CN112039666B - Quantum key distribution-based frequency locking and phase stabilizing method and system - Google Patents
Quantum key distribution-based frequency locking and phase stabilizing method and system Download PDFInfo
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- CN112039666B CN112039666B CN202010913841.7A CN202010913841A CN112039666B CN 112039666 B CN112039666 B CN 112039666B CN 202010913841 A CN202010913841 A CN 202010913841A CN 112039666 B CN112039666 B CN 112039666B
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- H04L9/00—Cryptographic mechanisms or cryptographic arrangements for secret or secure communications; Network security protocols
- H04L9/08—Key distribution or management, e.g. generation, sharing or updating, of cryptographic keys or passwords
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Abstract
The application provides a frequency locking and phase stabilizing method and system based on quantum key distribution, wherein the system comprises the following steps: a frequency locking system, the frequency locking system comprising: the ultra-stable laser is used for outputting two independent laser sources; the beat frequency module is used for acquiring beat frequency signals of the two independent laser sources; the spectrum analyzer is used for measuring the frequency difference between the two independent laser sources according to the beat frequency signals; the locking and tuning module is connected with the frequency standard instrument; the frequency standard instrument is used for providing peripheral frequency standard for the locking and tuning module; a locking and tuning module for tuning the frequency between the two independent laser sources to unity according to the frequency difference based on a frequency criterion and removing the amount of linear drift inherent in the output frequency of the ultra-stable laser. The application realizes the frequency locking between the two independent laser sources in the quantum key distribution process, so that the phases of the reference light signals and the quantum light signals of the two independent laser sources are stable.
Description
Technical Field
The application relates to the technical field of quantum secret communication, in particular to a frequency locking and phase stabilizing method and system based on quantum key distribution.
Background
Quantum secret communication is an unconditionally safe communication mode. Based on the quantum key distribution technology, the two communication parties can share an unconditionally safe key, and the key can encrypt classical information to be transmitted in a one-time-pad mode, so that absolute safety of the information is ensured.
To promote the practical use of quantum key distribution, it has been two critical issues to increase the key generation rate and increase the transmission distance. The novel double-Field (Tain-Field) quantum key distribution protocol is based on the single photon interference response of a third party Charlie, so that the two communication parties Alice and Bob establish an association relationship, the square root of the key generation rate and the distance form a linear relationship, and a theoretical basis is established for the novel quantum key distribution system with ultra-long distance and high key generation rate. However, the implementation condition of the dual-field protocol is very severe, so that in order to ensure the phase field interference with high visibility in a third party, on one hand, the frequency consistency of independent laser sources of two communication parties needs to be ensured, and on the other hand, the stability of the phase after the transmission of the ultra-long distance optical fiber needs to be ensured.
In the prior art, an optical phase-locked loop is used, the phase error between a master laser and a slave laser is extracted through an optical phase discriminator, and an output error signal is used as a feedback signal to tune the output frequency of the slave laser in real time, so that the two lasers are in the same frequency and phase. However, in this scheme, an optical fiber needs to be erected between the master laser and the slave laser to perform frequency locking, the system is complex, and in principle, the lasers of both communication parties are not independent of each other.
In the prior art, the phase change condition in long-distance optical fiber transmission is monitored by using an unmodulated reference optical signal through a time division multiplexing scheme, so that a phase modulator in a transmission link or a phase modulator at a coding end is adjusted, and the real-time calibration of the phase drift of a quantum optical signal is realized. In the scheme, a stronger reference optical signal can introduce certain interference noise, but the reduction of the light intensity of the stronger reference optical signal can reduce the number of particles detected by a third party after long-distance transmission, so that the accuracy of monitoring the phase change is reduced. This relationship of mutual constraints makes it impossible for the time division multiplexing scheme to guarantee the accuracy of phase calibration in longer-distance transmission.
Disclosure of Invention
The application aims to provide a frequency locking and phase stabilizing method and system based on quantum key distribution, wherein both communication parties respectively use an ultra-stable laser as a signal source, and can infinitely zoom out after frequency correction is completed, and service optical fibers are not needed to be used as a transmission medium for frequency locking. The phase stabilization scheme of frequency division multiplexing is adopted in the communication process, so that interference between a reference optical signal and a quantum optical signal can be avoided, the relative phase drift rate of the quantum optical signal can be greatly reduced, the realization of double-field quantum key distribution is facilitated, and the limit of transmission distance is broken through.
