JP6214093B2 - Quantum key distribution device - Google Patents

Description

  The present invention relates to a quantum key distribution device that supplies a secret key used for cryptographic communication to two remote parties.

  Research and development of quantum key distribution (QKD) for supplying a secret key used for cryptographic communication to two safely separated parties using the principle of quantum mechanics is in progress. There are several methods for quantum key distribution. For example, a method called differential phase shift (DPS) QKD is known.

  FIG. 1 shows a basic configuration of a conventional DPS-QKD system. The transmitter 10 includes a coherent pulse light source 11, a phase modulator 12, and an optical attenuator 13. The carrier phase of each pulse of the coherent optical pulse train output from the coherent pulse light source 11 is randomly phase-modulated by any one of {0, π} in the phase modulator 12, and the optical power is changed to 1 by the optical attenuator 13. The light is weakened to an average of less than 1 photon per pulse (for example, 0.2 photon / pulse) and transmitted to the optical transmission line. At this time, the transmitter 10 records phase modulation data. In the delay Mach-Zehnder interferometer 21, the receiver 20 branches the transmitted optical pulse train P1 into two branches, gives a time delay corresponding to the pulse interval to one of them, and then multiplexes them again by the 2 × 2 optical coupler. This configuration is called a delayed Mach-Zehnder interferometer. The propagation delay phase difference between the two paths in the delay Mach-Zehnder interferometer is assumed to be zero. Photon detectors 22 and 23 are provided at two output terminals of the optical coupler, respectively. Then, adjacent pulses interfere with each other in the multiplexing coupler, and as a result of the interference, photons are detected by one of the two photon detectors 22 and 23. Since the propagation delay phase difference between the two paths is 0, the photon is detected by the photon detector 22 if the phase difference between adjacent pulses is 0, and by the photon detector 23 if it is π. However, since the received light power is less than one photon / pulse, it is rare and random in time to detect a photon.

  Using the above device, the sender / receiver obtains a secret key by the following procedure. (1) After transmitting and receiving the optical pulse train, the receiver 20 notifies the transmitter 10 of the photon detection time. (2) The transmitter 10 knows from which photon detectors 22 and 23 the photon is detected from the photon detection time and its own phase modulation data. (3) In the transmitter 10 and the receiver 20, a photon detected event by the detector 22 is set to bit “0”, and a photon detected event by the detector 23 is set to bit “1”. Due to the above operating principle, both bits match. Here, only the photon detection time is disclosed to the outside, and the bit information is not disclosed. Therefore, the transmitter 10 and the receiver 20 use the bit string obtained as described above as a secret key.

  The security of the obtained secret key is ensured by the power of the transmitted optical pulse train being less than 1 photon / pulse. An eavesdropper cannot measure all the phase differences of such a low-power optical pulse train in terms of quantum mechanics, and can only eavesdrop on some key information. Some leaked parts can be erased by data processing called “enhancement of confidentiality”, whereby the sender / receiver obtains a completely secure secret key.

K. Inoue, et al., “Differential-phase-shift quantum key distribution using coherent light”, PHYSICAL REVIEW A 68, 022317 (2003). Hiroki Takesue, et al., “Quantum key distribution over a 40-dB channel loss using superconducting single-photon detectors”, nature photonics, VOL. 1, JUNE 2007.

  In the conventional configuration described above, means for phase-modulating the output light from the coherent pulse light source based on the random number data output from the random number generator is used to generate the {0, π} random phase pulse train. In order to realize secure quantum key distribution, a pseudo-random number that generates periodicity in the pattern cannot be used, and phase modulation must be performed using a completely random random number. Therefore, physical random number generators that generate random numbers based on random physical phenomena have been used so far. As the physical random number generator, for example, there is a method of detecting a physical quantity of noise and outputting binary data based on whether or not the detected physical quantity exceeds a predetermined threshold. That is, in the conventional configuration, in addition to the coherent pulse light source, a physical random number generator and a phase modulator are combined to generate a {0, π} random phase pulse train, so that the configuration becomes complicated.

  The present invention has been made in view of the above problems, and an object of the present invention is to provide a differential phase shift quantum key distribution system in which a generation part of a {0, π} random phase pulse train is greatly simplified. It is in.

