CN112688740B - Floodlight quantum key distribution method and system without phase feedback - Google Patents

Abstract

The invention provides a floodlight quantum key distribution method and a floodlight quantum key distribution system without phase feedback. By the method, the phase feedback operation in the key distribution process can be omitted, so that the distribution system is greatly simplified, and the code rate can be effectively extracted under the condition that the phase drift is continuous and random. Before the exclusive-or operation, the post-selection operation is additionally adopted, so that the code rate can be further greatly improved. As long as the appropriate light source spectral width and coding speed are ensured, the success code rate can be ensured to break the PLOB upper limit, and the method has great advantages in the metropolitan area range.

Description

Floodlight quantum key distribution method and system without phase feedback

Technical Field

The invention relates to the field of quantum communication, in particular to a floodlight quantum key distribution method and system without phase feedback.

Background

Because the existing classical key encryption mode is basically based on computational complexity, and a quantum computer which is being realized can provide far higher computing speed than a classical computer so as to crack a classical key, a novel key generation method becomes a key point of research of cryptography. Quantum key distribution is a key distribution method designed based on the basic principle of quantum mechanics, and the generated key is used for communication encryption according to the principle of one-time pad, so that unconditional safety guaranteed by an information theory can be obtained. This security is resistant to attack by quantum computers, and so a great deal of research resource investment is expected to develop and refine such key distribution methods.

However, quantum effects can be well embodied on a microscopic scale, so that most quantum key distribution protocols require instruments with extremely high precision, and practically, ideal instruments are not invented. The rate of finished code is always low due to imperfections in the instrumentation. Even though the BB84 protocol is developed completely by theory and experiment, the highest code rate of the prior art (the conventional system and high-rate quality distribution with time-bin estimates. Sci. adv. 3(11): e1701491) is only 7.71Mbps (Mbps =1 × 10)⁶Bits per second). It is even more annoying that the vast majority of quantum key distribution protocols, which utilize single-frequency light, storeThe upper limit of Pirandola-Laurenza-Ottavani-Banchi (PLOB) at the rate of coding constrained by the transmission efficiency (Fundamental limits of retransmission quality standards communications. Nat. Commun. 8, 15043). Although the two-field quantum key distribution protocol theoretically has the capacity of breaking the upper limit, the requirement on instruments is higher, and the effect is even inferior to that of the BB84 protocol in a metropolitan area range, namely about 50 km.

To overcome the above technical deficiencies, the industry proposed a protocol for quantum key distribution using a broad spectrum light source (flood light quantum key distribution: a positive to positive-per-second secret-keys rates, phys. rev. a 94, 012322, published in 10.2015 on preprinted journal arXiv and published in 2016 in 14.2016), and then proposed experiments using this protocol (flood light quantum key distribution: purifying a frame for high-rate security communication, phys. rev. a 95, 012332, published in 2016 in preprinted journal arXiv and published in 2017.1.26). In addition, patent publication No. CN 107453819 a proposes a high-speed quantum key distribution method, which is not inferior in principle to the above protocol principle. In general, the above protocol utilizes a broad spectrum light source and homodyne measurements, each of which produces results that are not constrained by the PLOB upper limit. The signal light is sent by one party, and the other party receives the code and then sends the code back to the sending party for measurement, so that the distance traveled by the quantum signal is twice as long as the distance between the two communication parties.

The above protocol has the following drawbacks:

on one hand, the coded information is on the phase basis vector of the signal light, and the homodyne measurement result is sensitive to phase change; on the other hand, the quantum signal passes through a very long optical fiber, while the local oscillator light passes through another optical fiber with the same length, and due to slight environmental changes, such as temperature, stress and the like, phase drift can be caused, so that finally, the encoded phase is completely random when the local oscillator light phase is taken as a reference phase, and the homodyne measurement cannot identify the encoded phase, thereby causing about half bit errors. Therefore, if no phase feedback is performed, the above protocol will result in zero final resultant code rate in implementation. The above patent does not suggest any particularly effective phase feedback or other solution to this problem. In the paper, a complex phase feedback method is used. However, the paper uses an attenuator instead of a real optical fiber, so that the phase drift occurring in real implementation cannot be reflected well, and thus the effectiveness of the phase feedback method in real implementation cannot be proved. In addition, the conventional phase feedback method for dealing with phase drift is complex, consumes much resources, requires many precise instruments for measurement and feedback modulation, and cannot realize perfect phase feedback.

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 floodlight quantum key distribution method and system without phase feedback. By using the method, the code rate can be effectively extracted under the condition that the phase drift is continuous and random. Before the exclusive-or operation, the post-selection operation is additionally adopted, so that the code rate can be further greatly improved. As long as the appropriate light source spectral width and coding speed are ensured, the success code rate can be ensured to break the PLOB upper limit, and the method has great advantages in the metropolitan area range.

