CN114900412A - Single-SPD-based QKD system public phase difference estimation method and system - Google Patents
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Abstract
The invention discloses a method and a system for estimating a public phase difference of a QKD system based on a single SPD (pulsed-light), wherein a sending end generates a periodic signal containing a plurality of specific strong reference light pulses, a plurality of specific modulation phases are loaded on the reference light pulses, and after a detection result corresponding to interference signals among the reference light pulses is obtained by detecting the reference light pulses through the single SPD at a Charlie end, the public phase difference in the QKD system is accurately estimated through a specific algorithm, so that the coding of the TF-type QKD system with the single SPD can be assisted under the condition of not increasing or changing any hardware composition, and the practicability and the production process of the TF-type QKD realized based on the single SPD are further promoted.
Description
Technical Field
The invention relates to the field of quantum optics and quantum information, in particular to an estimation method of a public phase difference in a double-field (TF) type QKD system based on a single-photon detector (SPD) and the TF type QKD system with the single SPD based on the estimation method.
Background
Due to the development of quantum computers, the security of the classical encryption technology is facing a huge threat, and with the development of science and technology, people pay more and more attention to information security problems in the fields of military affairs, government affairs, finance and the like. QKD has incomparable advantages over classical cryptography, and it can provide theoretically unconditionally secure key transport.
Significant progress has been made in the QKD field over the last decades, with QKD experiments with optical fibers over 400 km and satellite-to-ground links over 1000 km, fully demonstrating the feasibility of long-distance point-to-point QKD. At the same time, however, there are some fundamental limitations to the key transport capabilities of the unrepeatered QKD, namely the PLOB community proposed in 2017 by the university of york, uk and the college of london, uk. The PLOB boundary is an upper boundary of the coding rate of the non-relay QKD, and since the relationship between the coding rate and the channel transmittance is linear when the channel transmittance is small, the upper boundary is also referred to as an upper boundary of the linear coding rate of the non-relay QKD. The conventional protocols such as BB84 protocol and MDI-QKD protocol have no code rate exceeding PLOB boundary, TF-class QKD proposed in succession in 2018 breaks the PLOB boundary, and TF-class QKD has great potential in QKD for long-distance practical use. To date, TF-like QKD protocols can be broadly divided into three categories, namely transmit or not transmit TF-QKD schemes, phase-matched QKD schemes, phase-less post-selection QKD schemes, respectively.
In the prior art of performing TF-like QKD protocols, Charlie (the measurement end) typically uses two SPDs for measurement for completing the coding of the QKD system, and related studies show that the coding of the QKD system can also be completed by the measurement of a single SPD. However, one of the key techniques in TF-like QKD schemes is the estimation of the common phase difference between Alice (sender) and Bob (sender) optical pulses. In the prior art, two SPDs are required to complete the common phase difference estimation, that is, in the common phase difference estimation stage, Alice and Bob send strong reference optical pulses to Charlie for interference, and the common phase difference between Alice and Bob is obtained through the detection results of the two SPDs, as shown in fig. 1. In a system for realizing the TF-class QKD protocol by using a single SPD, no relevant research report is reported at present for estimating the common phase difference in the system.
Disclosure of Invention
Aiming at the problems in the prior art, the invention discloses a method for estimating the common phase difference in a TF-type QKD system based on a single SPD and the TF-type QKD system realized by the method. WhereinThe sending end generates a periodic signal containing a plurality of specific strong reference optical pulses, a plurality of specific modulation phases are loaded on the reference optical pulses, and after a detection result corresponding to interference signals between the reference optical pulses is obtained by detecting a single SPD at the Charlie end, a specific algorithm is used for accurately estimating and obtaining a common phase difference in the QKD system
Therefore, the code forming of the TF-type QKD system with the single SPD can be assisted without adding or changing any hardware composition, so that the practicability and the production process of the TF-type QKD realized based on the single SPD are further promoted.
