A phase-compensated TF-QKD system

Abstract

This invention proposes a TF-QKD system that achieves phase compensation without relying on a phase modulator within the detector. By post-processing the (interference) detection results of two phase reference lights of different wavelengths, an estimate of the relative phase difference between the light source and the transmission link when the two transmitters transmit quantum signal light is obtained. This estimate allows for compensation of additional phase differences caused by the light source and link, ensuring the relative phase stability between the quantum signal lights. Using this invention, a real-time computing module is unnecessary, reducing the computational burden. Furthermore, the need for a phase modulator at the detector for phase compensation is eliminated, greatly simplifying the system and reducing optical losses caused by additional phase modulation.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of quantum communication, and more particularly to a TF-QKD system capable of realizing phase compensation without relying on a phase modulator in a detection end. BACKGROUND

[0002] Two-field quantum key distribution (TF-QKD) is a new type of quantum key distribution protocol, which has higher anti-interference, longer distribution distance and higher code rate than MDI-QKD. TF-QKD includes two transmitting ends Alice and Bob, and a detection end Charlie. In the protocol, Alice and Bob respectively modulate local lasers for encoding, and the encoded lasers are transmitted to Charlie through an optical fiber link to enter an optical interference module for interference, and the interfered light enters a single-photon detector for measurement.

[0003] In the TF-QKD system, phase encoding is usually used, and to realize stable interference, the phase difference between Alice and Bob needs to be known. However, the phase is related to the length of the optical fiber link, and the length of the optical fiber link will change over time. Therefore, a phase reference light needs to be sent, and the phase difference is calculated or fed back through detection of the reference light. Most of the existing schemes feed back the phase difference in real time according to the detection result of the phase reference light. However, real-time feedback requires more calculation, so additional hardware support such as a PID operation module is needed, which makes the system more complex; real-time feedback needs to change the phase of the input light in real time, so additional optoelectronic modulation devices are needed, which will cause more optical loss; real-time feedback also has higher requirements for bandwidth and count rate of the reference light. SUMMARY

[0004] In view of the above defects of the prior art, the present application provides a TF-QKD system capable of phase compensation, wherein the detection results of two wavelengths of phase reference light are data-processed to obtain an estimation of the relative phase difference between the light sources and the transmission link when the two transmitting ends send quantum signal light, and then the additional phase difference caused by the light sources and the link is compensated according to the estimation of the relative phase difference, so as to ensure the stability of the relative phase between the quantum signal lights. With the scheme of the present application, no real-time calculation module is needed, the calculation burden is reduced, and no phase modulation for phase compensation needs to be set in the detection end, so that the system can be greatly simplified, and the optical loss caused by additional phase modulation can also be reduced.

[0005] Specifically, the present application relates to a TF-QKD system capable of phase compensation, which includes transmitting ends Alice and Bob, and a detection end Charlie.

[0006] The transmitters Alice and Bob are configured to generate a strong phase reference light of wavelength λ1 and combine it with a quantum signal light of wavelength λ2 using wavelength division multiplexing, and to generate a weak phase reference light of wavelength λ2 and time-division multiplex it with the quantum signal light. The strong phase reference lights from Alice and Bob are modulated with multiple known phase differences δθ. i The weak phase reference light emitted by Alice and Bob is modulated with multiple known phase differences.

[0007] The detection terminal Charlie includes a detection module and a data recording and processing module;

[0008] The detection module includes detectors A1, A2, B1 and B2, wherein detectors A1 and A2 are configured to detect strong phase reference light interference results, and detectors B1 and B2 are configured to detect weak phase reference light interference results and quantum signal light interference results.

[0009] The data recording and processing module is configured to record the detection counts of detectors A1, A2, B1, and B2 and their corresponding times; according to Calculation makes Take the minimum value This is denoted as the period T. A Corresponding strong phase reference optical link phase difference

[0010] p i =N1 / (N1+N2), where N1 and N2 are the statistical periods T, respectively. A The modulation phase difference δθ on internal detectors A1 and A2 i The corresponding count; according to Calculation makes Take the minimum value Set it to the statistical period T B Corresponding initial phase difference n Δψ For the statistical period T B The count on the internal detector B1 corresponding to the phase difference Δψ And, calculate the statistical period T. A Weak phase reference optical link phase difference And according to For the corresponding statistical period T A Phase compensation is performed on the quantum signal light inside, and c takes the value of 1 or λ1 / λ2;

[0011] Among them, the strong phase reference light has a stronger light intensity than the weak phase reference light, and the statistical period T B Greater than the statistical period T A .

