A reliability-enhanced bidirectional QKD system and optical fiber link monitoring method thereof

Abstract

The present invention discloses a bidirectional QKD system with enhanced reliability and a method for monitoring an optical fiber link thereof, wherein it is allowed to improve only the software control module of the bidirectional QKD system, realize the OTDR function while the QKD system normally executes the QKD process, and ensure the reliability of the QKD process in real time. Among them, by using its single-photon detector to detect the back Rayleigh scattered light signal, a higher time resolution and a longer dynamic range can be obtained, thereby effectively avoiding the detection blind area. Compared with the prior art, the present invention does not need to change the original optical path structure, and is easy to implement, and can improve the reliability of the QKD system at a very low cost and a simple control process.

Description

Technical Field

[0001] The present invention relates to the fields of quantum optics and quantum information, and in particular to a reliability-enhanced bidirectional QKD system and an optical fiber link monitoring method thereof. Background Art

[0002] With the development of science and technology and the progress of society, people pay more and more attention to information security issues in military, finance and even daily life, and have higher and higher requirements for confidential communication. QKD has advantages that classical cryptography cannot match. Its security is based on the basic principles of quantum mechanics and can provide theoretically unconditionally secure key transmission. At present, the research on quantum key distribution technology is developing in the direction of engineering and practical application. Practical QKD systems have high requirements for reliability. However, although QKD has theoretically unconditional security, its system is relatively fragile and does not mean absolute reliability. In the practical engineering of QKD, increasing attacks on real systems to improve its reliability and further realize the theoretical "unconditional security" is still one of the current research focuses in this field. In addition, the QKD system needs to be transmitted through long-distance optical fibers. During the external transmission process, the optical fiber may be damaged by natural disasters, man-made or birds and animals, causing the link to fail and resulting in communication interruption. Therefore, it is also very important to conduct effective optical fiber security monitoring during the communication process and to repair and maintain the communication system in a timely manner.

[0003] To solve the problem of real-time monitoring of optical fiber breakpoints, an optical time domain reflectometer (OTDR) is often used to achieve real-time monitoring of optical fibers. Figure 1 A method and device for pre-alarming optical cable problems are shown, wherein the OTDR curve of the optical fiber is compared with a pre-stored reference curve of the same optical fiber to determine whether the optical cable has problems. Figure 2 A device and method for anti-eavesdropping of optical cable network are shown, wherein a wavelength division multiplexing unit is used to multiplex monitoring signals with communication signals, and early warning is performed by comparing OTDR optical fiber line attenuation curve data. Currently, the conventional receiving end detectors are mostly photoelectric tubes working in linear mode, such as PIN tubes, avalanche photodiodes, photomultiplier tubes (PMTs), etc. These conventional detectors have their own high noise, which limits their maximum dynamic range, highest resolution and other performances that can be measured. Summary of the invention

[0004] In view of the above-mentioned problems existing in the prior art, the present invention discloses a bidirectional QKD system with enhanced reliability and a method for monitoring the optical fiber link of the QKD system. With the help of the present invention, it is allowed to improve the software control modules of the sender and the receiver in the bidirectional QKD system, and to realize the status monitoring of the optical fiber link based on the original hardware structure of the QKD system, and even to realize the OTDR function in a time-division manner while the QKD system normally executes the QKD process, so as to ensure the safety and reliability of the QKD process in real time. Among them, the bidirectional QKD system according to the present invention realizes the detection of the back Rayleigh scattered light signal with the help of its single-photon detector, and can obtain higher time resolution and longer dynamic range, thereby effectively avoiding the detection blind area. Compared with the prior art, the present invention does not need to change the original optical path structure, and is easy to implement, and can improve the safety and reliability of the QKD system at a very low cost and a simple control process.

