Quantum key distribution system and method tolerant to high-noise link

Abstract

Disclosed in the present invention are a quantum key distribution (QKD) system and method tolerant to a high-noise link. Particular ultra-narrow bandwidth configuration is performed in both a QKD device transmitting terminal and a QKD device receiving terminal, so that a capability of tolerating a high-noise link can be provided for the QKD system, especially in a scenario where a classical optical communication device and a QKD device have a small wavelength interval of 25 GHz or integer multiples thereof. Noise can be reduced without the need to lower the power of a classical optical signal or to configure the classical optical signal and a quantum optical signal on two wavebands having a relatively large interval, and there is no need to impose excessive restrictions on the power and wavelength of the classical optical communication device, greatly facilitating practical engineering applications; and both the quantum optical signal and the classical optical signal can achieve relatively small link attenuation, so that the present invention is applicable to long-distance scenarios.

Description

Quantum key distribution system and method tolerating high-noise link

[0001] The present application claims priority to the Chinese patent application No. 2024109617034, filed on July 17, 2024, entitled “Quantum key distribution system and method tolerating high-noise link”, the entire content of which is incorporated herein by reference. TECHNICAL FIELD

[0002] The present application relates to the field of quantum information technology, and in particular to a quantum key distribution system and method tolerating high-noise link. BACKGROUND

[0003] A quantum key distribution (QKD) system, based on the principles of quantum non-cloning and uncertainty, can provide high-level information security protection against high-power attacks. Due to the very weak signal light in the quantum key distribution process, such as single-photon level, the weak noise signal in the link has a significant impact on the signal light when transmitting from the transmitting terminal to the receiving terminal through the link. Therefore, early quantum key distribution systems often need to use dedicated fiber links separately to avoid the impact of other optical signals on the signal light.

[0004] In order to save link resources, related technologies for transmitting classical optical communication signals and quantum key distribution signals in the same link have been developed. Currently, the following measures are mainly used to realize quantum signal and classical signal multiplexing fiber transmission: first, attenuating the classical optical signal to reduce the noise it produces; second, increasing the wavelength interval between the classical optical signal and the quantum optical signal to reduce the noise; third, adding a narrowband filter at the receiving terminal to filter out part of the noise.

[0005] Figure 1 shows a quantum signal and classical signal multiplexing fiber transmission scheme in the prior art, in which an optical intensity self-adaptive adjustment device 110 is arranged in communication with the QKD receiving terminal 6 between the classical signal input module 1 and the first wavelength division multiplexer 104, so that the optical intensity self-adaptive adjustment device 110 automatically adjusts the attenuation coefficient to attenuate the classical signal according to the noise feedback information of the QKD receiving terminal 6.

[0006] Figure 2 shows another quantum signal and classical signal co-fiber transmission scheme in the prior art, in which a narrowband filter is designed at the receiving terminal to reduce noise; and a wavelength conversion device is designed at the transmitting terminal to convert the classical signal to a wavelength greatly separated from the quantum signal, for example, the quantum signal uses C-band 1550nm wavelength, and the classical signal uses S-band 1310nm or L-band 1625nm wavelength.

[0007] The purpose of transmitting classical optical communication signals and quantum key distribution signals in the same link is to save link resources, and in particular, the quantum key distribution signal hopes to be based on the optical fiber network laid by the classical optical communication, and for this purpose, the classical optical communication equipment should be modified as little as possible or even not modified. Therefore, the existing classical-quantum signal wavelength division multiplexing scheme cannot well meet this demand.

[0008] In the scheme of attenuating classical optical power, the requirement for the receiving sensitivity of the classical optical communication equipment is high, so it will be limited in many practical application scenarios, and there may not be enough power adjustment space in the existing network.

[0009] For the scheme of reducing noise by increasing the wavelength interval of the classical optical signal and the quantum optical signal, the classical optical communication backbone network often uses the C band, and part of it has also used the L band, and finally with the increase of bandwidth, it will be a trend to use the C band and the L band at the same time. In this way, if a large wavelength interval of the quantum optical signal and the classical optical signal is pursued, the quantum optical signal needs to be configured in the O band, and the O band optical signal has large transmission loss in the optical fiber, which will greatly reduce the coding rate distance of the quantum key distribution system. For example, for ordinary single-mode optical fiber, the transmission loss of the C band is about 0.2 dB / km, and the transmission loss of the O band is about 0.35 dB / km. For a system that can tolerate 20 dB loss, it can be transmitted for 100 km using the C band, and only 57 km using the O band, with a significant performance reduction.