[0012] Further, the transmitting end Alice and Bob each include a first and a second light source with wavelengths λ1 and λ2 respectively, a first and a second modulation light path, and a wavelength division multiplexing unit;

[0013] The first light source is configured to generate light signals with wavelength λ1;

[0014] The second light source is configured to generate light signals with wavelength λ2;

[0015] The first modulation light path of the transmitting end Alice and Bob is configured to modulate phases θ A and θ B on the light signals with wavelength λ1 synchronously according to a known phase sequence with a preset encoding period T1, to generate strong phase reference light;

[0016] The second modulation light path of the transmitting end Alice and Bob is configured to modulate phases θ and θ on the light signals with wavelength λ2 synchronously according to a known phase sequence with a preset encoding period T2, to generate weak phase reference light; and to modulate phases randomly on the light signals with wavelength λ2 synchronously, to generate quantum signal light;

[0017] The wavelength division multiplexing unit is configured to combine the strong phase reference light with the weak phase reference light and the quantum signal light.

[0018] Preferably, the phase difference δθ i and is selected from {0, π / 2, π / 2, 0}. Wherein θ A is selected from {0, 0, π, π), θ B is selected from {0, π / 2, π / 2, 0}; and / or, is selected from {0, 0, π, π), is selected from {0, π / 2, π / 2, 0}.

[0019] Further, the first modulation light path includes a first phase modulator and a first optical attenuator;

[0020] The second modulation light path includes a second phase modulator, a second intensity modulator and a second optical attenuator.

[0021] Preferably, the first modulation light path further includes a first intensity modulator configured to perform extinction processing on the strong phase reference light in a time period corresponding to the quantum signal light.

[0022] Further, the data recording processing module is further configured to take θ By iterating through the values, we can find the one that makes... Take the minimum value and / or make Take the minimum value

[0023] Preferably, the data recording and processing module is further configured to pre-calculate... and The lookup table, and use the lookup table to find the... Take the minimum value and / or make Take the minimum value

[0024] Furthermore, the data recording and processing module is also configured to be used when c takes the value λ1 / λ2, and two adjacent statistical periods T A Strong phase reference optical link phase difference and satisfy When, the i-th statistical period T A Strong phase reference optical link phase difference Set as m i The integer is m1 = 0. To estimate the i-th statistical period T A The phase difference of the strong phase reference optical link.

[0025] Furthermore, the detector Charlie does not contain a phase modulator.

[0026] Preferably, a time interval is provided between adjacent quantum signal lights and weak phase reference lights. Attached Figure Description

[0027] Figure 1 The transmitter for a TF-QKD system according to the present invention is shown schematically.

[0028] Figure 2 The probe end for a TF-QKD system according to the present invention is schematically shown;

[0029] Figure 3 An example of a data recording and processing module in the probe end according to the present invention is shown schematically;

[0030] Figure 4(a) and 4(b) A preferred example of the encoding period T1 for a strong phase reference light according to the present invention is shown;

[0031] Figure 5(a) and 5(b)An example of encoding periods T1 and T2 according to the present application is shown;

[0032] Figure 6 An example of statistical periods T A and T B according to the present application is shown. DETAILED DESCRIPTION

[0033] In the following, exemplary embodiments of the present application will be described in detail with reference to the accompanying drawings. The following embodiments are provided by way of example in order to fully convey the spirit of the present application to those skilled in the art to which the present application pertains. Therefore, the present application is not limited to the embodiments disclosed herein.

[0034] In the TF-QKD system of the present application, two wavelengths of phase reference light will be used to achieve compensation for the phase introduced in the transmission link, wherein a strong phase reference light of a different wavelength from the quantum signal light and with a higher light intensity is used for fast phase compensation, and a weak phase reference light of the same wavelength as the quantum signal light and with a lower light intensity is used for slow compensation for the remaining phase after compensation by the strong phase reference light, and there is no need to specially set a phase modulator in the detection end Charlie to provide the required phase compensation. In this way, not only can the problem of secondary Rayleigh scattering noise in the single reference light scheme and the strong post-pulse problem of the reference light caused by time division multiplexing be avoided, but also the Raman scattering noise introduced by the strong phase reference pulse can be avoided by setting a switch modulation of the same frequency as the quantum signal light for the strong phase reference pulse, and the optical loss caused by additional phase modulation can be reduced.

[0035] Specifically, Figure 1 An example of a transmitting end of the TF-QKD system according to the present application is shown.

[0036] As Figure 1 shown, each transmitting end Alice (Bob) is provided with first and second light sources with wavelengths λ1 and λ2, respectively, first and second modulation light paths corresponding to the two light sources, respectively, and a wavelength division multiplexing unit.