[0005] Specifically, a first aspect of the present invention relates to a bidirectional QKD system with enhanced reliability, which includes an Alice end, a Bob end, and an optical fiber link connected therebetween;

[0006] The Alice end includes a first sender and a first receiver, and the Bob end includes a second sender and a second receiver;

[0007] The first transmitter is configured to generate and send a first optical pulse sequence to the optical fiber link, wherein the first optical pulse sequence includes a first signal optical pulse and a first detection optical pulse;

[0008] The second receiver is configured to receive and decode the first signal light pulse;

[0009] The first receiver is configured to receive a first backscattered Rayleigh light signal formed by the first detection light signal on the optical fiber link, and obtain a photon count N of the first backscattered Rayleigh light signal by means of a single photon detector. i , and simultaneously obtain the formation position information P of the first backscattered Rayleigh light signal on the optical fiber link i , i = 1, ..., n, using the photon count N i and form position information P i A scattering relationship curve is calculated, and the state of the optical fiber link is monitored according to the scattering relationship curve.

[0010] Further, the second transmitter is configured to generate and send a second optical pulse sequence to the optical fiber link, wherein the second optical pulse sequence includes a second signal optical pulse and a second detection optical pulse;

[0011] The first receiver is configured to receive and decode the second signal light pulse;

[0012] The second receiver is configured to receive a second back-Rayleigh scattered light signal formed by the second detection light signal on the optical fiber link, and obtain a photon count N of the second back-Rayleigh scattered light signal by means of a single photon detector. j , and simultaneously obtain the formation position information P of the second backscattered Rayleigh light signal on the optical fiber link j , j = 1, ..., m, using the photon count N j and form position information P j A scattering relationship curve is calculated, and the state of the optical fiber link is monitored according to the scattering relationship curve.

[0013] Further, the transmitter includes a laser light source, an encoder and an attenuation module, wherein the laser light source is configured to generate laser pulses, the encoder is configured to encode the laser pulses and adjust the pulse width, and the attenuation module is configured to provide different attenuations for the laser pulses; and,

[0014] The receiver includes a time-to-digital converter and a single-photon detector, wherein the time-to-digital converter is configured to record the detection result of the single-photon detector to allow the receiver to calculate and obtain position information based on the time difference between the detection light pulse and the corresponding backscattered Rayleigh light signal.

[0015] Further, the first sender and the first receiver are connected to the optical fiber link via a first circulator, and the second sender and the second receiver are connected to the optical fiber link via a second circulator.

[0016] Furthermore, the signal light pulse and the detection light pulse have different light intensities and pulse widths.

[0017] Furthermore, the attenuation module is configured to make the power of the backscattered Rayleigh light signal reaching the single photon detector less than or equal to the typical value P of the single photon detector. eff ; and / or, the attenuation module includes an adjustable optical attenuator; and / or, the laser light source has a wavelength of 1550nm.

[0018] Further, the receiving side is configured to, if there is a peak on the scattering relationship curve, determine that there is a breakpoint or a connection degradation area on the optical fiber link, and / or if there is a slope change area on the scattering relationship curve, determine that the optical fiber link is attacked by eavesdropping; and / or, in the scattering relationship curve, the horizontal axis is the position information P formed i , the vertical axis is N i / N0, N0 is the background noise of the single photon detector.

[0019] A second aspect of the present invention relates to a QKD system optical fiber link monitoring method, which comprises the following steps:

[0020] Inputting a detection optical pulse into the optical fiber link from Alice and / or Bob;

[0021] The single photon detector is used to obtain the photon count N of the backscattered Rayleigh light signal formed by the detection light signal on the optical fiber link. i , and simultaneously obtain the formation position information P of the backscattered Rayleigh light signal on the optical fiber link i , i=1, ..., n;

[0022] Using the photon count N i and form position information P i Obtaining a scattering relationship curve, and monitoring the state of the optical fiber link according to the scattering relationship curve;

[0023] The power of the detection light pulse is selected so that the power of the backscattered Rayleigh light signal formed by it when reaching the single photon detector is less than or equal to the typical value P of the single photon detector. eff .

[0024] Furthermore, the pulse width of the detection light pulse can be adjusted by using an encoder in Alice's end and / or Bob's end, and the adjustable optical attenuator in Alice's end and / or Bob's end can be used to provide attenuation for the detection light pulse.

