JP2009147460A - Quantum encryption apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a quantum key delivery system for acquiring a high key generation rate. <P>SOLUTION: A quantum encryption apparatus includes: a single photon light source 110 for generating an optical pulse sequence which is one photon or less per pulse at a time interval T; an optical coupler 112 for branching an optical pulse from a light source; and two receivers A and B configured to receive the branched optical pulses from the optical coupler. Each receiver applies phase conversion to the optical pulse received from the single photon light source at random by 0 or π/2, and makes those optical pulses interfere each other at the time intervals T, and detects the photons by two photon detectors A1 and A2 or B1 and B2 according to the interfering state. Each receiver exchanges phase modulation information and photon detection time information to generate common secret key. Thus, the quantum key delivery system is provided with a high key generation rate. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to a quantum cryptography device.

  In recent years, research on quantum cryptography communication in which safety is physically guaranteed by using light of one photon level has been advanced. Quantum cryptography is a system that supplies a secret key for performing cryptographic communication to two parties at remote points, and is also called quantum key distribution. There are various types of quantum key distribution, but here, as a conventional technique, a quantum key distribution system using spontaneous emission parametric photons will be described (Patent Document 1).

  FIG. 1 shows a configuration example of a quantum key distribution system using spontaneous emission parametric photons. This system includes a quantum entangled light source 10 at an intermediate point between two parties (receivers A and B) who want to share a secret key. The quantum entangled light source 10 includes a pump light source 12 that outputs an optical pulse train, an optical nonlinear medium 14 that receives the pump light, an optical filter 16 that separates signal photons and idler photons generated from the optical nonlinear medium into two paths, and It is comprised by. In this configuration, the quantum entangled light source 10 outputs a signal / idler photon pulse train having a quantum correlation. The characteristics of the photon pulse train output in this way will be described later.

  The signal photon pulse train and idler photon pulse train output from the entangled light source 10 are respectively sent to the receivers A (20) and B (30) via the transmission line. Each receiver randomly modulates each transmitted optical pulse by 0 or π / 2 by the phase modulators 22 and 23, and then bifurcates them by the optical couplers C1 and C3. Then, after giving a time delay between the branched paths, the signals are multiplexed again by the second optical couplers C2 and C4. Here, it is assumed that the delay time given between the branch paths is equal to the pulse interval of the transmitted optical pulse train. Photon detectors A1 and A2, B1, and B2 are provided at two output ends of the multiplexing coupler, respectively.

  With the above configuration, the k-th pulse and the (k + 1) -th pulse overlap in the multiplexing coupler of the receiver. Then, two pulses interfere and a photon is detected by either of the two detectors according to the phase difference between the two pulses. More specifically, when the delay phase in the delay path is an integral multiple of 2π, if the phase difference at the input of the optical couplers C1 and C3 is 0, the detectors A2 and B2 are Detectors A1 and B1 detect photons, respectively. In other words, if detectors A2 and B2 detect photons, the phase difference between the corresponding pulses is 0, and if detectors A1 and B1 detect photons, the phase difference between the pulses is π. Become.

Here, the optical pulse train output from the entangled light source will be described. In the optical nonlinear medium 14 in the quantum entangled light source, pump photons are converted into signal photons and idler photons by a nonlinear phenomenon called optical parametric interaction. That is, a signal photon and an idler photon are generated from the pump light. Due to the nature of the interaction, signal photons / idler photons are always generated simultaneously in pairs, and there is a relationship of φ _s + φ _i = φ _p + φ ₀ between the phases. Here, φ _s is a signal light phase, φ _i is an idler light phase, φ _p is a pump light phase, and φ ₀ is a constant.

  The number of signal / idler photon generation pairs is probabilistic according to the Poisson distribution, and the average value depends on the pump light power. Here, the probability that two or more pairs of signal photons / idler photons are generated by setting the pump light power is negligibly small.

  Since the optical pulse train sent from the quantum entangled light source 10 to the receivers 20 and 30 has the above properties, when two receivers detect photons at the same time, a correlation appears in the detection result. This will be described below using equations.

