CN112887090A - Continuous variable four-state quantum key distribution method and system - Google Patents
Continuous variable four-state quantum key distribution method and system Download PDFInfo
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Abstract
The invention provides a continuous variable four-state quantum key distribution method and a system, wherein homodyne measurement is adopted at a communication receiving end to obtain regular components of signal light, appropriate data post-processing is combined, and published measurement results are appropriately combined and paired, so that a higher code rate can be obtained compared with the traditional receiving end adopting a homodyne measurement method; the system of the invention adopts the pulse laser module as the light source, thus reducing the complexity of the instrument; the system only has phase difference among four coherent states which need to be modulated, and does not need to additionally modulate amplitude when signal loading is carried out, so that the implementation difficulty is reduced.
Description
Technical Field
The invention relates to the technical field of quantum key distribution, in particular to a continuous variable four-state quantum key distribution method and system.
Background
The quantum key distribution technology (hereinafter, referred to as QKD) is mainly divided into discrete variable quantum key distribution (hereinafter, referred to as DV-QKD) and continuous variable quantum key distribution (hereinafter, referred to as CV-QKD) according to implementation. DV-QKD encodes information on a certain degree of freedom of a single photon, and a single photon detector is used for detection during detection; CV-QKD encodes information on the canonical component of the light field, and the transmitted optical signal is detected with a homodyne measurement device.
The CV-QKD can adopt a Gaussian modulation mode in specific implementation, and the modulation mode modulates the regular component of the coherent state to make the coherent state in Gaussian distribution; in addition, there are discrete modulation methods, which usually make coherent states symmetrically distributed in phase space.
For quantum key distribution of continuous variable discrete modulation, a receiving end needs to measure a regular component of a signal light field after receiving a signal. The measurement mode can be divided into homodyne measurement and heterodyne measurement, wherein the homodyne measurement can be realized by applying a phase measurement regular momentum component p or a regular coordinate component q to local oscillator light every time, and 1-bit information can be generated by one signal light pulse; heterodyne measurement is a method of splitting light and measuring a regular momentum component p and a regular coordinate component q of a signal light field at the same time, and one signal light pulse can generate 2-bit information at one time.
There is currently a system (CN105024809A) for implementing CV-QKD by using gaussian modulation, which introduces means of negotiation algorithm, low noise detector and compensation algorithm to implement continuous variable quantum key distribution by using gaussian modulation, but the following drawbacks exist in the patent: 1. gaussian modulation requires continuous modulation of regular components of light during preparation of signal light, and is difficult to implement; 2. in order to modulate the light signal, the patent uses a continuous laser and an intensity modulator, which increases the complexity and difficulty of the system. In addition, under the condition of high noise, the conventional discrete modulation quantum key distribution protocol can only obtain a very low code rate when a receiving end uses homodyne measurement, which limits the transmission distance of the protocol, so that the practicability of the protocol is greatly reduced.
Disclosure of Invention
The purpose of the invention is as follows: the invention aims to overcome the defects of the prior art and provides a method and a system for realizing continuous variable four-state modulation CV-QKD. The invention adopts homodyne measurement at a communication receiving end to obtain the regular component of the signal light, combines proper data post-processing, and performs proper combination pairing on the published measurement result, so that compared with the traditional receiving end adopting the homodyne measurement method, the invention can obtain higher code rate and improve communication distance under the condition of large noise; the system of the invention adopts the pulse laser module as the light source, thus reducing the complexity of the instrument; the system only has phase difference among four coherent states which need to be modulated, and does not need to additionally modulate amplitude when signal loading is carried out, so that the implementation difficulty is reduced.
The technical scheme is as follows: in order to achieve the purpose, the technical scheme provided by the invention is as follows:
a continuous variable four-state quantum key distribution method comprises the following steps:
(1) the method comprises the steps that local oscillation light is prepared at a sending end, signal light corresponding to four coherent states { | s >, | is > } is generated according to probability distribution [ q1, q2, q3 and q4], and then the signal light and the local oscillation light are sent to a receiving end; wherein q1 is q2, q3 is q4, q1+ q2+ q3+ q4 is 1, and s is a real number;
(2) the receiving end introduces a phase increment of 0 or pi/2 into the local oscillation light randomly and with equal probability through phase modulation, the phase increment is respectively used for measuring a regular coordinate component or a regular momentum component of the signal light, and then the receiving end carries out homodyne measurement to obtain a regular coordinate or a regular momentum of the signal light;
(3) repeating the steps (1) to (2) for N times to obtain data required by one round of code forming;
(4) the transmitting end and the receiving end communicate through an authenticated classical channel, the receiving end informs the transmitting end of selecting a regular component during measurement, then the transmitting end regards the situation that the same signal state is transmitted and the receiving end respectively measures regular coordinates and regular momentum as a same group according to the selection of the receiving end, and only tells the receiving end which two times of measurement can be regarded as the same group through the classical channel, but does not disclose the prepared coherent state per se;
(5) the sending end carries out key mapping on the sent signal state to obtain an initial key of the sending end; the receiving end carries out key mapping according to the measurement result to obtain an initial key of the receiving end;
(6) the sending end and the receiving end carry out reverse negotiation and related error correction and privacy amplification processes based on respective initial keys to obtain a final security key.
