CN113992323B - Chip-based measuring equipment independent quantum key distribution system and method - Google Patents
Chip-based measuring equipment independent quantum key distribution system and method 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
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Abstract
The invention discloses a measuring equipment irrelevant quantum key distribution system and a method, which can form a complete measuring equipment irrelevant quantum key distribution system by designing and realizing a silicon-based and superconducting material hybridization detection end chip, combining the chip with a laser, an intensity modulator, a phase modulator, an attenuator, a polarization controller and other traditional optical devices, and can obtain a key rate equivalent to a GHz clock frequency under a lower clock frequency by utilizing a time multiplexing technology due to the benefit of ultrafast detector response.
Description
Technical Field
The invention belongs to the technical field of quantum information, and particularly relates to a measuring equipment independent quantum key distribution (MDI-QKD) system and method based on a waveguide integrated superconducting nanowire single-photon detector.
Background
The security of quantum key distribution is guaranteed by the correctness of physical laws, and theoretically, quantum key distribution is an absolutely safe communication mode. Due to various vulnerabilities of an actual system, there are attacks against the source end and the detection end of a Quantum Key Distribution (QKD) system. Measuring device-independent quantum key distribution (MDI-QKD) was first proposed in 2012 by the Lo group with two significant advantages: firstly, the method comprises the following steps: it removes all detector-side vulnerabilities from quantum key distribution; II, secondly, the method comprises the following steps: after the spoofing state is added, the performance in the actual system is greatly improved. Zhou et al propose a four-strength method (one signal state strength and three spoof state strengths), which uses the Z-base for key generation and only the X-base for spoof state analysis, and has high efficiency at a small data scale. For the security analysis of the limited code length, the feasibility of long distance measurement device independent quantum key distribution (MDI-QKD) implementation in the reasonable time range of the prior art and signal transmission is proved.
Experimentally, liu et al reported the first demonstration of measuring device-independent quantum key distribution (MDI-QKD) using random modulation of the encoded and decoy states over 50km of fiber. Two milestone experiments measuring device-independent quantum key distribution (MDI-QKD) were reported in 2016. Yin et al extended the communication distance of the measuring device independent quantum key distribution (MDI-QKD) to 404km from scratch by optimizing the parameters and using ultra low loss fiber. In addition, the key rate achieved in this experiment at 100km distance was approximately 3kbps, sufficient to encode a voice message one-time pad. Thereafter, comandar et al increased the system clock frequency of the measuring device independent quantum key distribution (MDI-QKD) to 1GHz by using seed laser technology. In 2020, wei et al realized a 1.25GHz silicon chip transmitter-based polarization-encoded measurement device independent quantum key distribution (MDI-QKD) system. In 2021, woodward et al obtained considerable key rates at a clock frequency of 1GHz by laser injection locking technology.
The combination of the optical chip and the measuring device independent quantum key distribution (MDI-QKD) can realize a novel central network of untrusted relays. In this configuration, each user only needs a compact and small transmitter chip, and the relay has a large and expensive measurement system that can be shared by all users. A chipized measuring device independent quantum key distribution (MDI-QKD) network is one solution for future low cost, scalable untrusted relay QKD-based networks. The transmitting end of a measuring device independent quantum key distribution (MDI-QKD) system is well realized, but a detecting end chip is basically blank. Moreover, the technical difficulty of realizing GHz clock frequency is high, the cost is high, how to obtain higher key rate with lower clock frequency and realize a low-cost key distribution system is a difficult problem before the construction of a QKD network.
Disclosure of Invention
The purpose of the invention is as follows: in order to make up the technical vacancy of an irrelevant quantum key distribution (MDI-QKD) detection end chip of the measuring equipment and solve the problem of low key rate caused by low clock frequency, the invention provides a measuring equipment irrelevant quantum key distribution (MDI-QKD) system and a method based on a waveguide integrated superconducting nanowire single-photon detector, a silicon-based and superconducting material hybrid detection end chip is designed and realized, and the detection end chip is combined with traditional optical devices such as a laser, an intensity modulator, a phase modulator, an attenuator, a polarization controller and the like to form a complete measuring equipment irrelevant quantum key distribution system; the detection end chip of the invention is combined with a chip transmitting end, so that a low-cost and large-scale metropolitan quantum communication network can be realized.
The technical scheme is as follows: a detection end chip for a detection end of a measurement equipment irrelevant quantum key distribution system is used for receiving pulses from a transmission end of the measurement equipment irrelevant quantum key distribution system and carrying out Bell state measurement and result processing; the device comprises a grating coupler for receiving pulses from a transmitting end, a 50 beam splitter and a waveguide integrated superconducting nanowire single photon detector for detecting photons; the grating coupler is connected with a 50 beam splitter through an optical waveguide, and the waveguide integrated superconducting nanowire single photon detector is connected with the output end of the optical waveguide through the optical waveguide.
The invention also discloses a measuring equipment irrelevant quantum key distribution system, which comprises a transmitting end Alice, a transmitting end Bob and a detecting end Charlie; the transmitting end Alice and the transmitting end Bob encode the secret key by using the modulated weak coherent pulse set and the time box quantum bit to obtain an encoded time box quantum bit, and the encoded time box quantum bit is sent to the detecting end Charlie; the detection end Charlie comprises a detection end chip and a post-processing system, wherein the detection end chip is used for carrying out optimal Bell state measurement according to an incoming quantum bit code, the detection end chip is used for measuring the detection end of the equipment irrelevant quantum key distribution system, and the post-processing system is used for receiving an electric signal output by the detection end chip and processing the electric signal to obtain a security key.
