JP5475837B2 - Optical pulse generator - Google Patents

Description

  The present invention relates to an optical pulse generator, and more particularly to an optical pulse generator capable of measuring a phase modulation value in a phase modulator used for quantum cryptography communication.

  Cryptographic technology that can exchange data secretly only between legitimate communicators without leaking information to an eavesdropper is one of the cores of modern communication technology. As an existing encryption method, a public key encryption method and a secret key encryption method are known. It is known that the public key cryptosystem is broken by a computer having a very high calculation speed. Public key cryptosystems can also be broken by very sophisticated mathematical algorithms. On the other hand, the secret key cryptosystem is known to be safe if the sender and the receiver share a random number table of bit values not known to the eavesdropper, that is, the secret key. The problem here is how to share the secret key. Recently, quantum cryptography communication has attracted attention as a method for solving this problem.

  In the theory of studying the security of quantum cryptography communication, a mathematical model of communication equipment is assumed. And under the assumption that the mathematical model is correct, we can show that quantum cryptography is unconditionally secure. Since the mathematical model of a communication device has parameters that characterize the device, it is necessary to examine what parameter value the device actually used takes for safe communication.

  In quantum cryptography communication performed by transmitting an optical pulse, the phase modulator of the optical pulse generator is the most important element, and the value of the phase modulation error is an important parameter. In an actual phase modulator, it is impossible to make it equal to a theoretical phase modulation value, and a phase modulation error always occurs. It is known that if a value of this phase modulation error, in other words, a value of phase modulation actually applied is given, a secure secret key can be generated by using the method described in Non-Patent Document 1. Therefore, in actual quantum cryptography communication, it is necessary to accurately measure the phase modulation value in real time in order to ensure the security of quantum cryptography communication.

Daniel Gottesman, Hoi-Kwong Lo, Norbert Lutkenhaus, John Preskill, "Security of quantum key distribution with imperfect devices", Quant. Inf. Comput. 5 (2004) 325-360 M. Sasaki, M. Fujiwara, H. Ishizuka, et. Al., "Field test of quantum key distribution in the Tokyo QKD Network", Optics Express Vol.19, Iss.11, pp.10387-10409 (2011)

  However, in the conventional quantum cryptography communication, the accuracy of the phase modulator of the optical pulse generator is measured offline before performing the quantum cryptography communication, and the quantum cryptography communication is performed based on the accuracy. It was done in the original. That is, in the optical pulse generator, the phase modulation value in real time has not been measured (for example, see Non-Patent Document 2).

  An object of the present invention is to provide an optical pulse generator capable of accurately monitoring in real time the value of phase modulation when performing quantum cryptography communication.

In order to achieve such an object, an embodiment of the present invention is an optical pulse generator used in quantum cryptography communication, a beam splitter that outputs an optical pulse output from a light source to a Sagnac interferometer, A phase modulator that is inserted into the optical path of the Sagnac interferometer and performs phase modulation on the optical pulse in any one of the optical pulses; and an optical pulse that is inserted into the optical path of the Sagnac interferometer and is phase-modulated. Between the light source and the beam splitter, an optical attenuator that outputs the phase-modulated optical pulse output from the optical coupler to a single photon level and output to the transmission path An inserted circulator, which is output from the beam splitter and circulates along the optical path of the Sagnac interferometer and interferes with the optical path. Comprising a circulator for branching the scan, and a photodetector for converting an optical pulse after interference output from the circulator to an electrical signal, calculating the value of the phase modulation from the intensity of the light pulses detected by the photodetector characterized in that it.

Another embodiment of the present invention is an optical pulse generator for use in quantum cryptography communication, a first beam splitter for outputting an optical pulse output from a light source to two optical paths having different optical path lengths; A second beam splitter in which the two optical paths are connected to an input port; an output of the second beam splitter; and an optical pulse that has passed through one of the two optical paths. A phase modulator for performing phase modulation, a third beam splitter connected to the output of the phase modulator, a mirror connected to one output of the third beam splitter, and a third beam splitter An attenuator for attenuating the phase-modulated optical pulse output from the other output to a single photon level and outputting it to the transmission line; the light source; A circulator which is inserted between the beam splitter, reflected in the mirror, the circulator for branching the optical pulses interfere outputted from the first beam splitter, after interference output from the circulator A light detector for converting the light pulse into an electrical signal, and calculating a phase modulation value from the intensity of the light pulse detected by the light detector .