To achieve the above object, the present application provides a frequency locking and phase stabilizing system based on quantum key distribution, the system comprising: a frequency-locked system based on two-way independent laser sources, the frequency-locked system comprising: the system comprises an ultra-stable laser, a beat frequency module, a spectrum analyzer, a locking and tuning module and a frequency standard instrument; the ultra-stable laser is used for outputting two independent laser sources; the beat frequency module is connected with the ultra-stable laser and is used for acquiring beat frequency signals of the two independent laser sources; the spectrum analyzer is connected with the beat frequency module and is used for measuring the frequency difference between the two independent laser sources according to beat frequency signals; the locking and tuning module is connected with the frequency standard instrument; the frequency standard instrument is used for providing frequency standard of the peripheral equipment for the locking and tuning module; the locking and tuning module is used for tuning the frequency between the two independent laser sources to be consistent according to the frequency difference between the two independent laser sources based on the frequency standard and removing the inherent linear drift amount in the output frequency of the ultra-stable laser.
As above, wherein the locking and tuning module comprises an electro-optic modulator for tuning the frequency between two independent laser sources to be uniform according to a frequency difference between the two independent laser sources based on a frequency standard and an acousto-optic modulator; the acousto-optic modulator is used for removing the inherent linear drift in the output frequency of the ultra-stable laser.
As above, the system further comprises a phase stabilization system based on wavelength division multiplexing, the phase stabilization system comprising: a generation system for stabilizing the side frequency component and a feedback control system for phase compensation; the stable side frequency component generation system is used for respectively generating reference light signals and quantum light signals of two grounds according to the independent laser sources of the two grounds. The feedback control system of phase compensation includes: a fast phase compensation feedback system and a slow phase compensation feedback control system; the fast phase compensation feedback system is used for carrying out phase compensation on the reference optical signals at two places; the slow phase compensation feedback control system is used for carrying out phase compensation on the quantum optical signals at two places.
As above, the system for generating the stable side frequency component includes: broadband phase modulator, analog signal generator, power amplifier and optical filter; the analog signal generator is used for generating a radio frequency driving signal and sending the radio frequency driving signal to the broadband phase modulator; the power amplifier is connected with the analog signal generator and is used for increasing the amplitude of the radio frequency driving signal; the broadband phase modulator is used for modulating variable frequency components at two sides of a carrier based on light sources of two independent laser sources as the carrier after receiving radio frequency driving signals; the optical filter is connected with the broadband phase modulator and is used for filtering quantum optical signals and reference optical signals with different wavelengths from side frequency components respectively.
As above, the phase compensation feedback control system includes a wavelength division multiplexer, a wavelength division demultiplexer, a beam splitter, a single photon detector, a phase modulator, and a board card; the wavelength division multiplexer is used for carrying out combined transmission on the reference optical signal and the quantum optical signal; the wavelength division demultiplexer is used for carrying out branching transmission on the reference optical signal and the quantum optical signal; the beam splitter is used for carrying out single photon interference based on a phase field on the reference optical signal and the quantum optical signal at a third party end respectively; the single photon detector is used for detecting error photon signals generated by relative phase drift in a feedback link of the reference optical signals and the quantum optical signals; the phase modulator is used for carrying out rapid phase compensation based on the feedback result of the reference optical signal; performing slow phase compensation based on a feedback result of the quantum optical signal; and the board card is used for regulating and controlling the phase modulator to finish phase compensation on the transmission link according to the output result of the single photon detector.
The frequency locking and phase stabilizing system based on quantum key distribution further comprises an encoding module for encoding the quantum optical signals.
The application also provides a frequency locking and phase stabilizing method based on quantum key distribution, which comprises the following steps of: acquiring a frequency difference between the first independent laser source and the second independent laser source; tuning the frequencies of the two independent laser sources to be consistent according to the acquired frequency difference based on an external frequency standard; after the frequencies of the two independent laser sources are tuned to be identical, the inherent linear drift in the output frequencies of the first and second independent laser sources is removed.
As above, the frequency locking and phase stabilizing method based on quantum key distribution further comprises a phase stabilizing method based on wavelength division multiplexing: and carrying out fast phase compensation on the reference optical signals of the transmitting end and the receiving end, and carrying out slow phase compensation on the quantum optical signals of the transmitting end and the receiving end.
As above, the fast phase compensation method includes: interfering the reference light signals of the transmitting end and the receiving end through a first beam splitter of a third square end; a first detector is used for acquiring a phase difference between reference light signals of a transmitting end and a receiving end after interference; the first phase modulator at the third side is adjusted in real time to minimize the count rate of the first detector channel to accomplish fast phase compensation.
As above, the slow phase compensation method includes: the quantum optical signals of the transmitting end and the receiving end are interfered by a second beam splitter of the third square end; a third detector is used for obtaining the phase difference between quantum optical signals of the transmitting end and the receiving end after interference; the second phase modulator at the third side is adjusted in real time to minimize the count rate of the third detector channel to accomplish the slow phase compensation.