The technical scheme is as follows: in order to achieve the purpose, the invention provides the following technical scheme:

a floodlight quantum key distribution method without phase feedback comprises the following steps:

(1) a transmitting end prepares signal light and local oscillator light with the same center frequency, the same spectrum width and synchronous phase;

(2) the method comprises the steps that a sending end generates first associated light and second associated light with relevance, the first associated light is measured immediately to determine existence and calibrate measuring time, the second associated light and signal light are combined, the signal light ratio is far larger than the second associated light ratio, and the center frequency and the spectrum width of the signal light and the second associated light are the same; the sending end divides the combined light beam into tiny parts to measure immediately so as to determine the existence of second associated light and mark the measuring time, and sends the residual light beam to the receiving end through an untrusted quantum channel;

(3) after receiving the light beam, the receiving end separates out a minimum part and immediately measures the minimum part to determine the existence of second associated light and calibrate the measuring time; then a string of bit strings is randomly generated, the bit strings are carried on the residual light beams through phase modulation, and then the bit strings are sent back to the sending end through an untrusted quantum channel;

(4) after receiving a return beam carrying a bit string, a sending end performs homodyne measurement on the return beam and local oscillation light generated by the sending end to obtain a string of real number strings, and then the real number strings are mapped into a string of bit strings;

(5) the sending end and the receiving end respectively carry out near-bit XOR processing on the held bit strings to obtain an initial key string of the local end;

(6) the transmitting end/receiving end randomly discloses initial key information with a certain proportion based on the measured first and second associated light existence result data, calculates a safe code forming rate and publishes the safe code forming rate to the other party through a classical channel; and on the premise that the safe code rate meets the preset requirement, the transmitting end and the receiving end carry out classical error correction and privacy amplification, and finally the same and safe secret key is extracted.

Several alternatives are provided below for the method, 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, before the receiving end returns the light beam carrying the key string to the transmitting end, the receiving end further performs amplification processing on the light beam.

Optionally, the near-bit xor processing in step (5) specifically includes:

determining the maximum continuous coding bit number N of which the phase drift is smaller than a preset threshold value when the system continuously codes;

performing M rounds of XOR operation, generating a new bit by each round of XOR, and uniformly distributing the bits participating in the XOR in the original key string to finally obtain a new bit string; the new bit string simultaneously satisfies the following conditions:

a. the length of the new bit string must not exceed the length of the original bit string;

b. each bit of the original bit string participates in the exclusive-or operation only or is directly reserved.

Optionally, in step (4), before the sending end maps the real string into the bit string, the sending end further performs post-selection processing, that is: the sending end selects parameters according to preset

The measured value in the real number string is positioned

The results of the interval are discarded, the remainder is retained,

(ii) a The sending end informs the discarded real number result position to the receiving end through a classical channel, and the receiving end removes bits at corresponding positions from the held bit string and reserves the residual part.

In order to realize the method, the invention correspondingly provides a floodlight quantum key distribution system without phase feedback, which comprises a sending end and a receiving end, wherein the sending end and the receiving end adopt the method to distribute the floodlight quantum key.

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.

As an optional implementation manner of the system, the transmitting end includes a wide-spectrum, continuous and phase-randomized light source, and is configured to prepare continuous light and divide the continuous light into two beams to obtain the signal light and the local oscillator light.

Further, the transmitting end further comprises a coupler, a related light source, a first knocker, a delayer, a homodyne measurer, a first detector and a second detector; the detector measures the first associated light immediately; the coupler is used for combining the signal light and the second correlated light, so that the combined light beam enters the first knocker from the same light path; the first knocker is used for taking a small part of the combined light beam down and sending the small part of the combined light beam into the second detector for measurement, and after the operation of the knocker is finished, the rest light beam is sent to the receiving end; the delay device is used for delaying the local oscillator light, so that the local oscillator light and the return light beam can synchronously enter the homodyne measurer; the homodyne measurement is used for carrying out homodyne measurement by using the local oscillator light and the return light beam to obtain phase difference information between the local oscillator light and the return light beam;

the receiving end comprises a second knocker, a phase encoder, an amplifier and a detector III; the second knocker takes a small part of the received light beam down and sends the small part of the received light beam into the detector III for detection, and the rest light beam is sent to the phase encoder; the phase encoder carries out phase encoding on the residual light beam according to a bit string generated by a receiving end and loads a key string on the residual light beam; the amplifier is used for amplifying the coded light beam.

Further, the encoding method of the phase encoder is as follows: if the bit generated by the receiving end is 0, the phase of the light beam to be coded is not changed, and if the bit generated by the receiving end is 1, the phase of the light beam to be coded is increased

。

As another optional implementation of the system, the transmitting end includes a first amplified spontaneous emission source, a correlated light source, first to third beam splitters, a delay coil, a homodyne measurer, and first to second single photon detectors; wherein the associated light source is used for generating the first and second associated lights; the amplified spontaneous emission source is used for generating continuous light; the first beam splitter divides the continuous light into signal light and local oscillator light; the second beam splitter combines the second associated light and the signal light and sends the combined light to a third beam splitter; the third beam splitter splits a minimum part of the combined beam light for monitoring the existence of second associated light, and the rest part of the combined beam light is sent to a receiving end; the local oscillator light is delayed by a delay coil and then is synchronously sent to a homodyne measurer with a return light beam sent by a receiving end to carry out homodyne measurement;

the receiving end comprises a fourth beam splitter, a binary phase shift keying module, a second amplified spontaneous radiation source and a third single-photon detector; the fourth beam splitter divides a very small part from the received light beam, sends the very small part to the third single-photon detector to monitor the existence of second associated light, and sends the rest light beam to the binary phase shift keying module; the binary phase shift keying module performs phase modulation on the received light beam according to the bit string generated by the receiving end; the amplified spontaneous radiation source amplifies the two modulated light beams and feeds the amplified light beams back to the transmitting end.