Preferably, the transmitting terminal Alice and the transmitting terminal Bob both comprise encoding modules with time box encoding functions; each of the encoding modules includes:
a laser for emitting continuous laser light;
the polarization beam splitter is used for splitting incident laser and partially outputting light to the first intensity modulator;
a first intensity modulator for chopping the incoming portion of light into pulses;
the second intensity modulator is used for carrying out intensity modulation for realizing the decoy state protocol;
a phase modulator for applying a pi phase to the | - > state and a 0 phase to the | + > state in the X basis, as well as phase randomization;
an attenuator for adjusting the average photon number of the weak coherent pulse and simulating propagation loss in the optical fiber;
the electric polarization controller is used for adjusting the polarization of the pulse input to the detection end Charlie;
the random number generator is used for generating random numbers and storing the random numbers to the PC;
the PC is used for generating a coding sequence by using a random number and sending the coding sequence to an arbitrary wave generator;
an arbitrary wave generator for generating an electrical signal applied to the first intensity modulator, the second intensity modulator and the phase modulator according to the code sequence.
Preferably, the frequency of the other part of the output light of the polarization beam splitter of the transmitting end Alice and the other part of the output light of the polarization beam splitter of the transmitting end Bob is beat-frequency on the first beam splitter, a frequency difference between the laser of the transmitting end Alice and the laser of the transmitting end Bob is obtained through the measurement of a first photoelectric detector, and the frequency difference between the two lasers is within 10MHz through the feedback adjustment of the lasers of the transmitting end Alice and the transmitting end Bob.
Preferably, the laser, the polarization beam splitter, the first intensity modulator, the second intensity modulator and the phase modulator are connected in sequence through a polarization maintaining optical fiber.
Preferably, each of the encoding modules further includes a second beam splitter and a second photodetector, an input end of the second beam splitter is connected to the phase modulator, and an output end of the second beam splitter is connected to the second photodetector and the attenuator, respectively.
Preferably, the following components: the phase modulator, the attenuator and the electric polarization controller are connected in sequence through a single-mode optical fiber.
Preferably, in the PC of the transmitting terminal Alice and the transmitting terminal Bob, the coding sequence A is generated by using random numbers 1 ,A 2 ,...,A n And B 1 ,B 2 ,...，B n Wherein, the coding sequence A 2 Comparison code sequence A 1 Delay time tau, code sequence A 3 Comparison code sequence A 2 Delay time tau by whichAnd so on; analogously, code sequence B 2 Comparison coding sequence B 1 Delay time tau, code sequence B 3 Comparison coding sequence B 2 Delaying time tau, and so on; coding sequence A 1 ，A 2 ，...，A n Combining the sequences into a group of sequences according to time, and sending the combined sequences to an arbitrary wave generator of the transmitting end Alice to generate an electric signal; coding sequence B 1 ，B 2 ，...，B n Are combined into a group of sequences according to time, and the combined sequences are sent to an arbitrary wave generator of the transmitting terminal Bob to generate an electric signal.
The invention also discloses a quantum key distribution method, which comprises the following steps:
determining the time interval t of coding according to the recovery time of the superconducting nanowire single photon detector, and generating a coding sequence A by using a random number 1 ，A 2 ，...，A n And B 1 ，B 2 ，...，B n Wherein, the coding sequence A 2 Comparison code sequence A 1 Delay time tau, code sequence A 3 Comparison code sequence A 2 Delaying time tau, and so on; analogously, code sequence B 2 Comparing coding sequence B 1 Delay time tau, code sequence B 3 Comparison coding sequence B 2 Delaying time tau, and so on; coding sequence A 1 ，A 2 ，...，A n Combining the sequences into a group of sequences according to time, and obtaining an electric signal of a quantum bit of a time box generated and coded by a modulation transmitting terminal Alice based on the combined sequences; coding sequence B 1 ，B 2 ，...，B n Combining the sequences into a group of sequences according to time, and obtaining an electric signal of a quantum bit of a time box generated and coded by a modulation transmitting terminal Bob based on the combined sequences;
based on the obtained electric signal, controlling a first intensity modulator to chop the continuous wave into pulses and carrying out Z-based coding to obtain | e>And | l>(ii) a Controlling a second intensity modulator to perform decoy state modulation, and modulating three decoy states with different intensities; controlling the phase modulator to apply a phase generating X-base encoding, represented as:
and
and phase randomization, which adjusts the average photon number of weak coherent pulses output by the phase modulator through an attenuator and simulates the propagation loss in the optical fiber, and adjusts the polarization of the pulses input to the probe end through an electric polarization controller.
Preferably, the step of obtaining the recovery time of the superconducting nanowire single photon detector is as follows:
generating a pulse sequence with a fixed period, and extracting detection events with two continuous responses;
gradually increasing the pulse sequence period, and recording the change trend of the event number, wherein the saturation point of the event number is the recovery time of the superconducting nanowire single-photon detector;
and the time interval t of the coding = the recovery time of the superconducting nanowire single photon detector.