The beneficial effects achieved by the application are as follows:
(1) The two communication parties respectively use the ultra-stable laser as a signal source, and can infinitely zoom out after finishing frequency correction, and service optical fibers are not needed to be used as a transmission medium for frequency locking.
(2) The phase stabilization scheme of frequency division multiplexing is adopted in the communication process, so that interference between the reference optical signal and the quantum optical signal can be avoided, and after the phase drift of the reference optical signal is calibrated, the relative phase drift rate of the quantum optical signal can be greatly reduced, and the realization of double-field quantum key distribution is facilitated, thereby breaking through the limit of transmission distance.
Drawings
In order to more clearly illustrate the embodiments of the present application or the technical solutions in the prior art, the drawings used in the embodiments or the description of the prior art will be briefly described below, and it is obvious that the drawings in the following description are only some embodiments described in the present application, and other drawings may be obtained according to these drawings to those skilled in the art.
Fig. 1 is a schematic diagram of a frequency locking system based on two independent laser sources according to an embodiment of the present application.
Fig. 2 is a schematic structural diagram of a phase stabilization system based on wavelength division multiplexing according to an embodiment of the present application.
Fig. 3 is a schematic structural diagram of an encoding module according to an embodiment of the application.
Reference numerals: a 10-ultrastable laser; 20-beat frequency module; 30-spectrum analyzer; 40-locking and tuning module; 50-frequency standard instrument; a 60-wideband phase modulator; 70-an analog signal generator; an 80-power amplifier; a 90-ray filter; 91-an encoding module; 92-wavelength division multiplexer; 100-Alice end (transmitting end); 101-a first laser source; 102-a first ultra stable cavity; 200-Bob end (receiving end); 201-a second laser source; 202-a second ultra stable cavity; 300-Charlie end (third party end); 310-phase modulator; 320-wave-division multiplexer; 330-polarization controller; 340-beam splitter; 350-single photon detector; 360-FPGA board card; 400-coding module; 410-intensity modulator; 420-code phase modulator; 430-encoding a polarization controller; 440-optical attenuator.
Detailed Description
The following description of the embodiments of the present application will be made clearly and fully with reference to the accompanying drawings, in which it is evident that the embodiments described are some, but not all embodiments of the application. All other embodiments, which can be made by those skilled in the art based on the embodiments of the application without making any inventive effort, are intended to be within the scope of the application.
The application is based on a double-field quantum key distribution protocol, and the main factors influencing the interference visibility of a phase field in a third party are as follows:
wherein c represents the speed of light; f represents the carrier frequency of Alice and Bob encoded quantum signals and Δf represents the frequency difference between Alice and Bob encoded quantum signals; l represents the transmission distance between Alice and Bob and Charlie; Δl represents a transmission distance difference due to expansion and contraction of the optical fiber caused by a temperature change; delta ab Representing the relative phase difference of Alice and Bob for phase encoding. As can be seen from equation (1), in order to improve the visibility of interference and reduce the quantum bit error rate, it is necessary to reduce the relative phase difference between Alice and Bob.
The application provides a frequency locking and phase stabilizing system based on quantum key distribution, which comprises:
a frequency locking system based on two independent laser sources and a phase stabilizing system based on wavelength division multiplexing.
Example 1
As shown in fig. 1, the frequency locking system based on two-way independent laser sources includes an ultrastable laser 10, a beat frequency module 20, a spectrum analyzer 30, a locking and tuning module 40, and a frequency etalon 50. The ultrastable laser 10 includes a first ultrastable laser at a transmitting end (Alice end) 100 and a second ultrastable laser at a receiving end (Bob end) 200. The first ultrastable laser is used for outputting first monochromatic continuous light as a first independent laser source; the second ultrastable laser is used for outputting second monochromatic continuous light as a second independent laser source.
As shown in fig. 1, the first ultrastable laser includes a first laser source 101 and a first ultrastable cavity 102, and the first laser source 101 is locked on the first ultrastable cavity 102; the second ultra-stable laser comprises a second laser source 201 and a second ultra-stable cavity 202, wherein the second laser source 201 is locked on the second ultra-stable cavity 202; the first laser source 101 and the second laser source 201 have ultra-narrow linewidth and short-time ultra-high stability, and vibration isolation treatment is performed on the cabinet for placing the ultra-stable laser, so that external interference is reduced.
