JP4523365B2 - Quantum cryptographic communication device - Google Patents

Description

  The present invention relates to a quantum cryptography communication device, and more particularly, to a quantum cryptography communication device capable of supplying a quantum cryptography key to a transmitter and a receiver that are separated from each other by a longer distance than before.

  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 one of encryption methods for supplying a secret key for performing cryptographic communication between two communication devices located at distant points, and is also called quantum key distribution (Patent Document 1). Although there are various methods for quantum key distribution, here, a differential phase shift quantum key distribution method similar to the present invention will be described (Non-patent Document 1).

  FIG. 3 shows a basic configuration of a conventional differential phase shift quantum key distribution device. The transmitter 1 transmits a coherent optical pulse train 2 having a constant interval that is randomly phase-modulated with 0 or π as an average of less than one photon per pulse. This state where the average number of photons is less than one is realized by greatly attenuating normal laser light. When such a pulse train 2 is photon detected, a photon is detected with a certain pulse, but nothing is detected with a certain pulse. At which pulse photons are detected is quite probabilistic and uncertain until detected.

  The pulse train transmitted from the transmitter 1 reaches the receiver 4 through the transmission path 3. The receiver 4 branches the received pulse train into two by an optical branching device (optical coupler) 5, adds a delay to one by an optical delay circuit (optical delay line) 6, and then a 2 × 2 optical coupler (optical multiplexing). Combine) again by 7). The two output ports of the multiplexing coupler 7 are provided with a first photon detector 8 and a second photodetector 9, respectively.

  Here, the delay time given to one of the branching / combining circuits 5 to 7 is assumed to be equal to the time interval of the input pulse train. If it does in this way, in the multiplexing coupler 7, the front and back pulses will be overlapped and combined. The input pulse train is phase-modulated by 0 or π. Therefore, if the propagation phase of the branching / combining path is appropriate, the phase difference between the overlapping pulses is 0 or π. If the phase difference is 0 as a result of the interference, the first photon detector 8 detects the photon, and if the phase difference is π, the second photon detector 9 detects the photon.

  Using the above configuration, the transmitter 1 and the receiver 4 obtain a secret key by the following procedure. First, the receiver 4 detects photons by the above reception configuration. At this time, the detected time and the detector (either 8 or 9) are recorded. After the necessary number of photons are transmitted and received, the receiver 4 informs the transmitter 1 of the photon detection time. The transmitter 1 can know from which detector the receiver 4 has detected a photon from the notified detection time and its own phase modulation data.

  Therefore, if the first photon detector 8 is preliminarily determined to be bit “0” and the second photon detector 9 is preliminarily determined to be bit “1”, the transmitter 1 and the receiver 4 can obtain the same bit string. In this procedure, only the photon detection time is notified from the receiver 1 to the transmitter 4, and no bit information is output to the outside. Therefore, the bit information is not eavesdropped by another person through another receiver (not shown). Also, since the light that is being transmitted is an average of less than one photon per pulse, other receivers cannot branch part of the signal to obtain bit information. Because the photon is not divided into two, when another receiver detects the photon by branching, the photon does not reach the receiver 4, but the bit string transmitted by the transmitter 1 and the bit string received by the receiver 4 This is because does not match. Thus, the bit string obtained by the transceivers 1 and 4 by the above configuration and procedure is a bit string that is not wiretapped from the outside. Therefore, this bit string is a secret key for generating / reproducing the encrypted data.

JP 2004-187268 A K. Inoue, E. Waks, and Y. Yamamoto, “Differential-phase-shift quantum key distribution using coherent light”, Physical Review A, vol. 68, paper number 022317 (2003).

  In the conventional differential phase shift quantum key distribution system as described above, a single-photon level optical pulse is transmitted and received in order to avoid eavesdropping due to branching. By the way, there is a loss in the actual transmission path 3, and for this reason, some of the photons are lost in the transmission path 3 and do not reach the receiver 4. When the transmission line 3 is long, the loss is large, and the probability that the photon reaches the receiver 4 is small. On the other hand, in general, the photon detectors 8 and 9 sometimes operate as if photons were detected even though no photons were actually input. The malfunction of the photon detectors 8 and 9 can be ignored if the number of input photons is large to some extent. However, if the number of input photons is small, the number of erroneous signals due to malfunction increases relatively, and a correct secret key cannot be generated. That is, if the transmission distance is long and the loss is large, the quantum key distribution system does not operate correctly, and thus the transmission distance is limited.