Preferably, the time interval t of the coding is determined according to the recovery time of the superconducting nanowire single-photon detector, and the random number is used for generating the coding sequence A 1 ，A 2 ，...，A n And B 1 ，B 2 ，...，B n Before the step of "further comprising the steps of:
and acquiring the frequency difference between the laser of the transmitting end Alice and the laser of the transmitting end Bob, and adjusting the frequency difference between the two lasers within 10MHz by feedback.
The invention also discloses a quantum key distribution method, which comprises the following steps:
determining the time interval t of coding according to the recovery time of the superconducting nanowire single photon detector;
and (4) performing two Bell state measurements according to the time interval t, recording data, and transmitting the data to a PC (personal computer) for post-processing to obtain a security key.
Has the advantages that: compared with the prior art, the invention has the following advantages:
(1) The invention fills the technical blank of the chip detection end of the measuring equipment irrelevant quantum key distribution (MDI-QKD) system, the chip of the invention has a passive structure and an on-chip detection structure which are integrated by a full chip, can directly perform projection measurement on the quantum key coded by any time-bin (time-bin) with low error code and high efficiency, has the advantages of small size, low energy consumption, high precision and large expandable space compared with the existing detection end, can be used as a good solution for an untrusted relay, and is an important component part for realizing a low-cost and large-scale metropolitan area quantum communication network;
(2) The invention realizes twice promotion of the safe secret key rate: for time-bin (time-bin) coding, the Bell state | Ψ - >The corresponding detector responses are different detector responses at the front and back moments, the Bell state | Ψ + >The corresponding detector response is the same detector response at the front and back moments, the dead time of the conventional commercial superconducting nanowire single-photon detector is dozens of nanoseconds, only one Bell state can be detected, and the Bell state | psi cannot be detected + >The dead time of the waveguide integrated superconducting nanowire single-photon detector on the detection end chip is less than ten nanoseconds, so that two Bell states can be measured simultaneously due to the response of the ultrafast detector, namely, the Bell state | psi can be effectively measured + >Measurement is carried out, so that the improvement of twice of the security key rate can be realized;
(3) According to the invention, a time multiplexing technology is adopted, and under the clock frequency of nearly one hundred megahertz, by utilizing the best Bell State Measurement (BSM) and time division multiplexing, compared with a system without the two technologies, the key rate generation is improved by one order of magnitude, the key rate equivalent to that of the current GHz clock frequency system is obtained, the cost of the system is further reduced, and a foundation is provided for the large-scale establishment of an irrelevant quantum key distribution (MDI-QKD) system of the measuring equipment in the future.
Drawings
FIG. 1 is a schematic diagram of the system of the present invention;
FIG. 2 is a diagram illustrating the results of bin-bin (time-bin) coded qubit Bell State Measurement (BSM);
FIG. 3 is a schematic diagram of a user performing measurement device independent quantum key distribution (MDI-QKD) using time multiplexing;
FIG. 4 shows the result of the variation trend of the event number, and (a) in FIG. 4 shows the response signals of the waveguide integrated superconducting nanowire single photon detector with different lengths; (b) Continuously measuring the relationship between the normalized count of the front signal and the normalized count of the rear signal and the time interval of signal sending for a detector;
FIG. 5 (a) shows the user in embodiment 1 transmitting different or the same qubits projected into the Bell states | Ψ - >The coincidence count of (a) and the relative delay between users; (b) Sending different or same quantum bits for the user of embodiment 1, projecting to Bell state | Ψ + >The coincidence count of (a) and the relative delay between users; (c) For measuring Bell state | Ψ - >The relation between the error rate of the X-base quantum bit and the error rate of the Z-base quantum bit and the relative delay between the users is obtained; (d) For measuring Bell state | Ψ + >The relation between the error rate of the X-base quantum bit and the error rate of the Z-base quantum bit and the relative delay between the users is obtained; (e) To measure Bell state | Ψ - >The relation between the error rate of the X-base quantum bit and the error rate of the Z-base quantum bit and the frequency difference between users; (f) To measure Bell state | Ψ + >Relation between X-base and Z-base quantum bit error rate and frequency difference between users
FIG. 6 (a) is the relationship between the screening key number and the user insertion sequence number in example 2; (b) The relation between the bit error rate of X-base quantum bit and Z-base quantum bit and the number of inserted sequences;
FIG. 7 is a key rate of the system of the present invention;
FIG. 8 is a schematic table comparing the key rate of the system of the present invention to the key rate of the current GHz measurement device independent quantum key distribution (MDI-QKD) system.
Detailed Description
The technical solution of the present invention will be further explained with reference to the accompanying drawings and examples.
The time box qubit is used to encode bit information, is immune to random polarization rotation in optical fibers, and is suitable for fiber-based quantum communication. The transmitting end Alice and the transmitting end Bob encode the secret key by using the modulated weak coherent pulse set and using the time-box qubits. In the Pally Z basis, the key is encoded as | e with bit values of 0 and 1, respectively>And | l>，|e>And | l>The time interval in between is t. In the bladderIn X base, the key is encoded as | e>And | l>Coherent superposition state between:
and
representing 0 and 1, respectively. The Z-base code is used for key exchange and the X-base code is used for error detection. These coded time-bin (time-bin) qubits are then sent to Charlie, which uses a beam splitter BS and two single photon detectors (D) 1 And D 2 ) Bell State Measurements (BSMs) are performed on incoming time-bin (time-bin) qubits. The probability of success of Bell State Measurement (BSM) is bounded by 50% by the use of linear optical elements. For projection measurements, the best Bell State Measurement (BSM) corresponds to distinguishing two of the four bell states.