[0037] The first modulation light path is used to modulate the light signal output by the first light source with wavelength λ1 to generate time division multiplexed quantum signal light and weak phase reference light, and can include a first phase modulator PM for phase modulation of the light signal and a first optical attenuator ATT for attenuation of the light signal.

[0038] The first phase modulator can phase modulate the light signal according to a known phase sequence to generate the weak phase reference light, and randomly phase modulate the light signal to generate the quantum signal light, thereby achieving time division multiplexing of the weak phase reference light and the quantum signal light.

[0039] Preferably, an intensity modulator IM can also be arranged in the first modulation optical path for intensity modulating the optical signal, for example, implementing extinction processing on the optical signal so as to form a time interval between the adjacent weak phase reference light and the quantum signal light, and reducing the influence of the weak phase reference light on the quantum signal light.

[0040] The second modulation optical path is used for modulating the optical signal output by the second light source with wavelength λ2 to generate the strong phase reference light, and can include a second phase modulator PM, a second intensity modulator IM and a second optical attenuator ATT.

[0041] The second phase modulator can phase modulate the optical signal according to a known phase sequence so as to generate the strong phase reference light.

[0042] The second intensity modulator is used for intensity modulating the optical signal, for example, implementing extinction processing on the optical signal in the time period corresponding to the quantum signal light, so as to reduce the influence of the strong phase reference light on the quantum signal light.

[0043] The wavelength division multiplexing unit is used for combining the weak phase reference light and the quantum signal light with wavelength λ1 and the strong phase reference light with wavelength λ2 so as to be transmitted to the detection end Charlie through the same optical fiber line.

[0044] Figure 2 An example of the detection end of the TF-QKD system according to the present application is shown.

[0045] As shown in Figure 2 , the detection end can include two receiving optical paths respectively for the combined beams from Alice and Bob, an interference module, a detection module and a data recording processing module (not shown).

[0046] The first receiving optical path can include a first polarization control module for adjusting the polarization state of the combined beam from Alice, and a first polarizing module for polarizing the combined beam.

[0047] Similarly, the second receiving optical path can include a second polarization control module for adjusting the polarization state of the combined beam from Bob, and a second polarizing module for polarizing the combined beam.

[0048] The interference module is used for allowing the two combined beams output by the receiving optical paths to interfere and output the interference result. Since the combined beams include optical signals with two wavelengths λ1 and λ2, the interference result output by the interference module also correspondingly includes interference results with two wavelengths λ1 and λ2.

[0049] As an example, the interference module can include a beam splitter BS.

[0050] The detection module can include a first demultiplexing unit and detectors A1 and B1 connected thereto, and a second demultiplexing unit and detectors A2 and B2 connected thereto.

[0051] The first demultiplexing unit is configured to split the λ1 wavelength part and the λ2 wavelength part in one of the interference results output by the interference module according to the wavelengths, so that the detection result of the wavelength λ1 is detected by the detector A1 and the detection result of the wavelength λ2 is detected by the detector B1.

[0052] Similarly, the second demultiplexing unit is configured to split the λ1 wavelength part and the λ2 wavelength part in another of the interference results output by the interference module according to the wavelengths, so that the detection result of the wavelength λ1 is detected by the detector A2 and the detection result of the wavelength λ2 is detected by the detector B2.

[0053] As an example, the demultiplexing unit can include a dense wavelength division multiplexer (DWDM).

[0054] Figure 3 An example of a data recording processing module in a detection end according to the present application is shown, which includes a synchronization counter and a data processing module.

[0055] The synchronization counter is configured to collect the counts of the detectors A1, A2, B1 and B2, respectively, and is attached with a synchronization clock, and can record the time when the detection count occurs.

[0056] The data processing module is configured to perform corresponding phase compensation using the phase compensation method of the present application according to the counts of the detectors.

[0057] The phase compensation method for TF-QKD of the present application will be further described below with reference to FIGS. 4-6, so as to better understand the working principle of the above-mentioned TF-QKD system.

[0058] In the phase compensation method of the present application, first, the two kinds of wavelength optical signals in the transmission end need to be encoded by means of a reference light encoding step to generate the required strong phase reference light, weak phase reference light and quantum signal light. Those skilled in the art can understand that this step can be performed by the transmission end, for example, the required phase encoding can be realized by means of a modulation light path in the transmission end.