  In order to solve the above problem, an invention described in an embodiment is a transmitter used in a differential phase shift type quantum key distribution system, and a relative phase is randomly zero or 0 by degenerate optical parametric oscillation. A degenerate optical parametric oscillator that outputs an optical pulse train having a fixed period of π, branching means for bifurcating the optical pulse train from the optical parametric oscillator, and attenuating one optical pulse train of the branching means and sending it to the optical transmission line And a Mach-Zehnder interferometer that receives the other optical pulse train of the branching means, interferes the optical pulse train and outputs it from two output ends, and optical output intensities from the two output ends of the Mach-Zehnder interferometer. Detecting means for detecting; phase difference data storing means for storing the pulse position of the optical pulse detected by the detecting means and the phase difference thereof; Wherein said pulse position stored in the phase difference data storage means and by utilizing the phase difference which is a transmitter and generates a secret key.

  According to the present invention, the transmitter can eliminate the need for a random number generator and an optical phase modulator, and can provide a simpler quantum key distribution system than before.

This is a basic configuration of a conventional DPS-QKD system. It is a figure which shows the example of a basic composition of the quantum key distribution system of this invention embodiment. It is a figure which shows an example of a structure of a degenerate optical parametric oscillator. It is a figure which shows another example of a structure of a degenerate optical parametric oscillator.

  Hereinafter, embodiments of the present invention will be described in detail.

  The quantum key distribution system of this embodiment uses a continuous optical pulse train based on degenerate optical parametric oscillation as a light source of a transmitter in the differential phase shift quantum key distribution system. Each pulse of the continuous optical pulse train obtained by the degenerate optical parametric oscillation has a relative phase of 0 or π, and which phase is completely random.

  As described above, in the quantum key distribution system according to the present embodiment, since the continuous optical pulse train itself obtained by the degenerate optical parametric oscillation can be used as a light source, a random number generator and an optical phase modulator are not newly required, and compared with the conventional case. A differential phase shift quantum key distribution system with a simple configuration can be realized.

  FIG. 2 is a diagram illustrating a basic configuration example of the quantum key distribution system according to the embodiment of the present invention. The quantum key distribution system of this embodiment includes a transmitter 30 having a light source that outputs a continuous optical pulse train by degenerate optical parametric oscillation, and a receiver 20 having the same configuration as that of the prior art.

  The transmitter 30 includes a degenerate optical parametric oscillator 31, a beam splitter 32, a delay Mach-Zehnder interferometer 33, photodetectors 34 and 35, and an optical attenuator 36. The degenerate optical parametric oscillator 31 generates a continuous optical pulse train by degenerate optical parametric oscillation. The generated pulse train is branched into two by the beam splitter 32, one of which is attenuated to less than one photon / pulse by the optical attenuator 36, and then transmitted to the optical transmission line. The other branched part is input to the photodetectors 34 and 35 via the delay Mach-Zehnder interferometer 33 similar to that used in the receiver 20 in the conventional system. The photodetectors 34 and 35 are not photon detectors, but the same photodetectors as those used in a normal optical communication system can be used.

  The receiver 20 receives the transmitted optical pulse train with the same configuration as that of the conventional DPS-QKD system.

  Here, the degenerate optical parametric oscillator 31 will be described. The optical parametric oscillator 31 is an optical oscillator that uses an optical parametric amplification phenomenon that occurs when high-power pump light is incident on a second-order or third-order nonlinear optical medium as amplification means.

For example, second-order nonlinear optical medium to _{E p = A p exp (i2πf} p t) and the pump light of the optical frequency _{f p} represented by _{E s = A s exp (i2πf} s t) the optical frequency _{f s} which is represented by (A: complex amplitude, t: time). _{Then, P = ε 0 χ 2 E} p E s * = ε 0 χ 2 A p A s * exp [i2π (f p -f s) t] nonlinear polarization that occurs (epsilon _0: dielectric constant in vacuum, chi _2: 2-order nonlinear ^{susceptibility, *} represents complex conjugate), than _{this, f p} -f s = f _i new light on the frequency position of (this is referred to as idler light) is generated. This light to produce from the nonlinear polarization, the amplitude is proportional to _{ε 0 χ 2 A p A s} *. That is, it is proportional to the complex conjugate of the signal light amplitude. When idler light is generated, new light is generated at a frequency position of f _p −f _i = f _p − (f _p −f _s ) = f _s due to the parametric interaction between the light and the pump light. This light has the same frequency as the signal light and is superimposed on the signal light in the same phase. That is, the signal light increases. As the signal light increases, it causes more idler light, thereby further increasing the signal light. A phenomenon in which signal light is amplified in this way is called optical parametric amplification. Here, the signal light frequency to be amplified is determined by conditions for superimposing newly generated light on the original signal light in the same phase. This is called a phase matching condition.