Further, before the step (5) is executed, a numerical simulation convex optimization algorithm is adopted to estimate a code rate according to the measurement result of the step (4), if the code rate meets a preset safety condition, the step (5) is continuously executed, otherwise, the key distribution is finished.
Further, the specific step of estimating the bit rate by using the numerical simulation convex optimization algorithm includes:
the code rate calculation formula is constructed as follows:
wherein, R represents the key rate, σ represents the joint density matrix, S represents the condition that the joint density matrix should satisfy, H (rho | | | σ) is the mutual entropy, it is still unknown information content by the attacker after suffering attack to show, K establishes the association between density matrix and classical bit as mapping, P represents the effect of contracting the quantum channel, Δ represents the bit loss caused by the bit error correction of classical data, q represents the probability of keeping a pulse as the final generated key;
finding the lower bound of the mutual entropy by using a convex optimization algorithm according to a calculation formula of the code forming rate: first, from an initial joint density matrix σ0Initially, a joint density matrix σ 'close to the strongest attack is found through iteration, and the iterated σ' is substituted into the following formula to calculateCalculating the lower bound of the resultant code rate:
ε≥ξ(σ′)
where ε represents the minimum value of the mutual entropy when the joint density matrix satisfies the constraint S,
is a vector of expected values from the measured density matrix,
is the parameter to be optimized, S*Is that
The constraint that should be satisfied, superscript T represents the transpose of the matrix,
and expressing a gradient operator, wherein Tr expresses tracing of the matrix, the obtained xi (sigma') is the lower bound of the mutual entropy, and the calculated lower bound is substituted into the expression of the code rate to calculate the lower bound of the code rate.
In addition, the invention also provides a continuous variable four-state quantum key distribution system, which is used for realizing the method and comprises a sending end and a receiving end, wherein the sending end comprises a preparation module used for preparing local oscillation light and signal light which can be distinguished in the polarization direction; the receiving end includes: the device comprises a polarization control module, a beam splitting module, a phase modulation module and a homodyne measurement module; wherein the content of the first and second substances,
the polarization control module is used for adjusting the polarization states of the signal light and the local oscillator light in the quantum channel so as to recalibrate the polarization states of the signal light and the local oscillator light;
the beam splitting module is used for separating signal light and local oscillation light in different polarization directions;
the phase modulation module randomly and equiprobabilistically introduces a phase increment with an angle of 0 or pi/2 in the local oscillator light;
and the homodyne measurement module performs homodyne measurement on the signal light and the local oscillator light after phase modulation to obtain a regular coordinate or regular momentum.
Several alternatives are provided below for the system, but not as an additional limitation to the above general solution, but merely as a further addition or preference, each alternative being combinable individually for the above general solution or among several alternatives without technical or logical contradictions
Optionally, the sending end and the receiving end are respectively provided with a control module, and the control modules of the sending end and the receiving end respectively control each module inside the local end to realize corresponding functions, and perform reverse negotiation and related error correction and privacy amplification steps with the other party of communication, so as to negotiate a final security key.
Optionally, the sending end includes: the device comprises a pulse laser module, a beam splitting module, a phase modulation module and a beam combining module; wherein the content of the first and second substances,
the pulse laser module is used for generating laser pulses;
the beam splitting module is used for splitting the laser pulse into a first laser pulse and a second laser pulse with different intensities, and the second laser pulse is used as local oscillation light;
the phase modulation module performs phase modulation on the first laser pulse to generate the signal light;
the beam combining module combines the signal light and the local oscillator beam and transmits the combined beam to a receiving end through a quantum channel.
Optionally, the sending end includes: the laser device comprises a master laser module, a first slave laser module, a second slave laser module, a first optical transmission module, a second optical transmission module, a beam splitting module, a phase modulation module and a beam combining module; wherein the content of the first and second substances,
the main laser module is used for generating seed light and splitting the seed light into first seed light and second seed light through the beam splitting module;
the first seed light is injected into the first slave laser module through the first optical transmission module, and the first slave laser module generates original signal light pulse in a phase injection locking mode and sends the original signal light pulse into the phase modulation module to be modulated to generate the signal light;
the second seed light is injected into a second slave laser module through a second optical transmission module, and the second slave laser module generates local oscillator light in a phase injection locking mode;
the beam combining module combines the signal light and the local oscillator beam and transmits the combined beam to a receiving end through a quantum channel.
Optionally, the sending end includes: the laser device comprises a master laser module, a first slave laser module, a second slave laser module, a first optical transmission module, a second optical transmission module, a beam splitting module and a beam combining module; wherein the content of the first and second substances,
the main laser module is used for directly completing the loading of the phase of the signal light in an internal modulation mode to generate seed light with stable phases corresponding to four signal states, and the seed light generated by the main laser module is split into first seed light and second seed light through the beam splitting module;
the first seed light is injected into the first slave laser module through the first optical transmission module, and the first slave laser module generates the signal light in a phase injection locking mode;
injecting the second seed light into a second slave laser module through a second optical transmission module, wherein the second slave laser module generates the local oscillator light in a phase injection locking mode;
the beam combining module combines the signal light and the local oscillator beam and transmits the combined beam to a receiving end through a quantum channel.