[0059] The transmission end Alice / Bob end can respectively encode the strong phase reference light of wavelength λ1 according to an encoding period T1. In one encoding period T1, the transmission ends Alice and Bob can respectively use a phase modulator to phase modulate the strong phase reference light therein, so that a plurality of (relative) phase differences δθ i, i = 1, 2,.... Wherein, the strong phase reference light can be phase-modulated according to a known phase sequence, and thus the phase difference δθ i and the time of occurrence thereof are known.

[0060] Typically, in one encoding period T1, four phase differences δθ i , for example, {0, -π / 2, π / 2, π}.

[0061] In a preferred example, the transmitting end Alice can modulate the phase θ A = {0, 0, π, π} on the strong phase reference light, and the transmitting end Bob modulates the phase θ B = {0, π / 2, π / 2, 0} on the strong phase reference light, so as to realize four phase differences δ θ = θ A - θ B = {0, -π / 2, π / 2, π}, and vice versa. With this encoding mode, the transmitting ends Alice and Bob only need to encode two phases, and the encoded phase is not more than π, so as to avoid excessively high requirements on the modulation voltage of the phase modulator.

[0062] Preferably, the encoding period T1 can include a strong phase reference light segment and an extinction segment, wherein the required phase modulation is performed on the strong phase reference light segment, and the strong phase reference light is subjected to extinction processing by means of an intensity modulator on the extinction segment. Correspondingly, the weak phase reference light can have an encoding period T2 corresponding to the encoding period T1, wherein the encoding period T2 can include a weak phase reference light segment and a quantum signal light segment, the required phase modulation is performed on the weak phase reference light segment, and the light signal with a wavelength of λ2 is subjected to random phase encoding in the quantum signal light segment to generate quantum signal light. And in each encoding period, the strong phase reference light segment in the encoding period T1 corresponds to the weak phase reference light segment in the encoding period T2, and the extinction segment in the encoding period T1 corresponds to the quantum signal light segment in the encoding period T2, so as to reduce the interference of the strong phase reference light on the quantum signal light.

[0063] Typically, the encoding period T1 can be selected as 1 μs or 100 ns, etc. For example, when the encoding period T1 is 1 μs, the first 400 ns can be set as a strong phase reference light segment, and the last 600 ns can be set as an extinction segment. In the first 400 ns of each encoding period T1, the transmitting end Alice / Bob performs phase modulation on the light signal with a wavelength of λ1 to generate four phase differences, wherein each phase difference is in a span of 100 ns.

[0064] Alternatively, the entire coding period T1 can be used for phase modulation of the strong phase reference light. For example, four phase differences can be generated within each coding period T1, each phase difference occurring within a quarter of the coding period's time span.

[0065] Figure 4(a) and 4(b) A preferred example of the encoding period T1 for the strong phase reference light is shown, wherein the first 400 ns of the encoding period T1 is set as the strong phase reference light segment, and in the strong phase reference light segment, the transmitter Alice is based on θ A Phase modulation is performed on {0, 0, π, π}, and the transmitter Bob modulates the phase according to θ. B Phase modulation is performed using {0, π / 2, π / 2, 0}, thereby achieving four phase differences δ between the two strong phase reference beams. θ =θ A -θ B = {0, -π / 2, π / 2, π}.

[0066] The Alice / Bob transmitters can encode the optical signal with wavelength λ2 according to a coding period T2. Within one coding period T2, Alice and Bob can respectively use phase modulators to modulate the weak phase reference light, thereby generating multiple (relative) phase differences between the weak phase reference lights of the two transmitters. In this method, the weak-phase reference light can be phase-modulated based on a known phase sequence; therefore, the phase difference between the weak-phase reference lights... Its occurrence time is known.

[0067] Typically, within one coding period T2, four phase differences can be generated between weak phase reference lights. For example, {0, -π / 2, π / 2, π}.

[0068] In a preferred example, the transmitter Alice can modulate the phase on a weak phase reference light. Bob modulates the phase on the weak phase reference light at the transmitter. This allows for four phase differences {0, -π / 2, π / 2, π} between two weak-phase reference beams, and vice versa. With this encoding method, both Alice and Bob at the transmitter only need to encode two phases, and the encoded phase does not exceed π, thus avoiding excessively high requirements on the modulation voltage of the phase modulator.

[0069] Preferably, the encoding period T2 for the optical signal with wavelength λ2 can be set synchronously with the encoding period T1.

[0070] Within an encoding period T2, the weak phase reference light and the quantum signal light can be time-division multiplexed. Therefore, the encoding period T2 can include a weak phase reference light segment and a quantum signal light segment. The required phase modulation is performed on the weak phase reference light segment, and random phase encoding is performed on the optical signal with wavelength λ2 in the quantum signal light segment to generate the quantum signal light. The intensity of the weak phase reference light is typically set to be no less than that of the quantum signal light, for example, to ensure that at least 100 counts can be obtained within the test period at the detector. The intensity of the quantum signal light needs to be selected at the single-photon level according to the requirements of the quantum key distribution protocol.