As shown in fig. 1, the signal acquisition end of the beat frequency module 20 is connected to the output end of the first laser source 101 and the output end of the second laser source 201 through an optical fiber, and is used for acquiring beat frequency signals of the first laser source 101 and the second laser source 201. The spectrum analyzer 30 is connected to the beat frequency module 20, and the spectrum analyzer 30 is configured to determine a frequency difference between the first independent laser source and the second independent laser source according to the beat frequency signal acquired by the beat frequency module 20.
According to one embodiment of the present application, the lock and tune module 40 includes an electro-optic modulator and an acousto-optic modulator. An electro-optic modulator for tuning the frequency between the first and second independent laser sources to be uniform based on the frequency difference between the two independent laser sources. An acousto-optic modulator for removing the amount of linear drift inherent in the output frequencies of the first and second ultrastable lasers.
According to one embodiment of the application, the inherent linear drift amount is determined beforehand using an optical comb. To ensure accuracy of frequency tuning and linear drift removal, the lock and tune module 40 uses the peripheral frequency standard as a signal reference source. And finally, only the nonlinear drift quantity influences the relative frequency difference of two communication parties, and the requirement that the relative frequency difference of two ultra-stable lasers is smaller than 1KHz can be met within a certain time (within one day).
As shown in fig. 1, the frequency standard meter 30 is connected to the locking and tuning module 40, and is used for providing an external signal reference source for the locking and tuning module 40, the signal reference source provides a frequency standard for the locking and tuning module 40, and the method for providing the frequency standard by using the external signal reference source has the characteristics of high stability and high precision of adjusting the frequency standard, and the two independent laser sources are tuned to be consistent based on the frequency standard, so that the frequency adjusted per second is certain (for example, 150 millihertz is adjusted per second), no deviation occurs, and the precision of frequency adjustment is improved.
As an embodiment of the present application, the lock and tuning module 40 includes a first lock and tuning module disposed at Alice's end 100 and a second lock and tuning module disposed at Bob's end 200. The frequency etalons include a first frequency etalon disposed at Alice's end 100 and a second frequency etalon disposed at Bob's end 200.
According to one embodiment of the present application, the variation of the external temperature, such as a range of plus or minus 1 degree, is controlled so as to reduce the amount of nonlinear drift of the first and second ultrastable lasers.
Example two
The application provides a phase stabilization system based on wavelength division multiplexing, which comprises a generation system for stabilizing side frequency components and a feedback control system for phase compensation.
And the stable side frequency component generating system is used for generating the reference optical signal and the quantum optical signal. The system for generating the stable side frequency component comprises a system for generating the stable side frequency component of the Alice terminal 100 and a system for generating the stable side frequency component of the Bob terminal 200; the system for generating the stable side frequency component of the Alice terminal 100 is used for generating a reference optical signal and a quantum optical signal of the Alice terminal 100; the system for generating the stable side frequency component of Bob end 200 is used for generating the reference optical signal and the quantum optical signal of Bob end 100.
The phase compensation feedback control system comprises a fast phase compensation feedback system and a slow phase compensation feedback control system.
The fast phase compensation feedback system is used for performing phase compensation on the reference optical signals of the Alice terminal 100 and the Bob terminal 200; the slow phase compensation feedback control system is used for performing phase compensation on the quantum optical signals of the Alice terminal 100 and the Bob terminal 200.
As shown in fig. 2, the generating systems of the stable side frequency components of Alice end 100 and Bob end 200 each include: broadband phase modulator 60, analog signal generator 70, power amplifier 80, frequency etalon 50, and optical filter 90. The analog signal generator 70 is configured to generate a radio frequency driving signal (e.g., a radio frequency source generates a fundamental frequency signal of 25 GHz), and send the generated radio frequency driving signal to the wideband phase modulator 60. The analog signal generator 70 is connected to the frequency standard instrument 50, the frequency standard instrument 50 outputs a high-stability frequency reference signal, for example, 10MHz, the frequency standard instrument 50 provides a stable external frequency standard for the analog signal generator 70, the phase noise of the side frequency component is related to the phase noise of the ultra-stable laser, the phase noise of the radio frequency driving signal and the corresponding modulation order, and in order to reduce the phase noise, the analog signal generator 70 uses the peripheral frequency standard as a signal reference source, so that the line width of the side frequency signal is ensured to be similar to the line width of the carrier wave of the ultra-stable laser. The analog signal generator 70 is connected to the wideband phase modulator 60 through a power amplifier 80. The power amplifier 80 is used for increasing the amplitude of the radio frequency driving signal, thereby increasing the power of the higher-order side frequency component. The broadband phase modulator 60 is configured to, after receiving the rf driving signal, modulate the side frequency components at two sides of the carrier based on the light sources of the ultrastable lasers 10 at Alice end 100 and Bob end 200 as the input carrier, that is, the first laser source 101 of the first ultrastable laser and the second laser source 201 of the second ultrastable laser as the carrier. The broadband phase modulator 60 is connected to the optical filter 90, and the side frequency component modulated by the broadband phase modulator 60 is divided into a stable reference optical signal and a quantum optical signal with a narrow line width after passing through the optical filter 90. An optical filter 90 is configured to filter the quantum optical signal and the reference optical signal with different wavelengths from the side frequency component, respectively, where the quantum optical signal enters the encoding module 91.
