WO2017204440A1 - Code-based quantum cryptographic key distribution method, apparatus, and system - Google Patents

Abstract

The present invention relates to a quantum cryptographic key distribution method, apparatus, and system and, more particularly, to a quantum cryptographic key distribution method, apparatus, and system wherein, in order to implement a multi-qubit quantum cryptographic key distribution system, a quantum cryptographic key distribution is performed by dividing a single photon pulse signal into a plurality of chips, each having a period shorter than a pulse width, through fast phase modulation in a complex plane and coding the phase of each chip. A quantum cryptographic key distribution method, apparatus, and system according to an embodiment of the present invention performs quantum cryptographic key distribution between a sender (Alice) and a receiver (Bob) by dividing a light pulse into a plurality of chips and phase-modulating each of the chips on the basis of predetermined codes. Therefore, it is possible to increase data transmission efficiency and improve security while simplifying the structures of the receiver (Bob) and the like, and in addition, it is also possible to improve detection efficiency at a receiving end.

Description

Code-based Quantum Cryptographic Key Distribution Method, Apparatus and System

The present invention relates to a quantum cryptographic key distribution method, apparatus and system. More particularly, in implementing a multi-qubit quantum cryptographic key distribution system, a single photon pulse signal is shorter than the pulse width through high-speed phase modulation on a complex plane. The present invention relates to a quantum cryptographic key distribution method, apparatus and system for dividing a plurality of chips having a period and coding a phase of each chip to distribute quantum cryptographic keys.

Recently, as cases of damage caused by information leakage due to interception and eavesdropping have continued, interest in security has increased greatly. However, the security communication according to the prior art has a considerable risk of exposing the communication contents by an external attack, and as a next-generation security technology to compensate for this, quantum cryptography communication, which can guarantee very high security, has been spotlighted. I am getting it.

In this regard, research on quantum cryptographic key distribution among quantum cryptographic communication technologies has been actively conducted. Quantum cryptographic key distribution technology uses photon quantum mechanical properties to distribute and share cryptographic keys among remote users. In this case, when the eavesdropper attempts to obtain the encryption key information distributed among the users due to quantum mechanical properties, the encryption key information may be altered. Accordingly, the user who sends and receives the encryption key may detect the existence of the eavesdropper. Will be.

As one of the quantum cryptographic key distribution techniques, a technique for distributing a quantum cryptographic key by modulating the frequency or phase of an optical pulse has been proposed, but in this case, a modulation circuit should be provided not only for the sender (Alice) but also for the receiver (Bob). Etc., the structure may be complicated, and a disadvantage may occur in that loss of an optical pulse may occur.

In addition, a method for improving the security of quantum cryptographic key distribution while improving the reduction of data transmission efficiency due to the complicated structure of the receiver Bob and the loss of an optical pulse has been continuously requested.

Accordingly, there is a continuing need for a quantum cryptographic key distribution scheme capable of simplifying the structure of the receiver and the like, improving the data transmission efficiency and improving the security, and further increasing the detection efficiency at the receiving end. However, there is no proper solution yet.

The present invention was devised to solve the above problems of the prior art, and in the case of distributing a quantum cryptographic key by modulating the frequency or phase of an optical pulse according to the prior art, it modulates not only the sender Alice but also the receiver Bob. The structure may be complicated, such as the need for a circuit, and may cause a loss of optical pulses. The receiver may be simplified to improve data transmission efficiency, improve security, and improve reception. An object of the present invention is to provide a quantum cryptographic key distribution method, apparatus, and system that can also increase the detection efficiency in.

A quantum cryptographic key distribution system according to an aspect of the present invention for solving the above problems is a system in which a sender and a receiver distribute a quantum cryptographic key using a series of optical pulses, one optical pulse ^2N chips ( a transmitter that transmits an optical pulse carrying data of N qubits by dividing into chips and applying a first basis among a plurality of predetermined bases to the 2 ^N chips, wherein N is a natural number of 1 or more. ; And a receiver for receiving the optical pulse transmitted by the transmitter and applying a second basis among the plurality of basis to detect the optical pulse to restore some or all of the data. Is characterized by contrasting the first basis and the second basis and distributing encryption keys to each other using some or all of the data.

The receiver may include: an optical splitter configured to divide the received optical pulse with a predetermined probability and transmit the split optical pulse to the first path and the second path; A delay and summing filter which receives the optical pulses and divides the first optical pulses into second optical pulses, delays the first optical pulses by a predetermined chip interval, and then adds the optical pulses with the second non-delayed second optical pulses. ; And a photon detector for detecting an optical pulse output from the delay and summing filter.

In this case, the delay and summing filter may include a phase modulator for modulating the phase of the first light pulse or the second light pulse according to the second basis.

In addition, the receiver has a 1-chip delay and summing filter when the N = 1, and a 2-chip delay and summing filter and a 1-chip delay and summing filter when the N = 2, More generally, when N = n, 1-chip delay and summing filter, 2-chip delay and summing filter, ..., 2 ^{n -1} -chip delay and summing filter-where n is a natural number of 3 or more ; In this case, the optical pulses received by the receiver may be divided into a plurality of paths through one or more optical splitters, and sequentially passed through each type of delay and summing filters, and then detected by the optical detector.

The optical splitter may include a switch that divides a plurality of chips in the optical pulse and transfers some chips to the first path and the other chips to the second path.

Furthermore, the switch may divide the plurality of chips into a preceding half and a subsequent half to transfer the plurality of chips to the first path and the second path, respectively.