  As described above, according to the present invention, it is possible to monitor in real time the value of the phase modulation actually performed by the phase modulator used for quantum cryptography communication.

It is a figure which shows the structure of the optical pulse generator concerning the 1st Embodiment of this invention. It is a figure which shows the structure of the optical pulse generator concerning the 2nd Embodiment of this invention. It is a figure which shows the structure of the optical pulse generator concerning the 3rd Embodiment of this invention. It is a figure which shows the structure of the optical pulse generator concerning the 4th Embodiment of this invention.

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.

  FIG. 1 shows the configuration of an optical pulse generator according to the first embodiment of the present invention. It is an example of an optical pulse generator used for quantum cryptography communication, and the value of phase modulation can be monitored accurately in real time. An optical circulator 102 is connected to the output of the laser light source 101, and a photodetector 103 and a beam splitter 104 are connected to the output of the optical circulator 102. The two output ports of the beam splitter 104 are connected by a loop-shaped optical path, and a phase modulator 105 and an optical coupler 106 whose accuracy is to be measured are inserted into the loop-shaped optical path. The loop optical path constitutes a so-called Sagnac interferometer. Further, a variable delay line 107 for setting a counterclockwise and clockwise optical path difference from the beam splitter 104 to the optical coupler 106 is inserted in the loop-shaped optical path. For example, a heat controller, an electric delay line, or the like can be applied to the variable delay line.

  Pulse light having a relatively large intensity that is not a single photon level output from the laser light source 101 passes through the optical circulator 102 and is branched by the beam splitter 104. A clockwise light pulse (light pulse in any one direction) from the beam splitter 104 is subjected to phase modulation by the phase modulator 105, and returns to the beam splitter 104 through the optical coupler 106. The counterclockwise light pulse (light pulse in the other direction) from the beam splitter 104 is not subjected to phase modulation by the phase modulator 105, returns to the beam splitter 104, and interferes with the clockwise light pulse. The light pulse after the interference is detected by the photodetector 103 via the optical circulator 102.

  The intensity W of the light pulse input to this photodetector is

Given in. Where δ is the actual value of the phase modulation, T is the transmittance of the beam splitter 104, t is the transmittance of the looped optical path of the optical coupler 106, and α is the average number of photons of the light pulse from the laser light source 101. Represent.

  When the transmittances T and t are known, by measuring the intensity W for each light pulse, (Equation 1) becomes an equation with only δ. By selecting a plausible value from among a plurality of δ, the value of the phase modulation performed by the phase modulator 105 can be obtained.

  When the transmittances T and t are unknown, the set of phase modulation values targeted by the phase modulation value is

And the set of measured values is

And k = 0 represents a value when the phase modulator 105 is turned off. A plurality of optical pulses are generated from the laser light source 101 at certain time intervals, and the intensity fluctuation of each optical pulse, the detection efficiency fluctuation of the photodetector 103, and the performance fluctuation of the optical circulator 102 and the beam splitter 104 are statistically calculated. For processing, k is randomly selected for each light pulse.

Obtaining an average light intensity W _k of the respective k,

By calculating the relational expression of [delta] _k and [delta] ₀ is obtained. A plurality of δ _k are obtained as a function of δ _{0, and} the value of the phase modulation to be performed by the phase modulator 105 is selected by selecting a plausible value from among them.

However, how much the phase modulator 105 is different from that when the phase modulator 105 is off, that is, the average value of the phase modulation error is obtained.

  On the other hand, in the optical coupler 106, optical pulses having exactly the same relative phase as the counterclockwise optical pulse and the clockwise optical pulse are output from the two output ports, respectively. Depending on the purpose, light pulses from only one output port are used, or light pulses from both output ports are used. In addition, the intensity of the output optical pulse can be attenuated to a single photon level by the attenuators 108a and 108b and then sent to the transmission line. Furthermore, it can also be sent to the transmission line via means for adjusting the polarization of the output light pulse. Furthermore, an optical isolator may be introduced in order to prevent intrusion of light from an eavesdropper.