According to an embodiment of the present application, for example, the carrier wavelength of the ultrastable laser 10 is 1550.12nm, the rf driving signal is 25GHz, and a series of side frequency signals are formed on two sides of the carrier after external modulation, where two 1 st-order sidebands are spaced by 50GHz, which can be used as a reference optical signal and a quantum optical signal, respectively.
As shown in fig. 2, the quantum optical signals need to enter the encoding module 91 to be encoded separately, and finally, the reference optical signals and the quantum optical signals are combined by the wavelength division multiplexer 92 to realize co-fiber transmission, and the co-fiber transmission adopts one optical fiber for transmission, so that the optical fibers are saved, and meanwhile, the influence on the reference optical signals and the quantum optical signals in the transmission process is ensured to be identical.
As shown in fig. 2, the feedback control system for phase compensation includes a wavelength division multiplexer 92 and a wavelength division demultiplexer 320, a polarization controller 330, a beam splitter 340, a single photon detector 350, a phase modulator 310, and an FPGA board 360 for feedback control. The single photon detector 350 is preferably a multichannel superconducting nanowire single photon detector.
Wherein the wavelength division multiplexer 92 is a combiner of reference optical signals and quantum optical signals of different wavelengths; the wavelength division demultiplexer 320 is a splitter of the reference optical signal and the quantum optical signal of different wavelengths.
Wherein, the beam splitter 340 is a ratio of 50:50, and performs single photon interference based on the phase field on the reference optical signal and the quantum optical signal of the Alice end 100 and the Bob end 200 at the third party end, wherein, the reference optical signal and the quantum optical signal are separately performed on the third party end through single photon interference based on the phase field.
Wherein the single photon detector 350 is used for detecting error photon signals generated due to relative phase drift in a feedback link of the reference optical signal and the quantum optical signal.
Wherein, the phase modulator 310 is configured to perform fast phase compensation based on a feedback result of the reference optical signal; and performing slow phase compensation based on the feedback result of the quantum optical signal.
The FPGA board 360 is configured to regulate the phase modulator 310 to complete phase compensation on the transmission link according to the output result of the single photon detector 350.
As shown in fig. 2, the quantum optical signal filtered by the optical filter 90 at the Alice end 100 is encoded by the encoding module 91, the encoded quantum optical signal and the reference optical signal filtered by the optical filter 90 are transmitted to the phase modulator 310 by combining through the wavelength division multiplexer 92, and then are split by the wavelength division demultiplexer 320 to form the quantum optical signal and the reference optical signal, where the quantum optical signal interferes by the second beam splitter at the third end, and the reference optical signal interferes by the first beam splitter at the third end after passing through the polarization controller.
As shown in fig. 2, the quantum optical signal filtered by the optical filter 90 at the Bob end 200 is encoded by the encoding module 91, the encoded quantum optical signal and the reference optical signal filtered by the optical filter 90 are transmitted to the wavelength division multiplexer 320 by combining through the wavelength division multiplexer 92, and the quantum optical signal and the reference optical signal are formed by branching through the wavelength division multiplexer 320, wherein the quantum optical signal is modulated by the phase modulator 310 and then interfered by the second beam splitter at the third party end, and the reference optical signal is interfered by the first beam splitter at the third party end after passing through the polarization controller 330.
As shown in fig. 2, the first beam splitter at the third side is connected to the first detector (D1) at the third side through a polarizer, and the first detector (D1) is used to obtain the result of interference between the reference optical signals at Alice end 100 and Bob end 200 by the first beam splitter. The first detector is connected to a board 360, and the board 360 is connected to the phase modulator 310.
As shown in fig. 2, the second beam splitter at the third side is connected to the third detector (D3) at the third side through a polarizer, and the third detector (D3) is configured to obtain the result of interference of the quantum optical signals at the Alice end and the Bob end by the second beam splitter at the third side. The third controller (D3) is connected to the board 360, and the board 360 is connected to the phase modulator 310.
As shown in fig. 2, the first beam splitter at the third square end is connected with the second detector (D2) at the third square end through a polarization beam splitter, and the second detector (D2) is used for acquiring a feedback signal of polarization calibration in the reference optical signal transmission link; the first polarization controller of the Charlie terminal 300 is adjusted to minimize the count rate of the second detector (D2) so that the polarization of the two reference light signals of Alice terminal 100 and Bob terminal 200 are identical.