[0010] For the scheme using a narrowband filter, since the narrowband filter needs to be used at the receiving terminal of the quantum key distribution system, it is sensitive to loss, and the increase of the loss of the narrowband filter will directly cause the performance reduction of the quantum key distribution system. For this reason, the bandwidth of the narrowband filter in the existing QKD system is on the order of 10 GHz, and at the same time, measures such as reducing the classical optical power or configuring the classical optical and quantum optical signals on a relatively distant band need to be combined to reduce noise.

[0011] For example, in the narrowband filter scheme disclosed in “Dynes J., Tam W., Plews A., et al. Ultra-high bandwidth quantum secured data transmission. Sci Rep 6, 35149 (2016)”, the narrowband filter adopts a bandwidth of 25 GHz, and when using quantum optical signals and classical optical signals in the same C band (with a wavelength interval of about 20 nm), the classical optical power needs to be adjusted. According to the record, it needs to adjust the single-channel power to -25.5 dBm, and a total of 10 channels are used, with a total power of about -15.5 dBm. At present, the optical communication backbone network equipment often reaches 80 waves, 110 waves or even more, and the optical power entering the fiber reaches +21 dBm or even higher. Referring to the power limit of the paper, the power of the conventional optical communication equipment needs to be adjusted by about 36.5 dB (corresponding to about 4000 times), which is almost impossible to achieve in actual engineering applications.

[0012] For another example, in the narrowband filter scheme disclosed in “Yingqiu Mao, Bi-Xiao Wang, et al. Integrating quantum key distribution with classical communications in backbone fiber network. Opt. Express 26, 6010-6020 (2018)”, the narrowband filter used adopts a bandwidth of 20 GHz, which does not need to adjust the classical optical power, but needs to configure the quantum optical signal and the classical optical signal on a relatively far wave band to reduce noise (the quantum optical signal is in the O band, and the classical optical signal is in the C band, with a wavelength interval of about 240 nm).

[0013] In general, in the classical-quantum signal co-transmission system using a narrowband filter at present, it is generally recognized that a filter bandwidth of about 10 GHz is needed, and at the same time, the classical signal power needs to be adjusted or the classical optical signal and the quantum optical signal need to be configured on two wave bands with a large interval to reduce noise. Adjusting the classical signal power or configuring the wave band means that more requirements are put forward to the classical equipment, which hinders the practical application of the classical-quantum signal co-transmission technology. SUMMARY

[0014] To solve the above problems in the prior art, the application discloses a quantum key distribution system and method, which can provide the QKD system with the ability to tolerate a high-noise link, especially in a scenario where the classical optical communication equipment and the QKD equipment have a small wavelength interval of 25GHz or an integer multiple thereof, without the need to reduce the power of the classical optical signal or configure the classical optical signal and the quantum optical signal on two wave bands with a large interval to reduce noise, without the need to make too many restrictions on the power and wavelength of the classical optical communication equipment, and can greatly facilitate practical engineering applications, and the quantum optical signal and the classical optical signal can simultaneously achieve small link attenuation and are suitable for long-distance scenarios. Thus, the problems that the prior art needs to make too many restrictions on the classical optical communication equipment and hinder practical engineering applications, and the classical optical signal and the quantum optical signal cannot be in the same wave band at the same time, resulting in large link loss of one wave band and inability to support long-distance scenarios, can be well solved.

[0015] Specifically, the first aspect of the application relates to a quantum key distribution system capable of tolerating a high-noise link, which comprises an Alice end and a Bob end.

[0016] The Alice end comprises classical optical communication equipment, a QKD equipment sending terminal and a wavelength division multiplexing component.

[0017] The Bob end comprises classical optical communication equipment, a QKD equipment receiving terminal and a wavelength division demultiplexing component.

[0018] The QKD equipment sending terminal comprises an ultranarrow-band signal forming module and a quantum state modulation module, the ultranarrow-band signal forming module is configured to make the quantum optical signal have a first bandwidth lower than 1GHz.