Has the advantages that: the invention provides a continuous variable four-state quantum key distribution method and a system, which can realize the effect of adopting a homodyne measuring device to obtain an heterodyne measuring device, reduce the complexity of an instrument and the requirement on the instrument in design, improve the resultant code rate of a CV-QKD protocol of continuous variable four-state modulation in the whole effect and are embodied in the following aspects:
1. the invention adopts the processing method of the regular component front and back matching in the data post-processing process after the measurement is finished, can adopt the homodyne measuring device to obtain the effect of using the heterodyne measuring device, simultaneously ensures the simplicity of the device in implementation, obtains higher bit rate and transmission distance under the condition of large noise, and greatly improves the practicability of the device.
2. The invention simultaneously generates the signal light and the local oscillator light at the transmitting end in a light splitting mode, and the front and the back groups of the signal light and the local oscillator light do not have the necessity of phase correlation, so the system adopts the pulse laser module as a system light source instead of adopting the continuous laser module as the system light source, and the design reduces the requirement on the instrument.
3. According to the invention, only the phase difference exists between the four coherent states, so that when the laser pulse is modulated, the phase information to be coded is directly loaded to the signal light pulse through the signal loading module, and the signal light pulse does not need to be additionally subjected to amplitude modulation, so that the complexity is reduced and the implementation difficulty is reduced.
Drawings
Fig. 1 is a schematic structural diagram of a first embodiment of the present invention.
Fig. 2 is a schematic structural diagram of a second embodiment of the present invention.
Fig. 3 is a schematic structural diagram of a third embodiment of the present invention.
Fig. 4 is a schematic diagram of a third embodiment of the present invention in which an electric pulse is applied.
Detailed Description
It should be noted that the embodiments and features of the embodiments may be combined with each other without conflict. Furthermore, the terms "connected," "coupled," and "mounted" appearing in the description are to be construed broadly unless otherwise specified or limited. The specific meaning of the above terms in the present invention can be understood by those skilled in the art from the specific situation. Furthermore, it is to be understood that the terms "front", "rear", "left", "right", and the like, which refer to directions or reflect positional relationships, are presented in the description of the present invention only based on the directions or positional relationships presented in the drawings, and are intended to facilitate the description of the present invention, but do not indicate or imply that the referred devices or elements must have a specific direction, be constructed and operated in a specific direction, and thus, should not be construed as limiting the present invention. The present invention is not limited to the embodiments disclosed herein.
According to the convention in cryptography, the two parties participating in communication are respectively called Alice and Bob (also marked in the drawing), wherein Alice exclusively refers to the sending end of the information, and Bob exclusively refers to the receiving end of the information.
The invention provides a continuous variable four-state modulation quantum key distribution method, which comprises the following specific steps:
s1: preparation and delivery of the states. Modulating by an Alice terminal according to probability distribution [ q1, q2, q3 and q4] by methods of loading phase information and the like, and randomly preparing local oscillation light and four coherent signal light pulses which are only distinguished in phase; the four coherent states are { | s >, | is > }, q1 ═ q2, q3 ═ q4, q1+ q2+ q3+ q4 ═ 1, and s is a real number; after the preparation is finished, Alice combines the local oscillator light and the signal light through the quantum channel and sends the combined signal to Bob.
S2: and (4) measuring states. And the Bob end receives the signal light and the local oscillator light which are sent by the Alice end, and sends the signal light and the local oscillator light to a local homodyne measuring device to measure a certain regular component of the signal light.
S3: and (6) publishing a result. Repeating the steps in S1 and S2 for a plurality of rounds, then Alice and Bob communicate through the authenticated classical channel, Bob publishes that the selected canonical components are measured, according to the selection of the canonical components published by Bob, Alice considers the case of preparing the same coherent state and Bob measures the canonical momentum component p and the canonical coordinate component q respectively as the same group, and only telling Bob which two measurements can be considered as the same group through the classical channel and not publishing the prepared coherent state itself.
S4: and estimating the code forming rate. Estimating the safe code rate obtained by the steps, wherein the safe code rate is greater than 0, which indicates that a secret key can be generated, continuing the protocol, and performing subsequent secret key mapping; otherwise, no secure key generation is indicated, and the protocol is terminated. The secure code rate estimation method can adopt any existing method.
S5: and (6) post-processing the data. And after the key mapping is finished, carrying out error correction and privacy amplification on the initial key.
The invention also provides a continuous variable four-state modulation quantum key distribution system, which is used for realizing the method and is explained in detail by combining three specific embodiments.
Example one
Fig. 1 shows an exemplary embodiment of a continuous variable four-state modulation quantum key distribution system for implementing the present invention. Information transmission and interaction between Alice and Bob are realized through the quantum channel module 9. Single mode fibers may be chosen as the choice of quantum channels without loss of generality.
