CN113645027B - Key distribution system and method for air-sea cross-medium wireless quantum communication - Google Patents
Key distribution system and method for air-sea cross-medium wireless quantum communication Download PDFInfo
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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
- H04L9/0816—Key establishment, i.e. cryptographic processes or cryptographic protocols whereby a shared secret becomes available to two or more parties, for subsequent use
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Abstract
A secret key distribution system and method of wireless quantum communication comprise: the invention is based on a decoy state protocol, improves the modulation rate, contrast and stability of blue-green spectrum quantum light sources required by distributing empty mass sub-keys, and realizes high-precision time frame synchronization of quantum bits by using synchronous light and time-dependent photon counting equipment. Further, the underwater penetration distance and penetration depth of the distribution of the empty and massive sub-keys are expanded, and a foundation is laid for flexible and efficient unrepeatered secret communication among the underwater vehicle in the middle and far sea, the underwater monitoring node, the land platform and the satellite.
Description
Technical Field
The invention relates to a technology in the field of quantum communication, in particular to a key distribution system and a key distribution method suitable for wireless quantum communication under an air-sea cross-medium scene.
Background
The existing encryption algorithms such as the RSA public key algorithm generally utilize the mathematical problem which is difficult to solve reversely to encrypt, and as long as enough computing resources are available, the algorithms can be cracked theoretically, so that only the computing safety can be realized. The quantum communication utilizes the basic principle of quantum mechanics and combines a one-time pad mode, so that the information theory safety independent of the computational complexity can be realized. At present, quantum communication technologies of an atmospheric free space channel and an optical fiber wired channel based on real devices are mature, and a cross-medium quantum communication technology from atmosphere to underwater free space is yet to be developed. The seawater covers about 70% of the earth surface, and the underwater free space is used as a third quantum channel except the atmosphere and the optical fiber, so that a quantum link for connecting a satellite and an underwater terminal is explored, and the final jigsaw for constructing the global quantum communication network is formed.
Blue-green laser based optical communication technology is the best choice for cross-medium communication because it can utilize the common light-transmitting window of atmosphere and seawater. However, due to the huge attenuation of the seawater on the photon signals, the underwater penetration distance of the existing empty massive sub-key distribution is limited to a few meters.
Disclosure of Invention
Aiming at the defects in the prior art, the invention provides a key distribution system and a key distribution method for wireless quantum communication, which improve the modulation rate, the contrast and the stability of a blue-green spectrum quantum light source required by the distribution of an empty quantum key based on a decoy state protocol, and realize the high-precision time frame synchronization of quantum bits by using synchronous light and time-dependent photon counting equipment. Further, the underwater penetration distance and penetration depth of the distribution of the empty and massive sub-keys are expanded, and a foundation is laid for flexible and efficient unrepeatered secret communication among the underwater vehicle in the middle and far sea, the underwater monitoring node, the land platform and the satellite.
The invention is realized by the following technical scheme:
the invention relates to a key distribution system for wireless quantum communication, which adopts a three-strength decoy state protocol to carry out complete receiving and sending of air-sea cross-medium quantum key distribution and specifically comprises the following steps: the transmitting end that light source control system, optical coding device, classic light signal receiving system and beam expanding lens group constitute and receive collimating lens group, optical decoding device, classic light signal transmitting system and the receiving end that post processing system constitutes, wherein: the light source control system outputs three-intensity quantum optical signals and classical optical signals, the optical coding device couples and collects the three-intensity quantum optical signals and the classical optical signals, then attenuates the three-intensity quantum optical signals and the classical optical signals to required intensity to complete quantum light polarization coding and beam combination processing, the classical optical signal receiving system receives the classical optical signals, the classical optical signals are transmitted to the optical decoding device through the beam expanding lens group and the receiving collimating lens group, the optical decoding device separates and detects the three-intensity quantum optical signals and the classical optical signals and carries out polarization compensation and polarization measurement on the quantum light, the classical optical signal transmitting system transmits the classical optical signals to the classical optical signal receiving system, and the post-processing system records single photon counting and photon arrival time and carries out quantum key distribution post-processing calculation.
The three-intensity sub-light source and the classical light signal are generated by a multi-channel integrated laser in the light source control system after outputting an electric signal pulse by using a field programmable gate array based on a random number generated by a micro-processor in the light source control system.