[0019] The QKD equipment receiving terminal comprises a second ultranarrow-band filter, a quantum state demodulation module, a single-photon detection module and a control unit, the second ultranarrow-band filter has a second bandwidth lower than 1GHz and is configured to filter the quantum optical signal, and the control unit is configured to control the center wavelength of the second ultranarrow-band filter to match the center wavelength of the quantum optical signal.

[0020] Further, the frequency of the classical optical signal output by the classical optical communication equipment differs from the frequency of the quantum optical signal by 25GHz or an integer multiple thereof.

[0021] Preferably, the ultranarrow-band signal forming module comprises a narrow-line-width laser, which has the first bandwidth.

[0022] Preferably, the super-narrowband signal forming module comprises a laser light source and a first super-narrowband filter having the first bandwidth and arranged for filtering the quantum light signal to have the first bandwidth.

[0023] Preferably, the super-narrowband filter comprises a phase-shifted grating.

[0024] Preferably, the first bandwidth is set to 20MHz-1GHz and the second bandwidth is set to 20MHz-1GHz.

[0025] Further, the control unit is arranged for controlling the center wavelength of the second super-narrowband filter to make the single-photon detection count of the single-photon detection module about the quantum light signal reach a preset threshold or a maximum value.

[0026] Preferably, the second super-narrowband filter is integrated with a temperature controller, and the control unit is arranged for controlling the temperature controller.

[0027] The second aspect of the present application relates to a quantum key distribution method tolerable to high-noise link, comprising the following steps:

[0028] At the Alice end, a quantum light signal having a first bandwidth is generated by a QKD device sending terminal, and the quantum light signal and a classical light signal output by a classical light communication device are output to an optical fiber channel by wavelength division multiplexing components, wherein the first bandwidth is set to be lower than 1GHz; and,

[0029] At the Bob end, the classical light signal and the quantum light signal are demultiplexed by demultiplexing components and transmitted to a classical light communication device and a QKD device receiving terminal respectively, and the quantum light signal is filtered by a second super-narrowband filter having a second bandwidth before single-photon detection in the QKD device receiving terminal, wherein the second bandwidth is set to be lower than 1GHz; and,

[0030] According to the single-photon detection count of the single-photon detection module about the quantum light signal, the center wavelength of the second super-narrowband filter is controlled to make the detection count reach a preset threshold or a maximum value.

[0031] Further, the frequency difference between the classical light signal and the quantum light signal is set to 25GHz or an integer multiple thereof.

[0032] Preferably, the center wavelength of the second super-narrowband filter is controlled by controlling the temperature thereof.

[0033] Preferably, the quantum key distribution method of the present application can be realized by means of the above-mentioned quantum key distribution system. BRIEF DESCRIPTION OF DRAWINGS

[0034] The specific embodiments of the present application will be further described in details below with reference to the drawings.

[0035] In order to more clearly illustrate the technical solutions in the embodiments of the present application or the prior art, the drawings required to be used in the embodiments or the prior art description will be briefly introduced below. Obviously, the drawings in the following description are only some embodiments of the present application, and other drawings can also be obtained by those skilled in the art without any creative effort based on these drawings.

[0036] Fig. 1 schematically shows a quantum signal and classical signal multiplexing optical fiber transmission scheme in the prior art;

[0037] Fig. 2 schematically shows another quantum signal and classical signal multiplexing optical fiber transmission scheme in the prior art;

[0038] Fig. 3 schematically shows a high-noise-tolerable quantum key distribution system of the present application. DETAILED DESCRIPTION

[0039] In the following, the exemplary embodiments of the present application will be described in detail with reference to the drawings. The following embodiments are provided by way of example 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.

[0040] Fig. 3 schematically shows a high-noise-tolerable quantum key distribution (QKD) system of the present application, which includes an Alice end and a Bob end connected by an optical fiber channel.

[0041] As shown in Fig. 3, the Alice end is provided with a classical optical communication device, a QKD device sending terminal and a wavelength division multiplexing component. The classical optical signal output by the classical optical communication device and the quantum optical signal output by the QKD device sending terminal can be multiplexed by the wavelength division multiplexing component (for example, the classical quantum wavelength division component shown in Fig. 3) in a wavelength division multiplexing manner and transmitted to the Bob end through the optical fiber channel.