Has the advantages that: compared with the prior art, the invention has the following advantages:

(1) under the condition that the bit error rate is increased due to phase drift and the bit rate is possibly 0, according to the continuity characteristic of the phase drift (the environment change is slow relative to the bit coding speed, and the phase drifts of a plurality of signals which are sent in a similar way are almost the same in a certain time length), the bit error rate can be reduced by using the near bit XOR operation on the key string formed by mapping, and the key can be extracted from the key string;

(2) in the prior art, phase drift can cause the value ranges of two coded signal light measurement results to be overlapped, and further, the result falls into an overlapped value area, so that the two signal lights cannot be effectively distinguished, and a high error rate is generated. The method adds post-selection operation on the basis of near-bit XOR, thereby reducing the influence caused by the randomness of phase drift, effectively reducing the coincidence of the light measurement results of two code forming signals caused by the phase drift, reducing the error rate of the part and finally improving the code forming rate.

(3) Because the error caused by phase drift can be overcome by the near bit XOR operation and the post selection operation, the phase feedback can be omitted.

(4) The invention reduces the error rate without changing the safety of the system, and the invention still has the safety which can be protected by using the upper limit of Holevo. The invention reduces the error rate so as to realize the code forming rate higher than the PLOB upper limit.

Drawings

Fig. 1 is a block diagram of a floodlight quantum key distribution system without phase feedback according to embodiment 3;

fig. 2 is a structural diagram of a floodlight quantum key distribution system without phase feedback according to embodiment 4;

FIG. 3 is a comparison between the post-processing step according to example 1 and the post-processing step according to the comparison document, wherein FIG. 3 (a) shows the post-processing step according to the comparison document, and FIG. 3 (b) shows the post-processing step according to example 1;

FIG. 4 is a flowchart of the post-processing steps involved in example 2;

fig. 5 is a graph of the simulation result of the bit rate.

Detailed Description

The invention will be further described with reference to the accompanying drawings and specific embodiments. It is to be understood that the present invention may be embodied in various forms, and that there is no intention to limit the invention to the specific embodiments illustrated, but on the contrary, the intention is to cover some exemplary and non-limiting embodiments shown in the attached drawings and described below.

It is to be understood that the features listed below for different embodiments may be combined with each other, where technically feasible, to form further embodiments within the scope of the invention. Furthermore, the particular examples and embodiments of the invention described are non-limiting, and various modifications in the structure, steps, and sequence set forth below may be made without departing from the scope of the invention.

The invention aims to provide a floodlight quantum key distribution method and system without phase feedback, which can omit phase feedback, generate a key in a phase drift environment, and break the upper limit of PLOB by a key rate.

For clearly illustrating the implementation principle and technical effects of the present invention, the following embodiments are further described below.

Example 1:

the embodiment provides a floodlight quantum key distribution method without phase feedback, in the method, a sending end is marked as Alice, a receiving end is marked as Bob, and a listener is marked as Eve, and the method comprises the following steps:

first, Alice prepares local oscillator light and signal light, where the local oscillator light and the signal light may be prepared by the same light source or different light sources, and the same light source is taken as an example in this embodiment for description. Alice prepares phase random broad spectrum continuous laser, and divides the laser into two beams, wherein one beam with smaller light intensity is used as signal light for bearing key information, and one beam with larger light intensity is used as local oscillator light for carrying the key and performing homodyne measurement with the signal light to realize decoding and obtain the key, and the local oscillator light needs to be delayed to wait for the return trip of the signal light generated at the same time so as to realize that the local oscillator light and the signal light returned by Bob synchronously enter homodyne measurement.

Secondly, Alice utilizes a correlation light source to generate two beams of correlation light with correlation, wherein the first correlation light is measured immediately to determine existence and calibrate time; the second correlated light has the same spectral width and center frequency as the signal light, and is combined with the signal light for subsequent monitoring. The separated minimal part of the combined beam immediately monitors the presence of the second correlated light and calibrates the measurement instant, and the remaining part is sent to Bob via the untrusted quantum channel.

Thirdly, Bob immediately monitors the existence of second associated light for the minimum part of the received light beam separation and marks the detection time, and then applies corresponding phase modulation to the signal light according to the randomly generated bit string by adopting a phase coding mode; after modulation is completed, the light beam is sent to Alice through an untrusted quantum channel.

Preferably, Bob may also amplify the beam by a signal amplifier before transmitting the modulated beam to resist channel loss when transmitting back to Alice and to effectively obfuscate the information to reduce the ability of an attacker to eavesdrop.

And fourthly, after receiving the return light beam, Alice performs homodyne measurement on the signal light by using local oscillation light, the measurement result is a string of real number strings, and the generation speed and the coding speed are the same.