First, let the signal light phase of the kth pulse be φ _s ^(k) , the idler light phase be φ _i ^(k) , and the pump light phase be φ _p ^(k) . As described above, the relationship of the following equation is established among these three parties.
(1) φ _s ^(k) + φ _i ^(k) = φ _p ^(k) + φ ₀

In the receivers 20 and 30, the phase modulators 22 and 32 apply phase modulation to the transmitted pulse train. The modulation phase applied by the receiver A to the kth pulse is θ _a ^(k) , and the modulation phase applied by the receiver B is θ _b ^(k) . Then, the pulse phase at the input of the optical coupler C1 is φ _s ^(k) + θ _a ^(k) , and the pulse phase at the input of the optical coupler C2 is φ _i ^(k) + θ _b ^(k) . The phase difference of the (k + 1) th pulse is as shown in Equation (2a) at the receiver A and as shown in Equation (2b) at the receiver B.
(2a) Δφ _a ^(k) = φ _s ^{(k + 1)} + θ _a ^{(k + 1)} − (φ _s ^(k) + θ _a ^(k) )
= Φ _s ^{(k + 1)} −φ _s ^(k) + Δθ _a ^(k)
(2b) Δφ _b ^(k) = φ _i ^{(k + 1)} + θ _b ^{(k + 1)} − (φ _i ^(k) + θ _b ^(k) )
= Φ _i ^{(k + 1)} −φ _i ^(k) + Δθ _b ^(k)
However, Δθ _a ^(k) = θ _a ^{(k + 1)} −θ _a ^(k) and Δθ _b ^(k) = θ _b ^{(k + 1)} −θ _b ^(k) are set. Since each modulation phase is either 0 or π / 2, the phase differences Δθ _a ^(k) and Δθ _b ^(k) are either −π / 2, 0, or π / 2.

Here, Δφ _a ^(k) + Δφ _b ^(k) is considered. If the expressions (2a) and (2b) are substituted as they are, the following expression is obtained.
(3) Δφ _a ^(k) + Δφ _b ^(k)
= {Φ _s ^{(k + 1)} −φ _s ^(k) + Δθ _a ^(k) } + {φ _i ^{(k + 1)} −φ _i ^(k) + Δθ _b ^(k) }
= {Φ _s ^{(k + 1)} + φ _i ^{(k + 1)} } − {φ _s ^(k) + φ _i ^(k) } + {Δθ _a ^(k) + Δθ _b ^(k) }
Incidentally, the phase relationship of the k-th pulse is φ _s ^(k) + φ _i ^(k) = φ _p ^(k) + φ ₀ as shown in the equation (1). Substituting this into equation (3) yields:
(4) Δφ _a ^(k) + Δφ _b ^(k) = φ _p ^{(k + 1)} −φ _p ^(k) + {Δθ _a ^(k) + Δθ _b ^(k) }
Here, it is assumed that the coherence time of the pump light source 12 is sufficiently longer than the time interval of the pulse train. Then, φ _p ^{(k + 1)} −φ _p ^(k) = 0, and the equation (4) becomes the following equation.
(5) Δφ _a ^(k) + Δφ _b ^(k) = Δθ _a ^(k) + Δθ _b ^(k)
As described above, since the phase differences Δθ _a ^(k) and Δθ _b ^(k) are either −π / 2, 0, or π / 2, in the equation (5), (Δθ _a ^(k) + Δθ _b ^(k) ) is either -π / 2, 0, π / 2 or π.

  Now, in the receiver, photons are detected according to the phase difference between the kth pulse and the (k + 1) th pulse. If detected by the detectors A1 and B1, the phase difference is π, and if detected by the detectors A2 and B2, the phase difference is 0.

