CROSS REFERENCE TO RELATED APPLICATIONS

This Application is a National Stage of International Application No. PCT/JP2017/031456 filed Aug. 31, 2017, claiming priority based on Japanese Patent Application No. 2016-176364 filed Sep. 9, 2016, the disclosure of which is incorporated herein in its entirety by reference.

TECHNICAL FIELD

This invention relates to a transmitting device, a receiving device, a quantum key distribution method, and a quantum key distribution program for a quantum key distribution system.

BACKGROUND ART

In recent years, as a method for achieving encrypted communication having absolute safety against an eavesdropping action, quantum key distribution (Quantum Key Distribution, QKD) (Non-Patent Document 1) has been actively studied and developed for practical use.

Non-Patent Document 1 proposes to use a single-photon light source as a light source for the QKD. At the present stage, however, the single-photon light source is on the road to development and does not reach a practical level. Therefore, as a substitute for the single-photon light source, the mainstream is a method of using, as a pseudo-single-photon light source, a laser light source having a weakened intensity obtained by extremely weakening an intensity of a general laser light source.

However, in a case of the laser light source, which is different from an ideal single-photon light source, two or more photons are generated in one pulse at a constant probability. As a result, there is a possibility of being subjected to an eavesdropping attack called a photon number splitting attack (Photon Number Splitting attack, PNS attack) in which information corresponding to one photon is eavesdropped from two or more photons. Therefore, it is pointed out that, in a case where the laser light source is used, safety of the QKD is significantly detracted (Non-Patent Document 2).

Thus, as means for avoiding the PNS attack, a decoy method is proposed (Non-Patent Document 3). The decoy means a “lure” and the decoy method is a method which can detect presence or absence of the PNS attack by changing, at intervals, an intensity of optical pulses used in the QKD. It is considered that implementation of the decoy method is important in recent development of practical QKD.

In order to implement the decoy method into the QKD, it is necessary to use three or more types of optical intensities. It is known that, in the decoy method, the safety of the QKD is improved when a greater number of types of the intensities are used. However, taking the degree of difficulty of implementation into account, in general, the three types of optical intensities are frequently used in the decoy method. For that reason, description will hereinafter be made about the decoy method using the three types of optical intensities.

It is assumed that s, d, and v represent average photon numbers contained per one pulse in optical pulses of the three types of optical intensities, respectively. Herein, s is equal to about 0.5 [photons/pulse] in a case of a typical QKD system for 50 km transmission. d is equal to about 40% of s, namely, d is equal to about 0.2 and v is equal to 0 (vacuum). In this case, optical pulses of the average photon number s are used as signal lights and information obtained from the signal lights is used as a quantum encryption key. The optical pulses of the average photon numbers d and v are used as decoy lights (which may also be called decoy pulses) for detecting the eavesdropping. That is, in the decoy method, the optical pulses having the largest intensity among the three types of the optical pulses are used as the signal lights. Hereinafter, “optical pulses of the average photon number s (d or v)” will simply be represented as S (D or V). In the QKD system using the decoy method, most of transmission pulses are S and the eavesdropping is detected by mixing D and V as a part thereof. As a typical example, in the transmission pulses, S is about 90%, D is about 5%, and V is about 5%.

FIG. 1is a schematic view showing an example of a train of optical pulses transmitted in the decoy method. As shown inFIG. 1, the train of optical pulses comprises S as a large number of signal lights and V and D as a small number of decoy pulses.

PRIOR ART DOCUMENT(S)

Non-Patent Document(s)

Non-Patent Document 1: Bennett and Brassard, IEEE Int. Conf. on Computers, Systems, and Signal Processing, Bangalore, India, p. 175 (1984)Non-Patent Document 2: N. Lutkenhaus, Physical Review A, Vol. 61, 052304 (2000)Non-Patent Document 3: W. Y. Hwang, Physical Review Letters, Vol. 91, 057901 (2003)

SUMMARY OF THE INVENTION

Problem to be Solved by the Invention

In a case where the decoy method is used in the quantum key distribution, it is known that, when the intensity of the optical pulses fluctuates, an amount of information leakage to an eavesdropper increases and a generation efficiency of an encryption key significantly degrades. Specifically, when the intensity of the decoy pulses D fluctuates by 5%, the generation efficiency of the encryption key degrades to about 50% in comparison with a case of no fluctuation in the decoy pulses D.

On the other hand, a recent quantum key distribution system uses a high-speed electronic circuit operable with a clock frequency exceeding 1 GHz. Since a distortion occurs in a waveform of a modulation signal, a phenomenon called a pattern effect occurs. The pattern effect is a phenomenon in which a modulation signal for a pulse changes depending on a previous modulation pattern.