A quantum cryptographic key distribution method according to another aspect of the present invention is a method in which a transmitter and a receiver distribute a quantum cryptographic key using a series of optical pulses, wherein the transmitter divides one optical pulse into 2 ^N chips. And outputting an optical pulse carrying data of N qubits by applying a first basis among a plurality of predetermined bases to the 2 ^N chips, wherein N is a natural number of 1 or more. The receiver receives an optical pulse transmitted by the sender, applies a second basis among the plurality of basis to detect an optical pulse to restore some or all of the data, and the sender and receiver receive the The first base and the second base are collated and the encryption keys are distributed to each other using some or all of the data.

A quantum cryptographic key distribution method according to another aspect of the present invention is a method in which a sender and a receiver distribute a quantum cryptographic key using a series of light pulses, wherein the receiver has 2 ^N chips (one light pulse from the sender). chip), and applying a first basis among a plurality of predetermined bases to the 2 ^N chips, carrying out phase modulation by receiving data of N qubits, where N is 1 Natural numbers above); And by the receiver, applying a second basis of the plurality of basis to the received optical pulse to detect an optical pulse and recovering some or all of the data. The first base and the second base are collated and the encryption keys are distributed to each other using some or all of the data.

Here, the restoring step may include: a light splitting step of dividing the received light pulse with a predetermined probability and transferring the plurality of paths; Receives an optical pulse traveling in the plurality of paths, divides the first optical pulse into a second optical pulse, delays the first optical pulse by a predetermined wavelength, and adds the second optical pulse to the second non-delayed optical pulse. Delay and summing steps; And a photon detection step of detecting an optical pulse output from the delay and summing filter.

In the delay and summing step, the phase of the first light pulse or the second light pulse may be modulated according to the second basis.

In addition, the receiver has a 1-chip delay and summing filter when the N = 1, and a 2-chip delay and summing filter and a 1-chip delay and summing filter when the N = 2, More generally, when N = n, 1-chip delay and summing filter, 2-chip delay and summing filter, ..., 2 ^{n -1} -chip delay and summing filter-where n is a natural number of 3 or more ; In this case, the optical pulses received by the receiver may be divided into a plurality of paths through one or more optical splitters, and sequentially passed through each type of delay and summing filters, and then detected by the optical detector.

The optical splitting step may be implemented by using a switch that divides a plurality of chips in the optical pulse and transfers some chips to the first path and transfers the remaining chips to the second path.

The switch may divide the plurality of chips into a preceding half and a subsequent half, and transfer the plurality of chips to the first path and the second path, respectively.

According to still another aspect of the present invention, there is provided a transmitting apparatus, comprising: a transmitting apparatus for distributing a quantum encryption key with a receiving apparatus using a series of optical pulses, the apparatus comprising: an optical pulse generating unit generating an optical pulse; And splitting an optical pulse into 2 ^N chips and applying a first basis among a plurality of predetermined bases to the 2 ^N chips to phase modulate the optical pulse carrying N qubits of data. An optical pulse modulator for transmitting, wherein N is one or more natural numbers; and the receiving apparatus receives the optical pulse transmitted by the transmitting apparatus and applies a second basis among the plurality of bass. Detecting light pulses restores some or all of the data, and the transmitting device and the receiving device collate the first base and the second base and distribute cryptographic keys to each other using some or all of the data. Characterized in that.

A receiving device according to another aspect of the present invention is a receiving device for distributing a quantum cryptographic key using a series of light pulses received from a transmitting device, wherein one light pulse from the sender to the sender is 2 ^N chips. An optical pulse receiver configured to apply a first basis among a plurality of predetermined bases to the 2 ^N chips and to phase modulate the received data by receiving N qubits of data, wherein N is 1; Natural waters above; And an optical pulse detector configured to apply a second basis among the plurality of basis to the received optical pulses to detect an optical pulse to restore some or all of the data, wherein the transmitting apparatus and the receiving apparatus include: And comparing the first basis and the second basis and distributing encryption keys to each other using some or all of the data.

The quantum cryptographic key distribution method and apparatus system according to an embodiment of the present invention divides an optical pulse into a plurality of chips and phase-modulates each chip based on a predetermined code to distribute the quantum cryptographic key between the sender and the receiver. While simplifying the structure of Bob and the like, it is possible to improve data transmission efficiency, improve security, and further improve detection efficiency at a receiving end.

1 is a block diagram of a quantum cryptographic key distribution system according to an embodiment of the present invention.

2 is a flowchart of a quantum cryptographic key distribution method according to an embodiment of the present invention.

3 is a diagram illustrating a process of generating a plurality of chips from an optical pulse according to an embodiment of the present invention.

4 is a diagram illustrating the structure of a two-chip coded quantum cryptographic key distribution system according to an embodiment of the present invention.

5 is a diagram illustrating a chip signal (b) subjected to modulation (a) and delayed interference of a single photon signal according to an embodiment of the present invention.

6 is a view for explaining the structure of a two-chip quantum cryptographic key distribution system maximizing the detection efficiency according to an embodiment of the present invention.

FIG. 7 is a diagram illustrating an attack model of an eavesdropper (eve) using a two-chip switching & delay interferometer according to an embodiment of the present invention.

8 is a diagram illustrating a structure of a four-chip coded quantum cryptographic key distribution system according to an embodiment of the present invention.

9 is a diagram illustrating the structure of a four-chip quantum cryptographic key distribution system maximizing detection efficiency according to an embodiment of the present invention.