For example, in the receiver A, the detector A1 detects a photon. At this time, Δφ _a ^(k) = π. By the way, as described above, (Δθ _a ^(k) + Δθ _b ^(k) ) is any one of −π / 2, 0, π / 2, or π. For example, (Δθ _a ^(k) + Δθ _b ^(k) ) = 0. Substituting this into equation (5) and assuming Δφ _a ^(k) = π, the following equation is obtained.
Δφ _b ^(k) = −Δφ _a ^(k) + Δθ _a ^(k) + Δθ _b ^(k) = −π

The above equation indicates that in the receiver B, the detector B1 detects photons. Similarly, if (Δθ _a ^(k) + Δθ _b ^(k) ) = π, Δφ _b ^(k) = 0, and in this case, the detector B1 detects photons. Further, when (Δθ _a ^(k) + Δθ _b ^(k) ) = ± π / 2, Δφ _b ^(k) = ± π / 2 (note that −3π / 2 = π / 2). In this case, the phase difference is intermediate between 0 and π, and it is not determined which detector detects the photon. In some cases, B1 detects the photon, and in other cases, B2 detects the photon. Become.

  In the above description, in the receiver A, the case where the detector A1 detects photons has been described. The case where the detector A2 detects photons can be considered in the same manner. In summary, the relationship of photon detection in each case is as shown in Table 1.

  In Table 1, “B1” is an event in which the detector B1 positively detects photons, “B2” is an event in which the detector B2 positively detects photons, and “B1 / B2” Represents an event that is detected or stochastic.

Utilizing the above configuration and photon detection characteristics, a secret key is generated by the following procedure.
(1) The entangled light source 10 transmits a signal photon pulse train to the receiver A and an idler photon pulse train to the receiver B.
(2) Each of the receivers A and B detects a photon. At that time, the photon detection time, which detector detected the photon, and the modulation phase at that time are recorded.
(3) After transmission / reception of photons, the receivers A and B inform each other of the photon detection time and the modulation phase.
(4) The receivers A and B generate bits from the detection result of detecting photons at the same time as follows.
When (Δθ _a ^(k) + Δθ _b ^(k) ) = 0: Bits are generated as follows:
{A1 is photon detection, B1 is photon detection} = “0”
{A2 is photon detection, B2 is photon detection} = “1”
When (Δθ _a ^(k) + Δθ _b ^(k) ) = π: Bits are generated as follows:
{A1 is photon detection, B2 is photon detection} = "0"
{A2 is photon detection, B1 is photon detection} = “1”
When (Δθ _a ^(k) + Δθ _b ^(k) ) = ± π / 2: Nothing is done.

  Since the photon detection results of the receivers A and B have the correlation shown in Table 1, the bit values generated by the above procedure are the same for both. In the above procedure, since the receivers A and B do not exchange information on which detector has detected the photon, the generated bit value is not known to the outside. Therefore, this is a secret key bit.

JP 2006-179982 A

  In the above conventional example, in order to correctly generate the key bit, it is necessary that the signal / idler photon emitted from the entangled light source 10 is 1 pair or less per pulse.

  For example, if two pairs are output, it may happen that receiver A detects the first pair of signal photons and receiver B detects the second pair of idler photons. In this case, since the phase relationship shown in the equation (1) does not always hold between the signal / idler photons of different pairs, the above operating principle does not work, and the receivers A and B make the same key bit. I can't. In order to prevent this malfunction, the number of photon pairs per pulse must be 1 or less.

By the way, the number of photon pairs output from the entangled light source 10 follows a Poisson distribution. In this case, the probability number output pair is 2 or more, and the number average pair and mu, the approximate the mu ^2/2. In order to make this probability small enough to be ignored, it is necessary to make the average number of pairs μ sufficiently smaller than 1 (for example, 0.1 or less). In this case, the number of photons detected by the receiver is inevitably reduced, and the key bit generation efficiency is lowered. That is, in the conventional example, the malfunction probability and the key generation rate are in a trade-off relationship.

  It is also desirable if a quantum key distribution system can be constructed without using a quantum entangled light source.

  The present invention has been made in view of such problems, and an object of the present invention is to provide a quantum key distribution system capable of obtaining a high key generation rate while suppressing a malfunction probability using a quantum entangled light source. There is.

  In order to achieve the above object, the present invention provides a quantum cryptography device for sharing a secret key between two receivers, which is an optical pulse train having a time interval T. A light source that outputs an optical pulse train having one or less photons per pulse, an optical coupler that branches an optical pulse from the light source, and a branched optical pulse that is received from the optical coupler. Each of the receivers includes a phase modulation unit that performs phase modulation on the received optical pulse, and causes the optical pulse subjected to the phase modulation to interfere with each other at a time interval T, thereby causing an interference state. And a secret key for generating a secret key common to each other by exchanging phase modulation information in the phase modulation means and photon detection time information in the photon interference detection means Characterized in that a forming means.

