JP2009017277A - Quantum cryptography reception apparatus and quantum cryptography reception method employing the same - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a quantum cryptography reception apparatus which keeps high an interference sharpness degree and a baud rate regardless of random rotations of a polarization plane of a photon in the middle of transmission and does not require feedback polarization plane control using light of high intensity different from the transmission photon. <P>SOLUTION: A quantum cryptography reception apparatus comprises two unsymmetrical Mach-Zehnder interferometer for identifying the kind of a relative phase difference between two serial photon pulses, transmitted from a quantum cryptography transmission apparatus using the asymmetrical Mach-Zehnder interferometer, with four kinds of the relative phase difference and a time difference; and a polarization beam splitter for guiding to any one of the two asymmetrical Mach-Zehnder interferometer in accordance with a polarization state of transmitted and input photons. The quantum cryptography reception apparatus includes a polarization rotator, provided on a pre-stage of an input port of the polarization beam splutter, for rotating the polarization state of photons to be input in such a way as to incur the same number of detection events in both the two asymmetrical Mach-Zehnder interferometers. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to a quantum cryptography receiving apparatus for receiving a quantum signal which is a key for quantum cryptography and a quantum cryptography receiving method using the same.

A quantum cryptography system is generally composed of a quantum cryptography transmission device, a quantum cryptography reception device, a quantum communication channel that connects both devices, and a classical communication channel. When the BB84 protocol, which is a quantum cryptography protocol using a phase modulation method, is implemented, photons are transmitted from the quantum cryptography transmission device to the quantum cryptography reception device.
At this time, in the conventional quantum cryptography transmitter, an asymmetric Mach-Zehnder interferometer (hereinafter referred to as “AMZI”) having a different optical path length is used to form a double photon having a time difference from a single photon pulse. Generate a pulse. The relative phase difference of the double photon pulse is modulated into four of 0, π, π / 2, and −π / 2 and transmitted.

  On the other hand, a conventional quantum cryptography receiving device includes two AMZIs, and splits a transmitted photon by a beam splitter (hereinafter referred to as “BS”) and guides it to one of the two AMZIs. The requirements of the receiving device of the BB84 protocol for determining the observation base are satisfied (for example, see Non-Patent Document 1).

Specifically, photons transmitted to the quantum cryptography receiver are output to one of two output ports at a ratio of 50:50 at the BS. Each AMZI is connected to an output port of the BS. These two AMZIs have the same optical path length difference as the AMZI provided in the quantum cryptography transmission device, so that interference occurs.
In one AMZI, the optical path length difference is finely adjusted in the wavelength order, and the input double photon pulse having a phase difference of 0 is always guided to one specific output port. Also, the input double photon pulse having a phase difference of π is always guided to the other specific output port. On the other hand, it is undefined to which output port an input double photon pulse with a phase difference of ± π / 2 is led.
Similarly, in the other AMZI, the optical path length difference is finely adjusted in the wavelength order, and the input double photon pulse having a phase difference of π / 2 is always guided to one specific output port. The input double photon pulse having a phase difference of −π / 2 is always guided to the other specific output port. On the other hand, it is indeterminate which output port the input double photon pulses with phase differences of 0 and π are led to.

  If a photon detector is provided in each of the four AMZI total output ports, it is determined whether the bit value is “0” or “1” depending on which photon detector fires. Which of the two bases: + base (0, π) and × base (π / 2, −π / 2) is selected is determined by which AMZI the fired photon detector is connected to. The fact that the BS branches at a ratio of 50:50 corresponds to selecting a base at random.

However, in the conventional quantum cryptography receiving device, since the transmitted photon is guided to AMZI through the BS, the polarization plane of the photon is rotated during the transmission, and the photons in two orthogonal polarization modes are both Had entered AMZI. Since AMZI is generally composed of a birefringent medium, there is a problem in that the optical path length differs between the two polarization modes, resulting in reduced interference clarity, that is, increased error rate. .
Therefore, it is easy to prevent a decrease in the interference intelligibility by providing a polarizer in front of AMZI and allowing only a single polarization mode to be input, but discarding the other polarization mode that could not be input. Therefore, there was a problem that the communication speed decreased.