The coupling collection is realized through an adjustable optical fiber collimator and an adjustable angle half-wave plate.
The beam combination treatment is realized through a dichroic mirror.
The separation and detection are realized by a dichroic mirror, band-pass filters with different central wavelengths and a polarization-independent beam splitter.
Technical effects
The invention integrally solves the problems of low modulation rate, poor contrast, unstable brightness and the like of the blue-green spectrum quantum light source in the prior art; a key distribution system of water-air wireless quantum communication without transfer and across media based on a decoy state protocol realizes high-precision time frame synchronization of underwater sparse quantum bits by using a detection mode of synchronous optical signals and time-dependent photon counting. The underwater penetration depth of the lured state empty mass sub-key distribution is further expanded, and the method is suitable for the quantum key distribution of medium and long distance underwater free space channels and empty sea cross-medium channels.
Drawings
FIG. 1 is a schematic view of an overall system of the present invention;
FIG. 2 is a schematic diagram of a transmitting end device according to the present invention;
FIG. 3 is a diagram of an optical encoding device for an emitting end of the present invention;
fig. 4 is a schematic diagram of a receiving end device according to the present invention.
Detailed Description
As shown in fig. 1, the present embodiment relates to a system suitable for large-loss, air-seawater cross-medium channel quantum key distribution, which includes: the system comprises a transmitting end and a receiving end, wherein the transmitting end is used for generating a quantum light source and a classical light source, receiving and decoding a classical light signal while carrying out polarization coding on quantum light based on a decoy state protocol, and the receiving end is used for transmitting the classical light signal, receiving the quantum light signal and the classical light signal, carrying out polarization measurement analysis on the quantum light signal, decoding the classical light signal and carrying out post-processing to obtain a final key, and the system specifically comprises: the system comprises a light source control system, an optical coding device, a classical light signal receiving system, a transmitting end consisting of a beam expanding lens group, a receiving collimating lens group, an optical decoding device, a classical light signal transmitting system and a receiving end consisting of a post-processing system.
As shown in fig. 2, the transmitting end includes: the system comprises a light source control system for generating three intensities of 450nm quantum light sources and 520nm classical light signals required by a decoy state protocol, a classical light signal receiving system and an optical coding device for polarization coding of quantum light and beam combination.
The light source control system comprises: a Field Programmable Gate Array (FPGA) for generating electrical signal pulses, a first microprocessor CPU1 and a multiplexed integrated laser, wherein: the FPGA outputs five paths of electric signal pulses to the multi-path integrated laser, so that high-speed and high-contrast direct modulation of the blue-green spectrum quantum light source is realized, and the distribution requirement of the medium-distance and long-distance hollow mass sub-keys is met.
The five electric signal pulses comprise: four paths of pulse signals with 50MHz frequency and 5ns pulse width and one path of on-off keying modulation electric signals generated according to a decoy state protocol, wherein: the pulse signal has three pulse intensities, which respectively correspond to two decoy state intensities and one signal state intensity in the three-intensity decoy state protocol.
The multi-path integrated laser comprises: four 450nm blue laser diodes and a 520nm green laser diode packaged by a metal shell, a constant current driving chip and a temperature control chip, wherein: the electric signal input ports of the laser diodes are respectively matched with five electric signal pulses of the FPGA, five lasers are respectively output through the single-mode optical fibers, the constant current chip provides constant current for the five laser diodes to reach the light emitting threshold value of the five laser diodes, the temperature control chip conducts high-precision regulation and control on the spectrum and the brightness of the blue-green spectrum quantum light source, the temperature of the laser diodes is controlled within the range of the constant value, the light emitting intensity and the stability of the spectrum of the laser diodes are guaranteed, and the practical safety of air-sea quantum key distribution is improved.
In this embodiment, each laser diode is provided with a high-stability constant current source to ensure that the laser clock is in the excitation mode.