  The invention according to claim 2 is the quantum cryptography device according to claim 1, wherein the phase modulation means performs phase modulation of 0 or π / 2.

  The invention according to claim 3 is the quantum cryptography apparatus according to claim 1 or 2, wherein the photon interference detecting means includes an optical branching means for branching an optical pulse from the phase modulating means, An optical delay unit that gives a delay of a time interval T between the branched paths, an optical multiplexing unit that combines optical pulses from the optical delay unit, and a photon that detects photons from the multiplexing unit according to an interference state And a detector.

  The invention according to claim 4 is the quantum cryptography apparatus according to any one of claims 1 to 3, wherein each of the receivers is in phase modulation information in the phase modulation means and in the photon interference detection means. It further comprises wiretapping detection means for detecting presence / absence of wiretapping by exchanging photon detection time information to generate and collate test bits common to each other.

  The invention according to claim 5 is a quantum cryptography device for sharing a secret key between two receivers, which is an optical pulse train at a time interval T, and the number of photons per pulse is 1 or less. A light source that outputs an optical pulse train, an optical coupler that branches an optical pulse from the light source, and two receivers that are configured to receive the branched optical pulse from the optical coupler. The photon detector detects a photon by branching a part of the received optical pulse and detects the photon by interfering with the remaining optical pulse at a time interval T and detecting the photon according to the interference state. By exchanging photon detection time information in the photon interference detection means and an eavesdropping detection means for detecting the presence or absence of eavesdropping by exchanging photon detection time information in the photon detection means Characterized by comprising a private key generating means for generating a private key.

  The invention according to claim 6 is the quantum cryptography apparatus according to claim 5, wherein the photon interference detecting means includes an optical branching means for branching the remainder of the received optical pulse, and the branched path. An optical delay means for providing a delay of a time interval T between them, an optical multiplexing means for multiplexing optical pulses from the optical delay means, and a photon detector for detecting photons from the multiplexing means according to an interference state; It is provided with.

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. Throughout the drawings, the same constituent elements will be described with the same reference numerals.

(First embodiment)
FIG. 2 shows a configuration example of the quantum key distribution system according to the first embodiment of the present invention. In this system, a single photon light source 110 and an optical coupler 112 are placed at an intermediate point between two receivers A and B. The single photon light source 110 is a light source that emits only one photon per pulse, and is realized, for example, by taking out spontaneously emitted photons between specific levels with an optical filter or the like. From this single photon light source, a photon pulse train having a constant interval T is output. This photon pulse train is branched into two paths by the optical coupler 112, one being transmitted to the receiver A via the transmission path A and the other being transmitted to the receiver B via the transmission path B.

  Each receiver phase-modulates the transmitted photon pulse train at random by 0 or π / 2 by the phase modulators 122 and 132, and then bifurcates it by the optical couplers C1 and C3. Then, after giving a time delay between the branched paths, the signals are multiplexed again by the second optical couplers C2 and C4. Here, it is assumed that the delay time given between the branch paths is equal to the pulse interval T of the transmitted optical pulse train. Photon detectors A1 and A2, B1, and B2 are provided at two output ends of the multiplexing coupler, respectively.

In the above configuration, the following eight paths can be considered for the path of photons from the single photon light source 110 to the photon detector.
(1) Transmission path A → short path of receiver A → detector A1
(2) Transmission path A → short path of receiver A → detector A2
(3) Transmission path A → Long path of receiver A → Detector A1
(4) Transmission path A → Long path of receiver A → Detector A2
(5) Transmission path B → short path of receiver B → detector B1
(6) Transmission path B → short path of receiver B → detector B2
(7) Transmission path B → long path of receiver B → detector B1
(8) Transmission path B → long path of receiver B → detector B2

Here, the arrival time of the photon that has passed through each path to the detector will be considered. For convenience of explanation, assuming that the two transmission paths A and B have the same length, among the above, in the paths (1), (2), (5) and (6), the arrival times of the photons are equal, and the path In (3), (4), (7), and (8), the photon arrival time is delayed by one pulse (= delay time T in the receiver). From a different perspective, it emits a single photon light source at time t ₀ , emits a single photon light source at time t ₀ + T and a photon that has passed through path (3), (4), (7) or (8), Photons that have passed through path (1), (2), (5) or (6) reach the detector at the same time.