As shown in fig. 1, the sending end Alice may include four modules, including a pulse laser module 1, a beam splitting module 2, a phase modulation module 3, and a beam combining module 4. According to the protocol to be realized by the invention, the Alice terminal prepares and sends the signal light and the local oscillator light. When the laser is prepared, firstly, the pulse laser module 1 emits laser pulses, the laser pulses enter the beam splitting module 2 through the left port of the beam splitting module 2, and then the pulses are divided into first laser pulses and second laser pulses with different intensities and are respectively output from the upper port and the right port. The first laser pulse output by the upper port enters the phase modulation module 3, is modulated into one of the four coherent signal lights, and then enters the upper port of the beam combining module 4; the second laser pulse output by the right port is used as local oscillation light and directly enters the left port of the beam combining module 4, and at the moment, the local oscillation light and the signal light are not overlapped in time domain. Two laser pulses are output from the right port of the beam combination module 4 and enter the quantum channel module 9.
According to the principle of the continuous variable four-state modulation quantum key distribution protocol implemented by the present invention, the pulse laser module 1 is used for generating optical pulses with high extinction ratio. Preferably, an internal modulation laser or an electro-absorption modulation laser can be selected as the pulse laser light source, but an intensity modulator can be added after the pulse laser light source in order to further remove stray light carried in the light pulse emitted by the pulse laser and ensure that the amplitude of the light pulse generated by the pulse laser is stable.
And the beam splitting module 2 is used for splitting the pulse laser emitted by the pulse laser module 1 according to a larger proportion. But not limited to, a beam splitter (hereinafter BS) or a combination of a phase retarder plus a polarizing beam splitter (hereinafter PBS) with a splitting ratio of 999: 1. without loss of generality, a fixed attenuator may be added in the path that the signal light pulse travels, which serves to attenuate the intensity of the signal light pulse to a desired level.
The phase modulation module 3 is used for performing phase modulation on the signal light pulse, and can complete preparation of four coherent states without amplitude modulation.
And the beam combining module 4 is configured to combine the modulated signal light and the local oscillator light, and send the combined signal light and local oscillator light to a quantum channel for transmission. Preferably, a polarization-maintaining polarization beam splitter may be selected, but is not limited to, that may transmit the local oscillator light in the horizontal polarization directly into the quantum channel, while rotating the signal light in the horizontal polarization ninety degrees to the vertical polarization, which is reflected into the quantum channel. This process makes the local oscillator light distinguishable from the signal light in the degree of freedom of polarization.
The receiving end Bob may include four modules, including a polarization control module 5, a beam splitting module 6, a phase modulation module 7, and a homodyne measurement module 8. The signal light pulse and the local oscillation light pulse prepared by the sending end reach a Bob receiving end after being transmitted by the quantum channel module 9, firstly pass through the polarization control module 5, then reach a left port of the beam splitting module 6, according to the different polarization states of the signal light pulse and the local oscillation light, the signal light is output from the right port of the beam splitting module 6, the local oscillation light is output from the upper port of the module 6, enters the phase modulation module 7, and finally the signal light and the local oscillation light coincide again in the time domain and simultaneously reach the homodyne measurement module 8.
The polarization control module 5 is configured to adjust polarization states of the signal light and the local oscillator light in the quantum channel, so that the polarization states of the signal light and the local oscillator light are recalibrated. The polarization state can be feedback controlled by, but not limited to, an electrically controlled polarization controller.
And the beam splitting module 6 is used for splitting the signal light and the local oscillator light with different polarizations into two paths. Preferably, a polarization-maintaining polarization beam splitter may be selected, but not limited to, and the polarization-maintaining polarization beam splitter may split the signal light and the local oscillation light having different polarizations into two paths, and simultaneously rotate the polarization of the local oscillation light by ninety degrees, which makes the local oscillation light coincide with the polarization of the signal light.
The phase modulation module 7 is configured to add a phase 0 or pi/2 to the local oscillator light pulse at random, and measure different regular components in the homodyne measurement module correspondingly.
And the homodyne measurement module 8 is configured to perform homodyne measurement operation on the signal optical pulse and the local oscillator optical pulse from the sending end Alice.
The principle of implementing continuous variable four-state quantum key distribution in the present embodiment will now be described with reference to specific devices. In this example, module 1 comprises a pulsed laser (preferably an intensity modulator may be added), and module 2 comprises a beam splitting ratio of 999: 1 (an attenuator may preferably be added), module 3 comprises a phase modulator, module 4 comprises a polarization maintaining PBS, module 5 comprises an electrically controlled polarization controller, module 6 comprises a polarization maintaining PBS, module 7 comprises a phase modulator, module 8 comprises a set of homodyne measurement devices, and module 9 comprises single mode fiber. The method comprises the following implementation steps:
randomly preparing four coherent states by Alice. The preparation process comprises the following steps:
(1) firstly, a pulse laser generates laser pulses at an Alice end, and stray light in the generated laser pulses is removed through an intensity modulator;
(2) the outgoing light pulse passes 999: the BS of the 1 is divided into two paths of LO and signal light, and the signal light is further attenuated by the intensity of an attenuator;
(3) the signal light passes through a phase modulator, a random number generator generates random numbers according to probability distribution [ q1, q2, q3, q4], q1 is q2, q3 is q4, q1+ q2+ q3+ q4 is 1, the generated random numbers respectively correspond to four coherent states { | s >, | -s >, | is >, | -is > } (s is a real number, preferably 0.4 can be taken), coherent states needing to be prepared are determined according to the result of the random number generator, the signal light is modulated to prepare corresponding coherent states, the intensities of the four coherent states are the same, and the amplitude of the signal light does not need to be additionally modulated;
(4) the signal light and the local oscillator light are combined through the polarization-maintaining polarization beam splitter, and the polarization of the signal light and the local oscillator light is in a mutually vertical state at the moment, and then the signal light and the local oscillator light enter the single-mode optical fiber for transmission.