As shown in fig. 3, the optical encoding apparatus includes: four signal light paths L1-L4 and a classical signal light path L5, wherein: the light paths L1 and L2 are combined through a first polarization beam splitter PBS1 and then enter a transmission light path of a polarization-independent beam splitter NPBS 1; the light paths L3 and L4 are combined through a polarization beam splitter PBS2 and enter a reflection path of a polarization-independent beam splitter NPBS1 after passing through a half-wave plate with a fixed angle of 22.5 degrees;
the L1 optical path includes: the device comprises a first adjustable optical fiber collimator C1, a first angle-adjustable half-wave plate H1 and a first fixed horizontal angle polarization plate P1;
the L2 optical path includes: a second adjustable optical fiber collimator C2, a second adjustable angle half-wave plate H2 and a second fixed vertical angle polarization plate P2;
the L3 optical path includes: a third adjustable optical fiber collimator C3, a third adjustable angle half-wave plate H3 and a third fixed horizontal angle polarization plate P3;
the L4 optical path includes: a fourth adjustable optical fiber collimator C4, a fourth adjustable angle half-wave plate H4 and a fourth fixed vertical angle polarization plate P4;
the L5 optical path is emitted through a seventh adjustable optical fiber collimator C7;
the light of the L1-L4 light path is combined through the NPBS1, is coupled into the single mode fiber through the fifth adjustable fiber collimator C5, and the single mode coupling collection efficiency of the four paths of signal light entering the C5 is about 50% by adjusting the direction angle and the focal length of the C1-C5; the signal light output by the single-mode fiber is emitted through a sixth adjustable fiber collimator C6; through fine adjustment of the angle-adjustable half-wave plates H1-H4, the output intensity of each signal can be further adjusted, so that the four signal intensities after C6 transmission are completely balanced, and the signal light path transmitted by C6 is combined with the light of the L5 light path through the second dichroic mirror DM2 after passing through the optical attenuator ATT.
All devices of the optical coding device are packaged in a metal shell in an embedded fixing mode, and only five optical signal input interfaces and one optical beam output port are reserved outside the optical coding device.
The classical optical signal receiving system comprises: a sixth detector D6 and a first time dependent photon counting system TDC1.
The beam expanding lens group comprises: the first and second lenses Len1, len2.
The receiving collimating lens group comprises: third and fourth lenses Len3, len4.
As shown in fig. 4, the receiving end includes: the device comprises an optical decoding device, a post-processing system and a classical optical signal transmitting system, wherein the optical decoding device is used for separating and detecting classical optical signals and quantum optical signals and carrying out polarization compensation and polarization measurement on the quantum optical signals, the post-processing system is used for carrying out key distribution post-processing on basis, error code estimation, key negotiation, privacy amplification and the like and analyzing and decoding on the received classical optical signals, and the classical optical signal transmitting system is used for generating and transmitting the classical optical signals.
The optical decoding device comprises: the device comprises a wave plate group for polarization compensation, a dichroic mirror DM3 for blue-green light separation, a band-pass filter BP1 with the central wavelength of 520nm, a band-pass filter BP2 with the central wavelength of 450nm, and a sensor for randomly selecting a measurement basis vector, wherein the wavelength of the wave plate group is 50:50 polarization-independent beam splitters NPBS2 and two-way polarization measuring device, wherein: the 520nm classical optical signal is reflected by the DM2, separated from the quantum light and enters a single photon detector D5.
The wave plate set for polarization compensation comprises: two quarter-wave plates Q1, Q2 and a half-wave plate H6.
The two polarization detection devices respectively comprise: (1) the PBS3 detects horizontal and vertical polarization signals, a transmission path of the horizontal and vertical polarization signals enters a first single-photon detector D1, and a reflection path of the horizontal and vertical polarization signals enters a second single-photon detector D2; (2) the half-wave plate H7 and the polarization beam splitter PBS4 with the fixed angle of 22.5 degrees detect +/-45-degree polarization signals, a transmission path of the signals enters the third single-photon detector D3, and a reflection path of the signals enters the fourth single-photon detector D4.
The classical optical signal transmitting system comprises: a second microprocessor CPU2, a green laser LD and a third polarization independent beam splitter NPBS3.
The aftertreatment system comprises: a five-channel second time-dependent photon counting system TDC2 and a second microprocessor CPU2, wherein: the TDC2 records the single photon count and the photon arrival time, and performs quantum key distribution post-processing calculation through the second microprocessor CPU 2.