  The present invention has been made in view of the above points, and an object of the present invention is to provide a quantum encryption key to a transmitter and a receiver that are separated from each other by a distance longer than a conventional limit distance that enables quantum key distribution. An object of the present invention is to provide a quantum cryptography communication device that can be supplied.

To achieve the above object, a first aspect of the invention, and encrypts the data between a plurality of transceivers, in a quantum cryptography communication apparatus supplies a secret key for decoding, the nonlinear optical crystal pump light and quantum correlated photon pair generator for outputting a pair of photons consisting of a signal photon and idler photon with quantum correlation obtained simultaneously by irradiating a pulse train having a constant time interval, the quantum correlated photon pair generator or RaIzuru receiving a first transceiver of a receiver that receives a signal photon pulse train of photon pairs pulse train forces, the idler photon pulse train of the photon-to-pulse train which is the quantum correlated photon pair generator or RaIzuru force and a receiver of the second transceiver, the quantum correlated photon pair generator middle position between the receiver of the second transceiver and the receiver of the first transceiver Disposed, the first receiver of the receiver and the second transceiver transceivers, photons sent from said quantum correlated photon pair generator from photon detection events detected respectively at the same time, the secret A key is generated .

Here, the receiver of the first transmitter / receiver and the receiver of the second transmitter / receiver respectively include a branching unit that branches the input photon pulse train into two paths, and a photon branched by the branching unit. delay means for delaying one pulse to the other photon pulse train which is branched one pulse train, having input and output terminals of the 2 × 2, the other photon pulse train which is branched by the branching means that An optical coupler that inputs to the first input terminal and inputs the photon pulse train delayed by the delay means to the second input terminal, and a photon pulse train connected to the first output terminal of the optical coupler. First photon detection means for performing detection, and second photon detection means for detecting a photon pulse train connected to the second output terminal of the optical coupler may be provided.

Further, the quantum correlated photon pair generator, characterized in that said pump includes means for generating a light occurs simultaneously with said said signal photon constituting whether we said photon pair the pump light idler photons by the optical parametric process It can be.

  In general, the transmission distance of a signal transmission device is determined by the difference between transmission power and minimum reception sensitivity. Assuming that the photon pair generation efficiency of the quantum correlation photon pair generator according to the present invention and the photon transmission efficiency of the transmitter in the prior art (FIG. 3) are the same, from the quantum correlation photon pair generator to the receiver. The transmission distance is the same as the distance between the transmitter and the receiver in the prior art. Since the quantum correlation photon pair generator is placed at the intermediate point between the receivers A and B, the distance between the receivers A and B is twice the distance between the transmitter and the receiver in the prior art. That is, according to the present invention, the distance between two parties sharing a secret key can be made longer than before.

  The best mode for carrying out the present invention will be described below in detail with reference to the drawings.

  FIG. 1 shows a configuration of a quantum cryptography communication device according to an embodiment of the present invention. There is a pair of receivers A 41 and B 42 at remote points, and the quantum correlation photon pair generator 11 according to the present invention is disposed at an intermediate position between them. The configurations of the receivers A and B are almost the same as those of the conventional example described above with reference to FIG. 3, and are respectively an optical splitter (optical coupler) 5, an optical delay circuit (optical delay line) 6, and an optical multiplexer (optical coupler). 7. A pair of photon detectors 8 and 9 and an information exchange device 10 as means for exchanging information between the two receivers are provided. The information exchange device 10 is composed of a CPU and the like.

  The quantum correlation photon pair generator 11 has a pump light source 12 and an optical nonlinear medium 13, and always outputs two photons at the same time. Specifically, for example, two photons, that is, a signal photon and an idler photon, which will be described later, are output using an optical parametric process.