Although the time-bin qubit is well suited for quantum communication over optical fibers, optimal Bell State Measurements (BSM) of the time-bin qubit have not been achieved. The bottleneck to date is the lack of high speed single photon detectors. A bell-state measurement (BSM) scheme for time-bin (time-bin) qubits is shown in fig. 2. Coincidence counts of two different detectors at different time bins, corresponding to
(representing a Single photon Detector D 1 Detect photons during the e period) and
(representing a Single photon Detector D 2 Detect photons during the l period), or
And
coincidence count between. This coincidence detection projects two photons to
Above, this is a common scenario implemented in most time-bin (time-bin) bell measurement (BSM) schemes. In order to achieve optimal Bell State Measurements (BSM), it is also necessary to detect by measuring coincidence counts of a detector over different time periods
Correspond to
And
or
And
in between. This particular bell-state measurement (BSM) requires high-speed single photon detection, capable of detecting consecutive photons spaced apart by t.
Fig. 1 discloses a measuring device independent quantum key distribution (MDI-QKD) system of the present invention that provides short recovery times (< 10 ns) for single photon detection by the unique design of waveguide integrated SNSPD, enabling for the first time performing best bell-state measurements (BSM) of time-box coding between two independent lasers.
The invention discloses a measuring equipment independent quantum key distribution (MDI-QKD) system based on a waveguide integrated superconducting nanowire single-photon detector, which comprises a transmitting end Alice, a transmitting end Bob and a detecting end Charlie based on superconducting materials and silicon base.
The transmitting terminal Alice and the transmitting terminal Bob both comprise coding modules with time-bin (time-bin) coding functions. Each encoding module includes an optical path portion and an electrical circuit portion. Wherein the light path part comprises a narrow linewidth laser LS, a polarization beam splitter PBS and a first intensity modulatorIM1, a second intensity modulator IM2, a phase modulator PM, a second beam splitter BS 2 A second photodetector PD 2 A variable attenuator ATT and an electric polarization controller EPC. The circuit part is used for controlling the first intensity modulator IM1, the second intensity modulator IM2 and the phase modulator PM to carry out time-bin (time-bin) coding and specifically comprises a random number generator RNG, an arbitrary wave generator AWG and a computer PC. The functions of the devices at the transmitting end and the relationship between the devices will now be described as follows.
The first intensity modulator IM1 chops the continuous light into pulses, and the intensity modulator is an electro-optical modulator based on a mach-zehnder interferometer using lithium niobate, and the intensity of output light changes when a voltage is applied thereto, so that the continuous light can be controlled to pass at some times and not pass at some times. The second intensity modulator IM2 implements the intensity modulation of the trick-mode protocol. The phase modulator PM applies a pi phase to the | - > state and a 0 phase to the | + > state in the X basis. The phase modulator PM also implements the phase randomization required for measuring device independent quantum key distribution (MDI-QKD). All the above components are connected by polarization maintaining fiber.
Second beam splitter BS 2 50 beam splitter, second beam splitter BS 2 And the second photodetector PD 2 In combination, for monitoring the long term stability of the laser power in each encoder. The variable attenuator ATT adjusts the average photon number of the weak coherent pulse and simulates the propagation loss in the optical fiber, and the electric polarization controller EPC adjusts the polarization of the pulse input to the probe end. The connection of these components is a common single mode optical fiber. In addition, the electrical signals applied to the first intensity modulator IM1, the second intensity modulator IM2 and the phase modulator PM are generated by a computer PC calling a prestored random number to be transmitted to the arbitrary waveform generator AWG.
Bell State Measurements (BSM) are Hong-Ou-Mandel two-photon interferences, and to obtain a high-quality two-photon HOM interference, alice and Bob need to generate indistinguishable weak coherent pulses. The pulses must be indistinguishable in all degrees of freedom, including spectrum, time, and polarization. For spectral degrees of freedom, the unmodulated pulses of Alice and Bob are partially split into light at the first by the polarizing beamsplitter PBSBeam splitter BS 1 Up beat frequency, by the first photodetector PD 1 And measuring the frequency difference of the two narrow linewidth lasers LS, and adjusting the narrow linewidth laser LS at one transmitting end through feedback to enable the frequency difference between the frequency of the narrow linewidth laser LS and the frequency of the narrow linewidth laser LS to be within 10MHz. For polarization, two electrical polarization controllers EPC were used to optimize their polarization before coupling the two pulses to Charlie's chip. For the time degree of freedom, the relative electrical delay between the modulators of Alice and Bob is adjusted to ensure that their pulses arrive at the chip at the same time.