And 2, receiving the local oscillation light and the signal light which are sent by the Alice end and transmitted through the single-mode optical fiber by the Bob end, and measuring the coherent state prepared by the Alice. The measurement process is as follows:
(1) because the local oscillator light and the signal light can generate polarization drift in the single-mode optical fiber, the polarization of the local oscillator light and the signal light is aligned with a subsequent device by performing feedback control through the electric control polarization controller;
(2) the local oscillation light is reflected after entering the polarization-maintaining polarization beam splitter, and simultaneously the polarization is rotated by ninety degrees, the phase applied by the phase modulator is determined to be 0 or pi/2 according to the result of the random number generator, and then the regular component of the signal light is determined to be measured, if the applied phase is 0, the measured regular coordinate is shown, and if the applied phase is pi/2, the measured regular momentum is shown;
(3) the local oscillator light and the signal light enter the homodyne measuring device at the same time for measurement.
3. After repeating the above steps for N times (N is a positive integer), Alice and Bob communicate through an authenticated classical channel, Alice obtains the selection condition of the regular component sent by Bob, data when the Alice sends the same coherent state and Bob respectively measures the regular coordinate and the regular momentum are grouped into the same group, and Bob is only told through the classical channel which two groups of data can be regarded as the same group.
4. The method estimates the code rate by using a numerical simulation convex optimization algorithm through published information, if the code rate shows that a key can be generated, the protocol is continuously executed, otherwise, the execution of the protocol is interrupted, and the protocol is directly abandoned. If the protocol can be continued, Alice and Bob respectively complete key mapping to form an original key.
The specific steps of estimating the bit rate by adopting a numerical simulation convex optimization algorithm are as follows:
the code rate calculation formula is constructed as follows:
wherein, R represents the key rate, σ represents the joint density matrix, S represents the condition that the joint density matrix should satisfy, H (rho | | | σ) is the mutual entropy, it is still unknown information content by the attacker after suffering attack to show, K establishes the association between density matrix and classical bit as mapping, P represents the effect of contracting the quantum channel, Δ represents the bit loss caused by the bit error correction of classical data, q represents the probability of keeping a pulse as the final generated key;
finding the lower bound of the mutual entropy by using a convex optimization algorithm according to a calculation formula of the code forming rate: first, from an initial joint density matrix σ0Initially, a joint density matrix σ 'close to the strongest attack is found through iteration, and then the iterated σ' is substituted into the following formula to calculate the lower bound of the bit rate:
ε≥ξ(σ′)
where ε represents the minimum value of the mutual entropy when the joint density matrix satisfies the constraint S,
is a vector of expected values from the measured density matrix,
is the parameter to be optimized and is,S*is that
The constraint that should be satisfied, superscript T represents the transpose of the matrix,
and expressing a gradient operator, wherein Tr expresses tracing of the matrix, the obtained xi (sigma') is the lower bound of the mutual entropy, and the calculated lower bound is substituted into the expression of the code rate to calculate the lower bound of the code rate.
The specific steps of the numerical simulation convex optimization algorithm for estimating the bit rate are as follows:
and 5, further carrying out data post-processing operation between Alice and Bob, including reverse negotiation and related error correction and privacy amplification processes, and finally obtaining a security key for communication.
Example two
Fig. 2 presents a second exemplary embodiment of a continuous variable four-state modulated quantum key distribution system embodying the teachings of the present invention.
As shown in fig. 2, the sending end Alice includes a first slave laser module 1, a master laser module 2, a second slave laser module 3, a first optical transmission module 4, a beam splitting module 5, a second optical transmission module 6, a signal modulation module 7, and a beam combining module 8. According to the protocol to be realized by the invention, the Alice terminal prepares and sends the signal light and the local oscillator light. During preparation, laser pulses are emitted by the main laser module 2, enter the beam splitting module 5 through the left port, are output from the upper port and the right port according to a certain intensity ratio, and enter the first optical transmission module 4 and the second optical transmission module 6. The laser pulse passing through the first optical transmission module 4 enters the first slave laser module 1, and then the laser pulse with stable phase emitted by the first slave laser module 1 enters the signal modulation module 7 to be continuously variable-four-state modulated into signal light; the laser pulse passing through the second optical transmission module 6 enters the second slave laser module 3, and a phase-stabilized laser pulse is emitted from the second slave laser module 3 as local oscillation light. The two pulses respectively enter the beam combining module 8 through the upper port and the left port to be combined into the same quantum channel and sent to the receiving end Bob.