  Therefore, a polarization compensator is provided in the front stage of the quantum cryptography receiving apparatus, and the polarization plane is rotated so as to maximize the photons input to the AMZI. For example, in Non-Patent Document 1, the plane of polarization is adjusted using a static polarization rotator.

R T Thew, Eight others, “Low jitter up-conversion detectors for telecom wavelength GHz QKD”, New Journal of Physics, March 2006, vol. 8, no. 32, pp. 1-12

  However, practically, the plane of polarization of photons in the transmission path rotates randomly from moment to moment. Therefore, a combination of a polarization monitor and a polarization controller, and strong light that can operate the polarization monitor. There is a problem that it is necessary to perform complicated control such as polarization plane control that transmits and feeds back with photons.

  The object of the present invention is to maintain high intelligibility and communication speed regardless of random rotation of the polarization plane of a photon during transmission, and to provide feedback polarization using high-intensity light different from the transmitted photon. It is an object to provide a quantum cryptography reception device that does not require wavefront control and a quantum cryptography reception method that uses the quantum cryptography reception device.

  The quantum cryptography receiving device according to the present invention identifies the type of relative phase difference of a double photon pulse having four types of relative phase differences and having a time difference transmitted from the quantum cryptography transmission device using an asymmetric Mach-Zehnder interferometer. In the quantum cryptography receiving device comprising two asymmetric Mach-Zehnder interferometers and a polarization beam splitter that leads to one of the two asymmetric Mach-Zehnder interferometers according to the polarization state of the transmitted and input photons, the two asymmetric Mach-Zehnder interferometers The polarization state of the input photon is rotated so that the same number of detection events occur in both, and a dynamic polarization rotator is provided in front of the input port of the polarization beam splitter.

  The quantum cryptography reception method according to the present invention includes four types of relative phase differences transmitted from a quantum cryptography transmission device using an asymmetric Mach-Zehnder interferometer from the quantum cryptography transmission device, and relative phase differences between two photon pulses having a time difference. In a quantum cryptography receiving system that distinguishes two types by two asymmetric Mach-Zehnder interferometers, a transmitted photon having two polarization modes is branched into two by a polarization beam splitter by a polarization beam splitter, and a single polarization mode Only this is guided to the asymmetric Mach-Zehnder interferometer in the subsequent stage to measure photon interference.

The effect of the quantum cryptography receiver according to the present invention is that the photon transmitted using the polarization beam splitter is guided to one of the two asymmetric Mach-Zehnder interferometers, so that any photon having any polarization plane can be transmitted. This means that it is possible to prevent a decrease in communication speed (bit rate).
Further, since only a single polarization mode photon is guided to the asymmetric Mach-Zehnder interferometer, it is possible to prevent a reduction in interference clarity due to birefringence, that is, an increase in bit error rate.

FIG. 1 is a block configuration diagram showing the configuration of a quantum cryptography system including a quantum cryptography reception device according to an embodiment of the present invention.
As shown in FIG. 1, a quantum cryptography system according to an embodiment of the present invention includes a quantum cryptography transmission device 1, a quantum cryptography reception device 2, a quantum communication channel 3 mainly composed of optical fibers, and a classical communication channel 4. Consists of
In the quantum communication path 3, photons are transmitted from the quantum cryptography transmission device 1 to the quantum cryptography reception device 2. The classical communication path 4 transmits control information for performing quantum cryptography communication and classical information necessary for implementing a quantum cryptography protocol such as BB84.

FIG. 2 is a configuration diagram showing the configuration of the quantum cryptography transmission device according to the embodiment of the present invention.
As shown in FIG. 2, the quantum cryptography transmitter 1 according to the embodiment of the present invention includes a LASER 11 that is a photon source, a polarization controller 12 that rotates and adjusts the polarization plane of the output photon, and a phase that causes the photon to be phased. An asymmetric Mach-Zehnder interferometer (AMZI) 13 for converting into a double pulse having a time difference that can be modulated, and a phase modulator (hereinafter abbreviated as “PMA”) for performing phase modulation on the subsequent pulse of the double pulse output from the AMZI 13 14. Controls an attenuator (hereinafter abbreviated as “ATT”) 15, LASER 11 and PMA 14 for dimming the double pulse output from the PMA 14 to the intensity required by the quantum cryptography, and performs classical communication with the quantum cryptography receiver 2 A transmission side control device 16 is provided.