The key distribution method of the wireless quantum communication based on the system comprises the following steps:
step 1, debugging a light source: and controlling the front four paths of the FPGA to generate electric signal pulses with 50MHz repetition frequency by using the random number generated by the first microprocessor CPU1, and respectively inputting the electric signal pulses to the front four paths of 450nm lasers. Adjusting the threshold current of the multi-path integrated laser and the modulation voltage of the FPGA to ensure that the modulated laser pulse has three intensities, wherein the intensity ratio is 4:1:0, and the number of the three intensity pulses accounts for 2:1:1. and a CPU1 and an FPGA are utilized to generate a modulation signal of classical light and input the modulation signal into a fifth 520nm laser.
Step 2, optical coding calibration: inputting the L1-L5 light path laser into the optical coding device according to the figure 2, and sequentially adjusting each optical fiber collimator to ensure that the coupling efficiency from C1-C4 to C5 is about 50 percent approximately; then sequentially adjusting H1-H4 to enable the optical power of each path emitted from C6 to be equal; and adjusting the optical attenuator ATT to attenuate the three intensities of each path of output optical pulse to average 0.9 photon, 0.225 photon and 0 per pulse, thereby completing the quantum optical polarization encoding. Finally, C7 is adjusted to combine the classical light and the quantum light through DM 2.
Step 3, system calibration: preparing a signal with specific polarization at an emitting end, performing polarization detection at a receiving end, and adjusting three compensating wave plates Q1, Q2 and H6 to ensure that the error between the polarization state detected at the receiving end and the polarization state prepared at the emitting end is less than 2 percent, thereby completing system calibration.
Step 4, key distribution: the classical light at the transmitting end is a 100KHz modulated pulse signal and can be used as a time synchronization light source. The quantum light with different intensities generated based on the decoy state and the classical light are combined to reach a receiving end through an air-seawater channel. At the receiving end, the classical light and the quantum light are separated through DM3, the 520nm classical light is detected by a detector D5, and the time frame can be restored through a post-processing system. The 450nm quantum light is respectively detected by detectors D1-D4 through a polarization detection device, and a photon event and time are recorded through a time-dependent photon counting system.
And 5, post-treatment: the transmitting end and the receiving end carry out high-precision time frame synchronization through a classical optical transceiving system to finish the processes of base error estimation, error code estimation, key negotiation, privacy amplification and the like, and the method specifically comprises the following steps: quantum optical signals become extremely sparse photon streams after undergoing huge water body attenuation, and time frame information is difficult to extract and synchronize. Therefore, a path of classical optical signal is introduced at the transmitting end to serve as a synchronous reference. The classical light source is provided with a high-stability constant-current source to ensure that a laser clock is in an excitation mode. The FPGA generates a path of 100KHz low-frequency narrow pulse which is completely synchronous with the three-strength high-speed electric pulse according to a decoy state protocol, and the pulse width is 5ns. The detector D5 detects the classical optical signal of single photon magnitude and records the arrival time information of the photon by utilizing a time correlation single photon counting system TDC 2. And analyzing the arrival time of the photons of the classical light and aligning the arrival time with the arrival time of the quantum optical signal at the post-processing end, thereby realizing the high-precision time frame synchronization of the sparse quantum signal.
Through specific practical experiments, in Jerlovtype III (3C) water quality with attenuation of 450nm blue light signals of 0.8dB/m, a 450nm light source with repetition frequency of 50MHz is taken as a quantum light source, 520nm green light with repetition frequency of 100KHz is taken as a classical light source, and the average photon number per pulse of three intensity pulses in a decoy state is taken as: signal state mu s =0.9, decoy state μ 1 =0.225, vacuum state μ 2 =0, and the mixing ratio thereof is taken to be 2:1:1, the device can realize the distribution of empty mass sub-keys with the penetration depth of 30m underwater, and the following results are obtained: time frame synchronization accuracy<0.5ns, mean fidelity of quantum state>98.2 percent, the average sieve base rate is 48.9 percent, the final secret key rate is 220.5bit/s, and the average quantum error rate is 1.76 percent.
Compared with the prior art, the device can increase the repetition frequency of the blue-green spectrum quantum light source to 50MHz, the system synchronization precision to less than 0.5ns, and the empty quantum key distribution underwater penetration distance from 3m to 30m, which is equivalent to 345m clean seawater.