[0071] Furthermore, within the corresponding coding periods T1 and T2, the same phase sequence can be used on the strong phase reference light segment and the weak phase reference light segment, thereby allowing synchronous coding of the strong phase reference light and the weak phase reference light to be achieved using the same phase modulator.

[0072] Typically, the encoding period T1 can be selected as 1 μs, where the first 400 ns is set as a weak phase reference optical segment, used to phase modulate the optical signal with wavelength λ2 according to a known phase sequence, for example, Alice phase modulation. Bob modulation phase Each phase modulation spans 100 ns, correspondingly achieving four phase differences {0, -π / 2, π / 2, π}; the last 600 ns is designated as the quantum signal light segment, used to randomly modulate the phase of an optical signal with a wavelength of λ2.

[0073] Preferably, a certain time interval can be set between adjacent weak-phase reference lights and quantum signal lights within the same encoding period T2, for example, to separate the weak-phase reference lights from the quantum signal lights and avoid crosstalk between the reference lights and the quantum signal lights. For example, a waiting time can be inserted at the end of the weak signal reference light as a time interval.

[0074] Figure 5(a) and 5(b) An example of encoding periods T1 and T2 according to the present invention is shown, wherein: the strong phase reference light segment of encoding period T1 and the weak phase reference light segment of encoding period T2 use the same phase sequence to encode the strong phase reference light and the weak phase reference light synchronously; the optical signal with wavelength λ2 is randomly phase encoded in the quantum signal light segment of encoding period T2; a waiting time is provided between the quantum signal light and the weak phase reference light; and the optical signal with wavelength λ1 is extincted in the time period corresponding to the quantum signal light segment in encoding period T1.

[0075] After the transmitter generates and outputs a combined beam comprising a strong phase reference light, a weak phase reference light, and a quantum signal light, the detector (Charlie) needs to use a detection event recording step to detect and record the interference result between the quantum signal light and the phase reference light, as well as the corresponding detection events and their occurrence times. Those skilled in the art will understand that this step can be implemented by a detection module and a data recording and processing module.

[0076] As previously described, in the TF-QKD system of the present invention, detectors A1 and A2 detect the interference results of the strong phase reference light, and detectors B1 and B2 detect the interference results of the weak phase reference light and the interference results of the quantum signal light.

[0077] Since the intensity of the optical signal at wavelength λ1 is much higher than that at wavelength λ2, there are different requirements for the detection rates of detectors A (i.e., A1 and A2) and B (i.e., B1 and B2). Typically, the detection rate of detector A is required to be much higher than that of detector B. Specifically, the intensity of the optical signal at wavelength λ1 can be adjusted to allow for accurate estimation of the link phase difference using a strong phase reference light; for example, the detection rate of detector A can be controlled above 0.3 MHz. The intensity of the optical signal at wavelength λ2 can be determined jointly by the quantum signal light and the weak phase reference light, typically ensuring that detector B achieves a detection rate of over 100 for the weak phase reference light within a typical statistical time (e.g., 100 ms).

[0078] Since the strong phase reference light has high intensity and will be used for rapid estimation of the link phase, the detection event statistical period T for detector A is therefore... A The statistical period T is relatively short and can be selected to ensure that the phase difference caused by link phase changes within the same statistical period and between adjacent statistical periods is sufficiently small (e.g., π / 10 level) or that the impact on the system error rate is acceptable. In real-world fiber optic environments and under the set count rate conditions, the statistical period T... A The time can usually be selected between 5μs and 100μs, with 20μs being the preferred option.

[0079] Since the weak phase reference light has a small intensity and will be used to compensate for the remaining link phase difference, the detection event statistical period T for detector B is... B The relatively long duration can be selected to satisfy the condition that, within a statistical period, after phase compensation using a strong phase reference light of wavelength λ1, the change in the remaining phase difference (i.e., the phase difference of λ2 minus the phase difference of λ1) on the optical signal of wavelength λ2 is much smaller than π.

[0080] Statistical period T B It can typically be set to include multiple statistical periods T. A Optionally, the statistical period T B You can choose 100ms.

[0081] Figure 6 The statistical period T is shown. A and T B An example where the statistical period T B Includes multiple statistical periods T A Each statistical period T A It corresponds to multiple encoding periods T1 / T2.