The polarization controller 12 adjusts the polarization plane so that only a single polarization mode is input to the AMZI 13.
For example, AMZI composed of a quartz planar circuit (PLC) has birefringence, and the polarization plane is adjusted by the polarization controller so that only a single polarization mode is input to AMZI.
The transmission side control device 16 has a master clock 17 and controls the LASER 11 and the PMA 14 according to the master clock 17.
The PMA 14 performs quaternary modulation such as phase differences 0, π, π / 2, and −π / 2 according to the BB84 protocol. Which of the four values is selected depends on random number data generated by the transmission-side control device 16 for each photon pulse.
The transmission side control device 16 transmits transmission control information such as a clock signal and classical information required by the quantum cryptography protocol to and from the quantum cryptography reception device 2 via the classical communication path 4.

FIG. 3 is a configuration diagram of the quantum cryptography reception device according to the embodiment of the present invention.
The quantum cryptography receiving device 2 according to the embodiment of the present invention is intended to perform quantum cryptography communication with the quantum cryptography transmission device 1 in the quantum cryptography system as shown in FIG.
As shown in FIG. 3, the quantum cryptography receiving device 2 according to the embodiment of the present invention is a half as a static polarization rotator that statically rotates the polarization plane of a photon transmitted through the quantum communication path 3. A wave plate (hereinafter referred to as “HWP”) 21, a dynamic polarization rotator 23 that dynamically rotates the polarization plane of a photon that has passed through the HWP 21 under the control of the reception-side controller 22, and dynamic polarization rotation Two AMZIs 25, 26, AMZIs 25, 26 respectively connected to two output ports of a polarizing beam splitter (hereinafter referred to as “PBS”) 24 and a PBS 24 that branches the photons that have passed through the device 23 into two according to the plane of polarization. The four photon detectors 27a, 27b, 27c, 27d connected to the two output ports, respectively, receive the detection signals of the four photon detectors 27a, 27b, 27c, 27d, or the dynamic polarization rotator 23. The Control or, a receiving-side controller 22 which performs quantum cryptography transmitting apparatus 1 and the classical communication via a classical channel 4.

The HWP 21 is rotated by, for example, a person from the outside of the quantum cryptography reception device 2 such as when quantum cryptography communication is started or in the middle, and is stationary during that time. And static means this.
The polarization rotator 23 is composed of a half-wave plate or a quarter-wave plate. Even if these wave plates are mechanically rotated, polarization that electrically changes the birefringence of the medium by the electro-optic effect. A modulator may be used. Note that dynamic means that the receiving side control device 22 rotates randomly or regularly.

The PBS 24 guides a photon whose polarization plane is P-polarized light to the first AMZI 25.
When the first AMZI 25 has birefringence, the polarization axis is adjusted so that one of the two polarization modes corresponds to the P polarization of the PBS 24, and the PBS 24 and the first AMZI 25 are connected. Has been.
The PBS 24 guides photons having a polarization plane of S polarization to the second AMZI 26.
When the second AMZI 26 has birefringence, the polarization axis is adjusted so that one of the two polarization modes corresponds to the S polarization of the PBS 24, and the PBS 24 and the second AMZI 26 are connected. Has been.

  The AMZIs 25 and 26 are adjusted so that the optical path length difference is equal to the AMZI 13 provided in the quantum cryptography transmission device 1 so that interference occurs. The first AMZI 25 of the two AMZIs 25 and 26 interferes when a transmitted double photon pulse having a time difference is input, and when the phase difference is 0, the first photon detector 27a has a phase difference of π. At this time, the optical path length difference is finely adjusted in the wavelength order so as to be guided to the second photon detector 27b. When the phase difference is ± π / 2, it is undefined whether the light is guided to the first photon detector 27a or the second photon detector 27b.