The foregoing embodiments may be modified in many different ways by one skilled in the art without departing from the spirit and scope of the invention, which is defined by the appended claims and not by the preceding embodiments, and all embodiments within their scope are intended to be limited by the scope of the invention.
Claims (6)
1. A key distribution method of a key distribution system based on wireless quantum communication is characterized in that the key distribution system comprises: the device comprises a light source control system, an optical coding device, a classical light signal receiving system, a transmitting end consisting of a beam expanding lens group, a receiving collimating lens group, an optical decoding device, a classical light signal transmitting system and a receiving end consisting of a post-processing system;
the light source control system comprises: a field programmable gate array for generating pulses of an electrical signal, a first microprocessor and a multiplexed integrated laser, wherein: the FPGA outputs five paths of electric signal pulses to the multi-path integrated laser to realize the direct modulation of the blue-green spectrum quantum light source so as to meet the distribution requirement of the medium-distance and long-distance empty mass sub-keys;
the optical encoding device comprises: four signal light paths and a classical signal light path, wherein: the first signal light path and the second signal light path are incident to a transmission light path of the polarization-independent beam splitter after being combined by the first polarization beam splitter; the third signal light path and the fourth signal light path are combined through the polarization beam splitter and enter a reflection path of the polarization-independent beam splitter after passing through the half-wave plate;
the first signal light path comprises: the device comprises a first adjustable optical fiber collimator, a first adjustable angle half-wave plate and a first fixed horizontal angle polarization plate;
the second signal light path comprises: the second adjustable optical fiber collimator, the second adjustable angle half-wave plate and the second fixed vertical angle polarization plate;
the third signal optical path includes: a third adjustable optical fiber collimator, a third adjustable angle half-wave plate and a third fixed horizontal angle polarization plate;
the fourth signal optical path includes: the fourth adjustable optical fiber collimator, the fourth adjustable angle half-wave plate and the fourth fixed vertical angle polarization plate;
the classical signal light path is emitted by a seventh adjustable optical fiber collimator;
the classical optical signal receiving system comprises: a sixth detector and a first time-dependent photon counting system;
the beam expanding lens group comprises: first and second lenses;
the receiving collimating lens group comprises: third and fourth lenses;
the classical optical signal transmission system comprises: a second microprocessor, a green laser, and a third polarization independent beam splitter;
the aftertreatment system comprises: a five-channel second time-dependent photon counting system and a second microprocessor, wherein: the five-channel second time-dependent photon counting system records the single photon counting and the photon arrival time, and carries out the post-processing calculation of the quantum key distribution through the second microprocessor;
the optical decoding device comprises: the device comprises a wave plate group for polarization compensation, a dichroic mirror for blue-green light separation, a band-pass filter with the central wavelength of 520nm, a band-pass filter with the central wavelength of 450nm, a polarization-independent beam splitter for randomly selecting and measuring basis vectors and two polarization measuring devices, wherein classical light signals are reflected, separated from quantum light and enter a single-photon detector;
the key distribution method comprises the following steps:
step 1, debugging a light source: controlling the front four paths of the FPGA to generate electric signal pulses with 50MHz repetition frequency by using random numbers generated by the first microprocessor, and respectively inputting the electric signal pulses to the front four paths of laser diodes; adjusting the threshold current of the multi-path integrated laser and the modulation voltage of the FPGA to enable the modulated laser pulse to have three intensities, wherein the intensity ratio is 4:1:0, and the number of the three intensity pulses accounts for 2:1:1; a first microprocessor and an FPGA are used for generating a modulation signal of classical light and inputting the modulation signal into a fifth laser diode;
step 2, optical coding calibration: respectively inputting the laser of the first to fourth signal light paths and the laser of the classical signal light path into an optical coding device, and sequentially adjusting each optical fiber collimator to enable the coupling efficiency to be approximately 50% and enable the optical power of each path emitted from the sixth adjustable optical fiber collimator to be equal; adjusting the optical attenuator to attenuate the three intensities of each path of output optical pulse to average 0.9 photon, 0.225 photon and 0 photon per pulse, thereby completing quantum optical polarization encoding; finally, a seventh adjustable optical fiber collimator is adjusted, so that the classical light and the quantum light are combined through a second dichroic mirror;