Specifically, the first aspect of the invention relates to an estimation method of a common phase difference in a single SPD-based TF-type QKD system, which comprises a periodic signal generation step, a phase modulation step, an interference step and an estimation step;
in the periodic signal generating step, a first periodic signal is generated in an Alice terminal, a second periodic signal is generated in a Bob terminal, the first periodic signal comprises L first reference light pulses, the second periodic signal comprises L second reference light pulses, and L is larger than or equal to 3;
in the phase modulation step, phase modulation is carried out on a first reference light pulse in an Alice terminal, wherein M is arranged on the L first reference light pulses 1 Modulation phase
And modulates the phase
Within [0, 2 π); phase-modulating a second reference light pulse within Bob terminal, wherein the L second reference light pulses have M thereon 2 Modulation phase
M 1 *M 2 Not less than 3 and modulating phase
Within [0, 2 π);
in the interference step, enabling a first periodic signal and a second periodic signal to enter a Charlie end and generate interference to generate an interference signal, detecting the interference signal by using a single SPD, and outputting an interference result;
in the estimating step, a common phase difference is estimated using the result of the interference of the first reference light pulse and the second reference light pulse
Further, in the estimating step, the common phase difference is estimated using the interference result in one cycle
N 1l /k≈(1+cosγ ab (l) K is a constant),
for the modulation phase of the first reference light pulse,
for the modulation phase, N, of the l-th second reference light pulse 1l The responses of the interference signals of the first and second reference light pulses are counted, i.e. 1.
Further, in the estimating step, the common phase difference is estimated using the interference results in the N cycles
k n Is a constant number of times, and is,
the modulation phase of the first reference light pulse in one period,
the modulation phase of the l-th second reference light pulse in one period,
the responses of the interference signals of the first and second reference light pulses in the nth period are counted, i.e. 1,. and L, N > 1.
Preferably, L has a value of 4, and one of the first and second reference light pulses has four modulation phases, which are 0, pi/2, pi and 3 pi/2, respectively, and the other of the first and second reference light pulses has one modulation phase, which is one of 0, pi/2, pi and 3 pi/2, within one period.
More preferably still, the first and second liquid crystal compositions are,
and is
Or
The invention relates to a TF type QKD system based on a single SPD, which comprises an Alice end, a Bob end and a Charlie end;
the Alice terminal is configured to generate a first periodic signal having L first reference light pulses, and M is modulated on the L first reference light pulses 1 Modulation phase
Modulating the phase
Within [0, 2 π);
the Bob terminal is configured to generate a second periodic signal having L second reference light pulses with M modulated thereon 2 Modulation phase
Modulating phase
In [0, 2 π), wherein L is not less than 3, M 1 *M 2 ≥3；
The Charlie terminal is configured to interfere the first and second periodic signals to generate an interference signal, detect the interference signal with a single SPD to obtain an interference result, and estimate a common phase difference using the interference result of the first and second reference optical pulses
Further, the periodic signal also comprises a quantum optical signal.
Further, the Charlie end is configured to:
estimating common phase difference using interference results over one period
N 1l /k≈(1+cosγ ab (l) K is a constant),
for the modulation phase of the first reference light pulse,
for the modulation phase, N, of the l-th second reference light pulse 1l Counting responses of interference signals of the first and second reference light pulses, i.e. 1,. and L; alternatively, the first and second electrodes may be,
estimating common phase difference using interference results over N cycles
k n Is a constant number of times, and is,
the modulation phase of the first reference light pulse in one period,
the modulation phase of the l-th second reference light pulse in one period,
the responses of the interference signals of the first and second reference light pulses in the nth period are counted, i.e. 1,. and L, N > 1.
Preferably, L has a value of 4, and one of the first and second reference light pulses has four modulation phases of 0, pi/2, pi and 3 pi/2, respectively, and the other of the first and second reference light pulses has one modulation phase of one of 0, pi/2, pi and 3 pi/2, respectively, in one period.