10 is a block diagram of a transmitter according to an embodiment of the present invention.

11 is a block diagram of a receiving apparatus according to an embodiment of the present invention.

As the inventive concept allows for various changes and numerous embodiments, particular embodiments will be described in detail with reference to the accompanying drawings.

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

In describing the embodiments of the present invention, when 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. In addition, terms to be described below are terms defined in consideration of functions in the present invention, which may vary according to the intention or custom of a user or an operator. Therefore, the definition should be made based on the contents throughout the specification. The terminology used in the description is for the purpose of describing particular embodiments only and should not be limiting. Unless expressly used otherwise, the singular forms “a,” “an,” and “the” include plural forms of meaning. In this description, expressions such as "comprises" or "equipment" are intended to indicate certain features, numbers, steps, actions, elements, portions or combinations thereof, and one or more than those described. It should not be construed to exclude the presence or possibility of other features, numbers, steps, actions, elements, portions or combinations 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 used to distinguish one component from another component. Only used as

In the following, a general description of the quantum cryptographic key distribution technique will be made in order to understand the present invention, and then, various embodiments according to the present invention will be described in more detail.

In designing a quantum key distribution (QKD) system, the allowable error tolerance bound is an indicator of the security of the protocol. In many previous studies, a new protocol has been proposed to raise the limit of the allowable error rate, of which the limit of the allowable error rate in the multi-qubit quantum cryptographic key distribution protocol that encodes the multi-qubit information in one quantum state is determined by the number of qubits. It was found to increase accordingly. The present invention focuses on a method for implementing a multi-qubit quantum cryptographic key distribution system, dividing a single photon pulse signal into optical pulses of a unit of chip shorter than the pulse width through fast phase modulation in complex plane and coding the phase of each chip. By doing so, a new method for implementing a multi-qubit quantum cryptographic protocol is disclosed.

Quantum cryptographic key distribution is one of the cryptographic key distribution methods used to distribute keys for modulating or demodulating information for communication between two authenticated users. In contrast, quantum cryptographic key distribution allows sharing of secret keys in the presence of eavesdroppers (Eve) based on physical laws at the quantum level. In 1984, the BB84 protocol, first proposed by Bennett and Breassard, showed that quantum key distribution is possible using four quantum states that constitute two basis. The four quantum states are the base quantum states

,

Quantum states of and base 2

,

It is expressed as The sender Alice transmits any one of four quantum states to the receiver Bob, and the receiver Bob selects any one of the base 1 and the base 2 to measure. The receiver Bob can obtain the correct secret key information only when the measurement is made on the same basis as the basis of the transmitted quantum state. For this reason, after all transmissions are completed in the quantum channel, the sender (Alice) and the receiver (Bob) disclose their respective base information through the classical channel and use only the result of using the same base for secret key extraction. This process is called shifting. After the shifting is done, a part of the valid result is taken to calculate an error caused by the eavesdropper's attack to determine whether to use the result as a secret key. At this time, the limit value of the allowable error rate is determined, and if the error rate is larger than this value, all the result values are discarded. If the error rate is smaller than the limit value, the secret key can be successfully generated through an additional process.

The allowable error rate threshold is a good indicator in designing a more secure quantum key distribution system. For this reason, research has been conducted on protocols that raise the limit of acceptable error rates. A protocol using six quantum states has been proposed, and recently, quantum key distribution that encodes "single-photon multicubit" instead of one qubit per photon. In the protocol, when shifting and post-processing by combining several qubits in symbol units, the allowable error rate limit increases according to the number of qubits as shown in Equation 1 below.

 The present invention proposes a new implementation method that can be applied to a multi-qubit protocol based on the BB84 protocol. Various implementations have been proposed to implement the existing BB84 protocol, such as using phase modulation or frequency modulation, and two qubit single photon quantum key distribution systems using polarization modulation and phase modulation are proposed. The quantum key distribution system proposed by the present invention uses a quantum state in which phase coding is modulated on optical pulses of multiple chip units by performing a single complex pulse phase modulation of a complex plane at high speed.

In the present invention, we propose a code-based quantum cryptographic key distribution system to describe a photon transmission system composed of two chips capable of transmitting one qubit information and a photon transmission system composed of four chips capable of transmitting two qubit information. This paper proposes an implementation method of a multi-qubit quantum cryptographic key distribution system that can transmit N qubit information by extending it to 2 ^N chips.

In the following, exemplary embodiments of a code-based quantum cryptographic key distribution method, apparatus and system according to an embodiment of the present invention will be described in turn with reference to the accompanying drawings.

First, FIG. 1 illustrates a configuration diagram of a code-based quantum cryptographic key distribution system 100 according to an embodiment of the present invention. As shown in FIG. 1, a quantum cryptographic key distribution system 100 according to an embodiment of the present invention may include a sender 110 and a receiver 120, and the sender 110 and the receiver ( 120 may be connected using an optical network 130 including an optical channel capable of delivering optical pulses.

In addition, Figure 2 illustrates a flow chart of a quantum cryptographic key distribution method according to an embodiment of the present invention.

As can be seen in Figure 2, in the quantum cryptographic key distribution method according to an embodiment of the present invention, the transmitter 110 divides one optical pulse into 2 ^N chips, the 2 ^N chips And transmitting a light pulse carrying data of N qubits by applying a first basis among a plurality of predetermined bases to N phases (where N is a natural number of 1 or more) (S110) and the receiver 120 Receiving an optical pulse transmitted by the transmitter 110, applying a second basis of the plurality of basis to the received optical pulse to detect the optical pulse to restore some or all of the data ( S120) may be included.

