KR102263313B1 - Method and system for Quantum Key Distribution by Frequency-Domain Coding - Google Patents

Abstract

The present invention uses a high-frequency signal in the transmitter to amplitude-modulate photons in the frequency domain and transmits them, and the receiver uses an asymmetric delay interferometer with a short waveguide length without using a modulator to classify the quantum state of photons. By giving, it is possible to transmit a quantum key over a long distance by minimizing photon loss, and to a quantum cryptography transmission method and system using frequency domain coding that can improve stability to polarization and temperature.

Description

Quantum Key Distribution Method and System Using Frequency-Domain Coding {Method and system for Quantum Key Distribution by Frequency-Domain Coding}

The present invention relates to a quantum key distribution method and system using frequency domain coding, and more specifically, to a quantum key using frequency domain coding that can increase the transmission distance of a quantum key and improve stability to polarization and temperature. Dispensing methods and systems.

Recently, as wired and wireless communication services are widely distributed and social awareness of personal privacy is increasing, the security problem of communication networks is emerging as an important issue. In particular, the importance of security is being greatly emphasized because security in communication networks related to the state, corporations, finance, etc. can have so much importance that it can be extended to social problems beyond individual problems. On the other hand, since the existing secure communication has a significant theoretical weakness against external attacks, quantum cryptography communication, which theoretically guarantees unconditional security, is in the spotlight as a next-generation security technology to compensate for this. The background technology of the present invention is as follows.

1. BB84 Protocol and Coding Method

Among quantum cryptography communication technologies, technical research on quantum key distribution using photons is being actively conducted, and the BB84 protocol developed in 1984 is the most widely used. BB84 basically consists of two different non-orthogonal bases, and one base consists of two mutually orthogonal states. Various methods such as polarization coding and phase coding are being tried as a method of implementing each basis and state.

2. Limiting the average number of photons in a coherent source

In order for quantum key distribution using photons to be safe from eavesdroppers, the average number of photons must be kept below a certain level. This is because photons above a certain level generate too many multiple photons, which may provide an opportunity for eavesdroppers to secretly collect information. However, as weak coherent light is used, there are restrictions on long-distance transmission for quantum key distribution. Accordingly, it becomes an important issue to minimize loss of the receiving end to enable long-distance transmission.

3. Polarization dependence and complexity of the receiving end optics

Quantum key distribution systems according to the prior art typically distinguish the signal transmitted through the modulator in the receiver. However, since the modulator is an optical device that is highly polarization dependent, it has to bear the loss of a part of photons that undergo polarization drift due to long-distance transmission. That is, the receiver configured using the modulator according to the prior art causes a photon loss of a certain level or more, and the photon loss may increase depending on the characteristics of the interferometer and other optical devices of the receiver, so that the quantum key can be transmitted over a long distance. acts as a limiting point. In addition, such optical devices also have a problem in that they are greatly affected by polarization and temperature changes depending on their design.

4. Asymmetric delay interferometer and temperature stabilization

The asymmetric delay interferometer according to the prior art consists of one input and two outputs, and the input photons pass through the interferometer using two different paths. The difference in length between the two paths depends on the design of the interferometer, and a delay equal to the path difference causes interference. The two outputs have different characteristics, one with constructive interference and the other with destructive interference. However, in general, the interferometer has a characteristic that is sensitive to temperature, and as the difference in the length of the interferometer waveguide is small, the free spectral range is widened, and the change in the interference spectrum due to the temperature change is small. That is, as the length of the interferometric waveguide decreases, it is less affected by the temperature change. In addition, as the asymmetric delay interferometer interference path is shorter, the influence of the polarization dependence of the waveguide can be reduced. Accordingly, it is possible to reduce the temperature change and polarization dependence of the interferometer by reducing the waveguide length of the interferometer, but an appropriate method for implementing this has not yet been proposed.

The present invention has been devised to solve the problems of the prior art as described above, and to provide a quantum key distribution method and system capable of increasing the distance over which a quantum key can be transmitted and improving stability with respect to polarization and temperature.

The present invention for solving the above problems proposes a quantum key distribution method using a new type of frequency coding.