[0042] Correspondingly, the Bob end can be provided with a classical optical communication device, a QKD device receiving terminal and a wavelength division demultiplexing component (for example, the classical quantum wavelength division demultiplexing component shown in Fig. 3). Therefore, the classical optical signal and the quantum optical signal that reach the Bob end through the same optical fiber channel in a wavelength division multiplexing manner can be demultiplexed in the wavelength division demultiplexing component, so as to be transmitted to the classical optical communication device and the QKD device receiving terminal, respectively.

[0043] Unlike the existing manner of only setting a narrow-band filter with a bandwidth of the order of ten gigahertz at the receiving terminal of the QKD device, the present application proposes, for the first time, a manner of simultaneously configuring an ultra-narrow bandwidth at the transmitting terminal and the receiving terminal of the QKD device for the QKD scheme of co-channel transmission in a wavelength division multiplexing manner, thereby providing the QKD system with the ability to tolerate a high-noise link.

[0044] With reference to the example shown in FIG. 3, in the transmitting terminal of the QKD device of the present application, in addition to the quantum state modulation module for modulating a quantum state on an optical signal, an ultra-narrow-band signal forming module is specially provided for the quantum optical signal finally output by the transmitting terminal of the QKD device to have a first bandwidth. In the present application, the first bandwidth is set to an ultra-narrow bandwidth (with a spectral width of about 8 pm) lower than 1 GHz.

[0045] FIG. 3 shows a preferred example of the ultra-narrow-band signal forming module, which includes an optical source (such as a laser) and a first ultra-narrow-band filter, wherein the first ultra-narrow-band filter has the above-mentioned first bandwidth.

[0046] In the example of FIG. 3, the optical signal output by the optical source can, after being provided with a modulation function by the quantum state modulation module to carry corresponding quantum state information, be provided with frequency domain filtering processing of an ultra-narrow bandwidth by the first ultra-narrow-band filter to obtain an ultra-narrow bandwidth corresponding to the first bandwidth, thereby allowing the transmitting terminal of the QKD device to finally output a quantum optical signal with an ultra-narrow bandwidth.

[0047] In another example (not shown) of the present application, a narrow-linewidth laser with the first bandwidth can be used as the optical source to realize the ultra-narrow-band signal forming module of the present application by directly outputting an optical signal with the first bandwidth, thereby allowing the transmitting terminal of the QKD device to finally output a quantum optical signal with an ultra-narrow bandwidth.

[0048] In the receiving terminal of the QKD device of the present application, in addition to the quantum state demodulation module for demodulating the quantum optical signal and the single-photon detection module (in the form of a single-photon detector, for example) for detecting the quantum optical signal, a second ultra-narrow-band filter is provided, as shown in FIG. 3.

[0049] In the present application, the second ultra-narrow-band filter can have a second bandwidth and be configured to filter the quantum optical signal in the frequency domain. The second bandwidth can be set to an ultra-narrow bandwidth lower than 1 GHz accordingly.

[0050] Continuing to refer to FIG. 3, a control unit is further provided in the QKD device receiving terminal of the present application, which is configured to control the center wavelength of the second ultra-narrow band filter to be optimally matched (e.g., consistent) with the center wavelength of the quantum optical signal, thereby eliminating the system impact caused by the center wavelength variation of the optical signal in the QKD device transmitting terminal and the center wavelength drift of the ultra-narrow band filter in the QKD device receiving terminal.

[0051] As a preferred example, the control unit can control the second ultra-narrow band filter to adjust its center wavelength according to the detection result (e.g., the detection count of the quantum optical signal) of the single-photon detection module in a preset strategy, so as to eliminate the impact of the center wavelength variation of the optical signal in the QKD device transmitting terminal and the center wavelength drift of the ultra-narrow band filter in the QKD device receiving terminal on the system. For example, the control unit can monitor the detection count reported by the single-photon detector, and control the center wavelength of the second ultra-narrow band filter by using an optimization algorithm such as a hill climbing algorithm until the detection count of the single-photon detector reaches a set threshold or a maximum value, which means that the center wavelengths of the two are optimally matched (e.g., consistent).

[0052] Further, a temperature controller can be integrated in the second ultra-narrow band filter to allow the control of the center wavelength of the second ultra-narrow band filter by controlling the temperature. At this time, the control unit can control the center wavelength of the second ultra-narrow band filter by controlling the temperature controller of the second ultra-narrow band filter.