  In general, when an optical amplifying medium is arranged in an optical resonator, it operates as an optical oscillator, and coherent light is oscillated and output even if no signal light is incident from the outside. Similarly, in the case of optical parametric amplification, when a nonlinear optical medium is disposed in an optical resonator and pump light satisfying the optical parametric amplification condition is incident, coherent light is oscillated and output. This is an optical parametric oscillator. The oscillation light frequency is determined by the phase matching condition, and is adjusted as an optical oscillator to be used as a wavelength variable optical oscillator.

The degenerate optical parametric oscillator 31 has a special form in which the signal light and idler light of the optical parametric oscillator are degenerated. In the description of the optical parametric amplification phenomenon, the signal light and the idler light are assumed to be separate lights. However, when the signal light frequency is f _s = f _p / 2, the idler light frequency is f _i = f _p −f _s = _{f p} / 2 = f _s, and the signal light and the idler light overlap (degenerate). Here, as described above, idler light has the property of a complex conjugate wave of signal light. If the gain of the parametric amplification is sufficient, the signal light and the idler light are approximately equal in size. Then, the complex amplitude of the degenerate wave is E = Aexp [i (θ ₀ + θ _s )] + Aexp [i (θ ₀ −θ _s )] = Aexp (iθ ₀ ) {exp (iθ _s ) + exp (−iθ _s )} = ² cos (θ _s ) Aexp (iθ ₀ ), and the light intensity is expressed as | E | ² = 4A ² cos ² (θ _s ). Here, A is a real amplitude, θ _s is a signal light phase, and θ ₀ is a phase component other than θ _s .

The light intensity | E | ² = 4A ² cos ² (θ _s ) indicates that the signal light output intensity depends on the signal light phase and becomes maximum when θ _s = 0 or π. That is, in the degenerate optical parametric amplification, it can be seen that the amplification gain becomes maximum when the signal light phase is 0 or π. In general, in a resonator configuration, optical oscillation occurs under the maximum amplification gain condition. Accordingly, the degenerate optical parametric oscillator 31 outputs light having a relative phase of 0 or π. Which phase value is determined is determined by the phase of spontaneously emitted light that becomes the seed of oscillation, which is completely random in principle.

Here, the configuration of the degenerate optical parametric oscillator 31 will be described. FIG. 3 is a diagram illustrating an example of the configuration of the degenerate optical parametric oscillator 31. In Figure 3, transmitter 30 uses a continuous optical pulse train having a constant interval T _p as the pump light. That is, the degenerate optical parametric oscillator 31 (see FIG. 2) receives the continuous optical pulse laser 40 that generates the continuous optical pulse P3 having the optical frequency f _p and the pulse interval T _p , the optical switch 41, and the continuous optical pulse P3. The optical resonator 44, the nonlinear optical medium 42 provided in the optical resonator 44, and the optical filter 43 capable of transmitting light having the optical frequency f _p / 2 are provided. When the continuous light pulse P3 which is pump light propagates through the nonlinear optical medium 42, a continuous light pulse P4 having an optical frequency f _p / 2 and a pulse interval T _p is generated as signal light by the degenerate optical parametric amplification phenomenon. Here, the continuous light pulse P3 and the continuous light pulse P4 are synchronized in time. In the continuous light pulses P3 and P4 output from the nonlinear optical medium 42, the continuous light pulse P3 is removed by the optical filter 43. Therefore, only the continuous light pulse P4 that is signal light circulates in the optical resonator 44 (resonance). ) Here, in order to efficiently cause degenerate optical parametric amplification in the optical resonator 44, it is necessary that the continuous optical pulse P3 input into the optical resonator 44 and the circulating continuous optical pulse P4 overlap in time. is there. Due to the synchronized continuous optical pulses P3 and P4, as the continuous light pulse P3, the pulse interval T _p is the optical pulse train is one of the N frequency resonator round time (N is a natural number) may be set. The optical switch 41 provided at the output stage of the continuous light pulse laser 40 has a function of periodically stopping the pump light pulse incidence for a certain period of time in order to reset the oscillation state as necessary.