The transmitter applies frequency domain coding to a single photon to create different quantum states whose frequencies differ by several GHz. (Single photon maintains quantum mechanical single-photon properties even when modulated with fast pulses.) The receiver uses only an asymmetric delay interferometer with low input loss to distinguish the quantum state of photons, which significantly reduces optical loss compared to existing systems. can be lowered

In addition, while the existing frequency coding or phase coding quantum key distribution system has a problem in maintaining the stability of the interferometer of the receiver according to temperature and polarization changes, this frequency coding and detection system has a very short waveguide length difference of the delay interferometer, so it is sensitive to temperature and polarization. High stability is ensured.

A frequency-coded quantum key distribution system may be capable of eavesdropping by a spectrometer. In order to prevent this, in the present invention, a wave packet of one single photon is formed to be very short and designed to block eavesdropping by a spectrometer.

The present invention uses a high-frequency signal in the transmitter to amplitude-modulate photons in the frequency domain and transmits them, and the receiver uses an asymmetric delay interferometer with a short waveguide length without using a modulator to classify the quantum state of photons. By giving, it minimizes photon loss to enable long-distance transmission of a quantum key, and has the effect of providing a quantum cryptography transmission method and system using frequency domain coding that can improve stability against polarization and temperature.

1 is a comparison diagram of a frequency coding-based quantum key distribution protocol according to an embodiment of the present invention and polarization coding according to the prior art.
2 is a block diagram of a frequency coding-based quantum key distribution system according to an embodiment of the present invention.
3 is a frequency spectrum graph in a frequency coding-based quantum key distribution system according to an embodiment of the present invention.
4 is an explanatory diagram for frequency modulation in a frequency coding-based quantum key distribution system according to an embodiment of the present invention.
5 is an explanatory diagram for an operation in an asymmetric delay interferometer in a frequency coding-based quantum key distribution system according to an embodiment of the present invention.

The present invention can apply various transformations and can have various embodiments. Hereinafter, specific embodiments will be described in detail based on the accompanying drawings.

The following examples are provided to provide a comprehensive understanding of the methods, apparatus and/or systems described herein. However, this is merely an example and the present invention is not limited thereto.

In describing the embodiments of the present invention, if it is determined that the detailed description of the known technology related to the present invention may unnecessarily obscure the gist of the present invention, the detailed description thereof will be omitted. And, the terms to be described later are terms defined in consideration of functions in the present invention, which may vary according to the intention or custom of the user or operator. Therefore, the definition should be made based on the content throughout this specification. The terminology used in the detailed description is for the purpose of describing embodiments of the present invention only, and should not be limiting in any way. Unless explicitly used otherwise, expressions in the singular include the meaning of the plural. In this description, expressions such as “comprising” or “comprising” are intended to indicate certain features, numbers, steps, acts, elements, some or a combination thereof, and one or more other than those described. It should not be construed to exclude the presence or possibility of other features, numbers, steps, acts, elements, or any part or combination thereof.

In addition, terms such as first and second may be used to describe various components, but the components are not limited by the terms, and the terms are for the purpose of distinguishing one component from other components. used only as

The present invention is a novel implementation of the previously developed frequency coding or phase coding-based quantum key distribution technology, and has the effect of enabling long-distance transmission of the quantum key by minimizing the photon loss occurring in the receiver. In addition, it has stability to polarization and temperature compared to the existing system.

Hereinafter, a theoretical background and exemplary embodiments of a quantum key distribution method and system using frequency domain coding according to an embodiment of the present invention will be described in detail with reference to the accompanying drawings.

In the quantum key distribution method and system using frequency domain coding proposed by the present invention, a specific frequency means a quantum state. As can be seen in FIG. 1 , in the present invention, a specific frequency represents a frequency representing a basis and a state as follows. (At this time, the details of the frequency and modulation may vary slightly depending on the actual application environment and conditions.)

Base 1 State 1: Frequency

Base 1 State 2: Frequency

Base 2: State 1: Frequency

Base 2: State 2: Frequency

는 기저 2 광자의 캐리어 주파수이며, a는 캐리어 주파수로부터 변조되는 주파수의 크기이다. 또한 a << A를 만족한다. (도 3은 위와 같은 주파수 차이를 가지는 양자 상태들을 나타내는 주파수 스펙트럼 그래프이다.)where A is the carrier frequency of the base 1 photon,

is the carrier frequency of the base two photons, and a is the magnitude of the frequency modulated from the carrier frequency. Also, a << A is satisfied. (FIG. 3 is a frequency spectrum graph showing quantum states having the same frequency difference as above.)