More specifically, the quantum cryptographic key distribution method from the perspective of the sender 110 according to an embodiment of the present invention, the transmitter 110 and the receiver 120 distributes the quantum cryptographic key using a series of light pulses. For example, the transmitter 110 divides one optical pulse into 2 ^N chips, applies a first basis among a plurality of predetermined bases to the 2 ^N chips, and phase modulates the N qubits. Transmitting an optical pulse carrying data of (N, where N is a natural number of 1 or more) (S110), and the receiver 120 receives the optical pulse transmitted by the sender 110 and receives the plurality of bases. a second basis of the basis is applied to detect an optical pulse to restore some or all of the data, and the sender 110 and the receiver 120 collate the first and second bases and the data Split encryption keys between each other using some or all of It is. In this case, the data may be data for generating a quantum encryption key distributed between the sender 110 and the receiver 120.

In addition, the quantum cryptographic key distribution method from the perspective of the receiver 120 according to an embodiment of the present invention, in the method in which the transmitter 110 and the receiver 120 distributes the quantum cryptographic key using a series of light pulses. , receiver 120, to from a sender (110) to apply one of being light pulse is divided into 2 ^N chips (chip), the two first base of the ^N plurality of ground a predetermined in-chip (basis) phase Receiving the optical pulse transmitted by modulating the data of N qubits (where N is one or more natural numbers) (S120a) and the receiver 120 is the first of the plurality of basis to the received optical pulse (S120a) Applying a second basis to detect an optical pulse to restore some or all of the data (S120b), wherein the sender 110 and the receiver 120 collate the first basis and the second basis; And encrypting each other using some or all of the data Will be distributed.

Further, the restoring step (S120b) may include: a light splitting step of dividing the received light pulse with a predetermined probability and transferring the first and second paths; Receiving the optical pulse is divided into a first optical pulse and a second optical pulse, delaying and summing the first optical pulse by a predetermined chip interval, and then summing it with the second optical pulse that is not delayed ; And a photon detection step of detecting an optical pulse output from the delay and summing filter.

In particular, the receiver has a one-chip delay and summing filter when N = 1, and a two-chip delay and summing filter and a one-chip delay and summing filter when N = 2, More generally, when N = n, 1-chip delay and summing filter, 2-chip delay and summing filter, ..., 2 ^{n -1} -chip delay and summing filter-where n is a natural number of 3 or more ; In this case, the optical pulses received by the receiver may be divided into a plurality of paths through one or more optical splitters, and sequentially passed through each type of delay and summing filters, and then detected by the optical detector.

Here, the 1-chip delay and summing filter refers to a filter for delaying and summing an optical pulse including a plurality of chips by one chip, and more generally, the 2 ^n-1 -chip delay and summing filter is a plurality of chips. It refers to a filter that adds after delaying the optical pulse including a 2 ^n-1 -chip.

Hereinafter, a quantum cryptographic key distribution method and system according to an embodiment of the present invention will be described in detail with reference to FIGS. 1 and 2.

First, the transmitter 110 divides one optical pulse into a plurality of chips, and phase modulates the plurality of chips to correspond to a plurality of codes predetermined according to a basis, thereby optically communicating the optical network ( 130 is sent to the receiver (120).

In this regard, the receiver 120 receives a light pulse transmitted by the transmitter 110, passes through a beam splitter, and includes a plurality of delay and summing filters configured to correspond to a basis. After detecting the pulse of light by using a single photon detector.

Subsequently, the transmitter 110 informs the receiver 120 of a basis used to modulate an optical pulse, that is, a basis for measuring a code composed of chips constituting the phase-modulated optical pulse. The receiver 120 informs the sender 110 of the basis used to detect the optical pulse, that is, the phase delay value of the delay and add filter, so that After determining whether an attack is made, a quantum cryptographic key is calculated by applying an error correction technique and a privacy amplification technique.

Accordingly, the quantum cryptographic key distribution system 100 according to an embodiment of the present invention simplifies the structure such as configuring the receiver 120 without a modulator and improves data transmission efficiency. Furthermore, the attack of the eavesdropper 140 can be effectively suppressed.

3 is a diagram illustrating optical pulse modulation in the transmitter 110 according to an embodiment of the present invention. As shown in FIG. 3, an optical pulse (photon) generated by a light source such as a laser diode of the transmitter 110 or an optical pulse generator 112 may be an optical pulse modulator 114 such as an IQ modulator. While passing through may be divided into a plurality of chips (chip). In addition, the optical pulse modulator 114, such as the IQ modulator, may phase-modulate each chip and output the same.

That is, an optical pulse generated from a light source such as a laser diode has one phase (for example, 0 °). In an IQ modulator, the optical pulse is divided into a plurality of chips and the plurality of chips. Can be modulated at intervals of a predetermined phase, thereby forming a plurality of chips having a series of phase values.

4 illustrates a diagram for describing an implementation of the quantum cryptographic key distribution system 100 according to an embodiment of the present invention. As shown in FIG. 4, the quantum cryptographic key distribution system 100 according to an embodiment of the present invention may include a sender 110 and a receiver 120.

Hereinafter, the structure of a two-chip coded quantum cryptographic key distribution system 100 according to an embodiment of the present invention will be described in more detail with reference to FIG. 4.

