation converter 2.

  In step 102 (photon transmission step), the polarization / phase modulation converter 2 converts the horizontal and vertical polarization states into phase states as shown in the following equations.

  Here, s and l in the equation indicate a state in the preceding pulse and a state in the subsequent pulse of the time difference double photon pulse, respectively. The polarization information is converted into relative phase information of the preceding pulse and the succeeding pulse of the time difference double photon pulse.

  The photon 2 thus converted into a time difference double photon pulse is transmitted to the quantum cryptography receiver B through the optical fiber which is the quantum communication path 31. Although the preceding pulse and the succeeding pulse of the time difference double photon pulse are similarly disturbed by the disturbance caused by the optical fiber, the relative phase hardly changes, so that long distance transmission is possible.

  The photon 2 transmitted to the quantum cryptography receiver B is converted in the phase / polarization modulation converter 7 so that the state in the preceding pulse and the state in the subsequent pulse are each in the polarization state as shown in the following equation.

  Therefore, the photon 2 in the horizontal polarization state returns to the horizontal polarization state again, and the photon 2 in the vertical polarization state returns to the vertical polarization state again. As a result, the EPR pair can be shared between the quantum cryptography transmitting / receiving apparatuses A and B.

  However, since there is a possibility that a malicious attacker exists on the transmission line and breaks the quantum state, it is necessary to evaluate and confirm the quality of the EPR pair between the quantum cryptography transmitting / receiving apparatuses A and B. This is executed by the fidelity test means 3A, 3B.

  In step 103 (fidelity test step), if a photon is observed, its quantum state is destroyed. Therefore, a large number of EPR pairs to be evaluated are prepared, part of them are observed to obtain evaluation data of the EPR pair, and the remaining EPR Statistical methods using pairs in quantum cryptography are well known in conventional quantum cryptography. Specifically, 2N EPR pairs are prepared. It is assumed that these are arranged in the order of transmission. The fidelity test means 3A and 3B select and extract N EPR pairs at random between the transmitting and receiving apparatuses. For these N EPR pairs, the fidelity test means 3A and 3B further extract N / 2 at random, and measure whether they are in the horizontal polarization state or the vertical polarization state between the transmitting and receiving apparatuses. Next, the remaining N / 2 EPR pairs are measured between the transmitting and receiving apparatuses to determine whether they are in an oblique 45 degree polarization state or an oblique 135 degree polarization state. At this time, in the correct EPR pair, when the horizontal and vertical polarization states are identified, the states are opposite to each other between the transmission / reception devices, and when the oblique 45 ° / 135 ° polarization state is identified, the states are the same between the transmission / reception devices. The quality of the EPR pair can be evaluated by collating the measurement results between the fidelity test means 3A and 3B (PW Shor and J. Preskill, Physical Review Letters 85, pp.441-444, 2000). If the quality of the EPR pair is passed, the process proceeds to step 104 and subsequent steps in which meaningful message transmission is performed using the remaining N EPR pairs. If the quality is unacceptable, return to step 101 and repeat again.

  In step 104 (data modulation step), data to be transmitted is modulated bit by bit. First, a third photon is generated in the single photon source 4. This photon is in a horizontally polarized state. The polarization modulator 5 does not modulate when the transmission data b is “0”, and rotates the polarization plane by 90 degrees to make the state vertically polarized when the transmission data is “1”. Therefore, the quantum state of photon 3 can be written as:

  In step 105 (Bell measurement step), the Bell state measurement unit 6 measures the photon 1 that has passed the fidelity test unit 3A and the photon 3 polarized by the polarization modulator 5. Since 1-bit information is obtained as a measurement result, this is notified to the quantum cryptography receiver B. Specifically, it is known that the two-photon state can be expressed as an entangled state in addition to the direct product of two independent photon states. This is called a Bell state and there are four states represented by the following equations.

  Therefore, in the Bell state measurement, the quantum states of the photon 1 and the photon 3 are projected onto the four quantum states. However, since photon 1 and photon 2 are in an entangled state called an EPR pair, if the quantum states of photon 1, photon 2, and photon 3 are handled together, the projection of photon 1, 3 to the Bell state is It can be written as

This describes that the measurement of the Bell state of the photon 1 and the photon 3 shows that one of the four Bell states appears with an equal probability 1/4 as a measurement result. So far, this is a well-known fact in conventional quantum teleportation, but in the existing technology, it is a problem that among these four Bell states, | Φ ⁺ > and | Φ ⁻ > cannot be distinguished. When projected into these two states, correct quantum teleportation could not be performed. The probability that this event will occur is 50%.