The detection end Charlie based on the superconducting material and the silicon-based chip is used for receiving pulses from the encoding module, and carrying out Bell State Measurement (BSM) and result processing. The chip is arranged on a nanometer positioner in a closed-loop low-temperature thermostat with the base temperature of 2.1K, and devices on the chip comprise a grating coupler GC, a 50. Wherein the grating coupler GC is a grating coupler with a back mirror providing a 2.2dB coupling loss at a wavelength of 1536 nm. The silicon optical waveguide guides the pulses to a multimode interference (MMI) coupler, which acts as a 50. The waveguide integrated superconducting nanowire single photon detector is a superconducting nanowire single photon detector arranged on the top of a waveguide. At the output of the multimode interference (MMI) coupler, the SNSPDs of the two integrated waveguides operate simultaneously to detect photons. Both SNSPDs are biased with a constant voltage source and connected to an electronic readout circuit. The post-processing system comprises a signal acquisition module and a data processing module; the electrical signals are collected by the FPGA and post-processed by a computer. The superconducting nanowire in the detection end chip is 80nm wide, 80um long and the silicon optical waveguide is 500nm wide.
The key distribution relates to a transmitting end Alice, a transmitting end Bob and a detecting end Charlie based on superconducting materials and a silicon-based chip, a coding module codes a key in a time-bin (time-bin) and sends the key to the detecting end chip Charlie, a signal is attenuated through a long distance, and the defect of a detector is added (the defect is that the efficiency is not one hundred percent, and dead time exists) At a certain point in time, the detected signal is probabilistic. Therefore, the invention also provides a quantum key distribution method for improving the key rate through time multiplexing, and more coding signals are inserted into the middle of the coding signals by utilizing the time multiplexing, so that the utilization rate of the signals and the detector is improved. If only one set of time bin qubits is used, the system clock frequency will be limited to 1/(2 t), as shown in fig. 3. To maximize channel efficiency, time division multiplexing coding is used on sequence A 1 And B 1 E of>And | l>Insert independent sequence set (A) between 2 ，...，A n And B 2 ,...,B n ). Therefore, the system repetition rate is greatly increased to 1/(2 τ), where τ is t 1 And t 2 The time difference between them. This quantum key distribution process is carried out in three stages: the first stage is an electric signal generation stage: the second stage is an optical signal encoding stage, and the third stage is an optical signal detecting stage. The method specifically comprises the following steps:
| Ψ can be satisfied in order to determine the code pulse interval + >I.e. two consecutive responses before and after the same detector. Firstly, measuring the recovery time of the waveguide integrated superconducting nanowire single photon detector to generate a pulse sequence with a fixed period, extracting detection events which continuously respond twice through the detection of the detector, gradually increasing the period of the pulse sequence, and recording the change trend of the event number, wherein the result is as shown in fig. 4, the event number is saturated along with the increase of the pulse interval, and the saturation point is the real recovery time of the detector, namely the time interval t used for encoding later;
the narrow linewidth lasers of the users Alice and Bob emit a continuous light beam, part of the light is firstly split by the polarization beam splitter PBS to carry out beat frequency on the beam splitter BS, and after the frequency difference is detected by the photoelectric detector, one laser is fed back to enable the frequency of the other laser to approach the frequency of the other laser, and the difference is less than 10MHz.
The communication user determines the time interval t of the coding according to the recovery time of the superconducting nanowire single photon detector, and generates a proper coding sequence A by using the random number generated by the random number generator RNG 1 ,A 2 ,...,A n And B 1 ,B 2 ,...,B n . Coding sequence A 2 ,...,A n And B 2 ,...,B n Sequentially delaying time tau, combining all sequences into a group of sequences according to time, and sending the combined sequences to an arbitrary wave generator AWG to generate an electric signal;
the electric signal output by the arbitrary wave generator AWG controls the first intensity modulator IM1 to chop the continuous wave into narrow pulses and perform Z-based coding: | e>And | l>(ii) a The second intensity modulator IM2 carries out decoy state modulation to modulate three decoy states with different intensities; the phase modulator then applies the phase to generate the X-base code:
and
and phase randomization. The variable attenuator ATT adjusts the average photon number of the weak coherent pulse and simulates the propagation loss in the optical fiber, and the electric polarization controller EPC adjusts the polarization of the coded qubit input to the detection end.
The coded quantum bit enters a detection end chip Charlie based on superconducting materials and silicon substrates, the Charlie performs two Bell State Measurements (BSM) according to a time interval t, records data and transmits the data to a computer for post-processing. Charlie declares the measurement result of the bell state through the public channel, and the users Alice and Bob use the certified classical channel to perform the basis vector comparison. Based on the announcing result of Charlie, corresponding bits are screened and reserved by the users Alice and Bob, the bits operate according to rules, the Alice and Bob publish own basis vector selection to the opposite side through a public channel, the Alice and Bob reserve the same events selected by the basis vectors, and abandon the rest events, so that the screened key is obtained. Finally, they estimate the 'gain' (gain) and quantum bit error rate contributed by the single photon by using a decoy state method, and carry out error correction and privacy amplification to obtain a secure key.
Example 1
In this embodiment, the user sets and sends the specific coded qubit to realize the display and systematicness of two bell states of the systemCan display the stability. The system requirement that the measurement time-bin (time-bin) encodes two bell states, i.e. the recovery time of the detector is short, has been explained above. When the users Alice and Bob prepare the qubits simultaneously in the Paglie X basis, i.e.
And
these two states. To prepare these two states, two users are first required to modulate the first intensity modulator IM1 with the arbitrary wave generator AWG to chop the continuous light into pulses, then the second intensity modulator IM2 increases the extinction ratio of the pulses while modulating the intensity, where decoy states are not involved, so the intensity is set to be the same, then the phase modulator is passed to pair the two states
Applying 0 phase to
And applying a pi phase, enabling the count of the superconducting nanowire single-photon detector at the detection end to be maximum through a polarization controller, and adjusting an attenuator to enable the attenuation from two users to the detection end to be the same.