The first slave laser module 1 is configured to receive injection of the seed light generated by the master laser module 2, and further generate a signal light pulse with a stable phase.
The master laser module 2 is configured to generate seed light, inject the seed light into the slave laser modules 2 and 3, and generate signal light pulses and local oscillator light pulses with stable phases in an injection locking manner.
The action principle of the second slave laser module 3 is consistent with that of the first slave laser module 1, and both the second slave laser module and the first slave laser module are injection-locked, so that local oscillation light pulses with stable phases are finally obtained.
The first optical transmission module 4 is used for changing a transmission path of the pulse light in the quantum channel, and the change of the path is unidirectional and irreversible. The circulator may be selected, but is not limited to.
And the beam splitting module 5 is used for splitting the seed light generated by the main laser module 2 according to a certain intensity proportion.
The second optical transmission module 6 has the same function as the first optical transmission module 4.
And the signal modulation module 7 is used for loading phase information on the optical pulse and completing continuous variable four-state modulation. The phase modulator may be selected, but is not limited to.
And the beam combining module 8 is configured to combine the modulated signal light and the local oscillator light, output the combined signal light and local oscillator light to a quantum channel, and send the combined signal light and local oscillator light to a receiving end. A polarization maintaining PBS may be selected, but is not limited to.
The modules for implementing various functions of the quantum channel module and the receiving end Bob and the functions of the corresponding modules are the same as those in the first embodiment except for the module numbers, and are not described herein again.
The principle of the present embodiment for realizing continuous variable four-state quantum key distribution will now be described with reference to examples. In this example, modules 1, 2, 3 each comprise a pulsed laser, modules 4, 6 each comprise a circulator, module 5 comprises a BS (preferably a fixed attenuator may be added), module 7 comprises a phase modulator, module 8 comprises a polarization-maintaining PBS, and the remaining modules are illustrated as corresponding modules in the first embodiment. The method comprises the following implementation steps:
first, Alice randomly prepares four coherent states. The preparation process comprises the following steps:
(1) the pulse laser at the main laser module 2 generates laser pulses, which are split by the BS and respectively enter the two circulators;
(2) two beams of pulses are respectively injected into two slave lasers through a circulator, and the slave lasers are excited to generate laser pulses with stable phases;
(3) laser pulses generated from the laser module 1 enter a fixed attenuator, the intensity of the laser pulses is further attenuated, then the optical pulses pass through a phase modulator, a random number generator generates random numbers according to a probability distribution [ q1, q2, q3, q4] (q1, q2, q3, q4 ≧ 0. q1 ═ q2, q3 ═ q4, q1+ q2+ q3+ q4 ≧ 1), and the generated random numbers respectively correspond to the phase modulator to modulate four coherent states { | s >, | -s >, | is > } (s is a real number, preferably, may take 0.4), which does not need to perform additional intensity modulation;
(5) and combining the signal light and the local oscillator light which finish information loading into a quantum channel through a polarization maintaining PBS.
The subsequent steps are the same as those in the first embodiment, and are not described again.
EXAMPLE III
Fig. 3 shows a third exemplary embodiment of a continuous variable four-state modulation quantum key distribution system as described for implementing the present invention.
As shown in fig. 3, the sending end Alice includes a first slave laser module 1, a master laser module 2, a second slave laser module 3, a first optical transmission module 4, a beam splitting module 5, a second optical transmission module 6, and a beam combining module 7. According to the protocol to be realized by the invention, the Alice terminal prepares and sends the signal light and the local oscillator light. During preparation, laser pulses are emitted by the main laser module 2, enter the beam splitting module 5 through the left port, are output from the upper port and the right port according to a certain intensity ratio, and enter the first optical transmission module 4 and the second optical transmission module 6. The laser pulse passing through the first optical transmission module 4 enters the first slave laser module 1, and the optical pulse output from the module 1 is used as signal light; the laser pulse passing through the second optical transmission module 6 enters the second slave laser module 3, and the slave module 3 emits a laser pulse with a stable phase as local oscillation light. The two pulses enter the beam combining module 7 through the upper port and the left port respectively and are combined into the same quantum channel, and further sent to the receiving end Bob.
The first slave laser module 1 is configured to receive the injection of the seed light generated by the master laser module 2, and thereby generate the signal light whose phase is stable and whose phase modulation has been completed.
The master laser module 2 is configured to inject the seed light of the wide pulse into the slave laser in an internal modulation manner, and directly complete loading of the phase of the signal light without using an additional phase modulation module, and may generate pulse light with a stable phase.
And the second slave laser module 3 is configured to receive injection of the seed light generated by the master laser module 2, and further generate local oscillator light with a stable phase.
The first optical transmission module 4 is used for changing a transmission path of the pulse light in the quantum channel, and the change of the path is unidirectional and irreversible. Without loss of generality, the circulator may be selected, but is not limited to.
And the beam splitting module 5 is used for splitting the seed light generated by the main laser module 2 according to a certain intensity proportion. But is not limited to the use of a BS.
The second optical transmission module 6 functions as the first optical transmission module 4.