Preferably, the TF-like QKD system of the present invention may be arranged to estimate the common phase difference using the estimation method described above
Drawings
The following describes embodiments of the present invention in further detail with reference to the accompanying drawings.
In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings used in the description of the embodiments or the prior art will be briefly described below, it is obvious that the drawings in the following description are only some embodiments of the present invention, and for those skilled in the art, other drawings can be obtained according to the drawings without creative efforts.
Fig. 1 schematically shows a TF-like QKD system of the prior art, in which a Charlie end uses two SPDs to implement interferometric detection and estimation of common phase difference;
FIG. 2 schematically illustrates a TF-like QKD system with a Charlie end implementing interferometric detection and estimation of common phase difference using only a single SPD in accordance with the present invention;
fig. 3 shows an example of a periodic signal output by a transmitting end (Alice/Bob) according to the present invention.
Detailed Description
Hereinafter, exemplary embodiments of the present invention will be described in detail with reference to the accompanying drawings. The following examples are provided by way of illustration in order to fully convey the spirit of the invention to those skilled in the art to which the invention pertains. Accordingly, the present invention is not limited to the embodiments disclosed herein.
Fig. 2 schematically shows a TF-like QKD system according to the present invention, which includes an Alice terminal, a Bob terminal, and a Charlie terminal, wherein a single-photon detector (SPD) for detecting interference signals is disposed in the Charlie terminal.
The Alice terminal and the Bob terminal are used as sending terminals and are configured to send a first periodic signal and a second periodic signal to the Charlie terminal respectively.
The first periodic signal may include a first quantum light pulse, a first reference light pulse, and a vacuum state portion, wherein: the first quantum light pulse carries coding information for QKD coding; the first reference light pulse is used for estimating the common phase difference
The second periodic signal may include a second quantum light pulse, a second reference light pulse, and a vacuum state portion, wherein: the second quantum light pulse bears coding information and is used for QKD coding; the second reference light pulse is used for estimating the common phase difference
In the present invention, the first periodic signal may include L first reference light pulses, and the second periodic signal includes L first reference light pulsesL second reference light pulses are also included. And in the same period, the first reference light pulse hand interferes with the second reference light pulse at the Charlie end to generate an interference signal, so that the common phase difference is estimated through an interference result obtained by single photon detection on the interference signal
l＝1，...，L。
Estimating a common phase difference for interference results by means of a single SPD
The number L of reference light pulses in the periodic signal is set to L ≧ 3.
Further, in the transmitting ends (Alice and Bob) of the present invention, the reference light pulse is also modulated by means of the phase modulator PM.
In particular, in the Alice terminal, L first reference light pulses in the periodic signal may be each loaded with a modulation phase by means of the phase modulator PM
Wherein the modulation phase loaded on the L first reference light pulses in one period
Is M1 kinds, and modulates the phase
Within 0, 2 π).
Meanwhile, in Bob terminal, L second reference light pulses in the periodic signal may be loaded with modulation phases respectively by means of the phase modulator PM
Wherein the modulation phase loaded on the L second reference light pulses in one period
Is M 2 And modulate the phase
Within 0, 2 pi).
According to the invention, the Alice terminal and the Bob terminal load the number M of modulation phases on the reference light pulse by means of the phase modulators 1 And M 2 Should be set to satisfy M 1 *M 2 ≥3。
With continued reference to fig. 2, when the first and second periodic signals transmitted by the Alice terminal and the Bob terminal enter the Charlie terminal, the L-th (L ═ 1., L) of the L first reference optical pulses interferes with the L-th of the L second reference optical pulses at, for example, the beam splitter BS, and the interference signal is detected by a single SPD (e.g., SPD1 in fig. 2), so as to obtain an interference result N 1l (i.e., response count).