를 가지는 광자를 생성한다. 이 때, 상기 제1 레이저 생성부(211)에서 나오는 광자는 기저1을 의미하고, 제2 레이저 생성부(212)에서 나오는 광자는 기저 2를 의미한다. 제1 레이저 생성부(211)와 제2 레이저 생성부(212)의 다음에 위치하는 랜덤 레이저 선택부(213)는 스위치를 포함하여 구성될 수 있으며, 상기 랜덤 레이저 선택부(213)에서는 각 타임 슬롯(time slot)에서 제1 레이저 생성부(211)와 제2 레이저 생성부(212) 중 하나에서만 광자가 나오도록 스위칭을 수행하며, 이는 송신부(210)가 보내는 양자 상태의 기저를 선택하는 역할을 한다. 이어서 진폭 변조부(214)는 각 기저의 상태를 결정한다. 진폭 변조부(214)의 전기 신호 입력으로서 직류 혹은 상기한 a 주파수를 가지는 교류 신호를 가하게 되는데, 직류 신호는 양의 값을 가지도록 하고, 상기한 a 주파수를 가지는 교류 신호는 같은 크기의 양과 음의 값을 주기적으로 갖도록 한다. 이 때, 교류 신호는 시간 도메인에서 매우 빠른 속도로 단일 광자 웨이브 패킷(wave packet) 내부를 변조하게 되며, 직류 신호로 변조된 광자와 교류 신호로 빠르게 변조된 광자는 같은 특성을 가지고 단일 광자 검출기(224)에서 검출되는 특성을 가진다. As shown in FIG. 2 , the above frequency coding may be performed through the following process. First, the first laser generator 211 generates a photon having a center frequency A, and the second laser generator 212 generates a photon having a center frequency A.

generate a photon with In this case, the photon emitted from the first laser generator 211 means base 1, and the photon emitted from the second laser generator 212 means base 2. The random laser selection unit 213 positioned next to the first laser generation unit 211 and the second laser generation unit 212 may include a switch, and the random laser selection unit 213 may include a switch at each time. Switching is performed so that photons are emitted from only one of the first laser generator 211 and the second laser generator 212 in a time slot, which serves to select the basis of the quantum state transmitted by the transmitter 210 do Then, the amplitude modulator 214 determines the state of each base. As an electrical signal input of the amplitude modulator 214, DC or an AC signal having the above-described frequency a is applied. The DC signal has a positive value, and the AC signal having the above-mentioned frequency a has positive and negative values of the same magnitude. to have the value of . At this time, the AC signal modulates the inside of a single photon wave packet at a very high speed in the time domain, and the photon modulated with the DC signal and the photon rapidly modulated with the AC signal have the same characteristics, and a single photon detector ( 224).

만큼의 주파수 크기의 변화가 발생하며,

를 주기로 위상이 변화한다. 즉, A 주파수 광자에 직류 신호 변조를 가한 것이 기저1의 상태1, A 주파수 광자에 교류 신호 변조를 가한 것이 기저1의 상태2,

주파수 광자에 직류 신호 변조를 가한 것이 기저2의 상태1,

주파수 광자에 교류 신호 변조를 가한 것이 기저2의 상태2가 된다. 도 4에서는 상기 진폭 변조기(214)에서의 진폭 변조(amplitude modulation)를 설명하기 위한 도면이 제시되어 있다. 도 4에서 볼 수 있는 바와 같이, 제1 레이저 생성부(211) 또는 제2 레이저 생성부(212)에서 생성된 레이저 신호에 직류 신호가 진폭 변조되는 경우에는 레이저의 각 펄스가 모두 0도의 위상을 가지게 되고, 레이저 신호에 교류 신호가 진폭 변조되는 경우에는 각 펄스의 위상이 교류 신호의 위상에 따라 달라지게 되어, 수신부(220)의 비대칭 지연 간섭계(222, 223)에서 보강 간섭 또는 상쇄 간섭을 일으키게 된다.When a DC signal is applied as an electrical signal of the amplitude modulator 214, the frequency and phase of the input photon do not change. On the other hand, when applying the AC signal having the above-mentioned frequency a, from the center frequency

A change in the magnitude of the frequency occurs by

The phase changes with a period of In other words, a state 1 of base 1 is applied to the A frequency photon with DC signal modulation, and a state 2 of base 1 is that an AC signal modulation is applied to the A frequency photon.