  Light from the pump light source 12 is input to the optical nonlinear medium 13, and signal photons and idler photons output from the optical nonlinear medium 13 are transmitted to the receivers A, B 41, and 42, respectively. Signals from photon detectors A1, A2, B1, B2 8, 8, 9, 9 are input to information exchange units (CPU) 10, 10, where the photon detection time and the detected detector are recorded. . Then, the receivers A, B 41, 42 of the other party are notified of the photon detection time via the information exchange devices (CPU) 10, 10.

A type of optical nonlinear phenomena and said optical parametric process, for example, if you enter the signal light of the pump light and the optical frequency f _s of the optical frequency f _p which is generated by the pump light source 12 to the second-order optical nonlinear crystal 13, nonlinear polarization _P = _c 2 frequency light E _{p E} ^{s *} from the optical frequency _f i _{= f} p -f _s is newly generated _{(c 2:} nonlinear susceptibility, _{E p:} pump light electric field, _{E s:} signal photoelectric Field, * is a complex conjugate). Conventionally, the newly generated light is called idler light. In terms of quantum mechanics, this is a phenomenon in which an idler photon is generated from a pump photon and a signal photon, and there is a relationship as shown in FIG. This relationship can be regarded as a process in which one pump photon (f _p ) disappears and one signal photon (f _s ) and one idler photon (f _i ) are generated. The signal light input promotes this process, but if the pump light is sufficiently strong, this process occurs spontaneously even without the signal light input.

  That is, a signal photon and an idler photon are generated from an input of only pump light. At this time, in order to satisfy the energy conservation law, the signal photon and the idler photon are always generated in pairs. A pair of photons having such a correlation is called a quantum correlation photon pair.

The quantum correlation photon pair generator 11 in FIG. 1 is a device that generates such a correlated photon pair. Note that, when a correlated photon pair is generated using an optical parametric process, a certain relationship is established between phases of pump light, signal light, and idler light that interact with each other as described later. As described above, classically, idler light is generated from nonlinear polarization of P = c ₂ E _p E _s ^* . Looking at the phase of this expression, there is a relationship of φ _i = φ _p −φ _s (φ _i : phase of idler polarization wave = idler light phase, φ _p : pump light phase, φ _s : signal light phase) I understand. This relationship also holds true for the quantum correlation photon pair generated by the optical parametric process. In the present embodiment, the coherence time of the pump light used in the quantum correlation photon pair generator 11 is assumed to be sufficiently longer than the time interval of the pulse train described below.

  In the configuration of FIG. 1, the quantum correlation photon pair generator 11 emits the quantum correlation photon pair as described above, the signal photons to the receiver A 41, the idler photons to the receiver B 42, the transmission paths 31, 32. These photons to be transmitted are assumed to be a pulse train at regular time intervals. Further, it is assumed that the generated signal photon / idler photon is one pair or less per average pulse.

  The pulses sent to the receivers A and B indicated by 41 and 42 are branched into two by C1 and C3 of the optical coupler (optical splitter) 5, and given a time delay to one, 2 × 2 each. It is combined by C2 and C4 of the optical coupler 7. Here, it is assumed that the delay time given by La and Lb of the delay circuit 6 is equal to the time interval of the transmitted pulse train. The two output ports of the optical coupler C2 are provided with photon detectors A1 and A2, and the two output ports of the optical coupler C4 are provided with photon detectors B1 and B2.

Using the above configuration, the receiver A 41 and the receiver B 42 can share a secret key. The principle and procedure will be described below using equations. Using the quantum mechanical representation, the output state | Ψ ₀ > from the quantum correlation photon pair generator 11 is expressed as follows:

Here, | A, t> is a state in which signal photons are transmitted toward receiver A 41 at time t, and | B, t> is a state in which idler photons are transmitted toward receiver B 42 at time t. Represents. The two states are quantum correlated and occur as pairs | A, t> | B, t>. a exp (iφ) is the probability amplitude (complex number) of one pair, where a is the amplitude part (real number) and φ is the phase. More specifically, φ is the sum φ = φ _s + φ _i of the phase φ _s of the signal photon state | A, t> and the phase φ _i of the idler photon state | B, t>. The photon pair is output as a pulse train, and each pulse is identified by a subscript j.