As shown in fig. 2, the second beam splitter at the third square end is connected with the fourth detector (D4) and the fifth detector (D5) at the third square end through a polarizing beam splitter, and the fourth detector (D4) is used for obtaining a feedback signal of polarization calibration of the quantum optical signal; and adjusting the second polarization controllers of the encoding modules of the Alice end and the Bob end to enable the counting rate of the fourth detector (D4) to be minimum, so that the polarization of two paths of quantum optical signals of the Alice end and the Bob end are consistent. The fifth detector (D5) is used for generating the quantum key.
As shown in fig. 3, the encoding module 400 includes an intensity modulator 410, a code phase modulator 420, a code polarization controller 430, and an optical attenuator 440.
The intensity modulator 410 is configured to generate a narrow pulse signal from the quantum optical signal, and may also implement encoding of the decoy state.
A code phase modulator 420 for phase encoding in a two-field protocol.
The encoding polarization controller 430 is configured to calibrate the polarization states of the variable transmission link so that the polarization states of the quantum optical signals at Alice end 100 and Bob end 200 remain identical.
An optical attenuator 440 for attenuating the quantum optical signal to the level of a single photon.
Example III
The application provides a method for rapidly compensating the phase of reference optical signals of Alice end and Bob end, which comprises the following steps:
in step S1, reference optical signals at Alice end and Bob end interfere through a first beam splitter at a third party end (Charlie end).
In step S2, the first detector (D1) (multi-channel single photon detector) acquires an interference result, and the counting condition of the interference result can represent the phase difference between the reference optical signals at Alice end and Bob end.
Wherein the count of the first detector (D1) is as follows:
wherein C represents the count of the first detector (D1); c (C) 0 Is the dark count rate; c (C) 1 Half the particle count rate after complete interference;is the phase difference between Alice and Bob's reference optical signals after transmission through the optical fiber.
And S3, the FPGA board records the counting condition of the first detector (D1) in real time, and is used as a feedback signal of the rapid phase compensation, the FPGA board outputs an analog signal, and the first phase modulator at the Charlie end is regulated in real time to enable the counting rate of the detector channel to be minimum, so that the rapid phase compensation is completed.
Because the frequency interval between the reference optical signal and the quantum optical signal is very small, after the fast phase compensation of the reference optical signal is completed, the phase change of the quantum optical signal is converted from fast drift to slow drift.
Example IV
The application provides a method for carrying out slow phase compensation on quantum optical signals of an Alice end and a Bob end, which is carried out after the fast phase compensation is carried out on reference optical signals of the Alice end and the Bob end.
Specifically, the method of slow phase compensation is as follows:
in the step T1, quantum optical signals at the Alice end and the Bob end interfere through a second beam splitter at the Charlie end.
In step T2, the third detector (D3) (multi-channel single photon detector) acquires an interference result, and the counting condition of the interference result can represent the phase difference between the quantum optical signals at Alice end and Bob end.
And step T3, the FPGA board records the counting condition of the single photon detector in real time, and adjusts the second phase modulator at the Charlie end in real time to minimize the counting rate of the third detector (D3) channel, thereby completing the slow phase compensation.
After the rapid phase compensation is completed based on the reference optical signal, the phase change of the quantum optical signal is converted from rapid drift to slow drift. For example, the frequency interval between the reference optical signal and the quantum optical signal is 50GHz, and the frequency of the quantum optical signal with the wavelength of 1550nm is approximately 200THz, then the ratio of the obtainable relative phase drift rates according to equation (1) is:
(f-f′)ΔL/(fΔL)＝Δf′/f＝50GHz/200THz＝1/4000；
wherein f and f 'are the frequencies of the quantum optical signal and the reference optical signal, respectively, and Δf' is the frequency difference between the two.
Assuming that the rate of phase shift caused by 200km of fiber is 4rad/ms, the rate of phase shift of the quantum optical signal after rapid phase compensation based on the reference optical signal is 0.001rad/ms, which is greatly reduced compared to the previous rate of shift.
According to one embodiment of the present application, the quantum optical signal is an approximate single photon signal, and the transmission of a long distance (more than hundred kilometers) is strongly attenuated, so that the count of the third detector (D3) may be greatly affected by noise, and the effect caused by the phase difference between the quantum optical signals of Alice and Bob cannot be accurately reflected. At this time, a small amount of reference light signals with the same frequency can be inserted by a time division multiplexing method, so that the counting rate of the third detector (D3) is increased to be used as a feedback signal for slow phase compensation.