[0082] In the event recording step, the synchronization counter collects the detection counts (events) of detectors A1, A2, B1 and B2 respectively and records the occurrence time of the corresponding detection counts. Then, it accumulates them separately according to different time periods within the statistical period of the detection count (corresponding to the phase difference of different phase reference lights).

[0083] Specifically, for detectors A1 and A2, the statistical period T A It contains multiple coding periods T1, each coding period T1 containing different modulation phase differences δθ. i Multiple time periods, for example, corresponding to 0, -π / 2, π / 2, and π respectively. Therefore, the statistical period T can be taken. A All phase differences δθ i For time intervals of 0, -π / 2, π / 2, and π, the counts within each time interval are counted and summed to obtain the phase difference δθ on detector A1. i The corresponding detection count N1 i and the phase difference δθ on detector A2 i The corresponding detection count N2 i .

[0084] For detectors B1 and B2, the statistical period T B It contains multiple coding cycles T2, each coding cycle T2 containing a phase reference optical segment and a quantum signal optical segment. The phase reference optical segment contains segments corresponding to different modulation phase differences. Multiple time periods, for example, corresponding to four time periods: 0, -π / 2, π / 2, and π. Therefore, for the phase reference optical segment, the modulation phase difference corresponding to each detection event (count) can be recorded. (e.g., 0, -π / 2, π / 2, π) and the corresponding recording time, so as to subsequently look up and calculate the corresponding count and link phase difference of the strong phase reference light within the corresponding time period. For the quantum signal light segment, the statistical period T in which each detection event (count) occurs can be recorded. A and T B That is, in which statistical period T does the detection event occur? A and T B This is to facilitate subsequent phase compensation.

[0085] Based on the statistical data of the probe events, the phase changes on the link can be estimated using strong phase reference light estimation steps and weak phase reference light estimation steps, respectively. Those skilled in the art will understand that the estimation steps can be implemented by the data recording and processing module in the probe's Charlie.

[0086] Assume that when the optical signal (strong phase reference light) with wavelength λ1 from the transmitter Alice reaches the detector (before interference occurs), the link phase experienced is... The link phase experienced by the optical signal with wavelength λ2 (weak phase reference light / quantum signal light) is... The link phase experienced by the optical signal with wavelength λ1 from transmitter Bob is The link phase experienced by the optical signal with wavelength λ2 is When interference occurs, the link phase difference of the optical signal with wavelength λ1 is: The link phase difference of an optical signal with wavelength λ2 is

[0087]

[0088] Due to link phase difference and It changes with time t, therefore, at time t, and They can be denoted as and For an optical signal with a wavelength of λ2, in each statistical period T B At the initial time t=0, the initial phase difference of the two wavelength links

[0089] According to simple calculations based on interference theory, at the detector end Charlie, the total phase difference between the two optical signals from the transmitter end is: At that time, the normalized intensity of the interference output is:

[0090]

[0091] At this point, the normalized intensity This indicates that the total phase difference between the two interfering optical signals is... At that time, the probability that each photon will be detected by detector A1 or B1 after interference.

[0092] In one example of the present invention, at a certain moment, the link phase difference corresponding to the wavelength λ2 is... The following formula (2) can be used for estimation:

[0093]

[0094] In another preferred example, taking into account the wavelength difference between λ1 and λ2, the link phase difference corresponding to the wavelength λ2 is... A more accurate estimate can be made using the following formula (3):

[0095]

[0096] From formula (2) or 3 (3), it can be seen that in order to achieve the link phase difference The estimation of all requires the use of a strong phase reference light estimation step, within the statistical period T. A Internally, the link phase difference corresponding to wavelength λ1 (strong phase reference light) is estimated using a strong phase reference light. And by using the weak phase reference light estimation step, in the statistical period T B Internally, the weak phase reference light is used to adjust the initial phase difference. Make an estimate.

[0097] In the strong phase reference light estimation step, the error function will be used. The maximum probable phase difference is estimated using an optimization method, where:

[0098]

[0099] p i The phase difference modulated between the strong phase reference lights of Alice and Bob is δθ, respectively. i (For example, when δθ1=0, δθ2=π / 2, δθ3=π, δθ4=-π / 2), the probability of each λ1 photon being detected at detector A (i.e., As an example, if in the statistical period T... A Within, corresponding to a phase difference of δθ i If the detection count of detector A1 is N1 and the detection count of detector A2 is N2, then...

[0100] Guess the phase difference by adjusting / traversing. Calculate separately Find the function When taking the minimum value As corresponding to the statistical period T A Link phase difference The estimate.