And the beam combining module 7 is configured to combine the modulated signal light and the local oscillator light, output the combined signal light and local oscillator light to a quantum channel, and send the combined signal light and local oscillator light to a receiving end. A polarization maintaining PBS may be selected, but is not limited to.
The modules for implementing various functions of the quantum channel module and the receiving end Bob and the functions of the corresponding modules are the same as those in the first embodiment except for the module numbers, and are not described herein again.
The principle of the present embodiment for realizing continuous variable four-state quantum key distribution will now be described with reference to examples. In this example, modules 1, 2, 3 each comprise a pulsed laser, modules 4 and 6 comprise a circulator, module 5 comprises a BS, module 7 comprises a polarization maintaining PBS, and the remaining modules are illustrated as corresponding modules in embodiment one.
First, Alice randomly prepares four coherent states, and the probability distribution of the random numbers and the corresponding coherent states are the same as those in the previous embodiment. Here, the method of internal modulation is used for preparing the four coherent states, and the method directly performs phase modulation by applying perturbation to the loading voltage of the main laser without an additional phase modulation instrument. The process for preparing coherent state by internal modulation is as follows:
(1) the pulse laser at the master laser module 2 generates broad laser pulses, and fig. 4 shows the time domain relationship of the electrical pulse signals applied to the master laser and the two slave lasers in one period in an internal modulation case, and the figure only shows the principle qualitatively, and does not represent the real quantitative relationship. When the main laser is applied with voltage to generate light pulse, a wide pulse voltage is needed, and a small perturbation (shown as a main laser part in fig. 4) is applied to the middle of the applied pulse voltage, the voltage perturbation will cause the central frequency of the laser pulse generated by the main laser to generate offset, so that the phase of the laser pulse is changed along with the evolution of time, the offset can be controlled by changing the magnitude of the perturbation, and the phase difference generated by the offset corresponds to the phase of the continuous variable modulation four-state, namely 0, pi/2, pi, 3 pi/2;
(2) light pulse emitted by the main laser is split into local oscillation light and signal light by the BS, and the local oscillation light and the signal light respectively enter the two circulators;
(3) when a pulse voltage is applied to the slave laser at the module 1, a pulse voltage with a short pulse width should be selected, and this pulse voltage should be located after (or before) the perturbation of the wide pulse emitted by the master laser in the time domain, if before the perturbation, the coherent state corresponding to the modulated phase needs to be adjusted accordingly, because the phase difference between the signal light and the local oscillator light changes at this time), as in the slave laser module 1 of fig. 4, but the pulse voltage should still fall in the wide pulse in the time domain;
(4) the requirements in applying a pulse voltage to the slave laser at module 3 are similar to those in step (3), but this electrical pulse should be located before (or after) the wide pulse in the time domain, as in the slave laser module 3 part of fig. 4, in exactly tandem with the pulse in (3);
(5) and the signal light and the local oscillator light which complete information modulation enter a quantum channel through the polarization maintaining PBS and are transmitted to a receiving end Bob together.
The subsequent steps are the same as those in the first embodiment, and are not described again.
The above description is only of the preferred embodiments of the present invention, and it should be noted that: it will be apparent to those skilled in the art that various modifications and adaptations can be made without departing from the principles of the invention and these are intended to be within the scope of the invention.
Claims (8)
1.A continuous variable four-state quantum key distribution method is characterized by comprising the following steps:
(1) the method comprises the steps that local oscillation light is prepared at a sending end, signal light corresponding to four coherent states { | s >, | is > } is generated according to probability distribution [ q1, q2, q3 and q4], and then the signal light and the local oscillation light are sent to a receiving end; wherein q1 is q2, q3 is q4, q1+ q2+ q3+ q4 is 1, and s is a real number;
(2) the receiving end introduces a phase increment of 0 or pi/2 into the local oscillation light randomly and with equal probability through phase modulation, the phase increment is respectively used for measuring a regular coordinate component or a regular momentum component of the signal light, and then the receiving end carries out homodyne measurement to obtain a regular coordinate or a regular momentum of the signal light;
(3) repeating the steps (1) to (2) for N times to obtain data required by one round of code forming;
(4) the transmitting end and the receiving end communicate through an authenticated classical channel, the receiving end informs the transmitting end of selecting a regular component during measurement, then the transmitting end regards the situation that the same signal state is transmitted and the receiving end respectively measures regular coordinates and regular momentum as a same group according to the selection of the receiving end, and only tells the receiving end which two times of measurement can be regarded as the same group through the classical channel, but does not disclose the prepared coherent state per se;
(5) the sending end carries out key mapping on the sent signal state to obtain an initial key of the sending end; the receiving end carries out key mapping according to the measurement result to obtain an initial key of the receiving end;
(6) the sending end and the receiving end carry out reverse negotiation and related error correction and privacy amplification processes based on respective initial keys to obtain a final security key.
2. The continuous variable four-state quantum key distribution method according to claim 1, wherein before step (5) is performed, a numerical simulation convex optimization algorithm is further used to estimate a bit rate according to the measurement result of step (4), if the bit rate meets a preset safety condition, step (5) is continued, otherwise, the key distribution is ended.