  The other second AMZI 26 interferes when a transmitted double photon pulse having a time difference is input, and when the phase difference is π / 2, the third photon detector 27c has a phase difference of −π / 2. Sometimes, the optical path length difference is finely adjusted in the wavelength order so as to be guided to the fourth photon detector 27d. When the phase difference is 0, π, it is undefined whether the light is guided to the third photon detector 27c or the fourth photon detector 27d.

Next, the operation of the quantum cryptography reception device 2 according to the embodiment of the present invention will be described.
In general, in a quantum cryptography system as shown in FIG. 1, first, a photon phase-modulated from the quantum cryptography transmission device 1 to the quantum reception device 2 is transmitted via the quantum communication path 3. Along with this, control information such as a clock signal is also transmitted through the classical communication path 4. When a plurality of photon transmissions are completed, auxiliary information necessary to execute the quantum cryptography protocol, for example, the BB84 protocol, is transmitted through the classical communication path 4, and finally, after data processing is performed, The shared secret key is output by each of the quantum cryptography transmission device 1 and the quantum cryptography reception device 2.

The present invention relates to photon transmission in the preceding stage of the above-described process, and the operation related to photon transmission will be described below.
Generally, when passing through a quantum communication path 3 such as an optical fiber, the polarization plane of the photon rotates randomly due to birefringence, but the quantum cryptography receiver 2 rotates the HWP 21 randomly when starting photon transmission. And fix. By doing so, an eavesdropper cannot identify what polarization plane of the photon input to the quantum cryptography receiving device 2 corresponds to the P-polarized light or the S-polarized light of the PBS 24. For this reason, an eavesdropper cannot rotate all of the transmitted photons to one of the two AMZIs 25 and 26 by rotating the plane of polarization.

Next, the polarization plane of the photon that has passed through the HWP 21 is further rotated by the dynamic polarization rotator 23. The rotation of the polarization plane is controlled by the receiving side control device 22 and the polarization plane is dynamically rotated rather than fixed with respect to the passing photon. At this time, the polarization rotator 23 rotates the polarization plane of the photon at a unique timing asynchronously with the transmission timing of the transmission photon. Then, the magnitude of the polarization that rotates the polarization plane and the rotation timing are randomly changed.
In addition, since the receiving side control apparatus 22 is receiving the clock signal via the classical communication path 4, you may rotate the polarization plane of a photon synchronizing with the passage timing of a photon.
Further, the polarization rotator 23 may be constituted by a half-wave plate, and the half-wave plate may be rotated at a constant rotation speed in order to make the magnitude of the polarization rotating and the rotation timing regular. .

  By performing such processing, the polarization plane of the transmitted photon is completely different from the polarization plane when entering the quantum cryptography receiving device 2 when entering the PBS 24. As a result, the PBS 24 can sort at random whether the transmission photon is guided to the two AMZIs 25 and 26.

A photon guided to one of the two AMZIs 25 and 26 causes interference, and fires one of the four photon detectors 27a, 27b, 27c, and 27d.
When the first photon detector 27a is ignited, the reception-side control device 22 determines that “0” is received at the + base (0, π). When the second photon detector 27b is ignited, it is determined that “1” is received at the + base (0, π). When the third photon detector 27c is ignited, it is determined that “0” is received at the × base (π / 2, −π / 2). When the fourth photon detector 27d is ignited, it is determined that “1” is received by the × basis (π / 2, −π / 2).
With such reception information, the quantum cryptography receiving device 2 can execute processing subsequent to completion of photon transmission in a general quantum cryptography protocol. The received information does not require any information on how the polarization rotator 23 is controlled.
In addition, the reception-side control device 22 verifies the frequency deviation between the + basis and the × basis based on the reception information, and verifies the presence or absence of an attack in which an eavesdropper controls the polarization plane.

In the quantum cryptography receiving device 2 according to the embodiment of the present invention, the transmission photon is guided to one of the two AMZIs 25 and 26 using the PBS 24, so that any photon having any polarization plane is used for the photon transmission. And a decrease in communication speed (bit rate) can be prevented.
Further, since only photons of one polarization mode are guided to the AMZIs 25 and 26, it is possible to prevent a decrease in interference clarity due to birefringence, that is, an increase in bit error rate.