[0053] With the above-described structure of the QKD device transmitting terminal and the QKD device receiving terminal, it can be ensured that the quantum optical signal carrying quantum state information can pass through the second ultra-narrow band filter with low loss when it reaches the QKD device receiving terminal, while the tolerance of the system to noise is greatly improved, making it possible to tolerate high-noise links in the QKD system. Therefore, in the QKD system of the present application, a small wavelength interval can be used between the QKD device transmitting terminal and the classical optical communication device (i.e., between the quantum optical signal and the classical optical signal), without the need to make excessive restrictions on the power and wavelength of the classical optical communication device, thereby avoiding many problems existing in the prior art that hinder practical engineering applications.

[0054] On this basis, the applicant has further found that setting the wavelength interval between the QKD device transmitting terminal and the classical optical communication device (i.e., between the quantum optical signal and the classical optical signal) to 25 GHz (about 0.2 nm) or an integer multiple of 25 GHz is particularly suitable for the specific ultra-narrow bandwidth configuration in the above-described QKD device transmitting terminal and QKD device receiving terminal. At this time, the optical fiber channel in the QKD system can cause small attenuation to both the classical optical signal and the quantum optical signal, which is particularly suitable for long-distance application scenarios.

[0055] In a further preferred example, the first and second bandwidths can be set to 20MHz-1GHz.

[0056] As a preferred example, the first and / or second ultra-narrow band filter can be in the form of a phase-shifted grating.

[0057] Based on the technical information provided above, those skilled in the art can realize that the QKD system of the present application, on the basis of adopting a frequency domain filtering manner, no longer needs to reduce the power of the classical light signal or configure the classical light signal and the quantum light signal on two wavelength bands with a large interval to reduce noise, so as to realize a satisfactory quantum key distribution process. That is, when the quantum key distribution system of the present application is adopted, there is no need to make excessive restrictions on the power and wavelength of the classical light communication equipment, greatly facilitating practical engineering applications, and the wavelength interval of the QKD equipment and the classical light communication equipment does not need to be large, and both can simultaneously realize a small link attenuation, suitable for long-distance scenarios. Thus, the problems that the power of the classical light communication equipment needs to be attenuated or the classical light communication wavelength and the QKD equipment wavelength need to be configured on a large wavelength interval in the prior art, resulting in the need for many restrictions and modifications of the classical light communication equipment in practical applications, hindering practical engineering applications, and the problems that the classical light signal and the quantum light signal cannot be in the same wavelength band at the same time, resulting in a large link loss of one wavelength band and the inability to support long-distance scenario applications can be well solved.

[0058] At this point, those skilled in the art can realize that the present application also proposes a quantum key distribution method that can tolerate a high-noise link, which is particularly suitable for being implemented by means of the QKD system described above.

[0059] Specifically, the quantum key distribution method of the present application can include a signal generation and transmission process, a signal reception and demodulation process, and a feedback control process.

[0060] In the signal generation and transmission process, the QKD equipment sending terminal at the Alice end can generate a quantum light signal with a first bandwidth, and output the quantum light signal and a classical light signal output by the classical light communication equipment into the same optical fiber channel in a wavelength division multiplexing manner by means of a wavelength division multiplexing component to be transmitted to the Bob end, wherein the first bandwidth is set to an ultra-narrow bandwidth lower than 1GHz, preferably 20MHz-1GHz.

[0061] As a preferred example, the classical light signal and the quantum light signal can be set to have a wavelength interval of 25GHz or an integer multiple thereof.

[0062] In the signal receiving and demodulating process, when the quantum optical signal and the classical optical signal arrive at the Bob end in a wavelength division multiplexing manner, the classical optical signal and the quantum optical signal can be demultiplexed by a demultiplexing component and transmitted to a classical optical communication device and a QKD device receiving terminal respectively. Further, before single-photon detection of the quantum optical signal by a single-photon detection module, the quantum optical signal can be frequency domain filtered by a second ultra-narrow band filter with a second bandwidth, wherein the second bandwidth is set to an ultra-narrow bandwidth lower than 1GHz, preferably 20MHz-1GHz.