DC signal modulation applied to frequency photons is the basis 2 state 1,

State 2 of base 2 is obtained by applying AC signal modulation to frequency photons. 4 is a diagram for explaining amplitude modulation in the amplitude modulator 214 . As can be seen in FIG. 4 , when the DC signal is amplitude-modulated to the laser signal generated by the first laser generator 211 or the second laser generator 212, each pulse of the laser has a phase of 0 degrees. When the AC signal is amplitude-modulated to the laser signal, the phase of each pulse changes depending on the phase of the AC signal, causing constructive or destructive interference in the asymmetric delay interferometers 222 and 223 of the receiver 220. do.

만큼의 시간 지연을 가지도록 설계되어 있고, 두 경로의 보강 간섭과 상쇄 간섭을 출력으로 내보내는 두 개의 포트로 구성되어 있다. 두 개의 비대칭 지연 간섭계(222, 223)는 각각 기저1과 기저2에 대한 주파수 특성에 맞도록 설계되어 있는데, 자신의 설계와 맞는 기저 주파수를 가진 광자가 들어오는 경우 특정 포트에서 광자가 나가게 되지만, 자신의 설계와 맞지 않는 기저 주파수를 가진 광자가 들어오는 경우 광자가 두 포트 모두에서 같은 확률로 나가는 특성을 가진다. 즉, 광자와 비대칭 지연 간섭계(222, 223)의 기저 특성이 일치하는 경우 양자 상태 (주파수)를 정확히 구분할 수 있지만, 일치하지 않는 경우에는 양자 상태를 구분할 수 없게 된다.The receiver 220 detects the photon having the above frequency using two asymmetric delay interferometers 222 and 223 having a free spectral range. The asymmetric delay interferometers 222 and 223 show that the difference in the length of the waveguide is

It is designed to have as much time delay and consists of two ports that output constructive and destructive interference of two paths. The two asymmetric delay interferometers 222 and 223 are designed to match the frequency characteristics of the base 1 and the base 2, respectively. When a photon with a base frequency that does not match the design of , comes in, the photon exits from both ports with the same probability. That is, when the photons and the basis characteristics of the asymmetric delay interferometers 222 and 223 match, the quantum state (frequency) can be accurately distinguished, but when the photons do not match, the quantum state cannot be distinguished.

Figure 5 describes the operation in the asymmetric delay interferometer (222, 223) in the frequency coding-based quantum key distribution system 200 according to an embodiment of the present invention. Each of the asymmetric delay interferometers 222 and 223 corresponds to the first laser generator 211 and the second laser generator 212, respectively.

In addition, as can be seen in FIG. 5 in each of the asymmetric delay interferometers 222 and 223, when a DC modulated signal is incident, the signal is reinforced at the constructive interference terminal and detected by the single photon detector 224. and the signal is canceled at the destructive port so that the single photon detector 224 cannot detect the photon. On the other hand, when an AC-modulated signal is incident, the signal is canceled at the constructive port so that the single photon detector 224 cannot detect a photon, and the signal is reinforced at the destructive port. A single photon detector 224 may detect photons.

Accordingly, as an embodiment of the present invention, it is possible to configure a quantum key distribution system capable of detecting states 1 and 2 of base 1 and states 1 and 2 of base 2.

In addition, as shown in FIG. 2 , the single photon sent by the transmitter 210 is sent to arbitrary asymmetric delay interferometers 222 and 223 through a 50:50 splitter 221 located at the front end of the receiver 220 . will go in That is, half of the photons sent by the transmitter 210 can completely discriminate the quantum state, and half cannot discriminate the quantum state.

By using the above process, it is possible to implement the quantum key distribution system 200 using frequency domain coding according to the present invention. In the case of the receiver according to the prior art, there was a problem in that the input loss increases as it is configured to include a modulator, but in the quantum key distribution system 200 using the frequency domain coding in the present invention, the receiver 220 does not include a modulator and a spectrometer ( splitter) 221 and the asymmetric delay interferometers 222 and 223, so that the loss can be greatly reduced, and accordingly, the distance over which the quantum key can be transmitted can be greatly increased. In addition, the asymmetric delay interferometers 222 and 223 according to an embodiment of the present invention have a delay time of several hundred ps suitable for very fast a-frequency modulation compared to the prior art, which is asymmetric delay interferometers 222 and 223. This means that the difference in the length of the waveguide can be significantly reduced. Accordingly, it is possible to reduce the influence that may occur in different paths in the asymmetric delay interferometers 222 and 223 according to the change in temperature and polarization, and accordingly, to the change in polarization and temperature than other quantum key distribution systems according to the prior art. This allows for higher stability.