In the BB84 protocol using a phase modulation qubit, the transmitter (Alice) 110 generates a signal divided into two chips by a single photon pulse through complex plane fast phase modulation. The modulated phase of each chip has one of {0 °, 90 °, 180 °, 270 °}. 4 shows a system model using the above-described modulation scheme. 5 (a) shows a signal obtained by modulating a single photon pulse transmitted in a period T into two chips. When the phase of the first chip is referenced to 0 ° in the complex plane, all four possible quantum states are determined by chip 2's phase modulation value.

The receiver 120 uses a 50:50 beam splitter to divide the incoming signal into two paths and delay and summing filters DAF 1 and DAF 2 with delay paths of 1-chip periods in each path. Signal). The delay and summing filters DAF 1 and DAF 2 have phase path differences of 0 ° and 90 °, respectively, in the delay path.

In addition, Figure 5 (b) shows the interference signal generated by the interference of the two chips transmitted by the sender (Alice) in the constructive interference port and the cancellation interference port of the delay and summing filter. The interfering signal from each port is represented by chips located at t ₁ , t ₂ = t ₁ + t, and t ₃ = t ₁ + 2t on the time base, and measured by a single photon detector (SPD) connected. The receiver 120 may determine the quantum state received through the interference signals of the two chips sent by the transmitter 110, and since the actual interference chip is located at time t ₂ in the interference signal, it is detected at time t ₂ . Only the results can be used to extract key information.

Accordingly, the secret key can be generated as follows through the system structure of FIG. 4. The transmitter 110 modulates a single photon pulse into two chips and sends it to the receiver 120. Taking the time t ₂ = 0, the modulated signal can be expressed as Equation 2 below based on the single photon state.

: Qubit generation operation

Where | 1> _t represents a single photon state and the subscript represents the time position of the chip. If the phase of the first chip signal is set to 0 ° and the phase of the second chip signal is modulated by θ∈ {0 °, 90 °, 180 °, 270 °}, a total of four quantum states can be created according to θ. . Four quantum states are defined as in Equation 3 through the qubit generation operation of Equation 2.

Looking at the orthogonality of the four quantum states defined in equation (3) is as shown in equation (4).

According to Equation 4, the four quantum states defined in Equation 3 may be divided into two basis, the basis {| ψ ₁ >, | ψ ₂ >}, and the basis {| ψ ₃ >, | ψ ₄ >}. Therefore, as in the BB84 protocol, as shown in Equation 5 below, one quantum state can be represented as a superposition of two quantum states belonging to different bases, and it can be seen that an exact quantum state cannot be determined without using the same base. .

One of the four quantum states received from the transmitter 110 passes through the beam splitter of the receiver 120 and the delay and summing filters DAF1 and DAF2, and then of the delay and interference filters DAF1 and DAF2. It is detected at one of the single photon detectors 1, 2, 3, 4 (SPD1, SPD2, SPD3, SPD4) connected to the constructive interference port and the cancellation interference port. The detection probability of the detector will be described taking the case where the transmitter 110 sends | ψ ₁ > as an example.

When the transmitter 110 sends | ψ ₁ >, a signal entering the single photon detector i through the delay interferometer is | ψ _Di > and is represented by Equation 6 as follows.

When the transmitter 110 sends | ψ ₁ > by the state of Equation 6, the probability that a signal is measured in each single photon detector SPD is expressed by Equation 7 below.

| ψ _1> non-detection probability of a single photon detector 1, 2, 3, 4 ( SPD1, SPD2, SPD3, SPD4) 1 for 0: 1/2: 1/2, and similarly | ψ _2> a case 0: 1: 1/2: 1/2, | ψ 3> of the time 1/2: 1/2: 1: 0, | ψ 4> in the case of 1/2: 1/2: 1: Detection occurs at each detector at a ratio of one. In summary, if the basis of the set of quantum states {| ψ ₁ >, | ψ ₂ >} sent by the sender 110 is X and the basis of the set {| ψ ₃ >, | ψ ₄ >} is Z, the receiver ( The measurement basis of 120 corresponds to the base X when the detectors 1 and 2 (SPD1 and SPD2) are detected and the base Z when the detectors 3 and 4 (SPD3 and SPD4) are detected.

When all signals have been transmitted, the receiver 120 discloses to the sender 110 the time of the detected photons and the measured basis (X or Z) and gives the same basis for the measurement of the chip signal corresponding to the time t _{t -t} . Only use it as key information.

If sender 110 used basis X, it would generate bit "0" if measured at single photon detector 1 (SPD1), if it was measured at detector 2 (SPD2), generate bit "1", and sender 110 would base Using Z produces bit "0" if measured at detector 3 (SPD3) and bit "1" if measured at detector 4 (SPD4).

In the structure described above, when the optical signal consisting of two chip signals arrives at the detector SPD through a delay and summing filter DAF as shown in FIG. 5 (b), it corresponds to t ₁ , t ₂ , and t ₃ hours. Since the chip signals all have a probability of detecting photons, since the signal containing the valid information is the chip signal at time t ₂ , the chip signal corresponding to the time t ₁ and t ₃ other than t ₂ does not contain valid information. It reduces the detection efficiency by 50% and thus reduces the absolute secure transmission distance by as much as 3-dB. This is equivalent to a shorter transmission distance of 20 km when using ordinary communication fiber optics (SMF-28). Therefore, a new structure using a high speed switch is proposed in FIG. 6 in order to remove unnecessary signals and prevent a decrease in detection efficiency.