According to one embodiment of the application, polarization calibration is performed on the reference optical signal and the quantum optical signal simultaneously in the process of fast phase compensation and slow phase compensation.
The method for carrying out polarization calibration on the reference light signal comprises the following steps: acquiring a polarization-calibrated feedback signal in the reference optical signal transmission link using a second detector (D2); the first polarization controller at the Charlie end is adjusted to enable the counting rate of the second detector (D2) to be minimum, so that the polarization of two paths of reference light signals at the Alice end and the Bob end are consistent.
The method for carrying out polarization calibration on the quantum optical signals comprises the following steps: acquiring a quantum light signal polarization calibrated feedback signal using a fourth detector (D4); and adjusting the second polarization controllers of the encoding modules of the Alice end and the Bob end to enable the counting rate of the fourth detector (D4) to be minimum, so that the polarization of two paths of quantum optical signals of the Alice end and the Bob end are consistent. The polarization calibration of the reference optical signal and the quantum optical signal is beneficial to eliminating the influence of other factors, so that the interference result is only related to the phase difference between the reference optical signals at the Alice end and the Bob end or the quantum optical signal.
The beneficial effects achieved by the application are as follows:
(1) The two communication parties respectively use the ultra-stable laser as a signal source, and can infinitely zoom out after finishing frequency correction, and service optical fibers are not needed to be used as a transmission medium for frequency locking.
(2) The phase stabilization scheme of frequency division multiplexing is adopted in the communication process, so that interference between the reference optical signal and the quantum optical signal can be avoided, and after the phase drift of the reference optical signal is calibrated, the relative phase drift rate of the quantum optical signal can be greatly reduced, and the realization of double-field quantum key distribution is facilitated, thereby breaking through the limit of transmission distance.
Based on the above description, those skilled in the art will readily understand that the technical solution provided by the present application can realize frequency locking of two independent laser sources, the relative frequency difference is less than 1KHz to meet the experimental requirement, and the service optical fiber is no longer needed as a transmission medium for frequency locking, so that the system is simplified and the lasers at two remote places are ensured to be independent from each other. The frequency division multiplexing phase stabilization scheme provided by the application adopts two feedback control links, namely the fast phase compensation and the slow phase compensation, can greatly reduce the relative phase drift rate of the quantum optical signals, is beneficial to the realization of a double-field quantum key distribution scheme, and breaks through the limit of transmission distance.
The foregoing is merely exemplary of the present application and is not intended to limit the present application. Various modifications and variations of the present application will be apparent to those skilled in the art. Any modifications, equivalent substitutions, improvements, etc. which are within the spirit and principles of the present application are intended to be included within the scope of the claims of the present application.
Claims (7)
1. A frequency locking and phase stabilization system based on quantum key distribution, the system comprising:
a frequency-locked system based on two-way independent laser sources, the frequency-locked system comprising: the system comprises an ultra-stable laser, a beat frequency module, a spectrum analyzer, a locking and tuning module and a frequency standard instrument;
the ultra-stable laser is used for outputting two independent laser sources;
the beat frequency module is connected with the ultra-stable laser and is used for acquiring beat frequency signals of the two independent laser sources;
the spectrum analyzer is connected with the beat frequency module and is used for measuring the frequency difference between the two independent laser sources according to beat frequency signals;
the locking and tuning module is connected with the frequency standard instrument;
the frequency standard instrument is used for providing frequency standard of the peripheral equipment for the locking and tuning module;
the locking and tuning module is used for tuning the frequencies between the two independent laser sources to be consistent according to the frequency difference between the two independent laser sources based on frequency standards and removing the inherent linear drift amount in the output frequency of the ultra-stable laser;
the system further comprises a generation system for stabilizing the side frequency component,
the system for generating the stable side frequency component comprises: broadband phase modulator, analog signal generator, power amplifier and optical filter;
the analog signal generator is used for generating a radio frequency driving signal and sending the radio frequency driving signal to the broadband phase modulator;
the power amplifier is connected with the analog signal generator and is used for increasing the amplitude of the radio frequency driving signal;
the broadband phase modulator is used for modulating variable frequency components at two sides of a carrier based on light sources of two independent laser sources as the carrier after receiving radio frequency driving signals;
the optical filter is connected with the broadband phase modulator and is used for filtering quantum optical signals and reference optical signals with different wavelengths from side frequency components respectively.
2. The quantum key distribution-based frequency locking and phase stabilizing system according to claim 1, wherein the locking and tuning module comprises an electro-optic modulator and an acousto-optic modulator,
the electro-optic modulator is used for tuning the frequencies between the two independent laser sources to be consistent according to the frequency difference between the two independent laser sources based on the frequency standard;
the acousto-optic modulator is used for removing the inherent linear drift in the output frequency of the ultra-stable laser.