And fifthly, integrating the results of the three associated light monitoring measurements to perform conformance analysis, wherein Alice and Bob can determine the intrusion parameters of the attacker, and thus the information quantity acquired by the attacker is calculated. Alice performs post-processing on the generated real number result, and the specific steps are as shown in fig. 3, including:

mapping a real number string of a measurement result into a bit string (namely an original key string in a binary form), and then performing near-bit XOR operation to obtain an initial key;

bob and Alice synchronously realize near bit XOR, and communicate with Alice through a classical authoritative channel, and both parties randomly publish part of bit information for calculating error rate; calculating a safe code forming rate by Alice/Bob, and publishing the safe code forming rate to the other party through a classical channel;

and on the premise that the safe code rate meets the preset requirement, performing classical error correction and privacy amplification by Alice and Bob, and finally generating the same safe key string by the Alice and the Bob. This preset requirement refers to: the coding rate is greater than 0 or greater than a certain value that makes it meaningful to generate the key this time.

In the method, specifically, after Alice obtains a result through homodyne measurement, Alice maps the measurement result into a string of bit strings in a manner that greater than 0 is recorded as 0 and less than 0 is recorded as 1, and then performs a near-bit exclusive or operation. The specific method of the near bit exclusive or operation is as follows: considering the phase drift velocity, the system has a phase drift amount smaller than a preset threshold value in a time range of continuously encoding N bits, and therefore, the N-bit range is referred to as a near bit herein. Alice and Bob may negotiate to determine a selection rule that each time selects two bits that are no more than N bits apart to xor (identically denoted as 0 and differently denoted as 1) to generate a new key bit. The bit positions selected by both parties each time need to be kept consistent, that is, Alice selects to perform exclusive-or operation on the ith bit and the jth bit (i and j are not more than N bits apart), then Bob also needs to perform exclusive-or operation on the ith bit and the jth bit of his bit string. The operating rules need to meet two requirements: 1. the length of the finally generated new bit string must not exceed the length of the original bit string; 2. the bits participating in the XOR in the original bit string are uniformly distributed in the original key string, and the bits not selected to participate in the XOR are directly reserved in the new bit string.

Specifically, an alternative method of near-bit xor operation is: and sequentially carrying out exclusive OR on the ith bit and the jth bit of the original bit string, wherein j = i + N, and N is less than or equal to N, thereby generating a new bit string, namely the initial key string of Alice. If n is 1, the near bit XOR is also the adjacent bit XOR, namely the first bit and the second bit of the original bit string, and the first bit of the new bit string is generated according to the method of marking as 0 and marking as 1; generating a second bit of the new bit string according to the method of marking the second bit and the third bit of the original bit string as 0 and marking the second bit and the third bit as 1; and repeating the steps until the last two bits of the original bit string generate the last bit of the new bit string according to the rule. Note that the first and last bits of the original bit string of such near-bit exclusive-or are used only once in the process, since they have only one nearest bit. While every other bit of the original bit string is used twice because each bit has two neighboring bits before and after it. Bob operates on his bit string in the same manner as Alice.

Figure 3 shows a comparison of the prior art with the present invention. It can be seen that the exclusive or operation to which the present invention relates can be used solely in the classical information processing part. The post-processing part of Alice firstly obtains a measurement result from a homodyne measurer, wherein the measurement result is a real number and takes the local oscillator optical phase as a reference phase, and the measurement result and the phase difference between the signal optical phase and the reference phase have a functional relation.

In the above scheme, the mapping operation is the prior art, specifically, a measurement result greater than 0 is recorded as 0, and a measurement result less than 0 is recorded as 1, so that Alice side generates a binary original key string. The near-bit XOR operation is a new technology of the invention, and carries out near-bit XOR on the original key string according to a certain rule to generate a new key string. Then, Bob communicates with the classical authoritative channel to realize the existing technologies of bit error correction and privacy amplification, and a safe and consistent key string is obtained. The classical processing part of Bob first provides the encoded key string to binary phase shift keying and records the key string. There is an association between this key string and Alice's original key string. And Bob also performs near-bit XOR operation consistent with Alice on the own encoding key string to obtain a new encoding key string. Then, the secret key string is communicated with Alice through a classical authoritative channel to realize the prior art of bit error correction, privacy amplification and the like, and the safe and consistent secret key string is obtained.

Example 2:

this example is a further preferred embodiment of example 1. The remaining steps of this embodiment are the same as those of embodiment 1, and only one post-selection step is added to the fifth step, and after the addition, the whole post-selection process is shown in fig. 4. The post-selection step, also referred to as the screening step, is an operation performed after Alice finally obtains the measurement results. The operation method is that a screening parameter is preset

Taking the value of the real number result obtained by measurement by Alice exactly at the position

The result of the interval is discarded, the rest is reserved, then the discarded position is informed to Bob through a classical channel, and Bob removes the bit of the corresponding position from the bit string of the Bob to ensure that the initial key held by the Bob is consistent with that of Alice. Post-selection is thus in fact a sort of screening and selection. This screening parameter can be chosen to be different depending on the implementation conditions.