[0101] For example, with an accuracy of 1°, Set the angles sequentially to 0°, 1°, ..., 359°, calculate and record each... Corresponding Values, select the smallest one. of As a measure of link phase difference The estimate.

[0102] Preferably, it can be calculated in advance. hour The values ​​are used to create a lookup table. In the calculation function... When the value is obtained, the value is referenced from the lookup table, thereby speeding up the calculation.

[0103] In the weak phase reference light estimation step, it is assumed that the phase difference between the two λ2 wavelength light signals from Alice and Bob before interference is... It is the link phase difference corresponding to the λ2 wavelength. With modulation phase difference The sum of the total phase differences. Phase difference from the initial phase The difference is denoted as Δψ. By examining the number of detected events corresponding to different Δψ values, we can compare Δψ with... Relationship, to conduct The estimate.

[0104] based on It can be derived from formula (2):

[0105]

[0106] Alternatively, it can be derived from formula (3):

[0107]

[0108] Therefore, it can be seen that the link phase difference corresponding to wavelength λ1 at a certain time can be used as a basis. and the modulation phase difference corresponding to the λ2 wavelength The corresponding time interval Δψ is calculated.

[0109] Because of the estimation step obtained using a strong phase reference light Within the range [0, 2π), and since λ1 / λ2 is not equal to 1, when using formula (6), we can... Make corrections to The value of is expanded to the full range. Specifically, when the strong phase reference light estimation step is used, the statistical periods T of two adjacent periods (e.g., the i-th and i-1-th) are calculated. A Link phase difference The change between them is less than π, that is Then the phase domain can be extended, that is: if it corresponds to the i-th statistical period T A Estimated link phase difference for Then the i-th statistical period TA The link phase difference is set to m i The integer is m1 = 0.

[0110] Furthermore, by utilizing the link phase difference corresponding to wavelength λ1 and the modulation phase difference corresponding to wavelength λ2 After calculating Δψ according to formula (5) or (6), the portion of the calculated Δψ within [0, 2π) can be discretized (for example, with a precision of 1°), and then the result can be processed in the corresponding statistical period T. B Within each discretized Δψ (e.g., Δψ = 0, ..., 359°), the number of detection events of detector B1 is counted, and the counts are accumulated to obtain the detection count n corresponding to each Δψ. Δψ .

[0111] Based on this, the following error function can be used similarly. Estimating the initial phase difference

[0112]

[0113] Similarly, the phase difference can also be guessed by adjusting / traversing. Calculate separately Find the function When taking the minimum value As for the statistical period T B Inner initial phase difference The estimate.

[0114] For example, with an accuracy of 1°, Set the angles sequentially to 0°, 1°, ..., 359°, calculate and record each... Corresponding Values, select the smallest one. of As for the initial phase difference The estimate.

[0115] Preferably, it can be calculated in advance. hour The values ​​are used to create a lookup table. In the calculation function... When the value is obtained, the value is referenced from the lookup table, thereby speeding up the calculation.

[0116] Finally, in the phase compensation step, the estimated statistical period T can be used. B within and the statistical period T B The statistical periods T included A of According to formula (2) or (3), the statistical period T is obtained. A within The estimated value, and then used by The estimated value for the same statistical period T A Phase compensation is performed on the internal quantum signal light, for example, by correcting the measurement basis of the quantum signal light, thereby compensating for the detection result of the quantum signal light. This eliminates the need for a phase modulator for phase compensation within the detector end Charlie, simplifying the system structure and reducing losses caused by phase modulation.

[0117] In summary, this invention proposes a phase compensation scheme for TF-QKD systems using two wavelengths of phase reference light. By post-processing the (interference) detection results of the two wavelengths of phase reference light, an estimate of the relative phase difference between the light source and the transmission link when the two transmitters send quantum signal light is obtained. This estimate allows for compensation of the additional phase difference caused by the light source and the link, ensuring the relative phase stability between the quantum signal lights. Using this scheme, a real-time computing module is not required, reducing the computational burden. Furthermore, the need for a phase modulator at the detector end to provide the required phase compensation is eliminated, greatly simplifying the system and reducing optical losses caused by additional phase modulation.

[0118] Although the present invention has been described above with reference to the accompanying drawings and specific embodiments, those skilled in the art will readily recognize that the above embodiments are merely exemplary and used to illustrate the principles of the present invention. They do not limit the scope of the present invention. Those skilled in the art can make various combinations, modifications and equivalent substitutions to the above embodiments without departing from the spirit and scope of the present invention.