Since the detection efficiency of the SPD is known, the response probability P of the SPD in the Charlie end to the interference signal of the l-th reference optical pulse 1l Comprises the following steps:
N 1l /k≈P 1l ＝(1+cosγ ab )/2
wherein, γ ab For phase modulation of reference light pulses
The common phase difference between the rear Alice and Bob terminals,
is the common phase difference between the Alice and Bob terminals when the reference light pulse is not phase-modulated (i.e., the original common phase difference to be estimated); k is constant over one period, which is the total photon count in the presence of two SPDs (SPD1, SPD2), which can be measured in a TF-class QKD system with two SPDs, without obtaining its value in this application.
Since the number L of reference light pulses is set to L ≧ 3, and the modulation phases loaded on the first and second reference light pulses
Number M of 1 、M 2 Is set to satisfy M 1 *M 2 Greater than or equal to 3, therefore, can be based on
N 1l /k≈(1+cosγ ab (l) 2) uniquely determining the common phase difference
Wherein the content of the first and second substances,
for the modulation phase loaded on the l-th one of the first reference light pulses,
for the modulation phase, N, loaded on the l-th of the second reference light pulse 1l The response of the SPD to the interference signal of the ith first and second reference light pulses is counted, L1.
The number of photons detected by SPD1 may also fluctuate due to QKD system channel loss fluctuations, and so in a preferred example, this effect can be reduced/cancelled by accumulating and evaluating N cycles, and the common phase difference can be accurately estimated
N＞1。
In the preferred example, can be according to
Uniquely determining a common phase difference
Where kn is a constant and represents the total response count of the nth cycle, n is 1, 2,3......N；
The modulation phase loaded on the first reference light pulse in one period,
for the modulation phase loaded on the ith second reference light pulse in one period,
the responses of the SPD in the nth period with respect to the interference signals of the ith first and second reference light pulses are counted, L ═ 1.
In the invention, the value of N can be determined according to the phase fluctuation rate of a laser light source in a sending end, the channel phase fluctuation rate of a QKD system, the performance of a single-photon detector and other factors, the value can be usually measured by the specific QKD system, and in N time periods, the public phase difference is determined
No change occurs.
For a better understanding of the present invention, a specific embodiment of the present invention will be described further below with reference to fig. 3.
Fig. 3 shows a periodic signal for a certain transmitting end (e.g., Alice end) in a QKD system according to this particular embodiment.
As shown in fig. 3, the first periodic signal for Alice may include signal light pulses (i.e., quantum light signals), a first reference light pulse, and a vacuum state portion, wherein the number L of the first reference light pulses is 4, and M1 ═ 4 modulation phases 0, pi/2, pi and 3 pi/2 are loaded on the four first reference light pulses by the phase modulator PM, respectively; at the same time, Bob also generates L ═ 4 second reference optical pulses in its second periodic signal, and the same type (i.e., M) is loaded onto the four second reference optical pulses by means of phase modulator PM 2 1) modulates the phase pi.
Thus, in the first periodic signal,
and in the second periodic signal, while,
accordingly, in the Charlie end, interference result N generated by 4 first and second reference optical pulses, respectively, will be available by SPD1 11 、N 12 、N 13 And N 14 Wherein:
thus, according to N 11 And N 13 Can obtain
Because the cosine function is an even function, two functions can be obtained based on the relation
The value is obtained.
Further according to N 12 And N 14 Can obtain
At this time, it can be determined
And thus may be selected from the two mentioned
Determined to be unique in value
Value to thereby realize a common phase difference
And (4) estimating.
Similarly, the evaluation is preferably performed by means of an accumulation and evaluation of N periods
When it is worth, can be based on
And
uniquely estimating to obtain a common phase difference between the Alice terminal and the Bob terminal
Therefore, in the single-SPD-based TF-type QKD system, a transmitting end (Alice/Bob) generates a plurality of specific strong reference optical pulses, and a plurality of specific modulation phases are loaded on the reference optical pulses, so that the detection result obtained by detecting interference signals between the reference optical pulses by using the single SPD at the Charlie end by means of a specific algorithm can be used for accurately estimating the common phase difference in the QKD system
To assist in completing the coding of a TF-like QKD system with a single SPD. Therefore, the method fills the blank of realizing the key technology (public phase difference estimation) in the TF-type QKD by using a single SPD, is beneficial to the practicability and productization of the TF-type QKD system, and has lower complexity and cost compared with the prior TF-type QKD system realized by using two SPDs (which are superconducting nanowire single-photon detectors in certain use scenes with higher requirements on the code rate), and does not need to increase or change any hardware composition.