In this method, when the quantum state of the photon 2 after the projection of the Bell state divides the Bell state into two groups | Φ ⁺ > and | Φ ⁻ >, and | Ψ ⁺ > and | Ψ ⁻ >, Note that the only difference is the sign of the relative phase factor between the horizontal and vertical polarization states. That is, as a measurement result of the Bell state measurement, it is determined only whether it is a Φ group or a Ψ group, “1” is output for the Φ group, “0” is output for the Ψ group, and this value is output. The quantum cryptography receiver B is notified. What is important here is that such degenerate measurements can be carried out with a 100% success probability using the existing well-known Bell measurement technique (Physical Review Letters 76 pp.4656-4659, 1996). .

  In step 106 (quantum state measurement step), the quantum state measurement means 8 in the quantum cryptography receiver B waits for the degenerated Bell state measurement result in step 105 and measures the quantum state of the photon 2. If the quantum state of the photon 2 is in the horizontal polarization state, “0” is output, and if it is in the vertical polarization state, “1” is output. The transmission data can be correctly demodulated by taking an exclusive OR of this measurement result and 1 bit of the degenerated Bell state measurement result notified from the quantum cryptography transmission device A by the exclusive OR gate 9.

  From the projection formula for the Bell state, if the Bell state measurement result of photon 1 and photon 3 degenerated is x and the quantum state measurement result of photon 2 is y, transmission data b and x to y are uniquely Since it is determined, it is clear from the relationship shown in Table 1 below.

  Next, the polarization / phase modulation converter will be described with reference to FIG. FIG. 2 is a diagram showing the configuration of the polarization / phase modulation converter of the quantum cryptography communication system according to Embodiment 1 of the present invention. The polarization / phase modulation converter 2 is a photon that is polarization-modulated as follows:

  (Where H is the horizontal polarization state, V is the vertical polarization state, and information is encoded into the complex amplitudes α and β), the phase modulated photon:

  (Where s is a state in which a preceding photon pulse of a time difference double photon pulse has a photon, l is a state in which a subsequent photon pulse has a photon, and information is encoded as a phase difference between the two photon pulses.) As a result, a time difference double photon pulse is output.

  In FIG. 2, a polarization / phase modulation converter 2 includes a polarization beam splitter (first polarization beam splitter) 21 that separates horizontally polarized light and vertically polarized light, and a half-wave plate (HWP) that rotates vertically polarized light to horizontally polarized light. A half wave plate (first half wave plate) 22, a beam splitter (first beam splitter) 25 that combines, separates, and separates two photons separated in a polarization state, and the two photons. Each of the optical paths is changed and adjusted, and a mirror (first mirror) 23 and a mirror (second mirror) 24 installed to input the beam splitter 25 at the same time, and 1 of the photons re-separated by the beam splitter 25 A beam splitter (second beam splitter) 28 in which one of the two photons is input with a delay, and a mirror (third mirror) 2 installed to give an appropriate delay. , And a mirror (fourth mirror) 27..

  By adopting such a configuration, for example, when a photon in a horizontally polarized state is input, it is reflected by the polarizing beam splitter 21 and enters the beam splitter 25 through the mirror 24 from the horizontal direction. The beam is split 50:50 by the beam splitter 25, and one photon goes straight to the beam splitter 28, and the other photon enters the beam splitter 28 through the mirrors 26 and 27. When these two photons are incident on the beam splitter 28, they are incident with a time difference so that they are split 50:50 by the beam splitter 28 without interfering with each other. Accordingly, the photons output in the horizontal direction are output as a time difference double photon pulse.

  On the other hand, when a photon that is in a vertically polarized state is input, it passes through the polarizing beam splitter 21, and the polarization plane is rotated 90 degrees by the half-wave plate 22 to become a horizontally polarized state. Incident from. The beam is split 50:50 by the beam splitter 25, and one photon goes straight to the beam splitter 28, and the other photon enters the beam splitter 28 through the mirrors 26 and 27. When these two photons are incident on the beam splitter 28, they are incident with a time difference so that they are split 50:50 by the beam splitter 28 without interfering with each other. Also in this case, photons output in the horizontal direction are output as a time difference double photon pulse.

  At this time, the relative phase of the preceding pulse and the succeeding pulse is inverted due to the difference in the input direction of the beam splitter 25. That is, polarization information is converted into phase information.