It is noted that in the fifth step, only an exclusive or operation may be used, i.e. the post-selection step in the two-party post-processing is omitted. The xor operation itself can reduce the total error rate to be less than 0.5, so that the mutual information between the two communication parties is greater than 0, and the capability of generating the same key is provided. The post-selection operation can further reduce the error rate and improve the code rate. But the post-select operation cannot be used alone and needs to be used with an exclusive or operation. When used together, the post-selection needs to be done before the exclusive-or operation.

Example 3:

the present embodiment proposes a system for implementing the above method, and its functional architecture is shown in fig. 1. The system can be functionally divided into three parts: a key distribution optical path portion divided by three solid circles; a key distribution monitoring part which is divided by a dotted line circle, and a key distribution classical information processing part which is divided by a dot-dash line circle.

For the optical path part divided by the solid circles, three solid circles from left to right represent the communication user Alice, the eavesdropper Eve and the communication user Bob respectively.

Alice has a correlation light source, a first detector, a second detector, a wide spectrum light source, a coupler, a first knocker, a delayer and a homodyne measurer.

The purpose of the correlated light source is to generate a pair of correlated lights, i.e. a first correlated light and a second correlated light, which must be generated simultaneously, that can be used to monitor the attack intensity.

The wide-spectrum light source is used for generating signal light with a large spectrum width and capable of loading key information and local oscillator light with the same spectrum width, the same center frequency and synchronous phase as the signal light. The signal light is weaker than the local oscillator light, and the signal light and the local oscillator light are finally used for measuring the key information coded on the return light beam.

The coupler is used for combining the signal light and the second correlated light and enabling the signal light and the second correlated light to advance from the same optical path so as to carry out real-time monitoring on the second correlated light.

The first rapper is used to remove a small portion of the combined beam for second correlated light monitoring. After the first rapper operation is completed, the remaining light beam is directed from Alice to Bob.

The delayer is used for delaying the local oscillator light properly, so that the local oscillator light and the return light carrying the key information can synchronously enter the homodyne measurer.

The homodyne measurer can obtain phase difference information between the local oscillation light and the return light by utilizing measurement of the local oscillation light and the return light.

An eavesdropper Eve can completely control the quantum channels from Alice to Bob and from Bob to Alice, more precisely, the whole quantum channel is controlled by Eve from the beginning of the emission from Alice side to the receiving from Bob side and then from the beginning of the emission from Bob side to the receiving from Alice side, and Eve can do any collective attack without operation.

The communication user Bob has a second rapper, a third detector, a phase encoder and an amplifier. Wherein the second rapper removes a small portion of the received light for second correlated light monitoring. And the phase encoder performs phase encoding on the received light beam according to the key string generated by the receiving end. The amplifier amplifies the coded light, and according to the quantum mechanics principle, a large amount of noise is introduced while amplifying the light, and the noise has the capacity of confusing an attacker to distinguish signal components. After processing by the amplifier, the return light is sent back to Alice through an untrusted quantum channel for homodyne measurement.

The monitoring part, which is divided by a dashed circle in fig. 1, is a part that cannot be controlled by an eavesdropper Eve, and includes associated light sources, detectors one to three, and a conformance analyzer.

The first correlated light is detected by the detector-to verify presence (whether two correlated lights were successfully generated) and time is recorded for coincidence analysis. The center frequency of the second associated light is the same as the spectrum width of the signal light generated by the wide-spectrum light source, the associated light is sent to a coupler in an Alice light path and is combined with the signal light generated by the wide-spectrum light source to enter a first knocker, and the first knocker separates a minimum light beam from the received light beam and sends the minimum light beam to a second detector for second associated light detection and time recording. The resulting beamlets from the second rapper are fed into a third detector which is also used to detect the presence of a second correlated light and record the time for a coincidence analysis. By the conformity analysis, the amount of information acquired by the eavesdropper can be determined.

The associated light source, detector one, and detector two are typically on the Alice side, while the associated light monitoring measurements obtained by the second tap are typically made on the Bob side using detector three. The conformity analysis after the measurement can be performed by the Alice side or the Bob side. When a party is acting as an analysis, the other party should send its own measurement results to the party acting as an analysis through the classical authoritative channel.

The classical information processing part divided by the dot-dash line circle is the classical information processing after the measurement is finished. In a comparison file (Floodlight rectangle key distribution: A positive route to gigabit-per-connected secret-keys rates, Phys. Rev. A94, 012322), Alice needs to perform post-processing after performing the homodyne measurement, namely, analysis is performed according to the coincidence analysis result and the homodyne measurement result, and finally the key is extracted. It is this step in the comparison file that the present invention improves. The invention introduces near-bit XOR and post-processing operation in the post-processing step, thereby realizing the breakthrough that the phase feedback is not needed in the system. The classical processing part belongs to the side of Bob and carries the task of randomly generating a bit string to be encoded on signal light, so that the classical processing part needs to be in contact with a phase encoder in a real coil of Bob in the operation of an optical path, and then the classical processing part needs to be in agreement with the post-processing part of Alice in steps of error correction, privacy amplification and the like through a classical channel, so that the final security key strings generated by Alice and Bob respectively are safe and identical. We introduce near-bit xor and post-processing operations also in the classical processing part and have agreed upon requirements.