  By arranging the HWP 21 and the polarization rotator 23 in front of the PBS 24 in this way, the transmission photons in which polarization state are guided to which AMZI 25, 26, that is, which base is used for observation. Therefore, safety can be maintained.

  Since the polarization rotation control method as described above is used, the polarization rotation control can be performed independently by the quantum cryptography receiving device 2, and a strong control signal light is mixed in the quantum communication path 3 to perform feedback control. Since polarization rotation control is not required, simple polarization rotation control is sufficient.

It is a block block diagram which shows the structure of the quantum cryptography system containing the quantum cryptography receiver based on embodiment of this invention. It is a block diagram which shows the structure of the quantum cryptography transmitter based on Embodiment of this invention. It is a block diagram of the quantum cryptography receiver based on embodiment of this invention.

Explanation of symbols

  DESCRIPTION OF SYMBOLS 1 Quantum cryptography transmitter, 2 Quantum cryptography receiver, 3 Quantum communication channel, 4 Classical communication channel, 11 LASER, 12 Polarization controller, 13, 25, 26 Asymmetric Mach-Zehnder interferometer (AMZI), 14 Phase modulator, 15 Attenuator (ATT), 16 Transmission side control device, 17 Master clock, 21 Half-wave plate (HWP), 22 Reception side control device, 23 Polarization rotator, 27a, 27b, 27c, 27d Photon detector.

Claims (10)

Two asymmetric Mach-Zehnder interferometers and transmissions for identifying the type of relative phase difference of a double photon pulse having four types of relative phase differences and having a time difference transmitted from a quantum cryptography transmitter using an asymmetric Mach-Zehnder interferometer In a quantum cryptography receiving device including a polarization beam splitter that guides to one of the two asymmetric Mach-Zehnder interferometers according to the polarization state of the input photons.
A dynamic polarization rotator that rotates the polarization state of a photon that is input so that the same number of detection events occur in both of the two asymmetric Mach-Zehnder interferometers and that is provided in front of the input port of the polarization beam splitter. A quantum cryptography receiving device comprising:   The quantum cryptography receiver according to claim 1, further comprising a static polarization rotator provided in front of the dynamic polarization rotator. Using two asymmetric Mach-Zehnder interferometers, there are four types of relative phase differences transmitted from the quantum cryptography transmission device using the asymmetric Mach-Zehnder interferometer. In the quantum cryptography reception method to identify,
The transmitted photon having two polarization modes is branched into two by the polarization mode by the polarization beam splitter, and only the single polarization mode is guided to the asymmetric Mach-Zehnder interferometer in the subsequent stage to measure the photon interference. A quantum cryptography receiving method characterized by the above.   The above transmission cannot be specified from the plane of polarization when it is input to the quantum cryptography receiver of the transmitted photon which is guided to one of the two asymmetric Mach-Zehnder interferometers located at the subsequent stage of the dynamic polarization rotator 4. The quantum cryptography reception method according to claim 3, wherein the polarization plane of the generated photon is rotated by the dynamic polarization rotator.   The polarization plane of the input photon is randomly selected at the beginning of quantum cryptography communication so that the polarization plane when the transmitted photon is input to the quantum cryptography receiver is separated by the polarization beam splitter. The quantum cryptography reception method according to claim 4, wherein the quantum cryptography reception method rotates statically.   5. The quantum cryptography reception method according to claim 4, wherein a polarization plane of the transmitted photon is rotated asynchronously with a transmission timing of the transmitted photon by a static polarization rotator.   5. The quantum cryptography reception method according to claim 4, wherein a polarization plane of the transmitted photon is rotated in synchronization with a transmission timing of the transmitted photon by the dynamic polarization rotator.   5. The quantum cryptography reception method according to claim 4, wherein the plane of polarization is rotated by the dynamic polarization rotator at random polarization magnitude or at random timing.   5. The quantum cryptography reception method according to claim 4, wherein the plane of polarization is rotated by the dynamic polarization rotator at a regular polarization magnitude or regular timing.   The quantum cryptography reception method according to claim 3, wherein the frequency bias of the photon detection event generated by the two asymmetric Mach-Zehnder interferometers is tested, and if there is a significant bias, it is determined that an eavesdropper has detected. .

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