After the above operations are completed, two users start to send specific qubits, and the detection end Charlie performs Bell State Measurement (BSM). Two users modulate the same state separately, e.g., both generate | +++> AB Or | -> AB Scanning the relative delay between two users by an arbitrary wave generator to obtain the change of two Bell State Measurement (BSM) coincidence counts along with the relative delay of the two users; also, the two users can modulate different states, such as generating | + -> AB Or | - +> AB Scanning the relative delay between two users by an arbitrary wave generator to obtain the change of two Bell State Measurement (BSM) coincidence counts along with the relative delay between the two users; meanwhile, the change of the bit error rate of the Z-base quantum and the X-base quantum along with the relative delay between the two users can be obtained; by changingThe frequency of the laser can obtain the change of the quantum bit error rate along with the frequency difference between two users. The results are shown in FIG. 5, which shows the two-photon coincidence count of a Bell State Measurement (BSM) as a function of the relative electronic delay between Alice and Bob pulse sequences, where Charlie projects two photons transmitted by Alice and Bob, respectively, to | Ψ - >And | Ψ + >The above. The dependence of coincidence counting on delay is a result of Bell State Measurement (BSM), showing coherent two-photon superposition. Due to | Ψ - >And | Ψ + >Symmetry of (c), when Alice and Bob send the same state at X base | +++> AB Or | -> AB Then, the resulting Bell state | Ψ - >/|Ψ + >The resulting destructive/constructive interference pattern is measured. When Alice and Bob send the orthogonal state | + -> AB Or | - +> AB The opposite result is obtained.
And (3) enabling Alice and Bob to send a sequence of Z-base and X-base codes, obtaining a security key from Z-base measurement, and verifying the reliability of the quantum key distribution system in X-base. To quantify the performance of the system, the Quantum Bit Error Rate (QBER) was analyzed. For example, when Alice and Bob send the same/orthogonal states, alice and Bob conditionally exchange their keys, provided that Charlie obtains | Ψ from his Bell State Measurement (BSM) - >And | Ψ + >. For the X base, QBER is affected by the relative delay of the two users, and for the Z base, QBER is unaffected by the relative delay of the two users. With a relative delay of 0 between Alice and Bob, the X-base QBER has a minimum value close to 0.25. For the Z basis, QBER measured was close to zero, indicating that the quality of the system of the present invention is high. The relation between the relative central wavelength of the lasers of Alice and Bob and the relative wavelength of the two users is obtained by changing the relative central wavelength between the lasers of Alice and Bob, which shows that the wavelengths of the two lasers are required to be very close to obtain a lower QBER, and this is also the reason for performing the beat frequency feedback adjustment of the two lasers in the prior art.
Example 2
In this embodiment, the user sends a time-bin (time-bin) encoded qubit sequence containing a Z-base and an X-base, the sequence is generated by controlling an arbitrary wave generator by a computer, andthe first intensity modulator IM1 is controlled to chop the continuous light emitted by the laser into pulses, the second intensity modulator IM2 modulates the intensity of the respective fundamental light, here not involving decoy states, so the intensity is set to be the same, and then through the phase modulator, the pair
Applying a 0 phase to
And applying a pi phase, enabling the count of the superconducting nanowire single-photon detector at the detection end to be maximum through a polarization controller, and adjusting an attenuator to enable the attenuation from two users to the detection end to be the same.
In this embodiment, the communication user in the quantum key distribution method determines the time interval t of encoding according to the recovery time of the superconducting nanowire single-photon detector, and generates an appropriate encoding sequence a by using the random number generated by the random number generator 1 ,A 2 ,...,A n And B 1 ,B 2 ,...,B n . Coding sequence A 2 ,...,A n And B 2 ,...,B n Sequentially delaying time tau, and then combining all sequences into a group of sequences according to time; for further explanation.
Varying the number of inserted sequences in this step, the test sequences were as follows:
(1)Alice：A 1 ；Bob：B 1 (ii) a Number of inserted sequences: 0;
(2)Alice：A 1 ，A 2 ；Bob：B 1 ，B 2 (ii) a Number of inserted sequences: 1;
(3)Alice：A 1 ，A 2 ，A 3 ；Bob：B 1 ，B 2 ，B 3 (ii) a Number of inserted sequences: 2;
(4)Alice：A 1 ，A 2 ，A 3 ，A 4 ；Bob：B 1 ，B 2 ，B 3 ，B 4 (ii) a Number of inserted sequences: 3;
(5)Alice：A 1 ，A 2 ，A 3 ，A 4 ，A 5 ，A 6 ；Bob：B 1 ，B 2 ，B 3 ，B 4 ，B 5 ，B 6 (ii) a Number of inserted sequences: 5;
as shown in fig. 6, up to 5 sequences are inserted between 12ns, and the time interval becomes 2ns. First, corresponding to the same number of insertion sequences, the screening key for measuring two Bell states is twice as high as the screening key for measuring only one Bell state. As the insertion sequence increases, the screening key also increases linearly. By combining this time division multiplexing technique with optimal bell state measurements, the screened-out key rate is improved by almost an order of magnitude. Meanwhile, the two technologies have little influence on the Quantum Bit Error Rate (QBER) of the X base and the Z base. Time multiplexing techniques are utilized to increase the key rate by inserting more time bin pulse pairs. This is particularly useful in high loss communication applications.