Furthermore, the eavesdropper (Eve) obtains information by spectroscopically analyzing the signal transmitted from the transmitter 210 and retransmits it to the receiver 220 or replicates the signal transmitted from the transmitter 210 to approximate quantum information. In the case of post-post-spectral analysis, it is preferable to generate the frequency coding as a pulse of a short length in order to cause spectral overlap of each frequency coding state so that accurate information cannot be obtained if the base frequency of the transmitter 210 information is not known. At this time, the length of the pulse may vary somewhat depending on the shape of the envelope function of the pulse, but it is more appropriate to generate and send a photon pulse so that it is generally 2/a (sec).

Accordingly, in the frequency-coded photon signal according to an embodiment of the present invention, the length of one photon pulse is approximately 2/a when viewed in time space, and the optical phase of up to 720 degrees with a temporal resolution of 1/2a in this. It means the code in which the modulation is made. In addition, when the receiver 220 uses the imperfect asymmetric delay interferometers 222 and 223 in reality, the length of the photon pulse may be made somewhat longer than 2/a in order to increase the visibility of the frequency coding state. However, in this case, the pulse length must be limited so that the spectral overlap occurs sufficiently through the Fourier transform of the length so that the eavesdropper Eve cannot succeed in spectral analysis.

In addition, when using the asymmetric delay interferometers 222 and 223 according to an embodiment of the present invention, the frequency code may generate a code to have an extended repetition spectrum as follows:

, k는 정수 Base 1 State 1: Frequency

, k is an integer

, k는 정수 Base 1 State 2: Frequency

, k is an integer

, k는 정수 Base 2: State 1: Frequency

, k is an integer

, k는 정수. Base 2: State 2: Frequency

, k is an integer.

의 속도로 반복되는 return-to-zero (RZ) 광펄스에서 각각의 광펄스에 다음과 같은 위상 변조를 인가함으로서 얻을 수 있다: These repeating spectral codes are usually

In return-to-zero (RZ) light pulses repeated at a rate of , it can be obtained by applying the following phase modulation to each light pulse:

State 1 of Base 1: Continuous light pulses of the same phase

Base 1 State 2: Continuous light pulses with the same phase modulation of 0°, 180°, 0°, and 180° respectively

State of Base 2: Continuous light pulses with the same phase modulation of 0°, 90°, 180°, and 270° respectively

State 2 of Base 2: Continuous light pulses with phase modulation of 0°, 270°, 180°, and 90° respectively.

At this time, like the method described above, the maximum values of several pulses follow the shape of the occlusion function, and the length of the occlusion function is preferably approximately 2/a to generate the spectral code so that the spectral analysis becomes uncertain.

The above description is merely illustrative of the technical spirit of the present invention, and various modifications and variations will be possible without departing from the essential characteristics of the present invention by those skilled in the art to which the present invention pertains. Accordingly, the embodiments described in the present invention are not intended to limit the technical spirit of the present invention, but to explain, and are not limited to these embodiments. The protection scope of the present invention should be construed by the following claims, and all technical ideas within the scope equivalent thereto should be construed as being included in the scope of the present invention.

200: quantum key distribution system using frequency domain coding
210: transmitter
211: first laser generating unit
212: second laser generating unit
213: random laser selection unit
214: amplitude modulator
220: receiver
221: spectroscopy
222: first asymmetric delay interferometer
223: second asymmetric delay interferometer
224: single photon detector

Claims (10)