[0025] Preferably, the monitoring method of the present invention can be implemented with the aid of the above-mentioned bidirectional QKD system. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] The specific implementation modes of the present invention are further described in detail below in conjunction with the accompanying drawings.

[0027] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings required for use in the embodiments or the description of the prior art will be briefly introduced below. It is obvious that the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative work.

[0028] Figure 1 A method and device for early warning of optical cable problems in the prior art are shown;

[0029] Figure 2 A device and method for anti-eavesdropping of an optical cable network in the prior art are shown;

[0030] Figure 3 The structure of a two-way QKD system is shown in principle;

[0031] Figure 4 A typical example of a two-way QKD system is shown;

[0032] Figure 5 A preferred example of an optical pulse sequence used in the transmitter of the present invention is shown;

[0033] Figure 6 The scattering relationship curve of the optical fiber link under the ideal state is shown;

[0034] Figure 7 and 8 The scattering relationship curves when different problems occur in the optical fiber link are shown respectively. DETAILED DESCRIPTION

[0035] Hereinafter, exemplary embodiments of the present invention will be described in detail with reference to the accompanying drawings. The following embodiments are provided by way of example so as to fully convey the spirit of the present invention to those skilled in the art to which the present invention belongs. Therefore, the present invention is not limited to the embodiments disclosed herein.

[0036] Based on the existing two-way QKD system, the present invention proposes a solution for monitoring its optical fiber link, wherein there is no need to make any changes to the hardware structure of the original QKD system. Only by adding corresponding functions to its software control part, the optical fiber link monitoring function can be implemented in the two-way QKD system, and the occurrence of optical fiber breakages, eavesdropping attacks and other events can be monitored in real time, thereby realizing a two-way QKD system with enhanced security.

[0037] Figure 3 A two-way QKD system is shown in principle. Figure 4 A typical example of a two-way QKD system is shown.

[0038] As shown in the figure, the two-way QKD system includes Alice (User1) and Bob (User2), wherein both Alice and Bob are provided with a transmitter for transmitting a signal light pulse and a receiver for receiving and decoding the signal light pulse. Therefore, both user terminals in the two-way QKD system have QKD encoding and decoding functions at the same time, allowing a two-way quantum key distribution process.

[0039] The transmitter may include a laser light source, an encoder, and an attenuation module.

[0040] Laser light sources are used to generate laser pulses. Figure 4 In an example, the laser light source may include four lasers, such as a distributed feedback laser (DFB), for outputting four polarized light pulses. The laser may also generate light pulses of different intensities for realizing a signal state, a decoy state, and a vacuum state.

[0041] In a preferred example, the laser light source may have a central wavelength of 1550 nm.

[0042] The encoder is used to perform QKD encoding (such as polarization encoding, phase encoding and / or intensity encoding) on ​​the laser pulse according to the quantum key distribution (such as BB 84) protocol to form a signal light pulse carrying the encoded information. Figure 4 In the example, the encoder may include two polarization beam splitters PBS, a polarization controller PC and a beam splitter BS, wherein the polarization beam splitter and the polarization controller are configured to modulate four polarized light pulses output by the laser light source, and the beam splitter is configured to combine the modulated light pulses into signal light pulses.

[0043] The attenuation module is used to provide corresponding attenuation for the signal light pulse to meet the requirements of the BB84 protocol, such as enabling the signal light pulse to be emitted into the optical fiber link at the single photon level. Figure 4 In an example, the attenuation module may include an adjustable optical attenuator.

[0044] Continue to see Figure 3 and 4 , the receiver may include a decoder, a light detection module and a TDC (time to digital converter).

[0045] The decoder is used to decode the received signal light pulse. Figure 4 In the example of , the decoder can have the same structure as the encoder.

[0046] The optical detection module is used to detect the optical pulses output by the decoder. Figure 4 In an example, the optical detection module may include four single-photon detectors, such as InGaAs single-photon detectors.

[0047] TDC is used to record the detection results of the light detection module. Figure 4 In the example shown in FIG. 1 , the TDC is arranged after the single photon detector to record the detection result of the single photon detector.