  Next, the phase / polarization modulation converter will be described with reference to FIG. FIG. 3 is a diagram showing the configuration of the phase / polarization modulation converter of the quantum cryptography communication system according to Embodiment 1 of the present invention. The phase / polarization modulation converter 7 is a photon that is phase-modulated as follows:

  When a time-difference double photon pulse is input (where the information is encoded in the complex amplitudes γ and δ), a polarization-modulated photon:

  Is output as

  The phase / polarization modulation converter 7 includes a beam splitter (third beam splitter) 71 that divides each input time difference double photon pulse into 50:50, and a photon pulse divided by the beam splitter 71 again. The beam splitter (fourth beam splitter) 74 for multiplexing / interfering / separating, the time difference between the preceding pulse and the succeeding pulse of the input time difference double photon pulse, and the time difference between the two optical paths from the beam splitter 71 to the beam splitter 74 are A mirror (fifth mirror) 72, a mirror (sixth mirror) 73, and a polarization beam splitter (second polarization beam splitter) in which the photon pulses separated by the beam splitter 74 are combined again. 78 and the polarization plane of one photon pulse from the beam splitter 74 toward the polarization beam splitter 78 A half-wave plate (HWP) (second half-wave plate) 77 rotating 90 degrees and a mirror (seventh mirror) installed so that two optical paths connecting the beam splitter 74 and the polarization beam splitter 78 are equal in length. ) 75 and a mirror (eighth mirror) 76.

  By adopting such a configuration, for example, a photon state in which the relative phase difference between the preceding pulse and the succeeding pulse is 0.

  Is split by the beam splitter 71 into four photon pulses, the preceding and succeeding. At this time, of the two photon pulses generated by dividing the preceding photon pulse, the pulse that goes directly to the beam splitter 74 is discarded. Of the two photon pulses generated by dividing the subsequent photon pulse, those that do not go straight to the beam splitter 74 are also discarded. Another photon pulse produced by splitting the preceding photon pulse is delayed through the optical path through the mirrors 72 and 73, so that the photon pulse that goes directly to the beam splitter 74 made by dividing the subsequent photon pulse is just Incidence and interference occur at the same timing in the beam splitter 74. As a result, a photon is emitted only at the port that is output in the horizontal direction of the beam splitter 74. This photon passes through the mirror 75 toward the polarization beam splitter 78, is reflected by the polarization beam splitter 78, and is in a horizontally polarized state.

  Will be output.

  On the other hand, a photon state in which the relative phase difference between the preceding and succeeding pulses is π

  Is similarly divided by the beam splitter 71 into four photon pulses including the preceding and succeeding. Also in this case, of the two photon pulses generated by dividing the preceding photon pulse, the pulse that goes directly to the beam splitter 74 is discarded. Of the two photon pulses generated by dividing the subsequent photon pulse, those that do not go straight to the beam splitter 74 are also discarded. Another photon pulse produced by splitting the preceding photon pulse is delayed through the optical path through the mirrors 72 and 73, so that the photon pulse that goes directly to the beam splitter 74 made by dividing the subsequent photon pulse is just Incidence and interference occur at the same timing in the beam splitter 74. Up to this point, it is exactly the same as the photon state with a phase difference of 0, but the result of the interference is completely different reflecting the phase difference. As a result of the interference, a photon is emitted only from the port output in the vertical direction of the beam splitter 74, and the photon passes through the mirror 76 and the polarization plane is rotated 90 degrees by the half-wave plate 77, and then the polarization beam splitter 78. Is incident on. Photons that are transmitted through the polarizing beam splitter 78 and are in a vertically polarized state

  Will be output.

  In this way, the phase information is converted into a quantum state having a different polarization plane due to the difference in phase information, and thus phase information is converted into polarization information.

  As described above, since a meaningful message can be directly used as transmission data, the communication cost for message transmission can be reduced. Quantum teleportation is used as the transmission method, but since the quantum state is optimized according to the data to be transmitted, transmission can be achieved with a success rate of 100% even using a conventional incomplete Bell measurement device. it can. In addition, the amount of classical transmission information may be one bit of transmission data per bit, which is half of the conventional quantum teleportation, and the quantum cryptography receiver B only needs to measure the quantum state, without performing quantum unitary transformation. Good. Furthermore, transmission of photons forming an EPR pair from the quantum cryptography transmission device A to the quantum cryptography reception device B is performed by using phase-modulated photons instead of fragile polarization modulation, so that long-distance transmission is possible.

Embodiment 2. FIG.
A quantum cryptography communication system according to Embodiment 2 of the present invention will be described with reference to FIGS. FIG. 5 is a block diagram showing a configuration of a quantum cryptography communication system according to Embodiment 2 of the present invention.

  In the first embodiment, when the fidelity test means 3A and 3B fail, all the photons to be tested are discarded. In the second embodiment, the fidelity test is performed. Even if the means 3A, 3B fail, the photons to be tested are not discarded but are reused with increased fidelity.