The invention also discloses an estimation method of the public phase difference in the TF type QKD system based on the single SPD, which is particularly suitable for being realized by the QKD system.
The estimation method of the present invention may include a periodic signal generation step, a phase modulation step, an interference step, and an estimation step.
The periodic signal generating step is used for generating a first periodic signal in the Alice terminal and a second periodic signal in the Bob terminal. The first periodic signal comprises L first reference light pulses, the second periodic signal comprises L second reference light pulses, and L is larger than or equal to 3.
In one embodiment, L may take the value of 4.
The phase modulation step is used for phase modulating the first reference light pulse by means of the phase modulator PM in the Alice terminal and phase modulating the second reference light pulse by means of the phase modulator PM in the Bob terminal.
Wherein M can be loaded on L first reference light pulses in one cycle 1 Modulation phase
Modulating phase
Within 0, 2 π). Similarly, in one cycle, M is loaded on L second reference light pulses 2 Modulation phase
Modulating phase
Within [0, 2 π) and M 1 *M 2 ≥3。
In a particular embodiment, one of the first and second reference light pulses (e.g., the first reference light pulse) may be made to have four modulation phases (e.g., M) 1 4) of 0, pi/2, pi and 3 pi/2, respectively, while causing the other of the first and second reference light pulses (e.g., the second reference light pulse) to have a modulation phase (e.g., M) 2 1) which is one of 0, pi/2, pi and 3 pi/2, for example pi.
The interference step is used for enabling a first periodic signal and a second periodic signal entering a Charlie end to generate interference signals through interference, and detecting the interference signals by using a single SPD to obtain an interference result N 1l ，l＝1、...、L。
An evaluation step for using the interference result N of the first and second reference light pulses 1l Estimating the common phase difference
In one example of the estimating step, the common phase difference may be estimated using the interference results within one cycle
Wherein:
N 1l /k≈(1+cosγ ab (l) Is 2, k is a constant,
for the modulation phase loaded on the l-th one of the first reference light pulses,
for modulation loaded on the l-th of the second reference light pulseSystem phase, N 1l The response of the interference signal of the L-th one of the first and second reference light pulses is counted, L-1.
In a preferred example of the estimating step, the common phase difference may be estimated using the interference results over a plurality (N) of cycles
Wherein:
kn is a constant number, and the number of the n,
the modulation phase loaded on the l-th one of the first reference light pulses in one period,
the modulation phase loaded on the ith of the second reference light pulse in one period,
the response of the interference signal of the L-th one of the first and second reference light pulses in the N-th cycle is counted, L ═ 1,. and L, N > 1.
Thus, in one embodiment, may be based on
Estimating to obtain a common phase difference
Or further, may be based on
Estimating to obtain a common phase difference
Although the present invention has been described in connection with the embodiments illustrated in the accompanying drawings, it will be readily understood by those skilled in the art that the above embodiments are exemplary only, serve to explain the principles of the invention and not to limit the scope of the invention, and that various combinations, modifications and equivalents of the above embodiments may be made by those skilled in the art without departing from the spirit and scope of the invention.