Example 4:

the embodiment proposes another floodlight quantum key distribution system without phase feedback, and the structure of the floodlight quantum key distribution system is shown in fig. 2.

The system comprises a communication user Alice, an eavesdropper Eve and a communication user Bob.

The light path part of the communication user Alice comprises an amplified spontaneous emission source I, beam splitters I to III, a delay coil, a homodyne measurer, spontaneous parametric down-conversion, a single-photon detector I and a single-photon detector II.

The first amplified spontaneous emission source can emit continuous light with a wide spectrum and random phases, and then the continuous light is divided into a weaker beam serving as signal light and led to the second beam splitter through the first beam splitter, and a stronger beam serving as local oscillation light and led to the delay coil. Even a weak beam, the number of photons in it is greater than 1. The amplified spontaneous emission source one and the beam splitter constitute the broad spectrum light source part of the system.

In this embodiment, the associated light source employs spontaneous parametric down-conversion. The spontaneous parametric down-conversion is a correlated light source and can continuously generate correlated photon pairs for monitoring the attack intensity, wherein one beam is called as first correlated light, the other beam is called as second correlated light, the first correlated light is sent to a single photon detector for detecting and verifying the existence and recording the detection time, and the second correlated light is sent to a beam splitter II.

The second beam splitter is a coupler, the signal light and the second associated light sent by the first beam splitter can be combined into one beam and sent to the third beam splitter, and in the combined light, the spectral width and the spectral center of the signal light and the second associated light are consistent, so that an eavesdropper cannot distinguish the two lights, and the associated light has the capability of monitoring the eavesdropper.

And the third beam splitter is used as a knocker, and separates a tiny beam of light from the received light beam and sends the tiny beam of light to the second single-photon detector for second correlated light detection. In this embodiment, the splitting ratio of the third beam splitter may be 99:1, wherein a smaller beam is sent to the second single-photon detector, and a larger beam is emitted from the third beam splitter and sent to the fourth beam splitter on Bob side through a quantum channel.

The delay coil is a delayer, and the delay coil is very long, can make the transmission channel of local oscillator light prolong, realizes finally that local oscillator light and signal light walk through quantum channel as long, finally arrives homodyne caliber simultaneously. Preferably, in order to reduce the channel loss of the local oscillator light, an optical amplifier may also be connected to the delay coil, and an amplified spontaneous emission source may be generally used.

The homodyne measurer is an instrument which can be realized in the prior art, and the measurement result has a mathematical relation with the phase difference between two incident beams. And Bob coding key information is realized by phase modulation, so that the result of homodyne measurement can reflect the coded information, and therefore Alice can obtain the bit string coded by Bob.

The eavesdropper Eve is an unpredictable presence and behaves as described in example 3.

And the light path at one side of the communication user Bob is formed by sequentially connecting a beam splitter IV, a binary phase shift keying and an amplified spontaneous radiation source II. And the fourth beam splitter realizes the function of a knocker and also adopts a 99:1 beam splitting ratio, wherein a small beam is sent to the third single-photon detector to perform second correlated light detection, and a large beam is sent to binary phase shift keying.

Binary Phase Shift Keying (BPSK) is an instrument that adjusts the phase of light according to the binary bits that need to be encoded on the received light. If Bob generates a bit of 0, the phase is not changed, and if Bob generates a bit of 1, the phase inversion, or phase increment, is performed

. The received light is continuous light, and BPSK also has coding speed. The encoding speed and spectral width of the light affect the average number of photons and the optical mode in the light encoding a single bit. For the encoding speed R, the unit Hz (how many bits are encoded per second); the spectrum width W, unit Hz (Hertz, i.e. how many oscillation cycles are completed per second); the number of optical modes M = W/R in the encoded beam is therefore greater than 1, typically much greater than 1, which is more favorable for key generation; to ensure the security of the channel, the number of photons in each optical mode within a single bit is much less than 1, so the average number of photons transmitted must not exceed M, which constrains the signal light intensity.

After the encoding of the bit string is completed, the beam carrying the key information will be amplified by the amplified spontaneous emission source two, the amplification factor depending on the gain of the amplified spontaneous emission source two. Noise can be introduced while signal light is amplified, which is required by a quantum mechanics principle, otherwise, random amplification of the signal light without the introduction of the noise means that quantum information can be copied, a quantum state can be copied, a quantum key loses security, and the quantum unclonable principle is violated. The magnitude of the introduced noise depends on the noise index dependent properties of the amplified spontaneous emission source two. This noise also masks the signal light, so that the attacker cannot well distinguish the signal light component from the outgoing light on the Bob side. Meanwhile, the significance of amplifying the signal light is to offset the channel loss of the signal light from the Bob side back to the Alice side, so that information can be efficiently transferred. After the second spontaneous radiation source is amplified, the return light beam returns to the Alice side from the Bob side, and reaches the homodyne measurer with the local oscillation light split by the beam splitter to be measured.

The monitoring part divided by the dashed circle in fig. 2 is a part which cannot be controlled by an eavesdropper Eve, and comprises spontaneous parametric down-conversion, three single-photon detectors and a coincidence analyzer.