In FIG. 6, a fast switch and an interferometer are used in place of the delay and summing filter DAF (or a delay interferometer). In this case, it is known that quantum phase interference can obtain the same result as classical interference even in an interferometer having such a structure. The high-speed switch divides the chip signal constituting the transmit pulse into the up and down paths. The divided chip signal has a 1-chip time delay when it passes through the upper path and the chip signal in the lower path is 0 ° or 90 ° phase modulated. And enter the single photon detector through the interferometer. In the case of this structure, unlike the previous structure, since the detection is performed at one time, the detection efficiency does not decrease.

Likewise, using the structure of FIG. 6, the sender 110 and the receiver 120 may generate a secret key as follows.

1. The sender 110 randomly selects one of the quantum states of Equation (2) and transmits it to the receiver 120.

2. The receiver 120 divides the first chip signal into the upper path and the next chip signal into the lower path by using a switch. At this time, the chip signal entering the upper path undergoes a delay of one chip time without undergoing a change in phase, and the chip signal entering the lower path is modulated by the receiver 120 to any value of {0 °, 90 °}. (PM).

3. The signal by constructive interference of the chip signal of the upper path and the lower path enters the single photon detector 1 (SPD1) and the canceling interference signal enters the single photon detector 2 (SPD2) and is detected.

4. If the receiver 120 does not perform phase modulation, the measurement corresponds to the basis X, and if the receiver 120 detects the phase modulation of 90 °, it corresponds to the basis Z measurement.

5. The receiver 120 discloses the measured basis to the sender 110 and generates bit "0" or "1" according to the quantum state.

Furthermore, a four-chip coding system modulates one single photon pulse into four chip signals. As with a two-chip coding system, each chip signal is phase modulated with one of {0 °, 90 °, 180 °, 270 °}. Likewise, in order to apply a 4-chip coding system to a general BB84 protocol, four quantum states satisfying Equation 4 can be generated. When the phase value of the chip signal is represented as a complex number, Equation 8 is shown.

In general, in a BB84 protocol, a single photon pulse carries one bit of information. As shown in the two-chip coding system, one bit information can be transmitted using only two chip signals. If four quantum states are used as quantum information by applying it to four-chip coding as it is, two chip signals are divided into four chips. Will have duplicate information. Since the chip signal having the duplicated information increases the amount of information exposed to the eavesdropper 140, security cannot be guaranteed.

FIG. 7 is an example of an attack of the eavesdropper 140 when four bits of one bit information are transmitted. The eavesdropper 140 intercepts the signal sent by the transmitter 110 and divides the two chip signals into the upper path and the lower path through the two-chip delay switch to cause interference. In this case, only the quantum states {(1,1,1,1), (1, -1,1, -1)} are detected at the constructive interference port, and the quantum states {(1, i,- 1, -i), (1, -i, -1, i)} has a detection probability. Therefore, if the basis of the signal is automatically divided according to the constructive interference port and the destructive interference port, and the single photon detection for each base is performed, the eavesdropper 140 can accurately detect the signal sent by the sender 110 and By regenerating the same signal and sending it to the receiver 120, all key information can be found.

The problem described above may occur due to overlapping information while increasing the number of chip signals. In the present invention, the number of quantum states to be transmitted is increased according to the present invention to avoid duplication of information transmitted while increasing the number of chip signals. The problem was solved by increasing the amount of information transmitted in the quantum state. The four-chip coded quantum cryptography key distribution system 100 transmits two qubit information to one quantum state using a total of 16 quantum states.

A method of generating 16 quantum states may be represented by a qubit generation operation as shown in Equation 9 below.

here

The 16 quantum states produced by the combination of

...

Each quantum state is determined based on the θ value of the qubit generation operation.

If the base of the first qubit is X,

, The base of the first qubit matches Z. Likewise

If the base of the second qubit is X,

If the base of the second qubit is matched by Z, all quantum states are included in the base of {XX, XZ, ZX, ZZ}.

In the case of the four-chip coded quantum cryptographic key distribution system 100 having 16 quantum states, the inner products of the same base are orthogonal to each other and orthogonal to other base quantum states by calculating the inner product between the quantum states as shown in Equation 4. It is easy to see that it does not, and also, when the sender 110 and the receiver 120 use different basis, the superimposed set of Table 1 (quantum state information of the 4-chip coded quantum cryptographic key distribution system 100) below. As shown in the (superposition set) column, the quantum state of a particular base can be expressed as the superposition of the quantum states of another base, so that the exact quantum state cannot be determined without using the same base.

8 illustrates an operation structure of a 4-chip coded quantum key distribution system 100 according to an embodiment of the present invention. In FIG. 8, the sender 110 sends one of the 16 quantum states to the receiver 120. The signal received at receiver 120 is passed through a two-chip delay interferometer and through a one-chip delay interferometer (DAF) to be measured with a single photon detector (SPD). The receiver 120 bases the measurement on the phase path difference between the two-chip delay interferometer (DAF) and the one-chip delay interferometer (DAF). In other words, when expressed as (phase path difference of 2-chip delay interferometer (DAF), phase path difference of 1-chip delay interferometer (DAF)), detectors 1, 2, 5, 6 ( SPD1, SPD2, SPD5, SPD6) correspond to base XX, and detectors 9, 10, 13, 14 (SPD9, SPD10, SPD13, SPD14) corresponding to (90 °, 0 °) correspond to base XZ, and (0 Detectors 3, 4, 7, 8 (SPD3, SPD4, SPD7, SPD8) correspond to the base ZX and detectors 11, 12, 15, 16 correspond to (90 °, 90 °) (SPD11, SPD12, SPD15, SPD16) correspond to the base ZZ.