  The generated photons reach the receiver A 41 and the receiver B 42 through the transmission paths 31 and 32. The state at the receiver input stage is

Is described. Here, | IN _a , t> represents a state in which a signal photon is input to the receiver A at time t, and | IN _b , t> represents a state in which an idler photon is input to the receiver B at time t, t _j ″ is the time when the photon output from the photon pair source 11 reaches the receivers A and B at time t _j ′. Each state, the propagation phase phi _a of the photon pair source 11 to the receiver _A, the propagation phase phi _b from the photon pair source 11 to the receiver _{B, and} is added. For simplicity, the propagation loss was ignored. In order to consider the propagation loss, the coefficient a may be replaced with a corresponding value.

  Each state is branched by the optical couplers 5 and 7 and combined again. For example, when the one-photon state input to the receiver A is branched by the coupler C1, the state change due to the transmission of one optical coupler is:

It is expressed. | _S a, t> is state 1 photon short path _{S a} in the receiver A is outputted, _{| L} a, t> is state 1 photon long path _{L a} in the receiver A is output, It is. The same applies to each coupler. Further, in the path propagation, a propagation phase θ is added to each state. Furthermore, when each state change is described sequentially while considering the propagation time difference in the two-branch path, the state at the output end of the optical couplers C2 and C4 can be expressed as follows.

Here, | A1, t>, | A2, t>, | B1, t>, | B2, t> are states in which one photon is output to photon detectors A1, A2, B1, B2 at time t, in theta _Sa, short path _{S a} in theta _La each receiver a, propagation phase in the long path _{L a,} θ _Sb, θ _Lb is short path _{S b} of each receiver in the B, the propagation phase in the long path _{L b} is there. Time t _j is the time when the photon whose input time to the receiver is t _j ″ reaches the photon detector through the short path. Since the propagation time difference between the long and short paths is equal to the time interval of the input pulse train, the time when the photon whose input time to the receiver is t _j ″ reaches the photon detector through the long path is t _{j + 1.} ing. The above equation (4) consists of several terms. The first term time t _j to the receiver A, B are both state detecting photons, the second term time t _{j + 1} to the receiver A, B are both state detecting photons, the third term at time t _{j + 1} The receiver A is in a state in which the receiver B detects the photon at time t _j , and the fourth term represents the state in which the receiver A detects the photon at time t _j and the receiver B detects the photon at time t _{j + 1.} ing.

Here, attention is paid to an event in which the receivers A and B both detect photons at time t _j0 , that is, a state of | A, t _j0 > | B, t _j0 >. If j = j0 with reference to the first term of the above equation (4), this photon detection event occurs when j = j0 in the first term and j = j0-1 in the second term. It is a certain state. When these are extracted and the parentheses are further expanded, the following expression (5) is obtained.

In this _{case, Δφ j0 = φ j0-1 -φ j0} , Δθ a = θ La -θ Sa, are replaced by Δθ _b = θ _Lb -θ _Sb. In the above equation (5), φ _j0 is the phase of the photon pair state emitted from the photon pair source 11, more specifically, the sum of the phase φ _s of the signal photon state and the phase φ _i of the idler photon state φ = φ _s + φ _i . As described in the description of the quantum correlation photon pair generator 11, when generating a photon pair by the optical parametric phenomenon, φ _s + φ _i is equal to the pump light phase. In this embodiment, the coherence time of the pump light is sufficiently longer than the pulse interval of the generated photon pair. In this way, φ _s + φ _i for adjacent pulses has the same value, that is, φ _j0-1 = φ _j0 . Therefore, Δφ _j0 = 0. Further, in the above equation (5), it is set that Δθ _a + Δθ _b = 0. Such a setting is possible by adjusting the propagation phase of the branching / combining circuits of the receivers A and B.

  If it sets as mentioned above, a formula (5) will turn into a following formula (6).

  The above equation (6) indicates that photon detection by the first photon detector A1 8 of the receiver A 41, photon detection by the first photon detector B1 8 of the receiver B 42, or second of the receiver A 41. This means that the photon detection by the photon detector A29 of the photon and the photon detection by the second photon detector B29 of the receiver B42 are necessarily paired.