For example, it is assumed that the receiver A is the detector A1 and the receiver B is the detector B1, and the photon is detected at the same time. There are two possible patterns that cause this detection event. One is when the photons passing through the path (5) emits a single photon source photons and time t ₀ + T passing through the path (3) emits a single photon source at time t ₀ is detected, the other is In this case, a photon emitted from the single photon light source at time t ₀ + T and passed through the path (1) and a photon emitted from the single photon light source at time t ₀ and passed through the path (7) are detected. In the receiver, it cannot be distinguished which of the two patterns results in the simultaneous detection of photons.

  According to the principle of quantum mechanics, in such a situation, the probability amplitudes of the two patterns interfere with each other. For example, when Young's double slit interference experiment is performed at the one-photon level, the probability amplitudes when the photon passes through the first slit and the second slit interfere with each other. As the probability amplitude, the standardized complex display of the photoelectric field of normal light (classical light) is applied, and the photon detection probability follows the absolute value square of the sum of the two probability amplitudes. As a result, interference fringes appear on the photon detection histogram in accordance with the phase difference between the two probability amplitudes.

  Similar things happen here. That is, the probability amplitudes of the two patterns interfere with each other. The expression (expression method) in which the photoelectric field of normal light propagates is applied to the probability amplitude of each pattern. In the following, how interference of probability amplitude occurs will be described using equations. However, for the sake of simplicity, the loss of the transmission line and the optical circuit is not considered.

First, consider the probability amplitude for each pattern. First pattern, i.e., in the case of photons passing through a path (5) emits a single photon source photons and time t ₀ + T passing through the path (3) emits a single photon source at time t ₀ is detected Regarding the probability amplitude, the probability amplitude of the photon that has passed through the path (3) and the probability amplitude that has passed through the path (5) may be considered, and the two may be multiplied.

The probability amplitude a ₃ of the path (3) is expressed by the following equation.

Similarly, the probability amplitude a ₅ of the route (5) is expressed by the following equation.

Here, L _B is the length of the transmission line B, θ _b1 is the modulation phase applied by the receiver B to the photons emitted at time t ₀ + T, and L _b2 is the short path of the receiver B. Is the length of

From the equations (6) and (7), the probability amplitude a ₃₅ of the first pattern is given by the following equation as a product of the probability amplitude a ₃ of the route (3) and the probability amplitude a ₅ of the route (5).

Next, a second pattern, that is, a photon that has emitted a single photon light source at time t ₀ + T and passed through path (1) and a photon that has emitted a single photon light source at time t ₀ and passed through path (7) are detected. Consider the probability amplitude when First, the probability amplitude a ₁ of the route (1) is expressed by the following equation.

Here, θ _a1 is the modulation phase applied by the receiver A to the photons emitted at time t ₀ + T, and L _a2 is the length of the short path of the receiver A. On the other hand, the probability amplitude a ₇ of the route (7) is expressed by the following equation.

Here, θ _b0 is the modulation phase applied by the receiver B to the photon emitted at time t ₀ , and L _b1 is the length of the long path of the receiver B. Accordingly, the probability amplitude a ₁₇ of the second pattern is given by the following equation.

  As described in the example of Young's interference at the one-photon level, the two patterns interfere with each other, resulting in a simultaneous photon detection event. The appearance probability is represented by the square of the absolute value of the sum of the probability amplitudes of the two patterns, and is given by the following equation.

Here, k (L _a1 −L _a2 + L _b2 −L _b1 ) = 0 is set by setting the path state in the receiver. Then, the above equation becomes as follows.

In the above equation, Δθ _a = θ _a0 −θ _a1 and Δθ _b = θ _b0 −θ _b1 are set.