3. The continuous variable four-state quantum key distribution method according to claim 2, wherein the specific step of estimating the bit rate by using the numerical analog convex optimization algorithm comprises:
the code rate calculation formula is constructed as follows:
wherein, R represents the key rate, σ represents the joint density matrix, S represents the condition that the joint density matrix should satisfy, H (rho | | | σ) is the mutual entropy, it is still unknown information content by the attacker after suffering attack to show, K establishes the association between density matrix and classical bit as mapping, P represents the effect of contracting the quantum channel, Δ represents the bit loss caused by the bit error correction of classical data, q represents the probability of keeping a pulse as the final generated key;
finding the lower bound of the mutual entropy by using a convex optimization algorithm according to a calculation formula of the code forming rate: first, from the initial joint encryptionDegree matrix sigma0Initially, a joint density matrix σ 'close to the strongest attack is found through iteration, and then the iterated σ' is substituted into the following formula to calculate the lower bound of the bit rate:
ε≥ξ(σ′)
where ε represents the minimum value of the mutual entropy when the joint density matrix satisfies the constraint S,
is a vector of expected values from the measured density matrix,
is the parameter to be optimized, S*Is that
And (3) according to the constraint condition to be met, the superscript T represents the transposition of the matrix, ^ represents a gradient operator, Tr represents tracing on the matrix, the obtained ξ (σ') is the lower bound of the mutual entropy, and the calculated lower bound is substituted into the expression of the code rate to calculate the lower bound of the code rate.
4. A continuous variable four-state quantum key distribution system, for implementing the method according to any one of claims 1 to 3, comprising a transmitting end and a receiving end, wherein the transmitting end comprises a preparation module, for preparing the local oscillator light and the signal light; the receiving end includes: the device comprises a polarization control module, a beam splitting module, a phase modulation module and a homodyne measurement module; wherein the content of the first and second substances,
the polarization control module is used for adjusting the polarization states of the signal light and the local oscillator light in the quantum channel so as to recalibrate the polarization states of the signal light and the local oscillator light;
the beam splitting module is used for separating signal light and local oscillation light in different polarization directions;
the phase modulation module randomly and equiprobabilistically introduces a phase increment of 0 or pi/2 in the local oscillation light;
and the homodyne measurement module performs homodyne measurement on the signal light and the local oscillator light after phase modulation to obtain a regular coordinate or regular momentum.
5. The continuous variable four-state quantum key distribution system according to claim 4, wherein the sending end and the receiving end are respectively provided with a control module, the control modules of the sending end and the receiving end respectively control each module inside the local end to realize corresponding functions, and perform reverse negotiation and related error correction and privacy amplification steps with another party of communication to negotiate a final security key.
6. The continuous variable four-state quantum key distribution system according to claim 5, wherein the sending end comprises: the device comprises a pulse laser module, a beam splitting module, a phase modulation module and a beam combining module; wherein the content of the first and second substances,
the pulse laser module is used for generating laser pulses;
the beam splitting module is used for splitting the laser pulse into a first laser pulse and a second laser pulse with different intensities, and the second laser pulse is used as local oscillation light;
the phase modulation module performs phase modulation on the first laser pulse to generate the signal light;
the beam combining module combines the signal light and the local oscillator beam and transmits the combined beam to a receiving end through a quantum channel.
7. The continuous variable four-state quantum key distribution system according to claim 5, wherein the sending end comprises: the laser device comprises a master laser module, a first slave laser module, a second slave laser module, a first optical transmission module, a second optical transmission module, a beam splitting module, a phase modulation module and a beam combining module; wherein the content of the first and second substances,
the main laser module is used for generating seed light and splitting the seed light into first seed light and second seed light through the beam splitting module;
the first seed light is injected into the first slave laser module through the first optical transmission module, and the first slave laser module generates original signal light pulse in a phase injection locking mode and sends the original signal light pulse into the phase modulation module to be modulated to generate the signal light;
the second seed light is injected into a second slave laser module through a second optical transmission module, and the second slave laser module generates local oscillator light in a phase injection locking mode;
the beam combining module combines the signal light and the local oscillator beam and transmits the combined beam to a receiving end through a quantum channel.
8. The continuous variable four-state quantum key distribution system according to claim 5, wherein the sending end comprises: the laser device comprises a master laser module, a first slave laser module, a second slave laser module, a first optical transmission module, a second optical transmission module, a beam splitting module and a beam combining module; wherein the content of the first and second substances,
the main laser module is used for directly completing the loading of the phase of the signal light in an internal modulation mode to generate seed light with stable phases corresponding to four signal states, and the seed light generated by the main laser module is split into first seed light and second seed light through the beam splitting module;
the first seed light is injected into the first slave laser module through the first optical transmission module, and the first slave laser module generates the signal light in a phase injection locking mode;
injecting the second seed light into a second slave laser module through a second optical transmission module, wherein the second slave laser module generates the local oscillator light in a phase injection locking mode;
the beam combining module combines the signal light and the local oscillator beam and transmits the combined beam to a receiving end through a quantum channel.
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