The recorded information obtained by the three detectors is used for the coincidence analysis. The coincidence analysis can be realized by a CPU board with a set program, and the purpose is to count the times of response of the two and three single-photon detectors, and the times of time alignment and time dislocation of the first single-photon detector and the second and third single-photon detectors respectively. These statistics can eventually generate an intrusion parameter that characterizes the own light-occupied component injected into the channel by the attacker Eve. The analysis of conformity here is prior art and the specific procedures are described in the article "Floodlight rectangle key distribution A reactive route to gigabit-per-second secret-key rates, Phys. Rev. A94, 012322".

In the system described in this embodiment, because there is no relevant part for phase feedback in the optical path, and each part of the whole optical path structure such as the quantum channel and the delay coil may be in different environmental conditions such as temperature, optical fiber stress, etc., the return beam and the local oscillator light pass through completely different optical paths, and finally, relative phase drift is caused. Note that the phase of the continuous light generated as soon as the amplified spontaneous emission source is random at each instant, and continuity of the initial phase is not required. However, the initial phase difference between the local oscillation light and the signal light obtained by splitting by the beam splitter is 0. Since the environmental changes are continuous, the changes in phase drift are continuous; the phase shift is slow relative to the encoding speed, so although the phase shift is random, the phase shift is continuous, the phase shifts of a plurality of (how many are represented by N in embodiment 1) bits of light of adjacent codes are basically the same, the difference is negligible, and therefore the xor operation can utilize the basic property of the phase shift to reduce the error rate.

In the embodiment, bit errors can be reduced through the near-bit exclusive-or operation, and the key is extracted from the bit errors, so that the whole system can be free from the requirement of phase feedback, therefore, the system can be as simple as that shown in fig. 1 and fig. 2, and the phase feedback operations such as monitoring phase drift, adjusting phase and the like can be realized without adding complex instruments and lines.

The technical effect is verified:

fig. 5 is a diagram of simulation results of the rate-forming rate obtained by using the vicinal exclusive-or operation, i.e., under the following parameters. The spectral width W of the wide-spectrum light source is

Bob side amplification gain of the second spontaneous emission source

Is composed of

Noise index

Characterization parameters of experimental imperfections

The device causes 4.7dB of loss before reaching Bob side amplified spontaneous emission source two. Transmission distance is 50km, and channel loss is calculated according to 0.2dB/km, one-way transmission efficiency is

. Efficiency of error correction

0.94, intrusion parameters should be analyzed by coincidence measurement, and selected during simulation

。

The upper limit of PLOB depicted by the horizontal solid line is for

The case (1). We show whenAnd when the coding speed R is 100MHz and 1GHz respectively, performing near bit XOR operation and then selecting the result. The ideal protocol refers to the protocol without exclusive-or operation and post-selection operation of the present invention, with the phase drift noted as 0 or with perfect phase feedback (Floodlight positive phase to positive-controlled secret-keys rates, Phys. Rev. A94, 012322). Therefore, the method has better coding rate. In practice, if a real fiber is used, it is impossible to achieve phase drift of 0 or perfect phase feedback. If no phase feedback is performed, the randomness of the phase drift will directly result in a bit rate of 0. According to the invention, the code rate is successfully obtained under the condition of no phase feedback through near bit XOR operation (or near bit XOR operation + post selection), and the code rate is within a certain coding speed and can completely exceed the PLOB upper limit. Under the above experimental parameter conditions, the encoding rate of 100MHz can reach the encoding rate of nearly 30Mbps, and 1GHz can generate the encoding rate of more than 150Mbps, which is far more than the 7.71Mbps achieved by the existing published experiments. Therefore, the invention successfully omits complex phase feedback, and can obtain the code rate exceeding the upper limit of PLOB, which is far more than the existing experimental result.

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 (9)

1. A floodlight quantum key distribution method without phase feedback is characterized by comprising the following steps:

(1) a transmitting end prepares signal light and local oscillator light with the same center frequency, the same spectrum width and synchronous phase;

(2) the method comprises the steps that a sending end generates first associated light and second associated light with relevance, the first associated light is measured immediately to determine existence and calibrate measuring time, the second associated light and signal light are combined, the signal light ratio is far larger than the second associated light ratio, and the center frequency and the spectrum width of the signal light and the second associated light are the same; the sending end divides the combined light beam into tiny parts to measure immediately so as to determine the existence of second associated light and mark the measuring time, and sends the residual light beam to the receiving end through an untrusted quantum channel;

(3) after receiving the light beam, the receiving end separates out a minimum part and immediately measures the minimum part to determine the existence of second associated light and calibrate the measuring time; then a string of bit strings is randomly generated, the bit strings are carried on the residual light beams through phase modulation, and then the bit strings are sent back to the sending end through an untrusted quantum channel;

(4) after receiving a return beam carrying a bit string, a sending end performs homodyne measurement on the return beam and local oscillation light generated by the sending end to obtain a string of real number strings, and then the real number strings are mapped into a string of bit strings;

(5) the sending end and the receiving end respectively carry out N-bit near-bit XOR processing on the held bit strings to obtain an initial key string of the local end; n is the maximum continuous coding bit number of the system with the phase drift smaller than a preset threshold value during continuous coding;

(6) the transmitting end/receiving end randomly discloses initial key information with a certain proportion based on the measured first and second associated light existence result data, calculates a safe code forming rate and publishes the safe code forming rate to the other party through a classical channel; and on the premise that the safe code rate meets the preset requirement, the transmitting end and the receiving end carry out classical error correction and privacy amplification, and finally the same and safe secret key is extracted.