FIG. 2is a schematic view for use in describing the pattern effect, (a) shows a modulation signal in an ideal electronic circuit without the pattern effect, and (b) shows a modulation signal in a real electronic circuit with the pattern effect. InFIG. 2 (a)andFIG. 2 (b), the axis of abscissas represents a time while the axis of ordinate represents a voltage.

FIG. 2 (a)illustrates a modulation signal201a, a first optical pulse202aand a second optical pulse203aas optical pulses to be modulated, a first modulation signal204afor the first optical pulse202a, and a second modulation signal205afor the second optical pulse203a. As shown inFIG. 2 (a), the first modulation signal204aand the second modulation signal205ahave waveforms of the same shape as each other.

FIG. 2 (b)illustrates a modulation signal201b, a first optical pulse202band a second optical pulse203bas optical pulses to be modulated, a first modulation signal204bfor the first optical pulse202b, and a second modulation signal205bfor the second optical pulse203b. As shown inFIG. 2 (b), in the real electronic circuit, distortion occurs in the modulation signal201bdue to band limitation of wiring and so on. Due to the distortion, when the modulation is performed on the first optical pulse202band the second optical pulse203bas the same optical pulse, the first modulation signal204band the second modulation signal205bas respective modulation signals thereof have waveforms which are different from each other. InFIG. 2 (b), the modulation signal changes depending on a preceding modulation pattern. Such phenomenon is called the pattern effect. Thus, in the decoy method, even if the optical pulses are subjected to the same intensity modulation, the intensity may fluctuate depending on which one of the three types has been a preceding pulse intensity. The fluctuation of the optical pulse intensity due to the pattern effect may reach about 20%. Therefore, a generation efficiency of the encryption key may possibly degrade significantly.

It is an object of this invention to provide a transmitting device, a receiving device, a quantum key distribution method, and a quantum key distribution program for a quantum key distribution system which are capable of suppressing degradation of a generation efficiency of an encryption key when a decoy method is used.

Means to Solve the Problem

A transmitting device for a quantum key distribution system according to a first aspect of the present invention comprises an encoding unit configured to encode a train of optical pulses; an intensity modulating unit configured to subject a train of encoded optical pulses to N types of intensity modulation having mutually different intensities, with different timings, where N is an integer which is not less than three; and a first key distillation processing unit configured to generate an encryption key based on a data sequence obtained by removing data obtained from optical pulses having a specific modulation pattern from a data sequence used when the encoding unit and the intensity modulating unit carry out encoding and intensity modulation.

A receiving device for a quantum key distribution system according to a second aspect of the present invention comprises a decoding unit configured to receive, from a transmitting device, a train of optical pulses which has been encoded and subjected to N types of intensity modulation having mutually different intensities, with different timings, where N is an integer which is not less than three, the decoding unit producing a data sequence obtained by decoding the train of optical pulses; and a second key distillation processing unit configured to generate an encryption key based on a data sequence obtained by removing data obtained from optical pulses having a specific modulation pattern from the data sequence obtained by the decoding.

A quantum key distribution method according to a third aspect of the present invention comprises generating an encryption key based on a data sequence obtained by removing data obtained from optical pulses having a specific modulation pattern from a data sequence used when a train of optical pulses are subjected to encoding and N types of intensity modulation having mutually different intensities, where N is an integer which is not less than three.

A quantum key distribution method according to a fourth aspect of the present invention comprises receiving, from a transmitting device, a train of optical pulses which has been encoded and subjected to N types of intensity modulation having mutually different intensities, with different timings, where N is an integer which is not less than three, to produce a data sequence obtained by decoding the train of optical pulses, and generating an encryption key based on a data sequence obtained by removing data obtained from optical pulses having a specific modulation pattern from the data sequence obtained by the decoding.

A quantum key distribution program according to a sixth aspect of the present invention makes a computer function as means for generating an encryption key based on a data sequence obtained by removing data obtained from optical pulses having a specific modulation pattern from a data sequence used when a train of optical pulses are subjected to encoding and N types of intensity modulation having mutually different intensities, where N is an integer which is not less than three.

A quantum key distribution program according to a sixth aspect of the present invention makes a computer function as means for generating an encryption key based on a data sequence obtained by removing data obtained from optical pulses having a specific modulation pattern from a data sequence obtained by decoding a train of optical pulses which has been encoded and subjected to N types of intensity modulation having mutually different intensities, with different timings, where N is an integer which is not less than three.

Effect of Invention

According to this invention, it is possible to suppress degradation of a generation efficiency of an encryption key when a decoy method is used.