Using this photon detection event, the receivers A and B can have a common secret key by the following procedure.
(1) First, the receivers A and B receive the required number of photons. At this time, the photon detection time when the photon is detected and the detector of whether the photon is detected by 8 or 9 are recorded.
(2) The receivers A and B inform the time when the photons are detected.
(3) For the event that the receivers A and B detect photons at the same time, bit “0” is detected when the detectors A1 and B1 detect photons, and bit “0” is detected when the detectors A2 and B2 detect photons. 1 ”.
(4) When both photons A and B detect photons at the same time, the detector for detecting photons has the above-described correlation, that is, if the detector A1 detects photons, the detector B1 always detects photons, The relationship is established that whenever the detector A2 detects photons, the detector B2 detects photons. Therefore, the receivers A and B obtain the same bit string.

  In the above procedure, it is the detection time that is exchanged between the receivers A and B, and the detector information is not known to the outside. Therefore, this bit string can be used as a secret key.

(Other embodiments)
In the above, the preferred embodiments of the present invention have been described by way of example. However, the embodiments of the present invention are not limited to the above-described examples, and the constituent elements thereof are within the scope of the claims. Various modifications such as replacement, change, addition, and design change such as increase / decrease in number are all included in the embodiment of the present invention.

It is a block diagram which shows the structure of the quantum cryptography communication apparatus in one Embodiment of this invention. It is a conceptual diagram explaining the principle of a secondary optical parametric process. It is a block diagram which shows the structure of the quantum cryptography communication apparatus by the conventional differential phase shift quantum key distribution system.

Explanation of symbols

1 Transmitter 2 Pulse train 3 Transmission path 4 Receiver 5 Optical splitter (optical coupler)
6 Optical delay circuit (optical delay line)
7 Optical multiplexer (optical coupler)
8 First photon detector 9 Second photon detector 11 Quantum correlation photon pair generator (photon pair source)
12 Pump light source 13 Optical nonlinear medium 14 Transmission path 31, 32 Transmission path 41, 42 Receiver

Claims (3)

In a quantum cryptography communication device that supplies a secret key for encrypting and decrypting data transmitted and received between a plurality of transceivers ,
A quantum correlation photon pair generator that outputs a photon pair of signal photons and idler photons with quantum correlation obtained simultaneously by irradiating the nonlinear optical crystal with pump light as a pulse train of a constant time interval;
A receiver of the first transceiver for receiving a signal photon pulse train of the photon-to-pulse train which is the quantum correlated photon pair generator or RaIzuru force,
And a receiver of the second transceiver for receiving the idler photon pulse train of the photon-to-pulse train which is the quantum correlated photon pair generator or RaIzuru force,
The quantum correlated photon pair generator is disposed at an intermediate position between the receiver of the first transceiver and the receiver of the second transceiver ;
The secret key is generated from a photon detection event in which the receiver of the first transceiver and the receiver of the second transceiver respectively detect the photons transmitted from the quantum correlation photon pair generator at the same time. To
A quantum cryptography communication device characterized by the above. The receiver of the first transceiver and the receiver of the second transceiver are respectively
Branching means for branching the input photon pulse train into two paths;
Delay means for delaying one of the photon pulse trains branched by the branching means by one pulse with respect to the other of the branched photon pulse trains;
2 × 2 input / output terminals, the other of the photon pulse trains branched by the branching means is input to the first input terminal, and the photon pulse train delayed by the delaying means is the second input An optical coupler that inputs to the terminal;
First photon detection means connected to the first output terminal of the optical coupler for detecting a photon pulse train;
2. The quantum cryptography communication device according to claim 1, further comprising: a second photon detection unit that is connected to a second output terminal of the optical coupler and detects a photon pulse train. The quantum correlated photon pair generator includes means for generating said pump light, characterized in that generated by optical parametric process and the signal photon constituting the pump light or al the photon pairs and said idler photons simultaneously The quantum cryptography communication device according to claim 1 or 2.

Images