  As an example of the configuration of the degenerate optical parametric oscillator 31, FIG. 3 illustrates the configuration example using the Fabry-Perot type optical resonator 44. However, the present invention is not limited to this, and instead, for example, as shown in FIG. Alternatively, a ring-type optical resonator 50 (optical ring resonator) may be used. When an optical fiber is used as the nonlinear optical medium 42, a configuration using the optical ring resonator 50 is preferable. FIG. 4 is a diagram illustrating another example of the configuration of the degenerate optical parametric oscillator 31. The ring resonator 50 includes an optical input unit 45, a nonlinear optical medium 42, an optical filter 43, and an optical output unit 46. Pump light P5 and P6 are input to the ring resonator 50, degenerate optical parametric amplification occurs, and degenerate optical parametric oscillator pulse P7 is output.

  Returning to FIG. 2, the degenerate optical parametric optical oscillator 31 outputs an optical pulse train whose relative phase is {0, π} completely at random according to the above principle. This optical pulse train is branched into two, one of which is attenuated to less than one photon / pulse and transmitted to the receiver 20. The transmitted optical pulse train P2 has the same form as the optical pulse train P1 transmitted by the transmitter 10 (see FIG. 1) in the conventional system. The other of the two branches is input to a delay Mach-Zehnder interferometer 33 having the same configuration as that of the delay Mach-Zehnder interferometer 21 of the receiver 20, and its output is detected by photodetectors 34 and 35. Since the optical pulse train input to the delayed Mach-Zehnder interferometer 33 is not attenuated, a received signal is output from one of the two photodetectors 34 and 35 for each pulse, and the phase difference of each pulse is known from this. The transmitter 30 records this information (phase difference data: pulse position and its phase difference) in recording means (not shown). That is, the transmitter 30 transmits a DPS-QKD signal and holds phase difference data of each pulse. The transmitter 30 generates a secret key using the phase difference data based on the detected pulse position information sent from the receiver 20. This is the same as the operation performed by the transmitter 10 using the optical phase modulator 12 driven by the coherent pulse light source 11 and the random number data in the conventional DPS-QKD system shown in FIG.

  Therefore, the transmitter 30 of the present embodiment can use a continuous optical pulse train having a relative phase of 0 or π obtained by degenerate optical parametric oscillation as a light source as it is, so that the differential phase with a simpler configuration than the conventional one can be used. A shift quantum key distribution system can be realized.

In the present embodiment, the degenerate optical parametric oscillator 31 uses a secondary optical nonlinear effect as an example, but is not limited to this, and other optical parametric amplification processes in which the signal light and idler light are degenerated, for example, The same effect can be obtained even if an optical oscillator using a degenerate four-wave mixing process due to third-order optical nonlinearity as an amplification means is used. In this case, as shown in FIG. 4, the degenerate optical parametric oscillator 31 includes two continuous optical pulse lasers 40 having different optical frequencies f _p1 and f _p2 as the continuous optical pulse laser 40 as the light source, and the two An optical resonator 50 to which two continuous optical pulses P5 and P6 output from the continuous optical pulse laser 40 are input, a third-order nonlinear optical medium 42 provided in the optical resonator 50, and the optical resonator 50 are also included. And an optical filter 43 that feeds back light of optical frequency (f _p1 + f _p2 ) / 2 to the third-order nonlinear optical medium.