  5, each of the quantum cryptography transmitting / receiving apparatuses A and B of FIG. 1 includes an EPR pair refining means (first EPR pair refining means) 10A, an EPR pair refining means (second EPR pair refining means) 10B, and a quantum memory. A (first quantum memory) 11A and a quantum memory (second quantum memory) 11B are added.

  Next, the operation of the quantum cryptography communication system according to the second embodiment will be described with reference to the drawings. FIG. 6 is a flowchart showing the operation of the quantum cryptography communication system according to Embodiment 2 of the present invention.

  The operation is almost the same as in the first embodiment. The difference from the first embodiment is that photons that have passed the fidelity test means 3A and 3B are stored in the quantum memories 11A and 11B. Rather than discarding all photons rejected by the fidelity test means 3A and 3B, they are sent to the EPR pair refining means 10A and 10B of each of the quantum cryptography transmitting and receiving devices A and B, and the EPR pair refining means 10A and 10B In accordance with a known entanglement purification protocol, the appropriate unitary transformation for the photon and the quantum measurement and the interclassical communication with the EPR pair purification means 10A, 10B in each of the quantum cryptography transmitting / receiving devices A, B are repeated. We will improve the fidelity of the selection and EPR pair. When the fidelity is sufficiently increased, the photons are returned to the fidelity test means 3A and 3B. The fidelity test means 3A, 3B performs the fidelity test again, and if the test passes, the photons are sent to the quantum memories 11A, 11B. If it fails, the photons are returned again to the EPR pair purification means 10A, 10B.

  The purification procedure well known in the EPR pair purification means 10A, 10B can be the one described in “C. H. Benett, et. Al., Physical Review Letters 76, pp. 722-725, 1996”. In this way, EPR pairs that have failed the fidelity test means 3A, 3B can also be reused.

It is a block diagram which shows the structure of the quantum cryptography communication system which concerns on Embodiment 1 of this invention. It is a figure which shows the structure of the polarization / phase modulation | alteration converter of the quantum cryptography communication system which concerns on Embodiment 1 of this invention. It is a figure which shows the structure of the phase / polarization modulation converter of the quantum cryptography communication system which concerns on Embodiment 1 of this invention. It is a flowchart which shows operation | movement of the quantum cryptography communication system which concerns on Embodiment 1 of this invention. It is a block diagram which shows the structure of the quantum cryptography communication system which concerns on Embodiment 2 of this invention. It is a flowchart which shows operation | movement of the quantum cryptography communication system which concerns on Embodiment 2 of this invention.

Explanation of symbols

  A quantum cryptography transmitter, B quantum cryptography receiver, 1 polarization state EPR pair generation source, 2 polarization / phase modulation converter, 3A fidelity test means, 3B fidelity test means, 4 single photon source, 5 polarization Modulator, 6 Bell state measurement means, 7 phase / polarization modulation converter, 8 quantum state measurement means, 9 exclusive OR gate, 10A EPR pair purification means, 10B EPR pair purification means, 11A quantum memory, 11B quantum memory , 21 Polarizing beam splitter, 22 Half-wave plate, 23 mirror, 24 mirror, 25 Beam splitter, 26 mirror, 27 mirror, 28 Beam splitter, 31 Quantum channel, 32 Classical channel, 71 Beam splitter, 72 mirror, 73 mirror , 74 Beam splitter, 75 mirror, 76 mirror, 77 half wave plate, 78 polarization beam sp Jitter.

Claims (6)