  FIG. 2 shows a configuration of an optical pulse generator according to the second embodiment of the present invention. Only differences from the optical pulse generator of the first embodiment will be described. A beam splitter 122 is connected to the two output ports of the optical coupler 106 via two optical paths. A variable delay line 122 is inserted in one of the optical paths.

  According to this configuration, it is possible to transmit a light pulse taking a counterclockwise path on a loop-shaped optical path and a light pulse taking a clockwise path to one transmission path with a time difference. . The variable delay line 122 can adjust the time difference between the counterclockwise light pulse and the clockwise pulse.

  Further, the intensity of the output optical pulse train can be attenuated by the attenuator 123 and then sent to the transmission line. Furthermore, it can also be sent to the transmission line via means for adjusting the polarization of the output optical pulse train. Furthermore, an optical isolator may be introduced in order to prevent intrusion of light from an eavesdropper.

  FIG. 3 shows a configuration of an optical pulse generator according to the third embodiment of the present invention. It is an example of an optical pulse generator used for quantum cryptography communication, and the value of phase modulation can be monitored accurately in real time. An optical circulator 102 is connected to the output of the laser light source 101, and a photodetector 103 and a beam splitter 131 are connected to the output of the optical circulator 102. The two output ports of the beam splitter 131 are connected to the two input ports of the beam splitter 132 via two optical paths having different optical path lengths, and constitute a so-called Mach-Zehnder interferometer. A phase modulator 133 whose accuracy is to be measured is connected to the output of the beam splitter 132, and phase modulation is performed only on the optical pulse that has passed through one of the optical paths. A beam splitter 134 is connected to the output of the phase modulator 133, and a mirror 135 is connected to one output of the beam splitter 134.

  Pulse light having a relatively large intensity that is not a single photon level output from the laser light source 101 passes through the optical circulator 102 and is branched by the beam splitter 131. The optical pulse that has passed through the two optical paths is input to the phase modulator 133, and phase modulation is performed only on the optical pulse that has passed through one of the optical paths. The output of the phase modulator 133 is returned to the beam splitter 132 by the beam splitter 134 and the mirror 135. At this time, the phase modulator 133 does not perform phase modulation on either optical pulse. Branched by the beam splitter 132, a total of four optical pulse trains propagate through two optical paths. At this time, two light pulses that have passed through different light paths on the way and the way back interfere with each other. The light pulse after the interference is detected by the photodetector 103 via the optical circulator 102.

  The intensity W of the interference light input to this photodetector is

Given in. Here, δ represents the actual value of phase modulation, T ₁ , T ₂ , and T ₃ represent the transmittances of the beam splitters 131, 132, and 133, respectively, and α represents the average number of photons of the light pulse from the laser light source 101.

When the transmittances T and t are known, by measuring the intensity W for each optical pulse, phase modulation can be performed by selecting a plausible value from a plurality of δ in which (Equation 3) becomes an equation of δ only. The value of the phase modulation applied by the device 133 can be obtained. When the transmittances T ₁ , T ₂ , and T ₃ are unknown, the method for measuring the phase modulation value is the same as in the first embodiment.

  From the output port of the beam splitter 134, an optical pulse having exactly the same relative phase as the two optical pulses demultiplexed by the beam splitter 131 is output. Depending on the purpose, light pulses that have passed through one optical path are used, or both light pulses are used. In addition, the intensity of the output optical pulse can be attenuated to a single photon level by the attenuator 136 before being sent to the transmission line. Furthermore, it can also be sent to the transmission line via means for adjusting the polarization of the output light pulse. Furthermore, an optical isolator may be introduced in order to prevent intrusion of light from an eavesdropper.

  FIG. 4 shows the configuration of an optical pulse generator according to the fourth embodiment of the present invention. Only differences from the optical pulse generator of the third embodiment will be described. The two output ports of the beam splitter 131 are connected to the two input ports of the beam splitter 132 via two optical paths having different optical path lengths. The phase modulator 141 whose accuracy is to be measured is inserted in the optical path having the longer optical path length. The operating principle is the same as that of the third embodiment except that the insertion position of the phase modulator 141 is different.