Example 3
The embodiment shows a complete measuring device independent quantum key distribution (MDI-QKD) system based on the waveguide integrated superconducting single-photon detector, which comprises a decoy state and phase randomization. Using a four strength coding protocol, consider the symmetric case where Alice and Bob to Charlie channel transmissions are equal. Experimentally, alice and Bob each have one of the signal strengths in the basis to generate the key and use three of the decoy strengths in the X basis for error testing. In practical implementations, the key size is limited and statistical fluctuations should be considered in different ways if the pulse pairs from Alice and Bob are in different intensity combinations. The key idea of finite size analysis is to estimate the lower limit of the single photon pair yield and the upper limit of the phase error rate of single photon pairs, and to perform the parameter estimation by applying the chernoff bound, and to obtain the gain of the pulse pairs on an X basis. By jointly considering the observed data, a stricter constraint is obtained from the joint constraint than the independent constraint, resulting in a higher key rate. And generating a coding waveform by using the stored random number and introducing the coding waveform into an arbitrary wave generator. A first intensity modulator IM1 controlled by it chops the continuous light into pulses, and a second intensity IM2 controller implements modulation of three intensities of the X-base spoof state, where three intensities in the X-base are used for spoof state analysis and one intensity in the Z-base is used for key generation. The limited long-term response is considered in the security key rate analysis, and the error probability is 10 -1 For statistical fluctuations, joint constraints are used, where the same observables are grouped together and processed together.
In this part of the experiment, the present invention inserts two more pairs of time-bin qubits evenly between 12ns intervals. Thus, the effective clock rate of the system is increased by a factor of two, up to 125MHz (1/8 ns). The different lossy security key rates are shown in fig. 7. At a clock rate of 125MHz, the present invention achieves a key rate of 6.166kbps with a loss of 24.0 dB. This loss includes the chip insertion loss 4.5dB. The actual transmission loss is about 19.5dB, corresponding to 98km standard fibre. This is the highest secure key rate obtained experimentally with 20dB transmission loss in measuring device independent quantum key distribution (MDI-QKD). Furthermore, the invention achieves a secure key rate of 170bps at a loss of about 35.0dB and 34bps at a loss of about 44.0dB. As shown in fig. 8, the secure key rate of the 125MHz clock system of the present invention is very close to the measuring device independent quantum key distribution (MDI-QKD) experiment for the GHz clock rate. And compared with the GHz clock rate measurement device independent quantum key distribution (MDI-QKD) experiment, the system of the invention does not need a complex injection locking technology, which obviously reduces the complexity.
Claims (12)
1. A chip-based measuring device independent quantum key distribution system is characterized in that: the device comprises a transmitting end Alice, a transmitting end Bob and a detecting end Charlie;
the transmitting end Alice and the transmitting end Bob encode the secret key by using the modulated weak coherent pulse set and the time box qubit to obtain encoded time box qubits, and send the encoded time box qubits to the detecting end Charlie;
the detection end Charlie comprises a detection end chip and a post-processing system, wherein the detection end chip is used for carrying out two Bell state measurements according to an incoming quantum bit code;
the detection end chip comprises a grating coupler for receiving pulses from a transmitting end, a 50; the grating coupler is connected with a 50-beam splitter through an optical waveguide, the waveguide integrated superconducting nanowire single-photon detector is a superconducting nanowire single-photon detector arranged at the top of the waveguide, and the waveguide integrated superconducting nanowire single-photon detector is connected with the output end of the optical waveguide through the optical waveguide;
the post-processing system is used for receiving and processing the electric signal output by the detection end chip to obtain a security key;
in an emission end Alice and an emission end Bob, determining the time interval t of coding according to the recovery time of the superconducting nanowire single-photon detector, and respectively generating a coding sequence A by utilizing random numbers 1 And coding sequence B 1 And in the code sequence A in the time interval t 1 Late insert coding sequence A 2 ,...,A n Wherein, the coding sequence A 2 Comparison code sequence A 1 Delay time tau, code sequence A 3 Comparison code sequence A 2 Delaying the time tau by the same analogy to obtain a combined sequence, and obtaining an electric signal of a quantum bit of a time box generated and coded by modulation transmitting end Alice based on the combined sequence; and in the code sequence B in the time interval t 1 Late insert coding sequence B 2 ,...,B n Wherein, the coding sequence B 2 Comparing coding sequence B 1 Delay time tau, code sequence B 3 Comparison coding sequence B 2 And delaying the time tau by the same analogy to obtain a combined sequence, and obtaining an electric signal for modulating the quantum bit of the time box generated by the transmitting end Bob and encoded on the basis of the combined sequence.
2. The chip-based measurement device independent quantum key distribution system of claim 1, wherein: the optical waveguide is a silicon optical waveguide.