  In the above, the phenomenon in which the detector A1 and the detector B1 simultaneously detect photons has been considered. Similarly, the simultaneous detection probability of A1 and B2, the simultaneous detection probability of A2 and B1, and the simultaneous detection probability of A2 and B2 can also be obtained. If only the result is written, the following formula is obtained.

By the way, in the receiver, each pulse is phase-modulated by 0 or π / 2. Therefore, Δθ _a is −π / 2, 0 or π / 2, Δθ _b is −π / 2, 0 or π / 2, and Δθ _a −Δθ _b is −π / 2, 0, π / 2 or π (note that −π = π). Substituting this into equations (12)-(15) gives the following simultaneous detection probabilities:
When Δθ _a −Δθ _b = 0:
(A1-B1 simultaneous detection) = (A2-B2 simultaneous detection) = 1/16
(A1-B2 simultaneous detection) = (A2-B1 simultaneous detection) = 0
When Δθ _a −Δθ _b = π:
(A1-B1 simultaneous detection) = (A2-B2 simultaneous detection) = 0
(A1-B2 simultaneous detection) = (A2-B1 simultaneous detection) = 1/16
When Δθ _a −Δθ _b = ± π / 2:
(A1-B1 simultaneous detection) = (A2-B2 simultaneous detection) = (A1-B2 simultaneous detection) = (A2-B1 simultaneous detection) = 1/32

The above results show that when receivers A and B detect photons at the same time, if Δθ _a −Δθ _b = 0, detectors A1 and B1 detect photons, or detectors A2 and B2 detect photons. If Δθ _a −Δθ _b = π, either detectors A1 and B2 detect photons or detectors A2 and B1 detect photons , Δθ _a −Δθ _b = ± π / 2 indicates that photons can be detected in all combinations.

Using the above configuration and photon detection characteristics, a secret key can be generated by the following procedure.
(1) The single photon light source 110 transmits a photon pulse train to the receivers A and B.
(2) Each of the receivers A and B detects a photon. At that time, the photon detection time, which detector detected the photon, and the modulation phase at that time are recorded.
(3) After transmission / reception of photons, the receivers A and B inform each other of the photon detection time and the modulation phase.
(4) The receivers A and B generate bits from the detection result of detecting photons at the same time and the modulation phase of the corresponding pulse as follows.
When Δθ _a −Δθ _b = 0: bits are generated as follows:
{A1 is photon detection, B1 is photon detection} = “0”,
{A2 is photon detection, B2 is photon detection} = “1”,
When Δθ _a −Δθ _b = π: Generate bits as follows:
{A1 is photon detection, B2 is photon detection} = “0”,
{A2 is photon detection, B1 is photon detection} = “1”,
When Δθ _a −Δθ _b = ± π / 2: Nothing is done.

  Since the photon detection results of the receivers A and B have the correlation as described above, the bit values generated by the above procedure are the same in both. In the above procedure, since the receivers A and B do not exchange information on which detector has detected the photon, the generated bit value is not known to the outside. Therefore, this can be used as a secret key bit.

Next, the security of this secret key against eavesdropping will be described. The simplest eavesdropping method is that an eavesdropper extracts a photon from a transmission path and steals information. However, this wiretapping method is not successful because key generation is performed by a single photon. In the present invention, one receiver detects a single photon emitted at time t ₀ , and the other receiver detects a single photon emitted at time t ₀ + T, thereby generating a secret key. ing. If an eavesdropper extracts either photon, the two receivers do not both detect the photon, and therefore no key bit is generated. Therefore, the secret key has not been eavesdropped.

  As a more advanced wiretapping method, a method in which an eavesdropper detects all photons on the transmission path and retransmits a camouflaged signal based on the detection result can be considered. This wiretapping is successful if the camouflaged signal gives the receiver the same detection result as normal. However, the photon detection result depends on the modulation phase for each pulse. Since the eavesdropper does not know the modulation phase of the receiver in advance, it cannot always send a camouflaged signal that is correct. If a key bit is generated from a camouflaged signal different from the normal one, a bit mismatch occurs between the two receivers. Therefore, by collating several test bits, the receivers A and B can confirm the presence or absence of eavesdropping. In other words, if the test bit is normal, it is guaranteed that the key is not tapped.