3. The quantum key distribution based frequency locking and phase stabilization system of claim 1, further comprising a wavelength division multiplexing based phase stabilization system comprising: a generation system for stabilizing the side frequency component and a feedback control system for phase compensation;
the stable side frequency component generation system is used for respectively generating two-ground reference optical signals and quantum optical signals according to two-ground independent laser sources;
the feedback control system of phase compensation includes: a fast phase compensation feedback system and a slow phase compensation feedback control system;
the fast phase compensation feedback system is used for carrying out phase compensation on the reference optical signals at two places;
the slow phase compensation feedback control system is used for carrying out phase compensation on the quantum optical signals at two places.
4. The quantum key distribution-based frequency locking and phase stabilization system of claim 3, wherein the phase compensated feedback control system comprises a wavelength division multiplexer, a wavelength division demultiplexer, a beam splitter, a single photon detector, a phase modulator, and a board card;
the wavelength division multiplexer is used for carrying out combined transmission on the reference optical signal and the quantum optical signal;
the wavelength division demultiplexer is used for carrying out branching transmission on the reference optical signal and the quantum optical signal;
the beam splitter is used for carrying out single photon interference based on a phase field on the reference optical signal and the quantum optical signal at a third party end respectively;
the single photon detector is used for detecting error photon signals generated by relative phase drift in a feedback link of the reference optical signals and the quantum optical signals;
the phase modulator is used for carrying out rapid phase compensation based on the feedback result of the reference optical signal; performing slow phase compensation based on a feedback result of the quantum optical signal;
and the board card is used for regulating and controlling the phase modulator to finish phase compensation on the transmission link according to the output result of the single photon detector.
5. The quantum key distribution-based frequency locking and phase stabilization system of claim 3, further comprising an encoding module for encoding the quantum optical signal.
6. The frequency locking and phase stabilizing method based on quantum key distribution is characterized by comprising the following steps of:
acquiring a frequency difference between the first independent laser source and the second independent laser source;
tuning the frequencies of the two independent laser sources to be consistent according to the acquired frequency difference based on an external frequency standard;
after the frequencies of the two independent laser sources are tuned to be consistent, removing inherent linear drift amounts in the output frequencies of the first independent laser source and the second independent laser source;
the method further comprises the steps of:
phase stabilization method based on wavelength division multiplexing: performing fast phase compensation on reference optical signals of the transmitting end and the receiving end, and performing slow phase compensation on quantum optical signals of the transmitting end and the receiving end;
the fast phase compensation method comprises the following steps:
interfering the reference light signals of the transmitting end and the receiving end through a first beam splitter of a third square end;
a first detector is used for acquiring a phase difference between reference light signals of a transmitting end and a receiving end after interference;
the first phase modulator at the third side is adjusted in real time to minimize the count rate of the first detector channel to accomplish fast phase compensation.
7. The quantum key distribution-based frequency locking and phase stabilization method of claim 6, wherein the slow phase compensation method comprises:
the quantum optical signals of the transmitting end and the receiving end are interfered by a second beam splitter of the third square end;
a third detector is used for obtaining the phase difference between quantum optical signals of the transmitting end and the receiving end after interference;
the second phase modulator at the third side is adjusted in real time to minimize the count rate of the third detector channel to accomplish the slow phase compensation.
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| CN116192366A (en) * | 2021-11-29 | 2023-05-30 | 科大国盾量子技术股份有限公司 | TF-QKD system and method |
| NL2030077B1 (en) | 2021-12-08 | 2023-06-22 | Univ Delft Tech | Method and system for matching frequencies of lasers in a quantum communication system |
| CN115529088A (en) * | 2022-08-18 | 2022-12-27 | 北京量子信息科学研究院 | Frequency difference calibration device of remote light source and quantum key distribution system |
| CN116633541B (en) * | 2023-07-19 | 2023-09-29 | 济南量子技术研究院 | Double-field light source frequency locking method and system based on air chamber frequency reference |
| CN117375833B (en) * | 2023-12-07 | 2024-03-08 | 济南量子技术研究院 | Transmitting end full-on-chip TF-QKD system based on chip optical comb |
| CN117459152B (en) * | 2023-12-20 | 2024-04-30 | 济南量子技术研究院 | TF-QKD implementation method based on radio frequency and optical frequency reference multiplexing of optical frequency comb |
| CN117914411A (en) * | 2024-03-19 | 2024-04-19 | 济南量子技术研究院 | Phase feedback method for TF-QKD system |
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