Claims

1. A TF-QKD system capable of phase compensation, comprising a transmitting end Alice and Bob, and a detecting end Charlie; The transmitting end Alice and Bob are arranged to generate strong phase reference light of wavelength λ1 and output in wavelength division multiplexing manner with quantum signal light of wavelength λ2, and generate weak phase reference light of wavelength λ2 and time division multiplex with quantum signal light, wherein, The strong phase reference light between the transmitting end Alice and Bob is modulated with multiple known phase differences δθ i The weak phase reference light between the transmitting end Alice and Bob is modulated with multiple known phase differences The detecting end Charlie comprises a detecting module and a data recording processing module; The detecting module comprises detectors A1, A2, B1 and B2, wherein the detectors A1 and A2 are arranged to detect the interference result of strong phase reference light, and the detectors B1 and B2 are arranged to detect the interference result of weak phase reference light and quantum signal light; The data recording and processing module is configured to record the detection counts of detectors A1, A2, B1, and B2 and their corresponding times; according to Calculation makes Take the minimum value This is denoted as the period T. A Corresponding strong phase reference optical link phase difference p i =N1 / (N1+N2), where N1 and N2 are the statistical periods T, respectively. A The modulation phase difference δθ on internal detectors A1 and A2 i The corresponding count; according to Calculation makes Take the minimum value Set it to the statistical period T B Corresponding initial phase difference n Δψ For the statistical period T B The count on the internal detector B1 corresponding to the phase difference Δψ And, calculate the statistical period T. A Weak phase reference optical link phase difference And according to For the corresponding statistical period T A Phase compensation is performed on the quantum signal light inside, and c takes the value of 1 or λ1 / λ2; Wherein, the strong phase reference light has stronger light intensity than the weak phase reference light, and the statistical period T B greater than the statistical period T A . 2.The TF-QKD system of claim 1, wherein, The transmitting end Alice and Bob each comprise a first light source and a second light source with wavelengths of λ1 and λ2 respectively, a first modulation light path and a second modulation light path, and a wavelength division multiplexing unit; The first light source is used to generate light signals with a wavelength of λ1; The second light source is used to generate light signals with a wavelength of λ2; The first modulation light paths of the transmitting end Alice and Bob are respectively arranged to modulate the phase θ on the optical signal with wavelength λ1 synchronously according to the known phase sequence in the preset encoding period T1 A and θ B to generate strong phase reference light; The second modulation light path in the transmitting end Alice and Bob is respectively arranged to modulate the phase on the optical signal with wavelength λ2 according to the known phase sequence synchronously according to the preset encoding period T2 and to generate a weak phase reference light; and modulate the phase on the optical signal with wavelength λ2 randomly synchronously to generate a quantum signal light; The wavelength division multiplexing unit is arranged to combine the strong phase reference light with the weak phase reference light and the quantum signal light.

3. The TF-QKD system of claim 2, wherein, Phase difference δθ i And is selected from {0, -π / 2, π / 2, π}. 4.The TF-QKD system of claim 3, wherein, θ A selected from {0, 0, π, π}, θ B selected from {0, π / 2, π / 2, 0}; and / or, selected from {0, 0, π, π}, selected from {0, π / 2, π / 2, 0}. 5.The TF-QKD system of claim 2, wherein, The first modulation light path comprises a first phase modulator and a first optical attenuator; The second modulation light path comprises a second phase modulator, a second intensity modulator and a second optical attenuator.

6. The TF-QKD system of claim 5, wherein, The first modulation light path further comprises a first intensity modulator arranged to perform extinction processing on the strong phase reference light in a time period corresponding to the quantum signal light.

7. The TF-QKD system of claim 1, wherein, The data recording and processing module is also configured to process data in the range of 0 to 2π. By iterating through the values, we can find the one that makes... Take the minimum value and / or make Take the minimum value 8. The TF-QKD system of claim 7, wherein, The data record processing module is further configured to calculate in advance a look-up table of and to use the look-up table to find the value of that minimizes and / or the value of that minimizes ​ 9. The TF-QKD system of claim 7 or 8, wherein, The data recording and processing module is also configured to handle situations where c takes the value λ1 / λ2, and two adjacent statistical periods T A Strong phase reference optical link phase difference and satisfy When, the i-th statistical period T A Strong phase reference optical link phase difference Set as m i The integer is m1 = 0. To estimate the i-th statistical period T A The phase difference of the strong phase reference optical link.

10. The TF-QKD system of claim 1, wherein, No phase modulator is arranged in the detecting end Charlie.

11. The TF-QKD system of claim 1, wherein, A time interval is arranged between adjacent quantum signal light and weak phase reference light.

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