3. The chip-based measurement device independent quantum key distribution system of claim 1, wherein: the 50.
4. The chip-based measurement device independent quantum key distribution system of claim 1, wherein: the transmitting terminal Alice and the transmitting terminal Bob both comprise encoding modules with time box encoding functions; each of the encoding modules includes:
a laser for emitting continuous laser light;
the polarization beam splitter is used for splitting incident laser and partially outputting light to the first intensity modulator;
a first intensity modulator for chopping the incoming portion of light into pulses;
the second intensity modulator is used for carrying out intensity modulation for realizing a decoy state protocol;
a phase modulator for applying a pi phase to the | - > state and a 0 phase to the | + > state in the X basis, as well as phase randomization;
an attenuator for adjusting the average photon number of the weak coherent pulse and simulating propagation loss in the optical fiber;
the electric polarization controller is used for adjusting the polarization of the pulse input to the detection end Charlie;
the random number generator is used for generating random numbers and storing the random numbers to the PC;
the PC is used for generating a coding sequence by utilizing the random number and sending the coding sequence to an arbitrary wave generator;
an arbitrary wave generator for generating an electrical signal applied to the first intensity modulator, the second intensity modulator and the phase modulator according to the code sequence.
5. The chip-based measurement device independent quantum key distribution system of claim 4, wherein: the other part of output light of the polarization beam splitter of the transmitting end Alice and the other part of output light of the polarization beam splitter of the transmitting end Bob beat on the first beam splitter, the frequency difference between the laser of the transmitting end Alice and the laser of the transmitting end Bob is measured by the first photoelectric detector, and the frequency difference between the two lasers is within 10MHz by adjusting the lasers of the transmitting end Alice and the transmitting end Bob in a feedback mode.
6. The chip-based measurement device independent quantum key distribution system of claim 4, wherein: the laser, the polarization beam splitter, the first intensity modulator, the second intensity modulator and the phase modulator are connected in sequence through the polarization maintaining optical fiber.
7. The chip-based measurement device independent quantum key distribution system of claim 4, wherein: each coding module further comprises a second beam splitter and a second photoelectric detector, wherein the input end of the second beam splitter is connected with the phase modulator, and the output end of the second beam splitter is respectively connected with the second photoelectric detector and the attenuator.
8. The chip-based measurement device independent quantum key distribution system of claim 4, wherein: the phase modulator, the attenuator and the electric polarization controller are connected in sequence through a single-mode optical fiber.
9. The method for quantum key distribution of the chip-based measurement device independent quantum key distribution system according to any one of claims 1 to 8, is suitable for a transmitting terminal Alice and a transmitting terminal Bob; the method is characterized in that: the method comprises the following steps:
determining the time interval t of the codes according to the recovery time of the superconducting nanowire single photon detector, and respectively generating the code sequences A by the transmitting end Alice and the transmitting end Bob by using random numbers 1 And coding sequence B 1 And within a time interval t in a code sequence A 1 Late insertion coding sequence A 2 ,...,A n Wherein, the coding sequence A 2 Comparing coding sequence A 1 Delay time tau, code sequence A 3 Comparison code sequence A 2 Delaying the time tau by analogy to obtain a combined sequence, and obtaining an electric signal for modulating a quantum bit of a time box generated by an emitting end Alice and encoding based on the combined sequence; and within a time interval t, within the code sequence B 1 Late insertion coding sequence B 2 ,...,B n Wherein, the coding sequence B 2 Comparing coding sequence B 1 Delay time tau, code sequence B 3 Comparison coding sequence B 2 Delaying the time tau by the same analogy to obtain a combined sequence, and obtaining an electric signal of a quantum bit of a time box generated and coded by a modulation transmitting terminal Bob based on the combined sequence;
based on the obtained electric signal, controlling a first intensity modulator to chop the continuous wave into pulses and carrying out Z-based encoding to obtain | e>And | l>(ii) a Controlling a second intensity modulator to carry out decoy state modulation, and modulating three decoy states with different intensities; controlling the phase modulator to apply a phase to generate an X-base code, expressed as:
and
and phase randomization, adjusting the average photon number of the weak coherent pulse output by the phase modulator through an attenuator and simulating the propagation loss in the optical fiber, and adjusting the polarization of the pulse input to the detection end through an electric polarization controller.
10. The quantum key distribution method of claim 9, wherein: the method for acquiring the recovery time of the superconducting nanowire single photon detector comprises the following steps:
generating a pulse sequence with a fixed period, and extracting detection events with two continuous responses;
gradually increasing the pulse sequence period, and recording the change trend of the number of events, wherein the saturation point of the number of events is the recovery time of the superconducting nanowire single photon detector;
and the time interval t of the coding = the recovery time of the superconducting nanowire single photon detector.
11. The quantum key distribution method of claim 9, wherein: determining the time interval t of coding, the transmitting end Alice and the transmitting end according to the recovery time of the superconducting nanowire single photon detectorEnd Bob respectively generates coding sequence A by using random numbers 1 ,A 2 ,...,A n And B 1 ,B 2 ,...,B n Before the step (2), further comprising the following steps:
and acquiring the frequency difference between the laser of the transmitting end Alice and the laser of the transmitting end Bob, and adjusting the frequency difference between the two lasers within 10MHz by feedback.
12. The quantum key distribution method of the chip-based measurement device independent quantum key distribution system according to any one of claims 1 to 8, being adapted to a probe end; the method is characterized in that: the method comprises the following steps:
determining the time interval t of coding according to the recovery time of the superconducting nanowire single photon detector;
and (4) performing two Bell state measurements according to the time interval t, recording data, and transmitting the data to a PC (personal computer) for post-processing to obtain a security key.
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