2. A floodlight quantum key distribution method without phase feedback according to claim 1, wherein before the receiving end returns the light beam carrying the key string to the transmitting end, the receiving end further amplifies the light beam.

3. A floodlight quantum key distribution method without phase feedback according to claim 1, wherein the specific steps of near-bit XOR processing in the step (5) are as follows:

determining the maximum continuous coding bit number N of which the phase drift is smaller than a preset threshold value when the system continuously codes;

performing M rounds of XOR operation, generating a new bit by each round of XOR, and uniformly distributing the bits participating in the XOR in the original key string to finally obtain a new bit string; the new bit string simultaneously satisfies the following conditions:

a. the length of the new bit string must not exceed the length of the original bit string;

b. each bit of the original bit string participates in the exclusive-or operation only or is directly reserved.

4. A floodlight quantum key distribution method without phase feedback according to claim 1, wherein in step (4), before mapping the real string to the bit string, the sending end further performs post-selection processing, that is: the sending end discards a result that the value of the measured real number string is positioned in a (-delta, delta) interval according to a preset screening parameter delta, and keeps the rest part, wherein delta is more than or equal to 0; the sending end informs the discarded real number result position to the receiving end through a classical channel, and the receiving end removes bits at corresponding positions from the held bit string and reserves the residual part.

5. A floodlight quantum key distribution system without phase feedback comprises a sending end and a receiving end, and is characterized in that the floodlight quantum key distribution is carried out between the sending end and the receiving end by adopting the method of any one of claims 1 to 4.

6. A floodlight quantum key distribution system without phase feedback as claimed in claim 5, wherein said transmitting end comprises a broad-spectrum, continuous and phase-randomized light source for preparing continuous light and splitting said continuous light into two beams to obtain said signal light and said local oscillator light.

7. A floodlight quantum key distribution system without phase feedback according to claim 6, wherein the transmitting end further comprises a coupler, an associated light source, a first knocker, a delayer, a homodyne measurer, a first detector and a second detector; the detector measures the first associated light immediately; the coupler is used for combining the signal light and the second correlated light, so that the combined light beam enters the first knocker from the same light path; the first knocker is used for taking a small part of the combined light beam down and sending the small part of the combined light beam into the second detector for measurement, and after the operation of the knocker is finished, the rest light beam is sent to the receiving end; the delay device is used for delaying the local oscillator light, so that the local oscillator light and the return light beam can synchronously enter the homodyne measurer; the homodyne measurement is used for carrying out homodyne measurement by using the local oscillator light and the return light beam to obtain phase difference information between the local oscillator light and the return light beam;

the receiving end comprises a second knocker, a phase encoder, an amplifier and a detector III; the second knocker takes a small part of the received light beam down and sends the small part of the received light beam into the detector III for detection, and the rest light beam is sent to the phase encoder; the phase encoder carries out phase encoding on the residual light beam according to a bit string generated by a receiving end and loads a key string on the residual light beam; the amplifier is used for amplifying the coded light beam.

8. A floodlight quantum key distribution system without phase feedback according to claim 7, wherein the encoding mode of the phase encoder is as follows: if the bit generated by the receiving end is 0, the phase of the light beam to be encoded is not changed, and if the bit generated by the receiving end is 1, the phase of the light beam to be encoded is increased by pi.

9. A floodlight quantum key distribution system without phase feedback according to claim 5, wherein the transmitting end comprises a first amplified spontaneous emission source, a correlated light source, a first to a third beam splitter, a delay coil, a homodyne measurer and a first to a second single photon detector; wherein the associated light source is used for generating the first and second associated lights; the amplified spontaneous emission source is used for generating continuous light; the first beam splitter divides the continuous light into signal light and local oscillator light; the second beam splitter combines the second associated light and the signal light and sends the combined light to a third beam splitter; the third beam splitter divides a minimum part of the combined beam into a second single-photon detector for monitoring the existence of second associated light, and the rest part of the combined beam is sent to a receiving end; the single-photon detector is used for detecting the existence of the first correlated light; the local oscillator light is delayed by a delay coil and then is synchronously sent to a homodyne measurer with a return light beam sent by a receiving end to carry out homodyne measurement;

the receiving end comprises a fourth beam splitter, a binary phase shift keying module, a second amplified spontaneous radiation source and a third single-photon detector; the fourth beam splitter divides a very small part from the received light beam, sends the very small part to the third single-photon detector to monitor the existence of second associated light, and sends the rest light beam to the binary phase shift keying module; the binary phase shift keying module performs phase modulation on the received light beam according to the bit string generated by the receiving end; the amplified spontaneous radiation source amplifies the two modulated light beams and feeds the amplified light beams back to the transmitting end.

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