  One of the features of this embodiment is that a single photon light source is used for optical pulse train generation. This light source ideally outputs only one photon per pulse. Therefore, unlike the prior art, there is no malfunction due to the generation of multiple photons, and therefore it is not necessary to reduce the average number of output photons to avoid the generation of multiple photons. For this reason, it is possible to obtain a higher key generation rate than in the prior art.

(Second Embodiment)
FIG. 3 shows a configuration example of a quantum key distribution system according to the second embodiment of the present invention. This system is basically the same as that of the first embodiment, but the receiver does not perform phase modulation, and instead, a part of the received light is branched by the optical couplers 222 and 232, and the photon detector A3. , B3 is different in that photon detection is directly performed on the optical pulse train. The photon detectors A3 and B3 are not used for generating a secret key but are used as a signal light monitor for detecting eavesdropping. In FIG. 3, the signal light is always monitored by the optical couplers 222 and 232, but the present invention is not limited to this, and the reception path and the monitoring path are intermittently replaced by an optical switch instead of the optical couplers 222 and 232. May be switched. The role of the monitor detectors A3 and B3 will be described later.

Since the configuration according to the second embodiment is basically the same as the configuration according to the first embodiment except that the phase of each pulse is not modulated with respect to the signal detection system, the detectors A1, A2 and B1, The photon detection characteristics of B2 are the same as when Δθ _a −Δθ _b = 0 in equations (12)-(15). That is, the simultaneous detection probability is as follows.
(A1-B1 simultaneous detection) = (A2-B2 simultaneous detection) = α / 16
(A1-B2 simultaneous detection) = (A2-B1 simultaneous detection) = 0

Here, α is a constant representing the rate of decrease for the portion branched for monitoring. Although the probability of simultaneous detection varies depending on the splitting of the monitor light, the correlation between the detectors that cause simultaneous detection is the same as in the case of Δθ _a −Δθ _b = 0 in the first embodiment.

Using this configuration and the photon detection characteristics, a secret key can be generated by the following procedure.
(1) The single photon light source 110 transmits a photon pulse train to the receivers A and B.
(2) Each of the receivers A and B detects a photon. At that time, the time when the photon was detected and the detector with which the photon was detected are recorded.
(3) After transmitting and receiving photons, the receivers A and B inform the photon detection time to each other.
(4) The receivers A and B generate bits as follows when photons are detected at the same time.
{A1 is photon detection, B1 is photon detection} = “0”
{A2 is photon detection, B2 is photon detection} = “1”
From the correlation characteristics at the time of simultaneous detection, the bits generated as described above are the same in the two receivers. Therefore, this can be used as a secret key.

  Next, the security against eavesdropping of this secret key will be described. An eavesdropping method in which an eavesdropper extracts photons from the transmission path is not successful for the same reason as in the first embodiment. On the other hand, for the eavesdropping method in which the eavesdropper detects all photons on the transmission path and retransmits the camouflaged signal based on the detection result, this embodiment does not perform phase modulation, and thus is the same as the first embodiment. This method cannot detect this. In order to cope with this eavesdropping method, the photon detectors A3 and B3 for monitoring are used in this embodiment.

  A pulse train in which the output from the single photon light source is branched into two is directly input to the monitoring detectors A3 and B3. In this case, since one photon is not split into two, if the detector A3 detects the photon, the detector B3 does not detect the photon with the corresponding pulse. If the detector B3 detects a photon, the detector A3 will not detect the photon with the corresponding pulse. That is, the simultaneous detection probability is zero.

However, the situation changes when the above-described wiretapping is performed. In the present invention, by utilizing the interference phenomenon of photon states in which emitted from the light source to the photon states and time t ₀ + T, which emitted from the light source at time t _0, since the generating the key bit, eavesdropper retransmits the disguise signal Thus, in order to produce the same interference result as normal, it is necessary to send a continuous pulse in which photons can exist in either case. However, in this case, it is a pulse train emitted from another light source that reaches each receiver.