[0048] In a bidirectional QKD system, the sender and receiver in the same user end can be connected to a fiber optic link through, for example, a circulator, so that the optical pulses generated in the sender can be input into the fiber optic link through the circulator, and at the same time, the optical pulses input by the fiber optic link can be input into the receiver through the circulator, thereby allowing the QKD process to be performed bidirectionally with the help of the same fiber optic link.

[0049] In the bidirectional QKD system of the present invention, by controlling the encoder and the attenuation module, the transmitter can generate not only signal light pulses for the QKD process, but also detection light pulses for performing the OTDR process and monitoring the optical fiber link.

[0050] Those skilled in the art will appreciate that the sender may transmit a detection light pulse separately within a period of time to specifically detect the optical fiber link. It may also be preferred to transmit the detection light pulse within the time interval of transmitting the signal light pulse, thereby monitoring the optical fiber link in real time while performing the QKD process normally.

[0051] Figure 5 A preferred example of the optical pulse sequence of the transmitter of the present invention is shown. As shown in the figure, the encoder and the adjustable optical attenuator in the transmitter, such as Alice, can be controlled to send the optical pulse sequence to the optical fiber link, wherein the optical pulse sequence includes both the signal optical pulse and the detection optical pulse. Figure 5 In a preferred example, both the signal light pulse and the detection light pulse are periodic signals and may have different pulse widths and light intensities (powers).

[0052] The signal light pulses in the light pulse sequence are transmitted via an optical fiber link to, for example, Bob's end, and are decoded by a receiver at Bob's end, thereby completing the QKD process.

[0053] The detection light pulse in the optical pulse sequence will generate a scattered light signal in the opposite direction of the optical transmission direction (i.e., "backward Rayleigh scattered light signal") during the transmission process of the optical fiber link, and its power satisfies the following relationship:

[0054]

[0055] Among them, P O，eff is the peak power of the detected optical pulse when it enters the optical fiber link, S is the backscattering factor, α s is the optical fiber attenuation coefficient, v g is the group velocity of the optical pulse, Δt is the pulse width, and L is the transmission distance in the optical fiber link.

[0056] In a two-way QKD system, the backscattered Rayleigh light signal formed by the detection light pulse in the optical fiber link will return to Alice along the optical fiber link. At Alice's end, the backscattered Rayleigh light signal will enter the receiver via the circulator, and the single-photon detector in the receiver will detect it and generate a detection result, so as to obtain the photon count N of the backscattered Rayleigh light signal.

[0057] By using TDC to record the detection results of the backscattered Rayleigh light signal by the single-photon detector, the time difference between the detected backscattered Rayleigh light signal and its corresponding detection light signal can be obtained, and then the transmission time of the backscattered Rayleigh light signal in the optical fiber link can be obtained. Therefore, combined with its transmission speed in the optical fiber link, the distance of the formation position of the backscattered Rayleigh light signal on the optical fiber link relative to the Alice end can be finally determined, that is, the formation position information P of the backscattered Rayleigh light signal on the optical fiber link can be obtained.

[0058] Based on the intensity information of the backscattered light signal (such as the photon count N of the scattered light signal reaching the single-photon detector) and its formation position information P, the distribution of the backscattered light signal on the optical fiber link, namely the "scattering relationship curve", can be obtained.

[0059] Preferably, the scattering relationship curve of the present invention can be formed with position information P (i.e., the distance relative to the input end) as the horizontal coordinate and N / N0 as the vertical coordinate, wherein N is the single photon detection count of the back Rayleigh scattered light signal and N0 is the background noise of the single photon detector.

[0060] In the present invention, it is necessary to control the attenuation value of the adjustable optical attenuator in the transmitter acting on the detection light pulse according to the length of the optical fiber link and the single photon detector of the receiver, so that the power of the backscattered Rayleigh light signal detected by the single photon detector is less than or equal to the typical value P of the single photon detector. eff (optical power corresponding to the maximum count rate), where the power P of the backscattered Rayleigh light signal can be calculated according to the following relationship:

[0061]

[0062] P0 is the power of the probe light pulse when it enters the fiber link (for example, through a circulator).