In the transmitter, the carrier frequency of the first base is set to A, and the carrier frequency of the second base is set to A + a/2.
The first state frequency of the first basis is frequency-coded as A, the second state frequency of the first basis is frequency-coded as A±a, the first state frequency of the second basis is A + a/2, and the second state frequency of the second basis is frequency-coded as A + a/2. Transmitting a photon signal including quantum key information generated by frequency coding the two-state frequency with A + 3a/2 and A - a/2; and
Detecting the photon signal using an asymmetric delay interferometer in a receiver,
The detecting step is
randomly branching the photon signal received from the transmitter;
and detecting the randomly branched photon signal by applying an asymmetric delay interferometer corresponding to the first basis and the second basis,
The asymmetric delay interferometer is a quantum key distribution method, characterized in that the length difference of the waveguide is an asymmetric delay interferometer having a delay of 1/2a. The method of claim 1,
The sending step is
a switching step of selectively blocking one of the first laser having the first basis carrier frequency and the second laser having the second basis carrier frequency;
Quantum key distribution method comprising the step of frequency coding the switched laser (first laser or second laser) with direct current or frequency a. 3. The method of claim 2,
In the frequency coding step,
Quantum key distribution method, characterized in that the switched laser (first laser or second laser) is frequency-coded by amplitude modulation with a DC signal or an AC signal of frequency a. delete delete In the transmitter, the carrier frequency of the first base is set to A, and the carrier frequency of the second base is set to A + a/2.
The first state frequency of the first basis is frequency-coded as A, the second state frequency of the first basis is frequency-coded as A±a, the first state frequency of the second basis is A + a/2, and the second state frequency of the second basis is frequency-coded as A + a/2. Transmitting a photon signal including quantum key information generated by frequency coding the two-state frequency with A + 3a/2 and A - a/2; and
Detecting the photon signal using an asymmetric delay interferometer in a receiver,
In the sending step,
For a return-to-zero (RZ) light pulse repeated at a rate of 1/2a,
A quantum key distribution method, characterized in that by applying a predetermined phase modulation to each light pulse to generate signals corresponding to the first and second basis and the first and second states. In the transmitter, the carrier frequency of the first base is set to A, and the carrier frequency of the second base is set to A + a/2.
The first state frequency of the first basis is frequency-coded as A, the second state frequency of the first basis is frequency-coded as A±a, the first state frequency of the second basis is A + a/2, and the second state frequency of the second basis is frequency-coded as A + a/2. Transmitting a photon signal including quantum key information generated by frequency coding the two-state frequency with A + 3a/2 and A - a/2; and
Detecting the photon signal using an asymmetric delay interferometer in a receiver,
The length of the photon signal is
Quantum key distribution method, characterized in that the spectral overlap is short enough to prevent eavesdropping when spectral analysis is performed on the photon signal. In the transmitter, the carrier frequency of the first base is A + 2ka, and the carrier frequency of the second base is A + (2k + 1/2)a,
The first state frequency of the first basis is frequency-coded as A + 2ka, the second state frequency of the first basis is frequency-coded as A + (2k + 1)a, and the first state frequency of the second basis is A + (2k + Transmitting a photon signal including quantum key information generated by frequency coding with 1/2)a, where the second state frequency of the second basis is A + (2k - 1/2)a (where k is an integer) ; and
Detecting the photon signal using an asymmetric delay interferometer in a receiver,
The detecting step is
randomly branching the photon signal received from the transmitter;
and detecting the randomly branched photon signal by applying an asymmetric delay interferometer corresponding to the first basis and the second basis,
The asymmetric delay interferometer is a quantum key distribution method, characterized in that the length difference of the waveguide is an asymmetric delay interferometer having a delay of 1/2a. Let A be the carrier frequency of the first base and the carrier frequency of the second base be A + a/2,
The first state frequency of the first basis is frequency-coded as A, the second state frequency of the first basis is frequency-coded as A±a, the first state frequency of the second basis is A + a/2, and the second state frequency of the second basis is frequency-coded as A + a/2. a transmitter for transmitting a photon signal including quantum key information generated by frequency coding a two-state frequency with A + 3a/2 and A - a/2; and
A receiver for detecting the photon signal using an asymmetric delay interferometer,
In the receiver,
Randomly branching the photon signal received from the transmitter, and detecting the randomly branched photon signal by applying an asymmetric delay interferometer corresponding to the first basis and the second basis,
The asymmetric delay interferometer is a quantum key distribution system, characterized in that the difference in the length of the waveguide has a delay of 1/2a. Assuming that the carrier frequency of the first base is A + 2ka and the carrier frequency of the second base is A + (2k + 1/2)a,
The first state frequency of the first basis is frequency-coded as A + 2ka, the second state frequency of the first basis is frequency-coded as A + (2k + 1)a, and the first state frequency of the second basis is A + (2k + A transmitter that transmits a photon signal including quantum key information generated by frequency coding with 1/2)a, where the second state frequency of the second basis is A + (2k - 1/2)a (where k is an integer) ; and
A receiver for detecting the photon signal using an asymmetric delay interferometer,
In the receiver,
Randomly branching the photon signal received from the transmitter, and detecting the randomly branched photon signal by applying an asymmetric delay interferometer corresponding to the first basis and the second basis,
The asymmetric delay interferometer is a quantum key distribution system, characterized in that the difference in the length of the waveguide has a delay of 1/2a.

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