As the nonlinear optical medium used for the degenerate optical parametric oscillator 31, LiNbO ₃ can be used as the second-order nonlinear optical medium, and an optical fiber can be used as the third-order nonlinear optical medium. However, when an optical fiber is used, since the parametric amplification gain per unit length is not large, the fiber length for parametric oscillation becomes long, and therefore the resonator length may become long. In this case, if a pump light pulse train having a pulse interval T _p equivalent to the round-trip time of the resonator is used, a fast repetitive signal pulse train cannot be obtained, and as a result, the operation speed as a quantum key distribution system is limited. become. To avoid this limitation, the pulse interval T _p is the pump optical pulse train is one of the N frequency resonator round time (N is a natural number other than 1) incident, a plurality of independent pulse oscillation in the cavity Just make it happen. In this way, an oscillating optical pulse train having a narrow time interval (short repetition period) is output from the resonator. Of course, in applications where the operating speed is not limited, a pump light pulse train whose pulse interval T _p is equivalent to the round-trip time of the resonator (that is, a pump light pulse train of N = 1) may be used. Further, if the pump light pulse train is continuously incident, an oscillation light pulse having the same phase is output every N pulses. Therefore, in order to reset the oscillation state, the pump light pulse incidence is periodically stopped for a certain period of time. An optical switch may be provided at the output stage of the continuous optical pulse laser.

DESCRIPTION OF SYMBOLS 10 Transmitter 11 Coherent pulse light source 12 Phase modulator 13 Optical attenuator 20 Receiver 21 Delayed Mach-Zehnder interferometer 22, 23 Photon detector 30 Transmitter 31 Degenerate optical parametric oscillator 32 Beam splitter 33 Delayed Mach-Zehnder interferometer 34, 35 Optical detection 36 Optical attenuator 40 Continuous optical pulse laser 41 Optical switch 42 Non-linear optical medium 43 Optical filter 44 Optical resonator 45 Optical input means 46 Optical output means 50 Ring resonator

Claims (6)

A transmitter used in a differential phase shift type quantum key distribution system,
A degenerate optical parametric oscillator that outputs an optical pulse train having a constant period whose relative phase is randomly 0 or π by degenerate optical parametric oscillation;
Branching means for bifurcating the optical pulse train from the optical parametric oscillator;
Means for attenuating one optical pulse train of the branching means and sending it to an optical transmission line;
A Mach-Zehnder interferometer that receives the other optical pulse train of the branching means and causes the optical pulse train to interfere and output from two output terminals;
Detecting means for detecting light output intensities from two output ends of the Mach-Zehnder interferometer;
Phase difference data storage means for storing the pulse position of the optical pulse detected by the detection means and its phase difference;
A transmitter characterized in that a secret key is generated using a pulse position stored in the phase difference data storage means and its phase difference. The degenerate optical parametric oscillator, a second-order nonlinear optical medium, the second-order nonlinear means for entering the pump optical pulse train of optical frequency f _p to optical medium, the two light output from the second-order nonlinear optical medium primary And an optical resonance means for performing optical resonance by feeding back to the nonlinear optical medium, wherein the second-order nonlinear optical medium amplifies the pump light pulse train to an optical pulse train having an optical frequency f _p / 2. Item 2. The transmitter according to Item 1. The degenerate optical parametric oscillator includes a third-order nonlinear optical medium, means for injecting two pump light pulse trains having optical frequencies f _p1 and f _p2 into the third-order nonlinear optical medium, and an output from the third-order nonlinear optical medium Optical resonance means for feeding back light to the third-order nonlinear optical medium to cause optical resonance, and the second-order nonlinear optical medium converts the pump light pulse train to an optical frequency (f _p1 + f _p2 ) / 2. The transmitter according to claim 1, wherein the transmitter is amplified to an optical pulse train.   The transmitter according to claim 3, wherein the third-order nonlinear optical medium in the degenerate optical parametric oscillator is an optical fiber.   The transmitter according to claim 4, wherein the pump light pulse train incidence to the nonlinear optical medium in the degenerate optical parametric oscillator is stopped for a predetermined time. A transmitter according to any of claims 1 to 5;
A receiving-side Mach-Zehnder interferometer that causes the optical pulse train transmitted from the transmitter to interfere and output from two output ends;
A receiver having photon detection means for photon detection of light output from two output ends of the receiving side Mach-Zehnder interferometer,
The receiver generates a quantum key based on a pulse position that can be observed by a photodetector and its phase difference, and sends the observed pulse position to the transmitter.
The quantum key distribution system, wherein the transmitter generates a secret key based on a phase difference of a pulse corresponding to a pulse position received from a receiver.

Images