[0063] Figure 6 The scattering relationship curve of the optical fiber link in an ideal state is shown. Figure 7 and 8 The scattering relationship curves when different problems occur in the optical fiber link are shown respectively.

[0064] contrast Figure 6-8 It can be seen that: when factors affecting QKD performance such as fiber breakpoints or partial connection deterioration occur in the fiber link, obvious peaks will appear on the scattering relationship curve obtained based on the present invention; when factors such as length changes or attenuation changes occur in the fiber link that may indicate the existence of an eavesdropping attack, the slope of the scattering relationship curve obtained based on the present invention will change due to changes in the fiber attenuation coefficient.

[0065] Therefore, with the help of this scattering relationship curve, the status of the optical fiber link can be monitored in real time, and optical fiber connection problems or eavesdropping attacks can be promptly discovered, thereby improving the security and reliability of the two-way QKD system.

[0066] In summary, by improving the software control modules of the sender and receiver in the bidirectional QKD system, the original hardware structure of the QKD system can be used to monitor the status of its optical fiber link, and even allow the OTDR function to be implemented in a time-division manner while the QKD system is normally executing the QKD process, thereby ensuring the safety and reliability of the QKD process in real time. Among them, the bidirectional QKD system according to the present invention uses its single-photon detector to detect the backscattered Rayleigh light signal, which can obtain a higher time resolution and a longer dynamic range, thereby effectively avoiding detection blind spots. Compared with the prior art, the present invention does not need to change the original optical path structure, and is easy to implement. It can improve the safety and reliability of the QKD system at a very low cost and with a simple control process.

[0067] Based on the above, it can also be noted that the present invention also discloses a method for optical fiber link monitoring implemented with the aid of a single-photon detector, which is particularly suitable for implementation with the aid of the above-mentioned bidirectional QKD system.

[0068] In the optical fiber link monitoring method of the present invention, firstly, the user end (eg, Alice end or Bob end) can be controlled to generate a detection light pulse, and the detection light pulse is input into the optical fiber link.

[0069] As described above, a probe optical pulse can be generated separately within a period of time to specifically perform the OTDR function, or an optical pulse sequence containing both a probe optical pulse and a signal optical pulse can be generated to simultaneously perform the QKD process and the OTDR process in the QKD system in a time division multiplexing manner.

[0070] Furthermore, the pulse width and intensity of the optical pulse can be adjusted by controlling the encoder and the adjustable optical attenuator in the transmitter to form a detection optical pulse different from the signal optical pulse.

[0071] Subsequently, the back-Rayleigh scattered light signal formed when the detection light signal propagates on the optical fiber link can be input to the receiver in the same user end, and detected by a single-photon detector to obtain the photon count Ni of the back-Rayleigh scattered light signal. At the same time, the detection result of the single-photon detector is recorded by TDC to calculate the formation position information P of the back-Rayleigh scattered light signal on the optical fiber link. i .

[0072] Therefore, the photon count N of the Rayleigh backscattered light signal can be used i Form position information P iA scattering relationship curve of the optical fiber link is obtained, and the change of the optical fiber link state is judged according to whether there is a peak or a slope change area on the scattering relationship curve.

[0073] The power of the detection light pulse can be adjusted by the transmitter so that the power of the backscattered Rayleigh light signal formed on the optical fiber link is less than or equal to the typical value P of the single photon detector when it reaches the single photon detector. eff .