  Then, the monitor detectors A3 and B3 of the two receivers can simultaneously detect photons. This cannot occur at the normal time when a photon is detected from a pulse train from one single photon light source. Therefore, if the monitor detector detects simultaneously, it can be determined that there is an eavesdropping. Thereby, eavesdropping can be detected. In other words, if there is no abnormality in monitor photon detection, it is guaranteed that the key is not tapped.

  According to the present invention, a high secret key generation rate can be realized without reducing the number of output photons from the light source.

  The present invention has been described above with respect to several embodiments. However, in view of the many possible embodiments to which the principles of the present invention can be applied, the embodiments described herein are merely illustrative, It is not intended to limit the scope of the invention. The configuration and details of the embodiment exemplified here can be changed without departing from the spirit of the present invention. Further, the illustrative components and procedures may be changed, supplemented, or changed in order without departing from the spirit of the invention.

It is a figure which shows the structural example of the conventional quantum key distribution system. It is a figure which shows the structural example of the quantum key distribution system which concerns on the 1st Embodiment of this invention. It is a figure which shows the structural example of the quantum key distribution system which concerns on the 2nd Embodiment of this invention.

Explanation of symbols

DESCRIPTION OF SYMBOLS 10 Quantum entangled light source 12 Pump light source 14 Optical nonlinear medium 16 Optical filter 20, 30 Receiver 22, 32 Phase modulator 110 Single photon light source 112 Optical coupler 120, 130, 220, 230 Receiver 122, 132 Phase modulator 222, 232 Optical coupler A1, A2, A3, B1, B2, B3 Photon detector C1, C2, C3, C4 Optical coupler

Claims (6)

A quantum cryptography device for sharing a secret key between two receivers,
A light source that outputs an optical pulse train that is an optical pulse train at a time interval T and that has one or less photons per pulse;
An optical coupler for branching a light pulse from the light source;
Two receivers configured to receive the branched light pulses from the optical coupler, each receiver comprising:
Phase modulation means for performing phase modulation on the received optical pulse;
Photon interference detection means for causing the optical pulse subjected to the phase modulation to interfere at a time interval T and detecting photons according to the interference state;
A quantum cryptography apparatus comprising: a secret key generation unit configured to generate a secret key common to each other by exchanging phase modulation information in the phase modulation unit and photon detection time information in the photon interference detection unit. The quantum cryptography device according to claim 1,
The quantum cryptography apparatus characterized in that the phase modulation means performs phase modulation of 0 or π / 2. The quantum cryptography device according to claim 1 or 2,
The photon interference detection means includes
Optical branching means for branching the optical pulse from the phase modulation means;
Optical delay means for providing a delay of a time interval T between the branched paths;
Optical multiplexing means for multiplexing optical pulses from the optical delay means;
A quantum cryptography apparatus comprising: a photon detector that detects a photon from the multiplexing unit according to an interference state. The quantum cryptography device according to any one of claims 1 to 3,
Each receiver detects the presence or absence of eavesdropping by exchanging the phase modulation information in the phase modulation means and the photon detection time information in the photon interference detection means, and generating and collating common test bits. A quantum cryptography apparatus further comprising an eavesdropping detection means. A quantum cryptography device for sharing a secret key between two receivers,
A light source that outputs an optical pulse train that is an optical pulse train at a time interval T and that has one or less photons per pulse;
An optical coupler for branching a light pulse from the light source;
Two receivers configured to receive the branched light pulses from the optical coupler, each receiver comprising:
Photon detection means for branching a part of the received light pulse to detect photons;
Photon interference detecting means for causing the remainder of the received light pulse to interfere at a time interval T and detecting photons according to the interference state;
Eavesdropping detection means for detecting the presence or absence of eavesdropping by exchanging photon detection time information in the photon detection means,
A quantum cryptography apparatus comprising: a secret key generation unit configured to generate a secret key common to each other by exchanging photon detection time information in the photon interference detection unit. The quantum cryptography device according to claim 5,
The photon interference detection means includes
Optical branching means for branching the remainder of the received optical pulse;
Optical delay means for providing a delay of a time interval T between the branched paths;
Optical multiplexing means for multiplexing optical pulses from the optical delay means;
A quantum cryptography apparatus comprising: a photon detector that detects a photon from the multiplexing unit according to an interference state.

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