step 3, system calibration: preparing a signal with specific polarization at an emitting end, performing polarization detection at a receiving end, and adjusting a compensation wave plate to enable the error between the polarization state detected at the receiving end and the polarization state prepared at the emitting end to be less than 2%, so that the system calibration is completed;
step 4, key distribution: the classical light at the transmitting end is a 100KHz modulated pulse signal serving as a time synchronization light source, and quantum light and classical light beams with different intensities generated based on a decoy state reach the receiving end through an air-seawater channel; at a receiving end, classical light and quantum light are separated through a third dichroic mirror, fifth channel classical light is detected by a detector, and a time frame is restored through a post-processing system; the quantum light of 450nm is respectively detected by the first detector, the second detector, the third detector and the fourth detector through a polarization detection device, and a photon event and time are recorded through a time-dependent photon counting system;
and step 5, post-treatment: the transmitting end and the receiving end carry out time frame synchronization through a classical optical transceiving system to finish the processes of base error estimation, error code estimation, key negotiation and privacy amplification, and the method specifically comprises the following steps: quantum optical signals are attenuated by huge water bodies and then become extremely sparse photon streams, and time frame information is difficult to extract and synchronize; therefore, a path of classical optical signal is introduced at the transmitting end to be used as synchronous reference; the classical light source is provided with a high-stability constant-current source to ensure that a laser clock is in an excitation mode; the FPGA generates a path of 100KHz low-frequency narrow pulse which is completely synchronous with the three-strength high-speed electric pulse according to the decoy state protocol, and the pulse width is 5ns; a fifth detector detects classical optical signals of single photon magnitude and records arrival time information of photons by utilizing a five-channel second time correlation photon counting system; and analyzing the arrival time of the photons of the classical light and aligning the arrival time of the photons with the arrival time of the quantum optical signal at the back-end processing end, thereby realizing the time frame synchronization of the sparse quantum signal.
2. The key distribution method of claim 1, wherein said five electrical signal pulses comprise: four paths of pulse signals with 50MHz frequency and 5ns pulse width and one path of on-off keying modulation electric signals generated according to the trap state protocol, wherein: the pulse signal has three kinds of pulse intensity, which respectively correspond to two kinds of decoy state intensity and one kind of signal state intensity in the three-intensity decoy state protocol.
3. The key distribution method of claim 2, wherein the multiplexed integrated laser comprises: four 450nm blue laser diodes and a 520nn green laser diode, constant current drive chip and temperature control chip packaged with metal casing, wherein: the electric signal input ports of the laser diodes are respectively matched with five electric signal pulses of the FPGA, five lasers are respectively output through the single mode fiber, the constant current chip provides constant current for the five laser diodes to reach the light emitting threshold value of the five laser diodes, the temperature control chip regulates and controls the spectrum and the brightness of the blue-green spectrum quantum light source, the temperature of the laser diodes is controlled within the range of the constant value, the light emitting intensity and the spectrum stability of the laser diodes are guaranteed, and the practical safety of air-sea quantum key distribution is improved.
4. A method as claimed in claim 3, wherein each laser diode is provided with a stable constant current source to ensure that the laser clock is in the active mode.
5. The key distribution method according to claim 1, wherein the first to fourth signal light paths are combined and coupled into the single-mode fiber through a fifth tunable fiber collimator, and the signal light output from the single-mode fiber is emitted through a sixth tunable fiber collimator by adjusting the direction angle and the focal length; the four paths of transmitted signals are completely balanced in intensity by adjusting each path of angle-adjustable half-wave plate, and the transmitted signal light path is combined with classical signal light path light through a second dichroic mirror after passing through an optical attenuator.
6. The key distribution method according to claim 1, wherein the two-way polarization measurement device comprises: (1) the polarization beam splitter detects horizontal and vertical polarization signals, a transmission path of the polarization beam splitter enters the first single-photon detector, and a reflection path of the polarization beam splitter enters the second single-photon detector; (2) the half-wave plate and the polarization beam splitter with the fixed angle of 22.5 degrees detect +/-45-degree polarization signals, a transmission path of the polarization signal enters the third single-photon detector, and a reflection path of the polarization signal enters the fourth single-photon detector.
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