A quantum cryptography communication system comprising: a quantum cryptography transmission device; and a quantum cryptography reception device connected to the quantum cryptography transmission device by a quantum communication channel and a classical communication channel,
The quantum cryptography transmission device includes:
A polarization state EPR pair source that generates a photon pair called a polarization state EPR pair;
A polarization / phase modulation converter that converts a quantum state encoded in a polarization state of one photon of a photon pair generated by the polarization state EPR pair generation source into a quantum state encoded in a phase state;
A first state in which the quantum state of the other photon of the photon pair generated by the polarization state EPR pair generation source is measured and the fidelity of the EPR pair is determined by classical communication with the same means on the quantum cryptography receiver side. Fidelity test means,
A single photon source that generates a single photon;
A polarization modulator that modulates photons generated by the single photon source into a polarization state according to the data to be transmitted;
The two-photon state called the Bell state of the photon passed by the first fidelity test means and the photon modulated by the polarization modulator was measured, and the four Bell states were degenerated and classified into two groups. And having a Bell state measuring means,
The quantum cryptography receiving device is:
A phase / polarization modulation converter for converting a photon forming an EPR pair transmitted from the polarization / phase modulation converter through the quantum communication channel into a polarization state;
A second state in which the quantum state of a photon output from the phase / polarization modulation converter is measured, and the fidelity of an EPR pair is determined by performing classical communication with the first fidelity test unit and the classical communication channel. Fidelity test means,
The transmission data is obtained by measuring the quantum state of the photon passed by the second fidelity test means and taking the exclusive OR of this measurement result and the degenerated Bell state measurement result transmitted from the Bell state measurement means. And a quantum state measuring means for decrypting. The quantum cryptography transmission device includes:
A first EPR pair refinement that improves fidelity by exchanging classical information with the same means on the quantum cryptography receiver side for a photon whose EPR pair fidelity fails in the first fidelity test means Means,
And further comprising a first quantum memory for storing a quantum state of a photon passed by the first fidelity test unit until the Bell state measurement unit performs measurement,
The quantum cryptography receiving device is:
A second method for improving fidelity by exchanging classical information by the first EPR pair refining means and the classical communication path for a photon whose EPR pair fidelity has failed in the second fidelity test means. EPR vs. purification means of
2. A second quantum memory for storing a quantum state of a photon that has passed the second fidelity test unit until the quantum state measurement unit performs measurement. Quantum cryptographic communication system. In the quantum cryptography transmitter, a polarization EPR pair generation step for generating a photon pair called an EPR pair in a polarization state;
A photon transmission step of converting a quantum state encoded in a polarization state of one photon of the photon pair into a quantum state encoded in a phase state and transmitting the quantum state to a quantum cryptography receiving device through a quantum channel;
After transmission, the quality of the EPR pair between the photon in the quantum cryptography transmission device and the photon in the quantum cryptography reception device is verified. If the quality is good, the process proceeds to the next step. If the quality is bad, the process returns to the first polarization EPR pair generation step. A fidelity test step;
In the quantum cryptography transmitter, a data modulation step of generating a photon using a single photon source and performing polarization modulation according to transmitted data;
In the quantum cryptography transmitter, degenerate Bell measurement is performed by degenerating Bell measurement, which divides four Bell states composed of the other photon of the photon pair and photons generated by the single photon source into two groups. Bell measurement step for notifying the quantum cryptography receiver of the result;
In the quantum cryptography receiver, after waiting for the Bell measurement result from the quantum cryptography transmitter, the quantum state of one photon of the photon pair is measured as horizontal polarization or vertical polarization, and the measurement result and the Bell measurement result are exclusive. And a quantum state measuring step of demodulating the transmitted data by taking a logical logic. The fidelity test step further includes an EPR pair purification step of performing EPR pair purification when the EPR pair quality is poor.
Repeat the fidelity test step and the EPR pair purification step until the EPR pair quality is good,
The quantum cryptography communication method according to claim 3, further comprising a quantum storage step of storing the photons until data transmission without destroying an EPR pair state of the photons passed in the fidelity test step. A first polarizing beam splitter that separates horizontally polarized light and vertically polarized light;
A first half-wave plate that rotates the plane of polarization of the vertically polarized photons emitted from the first polarizing beam splitter into horizontally polarized light;
A first beam splitter that combines a horizontally polarized photon emitted from the first half-wave plate and a horizontally polarized photon emitted from the first polarized beam splitter;
A first and a second mirror that simultaneously cause two photons of a horizontally polarized photon emitted from the first half-wave plate and a horizontally polarized photon emitted from the first polarized beam splitter to be incident on the first beam splitter; ,
A second beam splitter for recombining the two photons branched by the first beam splitter;
A polarization / phase modulation converter comprising: a third mirror and a fourth mirror that cause the two photons branched by the first beam splitter to enter the second beam splitter at different timings. A third beam splitter for injecting horizontally polarized photons at two different timings;
A fourth beam splitter for recombining the four photons branched by the third beam splitter;
Of the four photons branched by the third beam splitter, the photons that are incident at a subsequent timing and go straight to the fourth beam splitter from the third beam splitter, and the photons that are incident at a preceding timing are Fifth and sixth mirrors that simultaneously enter and interfere with the fourth beam splitter;
A second half-wave plate for rotating the plane of polarization of the horizontally polarized photons emitted from the fourth beam splitter into vertically polarized light;
A second polarization beam splitter that combines the vertically polarized photon emitted from the second half-wave plate and the horizontally polarized photon emitted from the fourth beam splitter;
Seventh and eighth mirrors that simultaneously cause two photons, a vertically polarized photon emitted from the second half-wave plate and a horizontally polarized photon emitted from the fourth beam splitter, to enter the second polarized beam splitter; A phase / polarization modulation converter.

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