[0063] Meanwhile, in the feedback control process, the second ultra-narrow band filter can be controlled according to the detection result of the single-photon detection module (such as the detection count of the quantum optical signal) to adjust the center wavelength, so as to eliminate the influence of the center wavelength change of the quantum optical signal and the center wavelength drift of the second ultra-narrow band filter. For example, the detection count reported by the single-photon detector can be monitored, and an optimization algorithm such as a climbing algorithm can be used to control and adjust the center wavelength of the second ultra-narrow band filter until the detection count of the single-photon detector reaches a set threshold or a maximum value, which means that the center wavelengths of the two are in an optimal matching state (such as consistent).

[0064] Similarly, a temperature controller can be integrated in the second ultra-narrow band filter to allow control of the center wavelength of the second ultra-narrow band filter by controlling the temperature.

[0065] Although the present application has been described in conjunction with the specific embodiments thereof with reference to the drawings, it is to be noted that the above-described embodiments are merely exemplary and illustrative of the principles of the present application, and that various modifications and equivalent arrangements can be made by those skilled in the art without departing from the spirit and scope of the present application.

Claims

1. A quantum key distribution system tolerable to high-noise link, comprising an Alice side and a Bob side; the Alice side comprises a classical optical communication device, a quantum key distribution (QKD) device sending terminal and a wavelength division multiplexing (WDM) component; the Bob side comprises a classical optical communication device, a QKD device receiving terminal and a wavelength division demultiplexing (WDMD) component; wherein the QKD device sending terminal comprises an ultra-narrowband signal forming module and a quantum state modulation module, the ultra-narrowband signal forming module is configured to make the quantum optical signal have a first bandwidth lower than 1 GHz; the QKD device receiving terminal comprises a second ultra-narrowband filter, a quantum state demodulation module, a single-photon detection module and a control unit, wherein the second ultra-narrowband filter has a second bandwidth lower than 1 GHz and is configured to filter the quantum optical signal, and the control unit is configured to control the center wavelength of the second ultra-narrowband filter to match the center wavelength of the quantum optical signal.

2. The quantum key distribution system of claim 1, wherein, The frequency of the classical optical signal output by the classical optical communication device is different from the frequency of the quantum optical signal by 25 GHz or an integer multiple thereof.

3. The quantum key distribution system of claim 1 or 2, wherein, The ultra-narrowband signal forming module comprises a narrow-linewidth laser having the first bandwidth; or the ultra-narrowband signal forming module comprises a laser light source and a first ultra-narrowband filter, the first ultra-narrowband filter has the first bandwidth and is configured to make the quantum optical signal have the first bandwidth by filtering.

4. The quantum key distribution system of claim 1 or 2, wherein, The ultra-narrowband filter comprises a phase-shifted grating.

5. The quantum key distribution system of claim 1 or 2, wherein, The first bandwidth is set to 20 MHz-1 GHz, and the second bandwidth is set to 20 MHz-1 GHz.

6. The quantum key distribution system of claim 1 or 2, wherein, The control unit is configured to control the center wavelength of the second ultra-narrowband filter to make the detection count of the single-photon detection module about the quantum optical signal reach a preset threshold or a maximum value.

7. The quantum key distribution system of any one of claims 1-2 and 6, wherein, The second ultra-narrowband filter is integrated with a temperature controller, and the control unit is configured to control the temperature controller. 8.A quantum key distribution method tolerable to high-noise link, comprising the following steps: At the Alice side, generating a quantum optical signal having a first bandwidth by using a QKD device sending terminal, and outputting the quantum optical signal and a classical optical signal output by a classical optical communication device to an optical fiber channel by using a WDM component, wherein the first bandwidth is set to be lower than 1 GHz; and At the Bob side, demultiplexing the classical optical signal and the quantum optical signal by using a WDMD component and transmitting them to a classical optical communication device and a QKD device receiving terminal respectively, and filtering the quantum optical signal by using a second ultra-narrowband filter having a second bandwidth before single-photon detection in the QKD device receiving terminal, wherein the second bandwidth is set to be lower than 1 GHz; and According to the detection count of the single-photon detection module about the quantum optical signal, the center wavelength of the second ultra-narrowband filter is controlled to make the detection count reach a preset threshold or a maximum value.

9. The quantum key distribution method of claim 8, wherein, a frequency difference between the classical optical signal and the quantum optical signal is set to 25 GHz or an integer multiple thereof; and / or, a center wavelength is controlled by controlling a temperature of the second ultra-narrow band filter.

10. The quantum key distribution method of claim 8, implemented with the quantum key distribution system of any one of claims 1-7.

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