[0074] Although the present invention has been described above through specific embodiments in conjunction with the accompanying drawings, those skilled in the art will readily recognize that the above embodiments are merely exemplary and are used to illustrate the principles of the present invention, and do not limit the scope of the present invention. Those skilled in the art may 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 bidirectional QKD system with enhanced reliability, comprising an Ali ce end, a Bob end and an optical fiber link connected therebetween; The Ali ce end includes a first sender and a first receiver, and the Bob end includes a second sender and a second receiver; The first transmitter is configured to generate and send a first optical pulse sequence to an optical fiber link, wherein: The first optical pulse sequence includes a first signal optical pulse and a first detection optical pulse; The second receiver is configured to receive and decode the first signal light pulse; The first receiver is configured to receive a first backscattered Rayleigh light signal formed by the first detection light signal on the optical fiber link, and obtain a photon count N of the first backscattered Rayleigh light signal by means of a single photon detector. i , and simultaneously obtain the formation position information P of the first backscattered Rayleigh light signal on the optical fiber link i , i = 1, ..., n, using the photon count N i and form position information P i Calculating a scattering relationship curve, and monitoring the state of the optical fiber link according to the scattering relationship curve; The second transmitter is configured to generate and send a second optical pulse sequence to the optical fiber link, wherein the second optical pulse sequence includes a second signal optical pulse and a second detection optical pulse; The first receiver is configured to receive and decode the second signal light pulse; The second receiver is configured to receive a second back-Rayleigh scattered light signal formed by the second detection light signal on the optical fiber link, and obtain a photon count N of the second back-Rayleigh scattered light signal by means of a single photon detector. j , and simultaneously obtain the formation position information P of the second backscattered Rayleigh light signal on the optical fiber link j , j = 1, ..., m, using the photon count N j and form position information P j A scattering relationship curve is calculated, and the state of the optical fiber link is monitored according to the scattering relationship curve.

2. The two-way QKD system of claim 1, wherein: The transmitter includes a laser light source, an encoder and an attenuation module, wherein the laser light source is configured to generate laser pulses, the encoder is configured to encode the laser pulses and adjust the pulse width, and the attenuation module is configured to provide different attenuations for the laser pulses; and, The receiver includes a time-to-digital converter and a single-photon detector, wherein the time-to-digital converter is configured to record the detection result of the single-photon detector to allow the receiver to calculate and obtain position information based on the time difference between the detection light pulse and the corresponding backscattered Rayleigh light signal.

3. The two-way QKD system of claim 1, wherein: The first sender and the first receiver are connected via a first circulator via an optical fiber link, and the second sender and the second receiver are connected via a second circulator via an optical fiber link.

4. The two-way QKD system of claim 1, wherein: The signal light pulse and the detection light pulse have different light intensities and pulse widths.

5. The two-way QKD system of claim 2, wherein: The attenuation module is configured to make the power of the backscattered Rayleigh light signal reaching the single photon detector less than or equal to the typical value P of the single photon detector. eff and / or, The attenuation module includes an adjustable optical attenuator; and / or, The laser light source has a wavelength of 1550 nm.

6. The two-way QKD system of claim 1, wherein: The receiving side is configured to, if there is a peak on the scattering relationship curve, determine that there is a breakpoint or a connection degradation area on the optical fiber link, and / or if there is a slope change area on the scattering relationship curve, determine that the optical fiber link is attacked by eavesdropping; and / or, In the scattering relationship curve, the horizontal axis is the position information P i , the vertical axis is N i / N0, N0 is the background noise of the single photon detector.

7. A QKD system optical fiber link monitoring method, comprising the following steps: Inputting a detection optical pulse into the optical fiber link from Alice and / or Bob; The single photon detector is used to obtain the photon count N of the backscattered Rayleigh light signal formed by the detection light signal on the optical fiber link. i , and simultaneously obtain the formation position information P of the backscattered Rayleigh light signal on the optical fiber link i , i=1,…,n; Using the photon count N i and form position information P i Obtaining a scattering relationship curve, and monitoring the state of the optical fiber link according to the scattering relationship curve; The power of the detection light pulse is selected so that the power of the backscattered Rayleigh light signal formed by it when reaching the single photon detector is less than or equal to the typical value P of the single photon detector. eff ; The QKD system optical fiber link monitoring method is implemented by means of a bidirectional QKD system as described in any one of claims 1-6.

8. The monitoring method according to claim 7, wherein: The pulse width of the detection light pulse is adjusted by using an encoder in the Ali ce end and / or the Bob end, and the adjustable optical attenuator in the Ali ce end and / or the Bob end is used to provide attenuation for the detection light pulse.

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