After the communication of both channels is finished, the transmitter 110 and the receiver 120 disclose the basis of each other and use only the signal when the same base is used for secret key extraction. In this case, the same basis refers to a case in which the basis of the first qubit and the second qubit is the same, and the two qubits are shifted and post-processed in one symbol unit as in quadrature phase shifting key. Proceed. In this way, when the shifting and post-processing processes are applied in symbol units, the limit of the allowable error rate of the system increases.

When the structure shown in FIG. 8 is used, the measurement efficiency of the signal decreases and as the number of chip signals increases, the number of single photon detectors (SPDs) also increases, as in the two-chip quantum cryptographic key distribution system 100. The two-chip switching & delay interferometer (DAF) and one-chip switching & delay interferometer (DAF) of FIG. 9 can be used to reduce the detection efficiency and reduce the number of single photon detectors (SPDs).

If the number of chip signals is increased to 2 ^N , the number of quantum states that we use is 4 ^N in total and can be represented by the qubit generation operation as shown in Equation 10 below.

As before

Can be combined, so that 4 ^N quantum states can be generated for N qubits as shown in Equation 11 below.

...

One qubit generation operation encodes one qubit information. Qubit Generation Operation

Can generate independent qubits, so commutators

to be. As shown in Table 1, quantum states may be generated through quantum superposition of non-orthogonal quantum states.

The basis of each quantum state

According to the base of one qubit by X and Z, there are a total of 2 ^N bases from XXX ... X, ZZZ ... Z, and the sender 110 and the receiver 120 are 4-chip quantum encryption keys. The secret key information is extracted from the signal when the basis of the quantum state sent by the transmitter 110 and the measurement basis of the receiver 120 are the same as the method used in the distribution system 100.

Further, the structure of FIG. 9 is a two-chip coding system. Not only 4-chip coding system but also ^2N -chip coding system can be applied according to the method described above.

In the present invention, one single photon pulse is subjected to complex plane fast phase modulation and divided into chips of small time units to make one quantum state. By looking at a two-chip coding system with one classical bit information and a four-chip system with two bits of information, a 2 ^N -chip coding system capable of having ^N classical bit information in one photon through the corresponding modulation scheme It was confirmed that it can be generalized to. Above all, the proposed method of encoding multiple qubits is simple and the structure of the receiver is composed of only a switch, a delay interferometer and a single photon detector, so that the actual quantum cryptographic key distribution protocol can be easily implemented.

In addition, FIG. 10 illustrates a configuration diagram of the transmission apparatus 110 according to an embodiment of the present invention, and FIG. 11 illustrates a configuration diagram of the reception apparatus 120 according to an embodiment of the present invention.

The transmission device 110 and the reception device 120 can be easily implemented with reference to the description of the quantum cryptographic key distribution system 100 and the method according to the present invention. Replace with

First, the transmitting apparatus 110 according to an embodiment of the present invention is a transmitting apparatus 110 that distributes a quantum cryptographic key with the receiving apparatus 120 by using a series of optical pulses, and generates an optical pulse. generator 112 and by dividing the my pulse with 2 ^N chips (chip), and the phase by applying the first base of the plurality of ground (basis) predetermined in the 2 ^N-chip modulation data of N qubits In fact, it comprises an optical pulse modulator 114 for transmitting the optical pulse (where N is a natural number of 1 or more), wherein the receiving device 120 receives the optical pulse sent by the transmitting device 110 And applying a second base among the plurality of bases to detect an optical pulse to restore some or all of the data, and accordingly, the transmitting device 110 and the receiving device 120 are connected to the first base. Collating the second basis and using some or all of the data It is possible to distribute encryption keys to each other.

In addition, the reception device 120 according to an embodiment of the present invention is a reception device 120 that distributes a quantum encryption key using a series of optical pulses received from the transmission device 110, and the transmission device 110. from one of the optical pulse is divided into 2 ^N chips (chip), the 2 ^N of applying a first base of the plurality of baseband predetermined (basis) on the chip to the phase modulation carries the transmission light data of the N qubits An optical pulse receiver 122 (where N is a natural number of 1 or more) for receiving a pulse and a second basis among the plurality of bases are applied to the received optical pulses to detect an optical pulse, thereby detecting a part of the data or And a light pulse detector 124 for restoring the entirety, wherein the transmitting device 110 and the receiving device 120 collate the first base and the second base and partially or all of the data. Distribution of encryption keys between It can be so.

The above description is merely illustrative of the technical idea of the present invention, and those skilled in the art to which the present invention pertains may make various modifications and changes without departing from the essential characteristics of the present invention. Therefore, the embodiments described in the present invention are not intended to limit the technical spirit of the present invention but to describe the present invention, and are not limited to these embodiments. The protection scope of the present invention should be interpreted 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.

Claims (15)

1. In a system where a sender and a receiver distribute a quantum cryptographic key using a series of light pulses,

   Dividing the one light pulse with 2 ^N chips (chip), said 2 ^N different to the chip applying the first base of the plurality of ground (basis) predetermined by the phase modulation actually the data of the N qubits transmitted light pulses Sender, where N is one or more natural numbers; And

   And a receiver for receiving the optical pulse transmitted by the transmitter and applying a second basis among the plurality of basis to detect the optical pulse to restore some or all of the data.