Claims (10)
1. A method for estimating public phase difference in a TF-type QKD system based on a single SPD comprises a periodic signal generation step, a phase modulation step, an interference step and an estimation step;
in the periodic signal generating step, a first periodic signal is generated in an Alice terminal, a second periodic signal is generated in a Bob terminal, the first periodic signal comprises L first reference light pulses, the second periodic signal comprises L second reference light pulses, and L is larger than or equal to 3;
in the phase modulation step, phase modulation is carried out on first reference light pulses in an Alice terminal, wherein M is arranged on the L first reference light pulses 1 Modulation phase
And modulates the phase
Within [0, 2 π); phase-modulating a second reference light pulse within Bob's end, wherein the L second reference light pulses have M thereon 2 Modulation phase
M 1 *M 2 Not less than 3 and modulating phase
Within [0, 2 π);
in the interference step, enabling a first periodic signal and a second periodic signal to enter a Charlie end and generate interference to generate an interference signal, detecting the interference signal by using a single SPD, and outputting an interference result;
in the estimating step, a common phase difference is estimated using the result of interference of the first reference light pulse and the second reference light pulse
2. The estimation method according to claim 1, wherein in the estimation step, the common phase difference is estimated using the interference result in one cycle
N 1l /k≈(1+cosγ ab (l) K is a constant),
for the modulation phase of the first reference light pulse,
for the modulation phase, N, of the l-th second reference light pulse 1l The responses of the interference signals of the first and second reference light pulses are counted, i.e. 1.
3. The estimation of claim 1A method wherein in the estimating step, the common phase difference is estimated using the interference results over N cycles
k n Is a constant number of times, and is,
the modulation phase of the first reference light pulse in one period,
the modulation phase of the l-th second reference light pulse in one period,
the responses of the interference signals of the first and second reference light pulses in the nth period are counted, i.e. 1,. and L, N > 1.
4. The estimation method according to claim 2 or 3, wherein L takes a value of 4, and one of the first and second reference light pulses has four modulation phases of 0, pi/2, pi and 3 pi/2, respectively, and the other of the first and second reference light pulses has one modulation phase of one of 0, pi/2, pi and 3 pi/2, within one period.
5. The estimation method according to claim 4,
l is 1, 2, 3, 4, and
or
6. A TF type QKD system based on a single SPD comprises an Alice end, a Bob end and a Charlie end;
the Alice terminal is configured to generate a first periodic signal having L first reference light pulses, and M is modulated on the L first reference light pulses 1 Modulation phase
Modulating phase
Within [0, 2 π);
the Bob terminal is configured to generate a second periodic signal having L second reference light pulses with M modulated thereon 2 Modulation phase
Modulating phase
In [0, 2 π), wherein L is not less than 3, M 1 *M 2 ≥3；
The Charlie terminal is configured to interfere the first and second periodic signals to generate an interference signal, detect the interference signal with a single SPD to obtain an interference result, and estimate a common phase difference using the interference result of the first and second reference optical pulses
7. The TF-like QKD system according to claim 6, wherein the periodic signal further includes a quantum optical signal.
8. The TF-like QKD system according to claim 6, wherein the Charlie end is configured for:
estimating common phase difference using interference results over one period
N 1l /k≈(1+cosγ ab (l) K is a constant),
for the modulation phase of the first reference light pulse,
for the modulation phase, N, of the l-th second reference light pulse 1l Counting responses of the interference signals of the first and second reference light pulses, i.e. 1,. and L; alternatively, the first and second electrodes may be,
estimating common phase difference using interference results over N cycles
k n Is a constant number of times, and is,
the modulation phase of the first reference light pulse in one period,
the modulation phase of the l-th second reference light pulse in one period,
the responses of the interference signals of the first and second reference light pulses in the nth period are counted, i.e. 1,. and L, N > 1.
9. The TF-like QKD system according to claim 6, wherein L has a value of 4 and, during a cycle, one of the first and second reference light pulses has four modulation phases, which are 0, pi/2, pi and 3 pi/2, respectively, and the other of the first and second reference light pulses has one modulation phase, which is one of 0, pi/2, pi and 3 pi/2.
10. The TF-like QKD system according to claim 6, arranged to estimate a common phase difference using an estimation method according to any of claims 1-5
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