  According to the present embodiment, it is possible to monitor in real time the value of the phase modulation actually performed by the phase modulator of the optical pulse generator used for quantum cryptography communication. In addition, if you want to find the average of phase modulation when performing phase modulation multiple times, even if the characteristics of factors such as the efficiency of the beam splitter, optical circulator, optical coupler, and photodetector are unknown, The average value can be obtained with high accuracy.

DESCRIPTION OF SYMBOLS 101 Laser light source 102 Optical circulator 103 Optical detector 104,121,131,132,134 Beam splitter 105,133,141 Phase modulator 106 Optical coupler 107,122 Variable delay line 108,123,136 Attenuator 135 Mirror

Claims (6)

An optical pulse generator used for quantum cryptography communication ,
A beam splitter that outputs a light pulse output from a light source to a Sagnac interferometer;
A phase modulator that is inserted into the optical path of the Sagnac interferometer and applies phase modulation to the optical pulse in any one direction of the optical pulses;
An optical coupler that is inserted into the optical path of the Sagnac interferometer and outputs a phase-modulated optical pulse;
An attenuator for attenuating the phase-modulated optical pulse output from the optical coupler to a single photon level and outputting it to a transmission line;
A circulator inserted between the light source and the beam splitter, the circulator for branching an optical pulse output from the beam splitter and interfering around the optical path of the Sagnac interferometer;
A photodetector for converting the optical pulse after interference output from the circulator into an electrical signal ;
An optical pulse generator characterized in that a value of phase modulation is calculated from the intensity of an optical pulse detected by the photodetector . The intensity W of the light pulse detected by the photodetector is

Where δ is the phase modulation value, T is the transmittance of the beam splitter, t is the transmittance of the optical path of the Sagnac interferometer of the optical coupler, and α is the average of the light pulses from the light source. 2. The optical pulse generator according to claim 1, which represents the number of photons. A beam splitter connected to two output ports of the optical coupler via two optical paths;
The optical pulse generator according to claim 1, further comprising: a variable delay line inserted into one of the two optical paths. An optical pulse generator used for quantum cryptography communication ,
A first beam splitter that outputs light pulses output from a light source to two optical paths having different optical path lengths;
A second beam splitter in which the two optical paths are connected to an input port;
A phase modulator connected to the output of the second beam splitter for phase-modulating an optical pulse that has passed through one of the two optical paths;
A third beam splitter connected to the output of the phase modulator;
A mirror connected to one output of the third beam splitter;
An attenuator for attenuating the phase-modulated optical pulse output from the other output of the third beam splitter to a single photon level and outputting it to the transmission line;
A circulator inserted between the light source and the first beam splitter, the circulator reflecting off the mirror and reflecting the interfering light pulse output from the first beam splitter;
A photodetector for converting the optical pulse after interference output from the circulator into an electrical signal ;
An optical pulse generator characterized in that a value of phase modulation is calculated from the intensity of an optical pulse detected by the photodetector . An optical pulse generator used for quantum cryptography communication ,
A first beam splitter that outputs light pulses output from a light source to two optical paths having different optical path lengths;
A second beam splitter in which the two optical paths are connected to an input port;
A phase modulator that is inserted into one of the two optical paths and applies phase modulation to the optical pulse;
A third beam splitter connected to the output of the second beam splitter;
A mirror connected to one output of the third beam splitter;
An attenuator for attenuating the phase-modulated optical pulse output from the other output of the third beam splitter to a single photon level and outputting it to the transmission line;
A circulator inserted between the light source and the first beam splitter, the circulator reflecting off the mirror and reflecting the interfering light pulse output from the first beam splitter;
A photodetector for converting the optical pulse after interference output from the circulator into an electrical signal ;
An optical pulse generator characterized in that a value of phase modulation is calculated from the intensity of an optical pulse detected by the photodetector . The intensity W of the interference light detected by the photodetector is

Where δ is the value of the phase modulation, T ₁ , T ₂ , T ₃ are the transmittances of the first, second and third beam splitters, respectively, and α is the average photon of the light pulse from the light source. The optical pulse generator according to claim 4 or 5 , wherein the optical pulse generator represents a number.

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