   And the sender and the receiver collate the first basis and the second basis and distribute cryptographic keys to each other using some or all of the data.
2. The method of claim 1,

   The recipient,

   An optical splitter that splits the received optical pulse with a predetermined probability and transmits the optical pulse to the first path and the second path;

   A delay and summing filter which receives the optical pulses and divides the first optical pulses into second optical pulses, delays the first optical pulses by a predetermined chip interval, and then adds the optical pulses to the second optical pulses which are not delayed. ; And

   And a photon detector for detecting the light pulses output from the delay and summing filters.
3. The method of claim 2,

   The delay and summing filter,

   And a phase modulator for modulating the phase of the first light pulse or the second light pulse in accordance with the second basis.
4. The method of claim 2,

   The recipient,

   When N = 1, provided with a 1-chip delay and summing filter,

   When N = 2, a 2-chip delay and summing filter and a 1-chip delay and summing filter are provided.

   More generally, when N = n, 1-chip delay and summing filter, 2-chip delay and summing filter, ..., 2 ^{n -1} -chip delay and summing filter-where n is a natural number of 3 or more ;

   At this time, the optical pulse received by the receiver,

   Split into multiple paths through one or more optical splitters,

   After sequentially passing through each type of delay and summing filter,

   And a quantum cryptographic key distribution system as detected by said photo detector.
5. The method of claim 2,

   The optical splitter,

   And dividing a plurality of chips in the optical pulse and transferring a plurality of chips to a first path and a second chip to a second path.
6. The method of claim 5,

   The switch,

   And dividing the plurality of chips into a preceding half and a subsequent half, and transferring the plurality of chips to the first path and the second path, respectively.
7. In a method in which a sender and a receiver distribute a quantum cryptographic key using a series of light pulses,

   Transmitter divides one optical pulse into ^2N chips, and applies a first basis among a plurality of predetermined bases to the ^2N chips to phase-modulate the optical pulse carrying N qubit data Sending a step (where N is a natural number of 1 or more),

   The receiver receives the light pulses sent by the sender, applies a second base among the plurality of bases, detects the light pulses, and restores some or all of the data,

   And the sender and the receiver collate the first basis and the second basis and distribute cryptographic keys to each other using some or all of the data.
8. In a method in which a sender and a receiver distribute a quantum cryptographic key using a series of light pulses,

   The receiver, to a light pulse 2 is divided into ^N chips (chip), applying a first base of said second plurality of ground a predetermined on ^N chips (basis) from the sender to the phase modulation data of N qubits Receiving a light pulse sent out, where N is a natural number of 1 or more; And

   And applying, by the receiver, the second basis of the plurality of basis to the received optical pulse to detect an optical pulse to restore some or all of the data.

   And the sender and the receiver collate the first basis and the second basis and distribute cryptographic keys to each other using some or all of the data.
9. The method of claim 8,

   Restoring the step,

   An optical division step of dividing the received optical pulse with a predetermined probability and transferring the received optical pulse to the first path and the second path;

   Receiving the optical pulse is divided into a first optical pulse and a second optical pulse, delaying and summing the first optical pulse by a predetermined chip interval, and then summing it with the second optical pulse that is not delayed ; And

   And a photon detection step of detecting an optical pulse output from the delay and summing filter.
10. The method of claim 9,

    In the delay and summing step,

    And modulating a phase of the first optical pulse or the second optical pulse in accordance with the second basis.
11. The method of claim 9,

    The recipient,

    When N = 1, provided with a 1-chip delay and summing filter,

    When N = 2, a 2-chip delay and summing filter and a 1-chip delay and summing filter are provided.

    More generally, when N = n, 1-chip delay and summing filter, 2-chip delay and summing filter, ..., 2 ^{n -1} -chip delay and summing filter-where n is a natural number of 3 or more ;

    At this time, the optical pulse received by the receiver,

    Split into multiple paths through one or more optical splitters,

    After sequentially passing through each type of delay and summing filter,

    Quantum cryptographic key distribution method, characterized in that detected by the photo detector.
12. The method of claim 9,

    The light splitting step,

    And dividing a plurality of chips in the optical pulse to transfer some chips to the first path and the other chips to the second path.
13. The method of claim 12,

    The switch,

    And dividing the plurality of chips into a preceding half and a subsequent half and transferring the plurality of chips to the first path and the second path, respectively.
14. A transmitting device for distributing a quantum cryptographic key with a receiving device using a series of light pulses,

    An optical pulse generator for generating an optical pulse; And

    Dividing the one light pulse with 2 ^N chips (chip), said 2 ^N different to the chip applying the first base of the plurality of ground (basis) predetermined by the phase modulation actually the data of the N qubits transmitted light pulses An optical pulse modulator, wherein N is one or more natural numbers;

    The reception device receives an optical pulse transmitted by the transmission device, detects an optical pulse by applying a second basis among the plurality of basis, and restores some or all of the data,

    And the transmitting apparatus and the receiving apparatus collate the first basis and the second basis and distribute encryption keys to each other using some or all of the data.
15. A receiving device for distributing a quantum cryptographic key using a series of light pulses received from a transmitting device,

    One optical pulse is divided into ^2N chips from the transmitter, and the ^2N chips are subjected to a phase modulation by applying a first basis among a plurality of predetermined bases, and carrying N qubits of data for transmission. An optical pulse receiver for receiving the received optical pulse, wherein N is one or more natural numbers; And

    And an optical pulse detector configured to apply a second basis among the plurality of basis to the received optical pulses, detect an optical pulse, and restore some or all of the data.

    And the transmitting apparatus and the receiving apparatus collate the first basis and the second basis and distribute encryption keys to each other using some or all of the data.

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