CN111431702A - Quantum encryption circuit system and method based on coincidence time - Google Patents
Quantum encryption circuit system and method based on coincidence time Download PDFInfo
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Abstract
The invention provides a quantum encryption circuit system and method based on coincidence time. The method comprises the following steps: 1. the global clock module ensures that all signals are kept synchronous, and the single-event photon light source emits a pulse light source; 2. the Si-APD photon detector detects the photon signals output by the optical attenuator, calculates the coincidence time, and sequentially performs a detection stage, a quenching stage and a recovery stage; 3. then the sender and the receiver can screen the associated result by comparing the basis vectors to obtain an original key; 4. and the photon detector at the sender and the photon detector at the receiver transmit the basis vector comparison information through a public channel, encrypt the basis vector comparison information by using a one-time pad encryption system and transmit a ciphertext to complete safe communication. The invention effectively promotes the counting of the incident single photon to realize the detection of the extremely weak target signal, thereby improving the precision of quantum encryption.
Description
Technical Field
The present invention relates to the field of quantum, and more particularly, to a quantum cryptography circuit system and method based on coincidence time.
Background
Quantum cryptography communication is generally referred to as Alice (sender of information) and Bob (receiver of information), A, B for short. The quantum cipher communication aims to establish a random number key between A and B in different places, the key is generated under the combined action of the A and B in the communication process, and the absolute safety of the key is ensured by the quantum optical basic principle. The information carrier used by the present quantum cryptography system is a single photon, and is encoded in a phase or polarization modulation mode.
The key aspect of quantum cryptography relies on physics as a security model rather than mathematics. Essentially, quantum cryptography is an indecipherable system developed based on the application of a pair of gamma photons and their inherent quantum properties, precisely because the quantum state of the system cannot be determined without disturbing the system. Other particles could be used instead in theory, except that photons have all the qualities required for quantum cryptography, their behavior being relatively well understood, and in addition being the most promising information carrier for high bandwidth communication media fiber optic cables. Because the information carrier of quantum communication is a single photon, how to accurately detect and record the arrival time of the single photon, and how to establish high-precision time synchronization among communication parties is of great importance for quantum communication. Time measuring instruments are an essential and important component of the system. The accuracy of time measurement and the accuracy of time synchronization of both communication sides directly determine the bit error rate, the code rate and even the success or failure of quantum communication.
The BB84 protocol is also called a four-state protocol, the basis of the four-state protocol is quantum complementarity, a four-quantum-state scheme of two conjugate bases is adopted, and the principle of uncertainty determination and unclonable of single-photon quantum states is utilized, so that the operation is simple and unconditional safety is realized. The BB84 protocol was proposed and appreciated by many scholars.
In the prior art, quantum key distribution mainly comprises a quantum generation module and a quantum receiving module which are distributed at two places and are matched with each other, and corresponding key transmission is completed together by connecting the quantum generation module and the quantum receiving module with optical fibers.
In a quantum communication experiment, only if a precise time measurement circuit is insufficient, a sending end and a receiving end are required to have synchronous time reference, and thus accurate basis vector comparison and other processing can be carried out. The two ends of the experiment are often far away from each other, the time references are different, and how to realize high-precision time synchronization at the two ends is also an important problem to be solved in quantum communication. Time synchronization is often coupled together because it is closely associated with time measurement. Time synchronization technology and time coincidence module are added in quantum communication experiment.
In addition, the existing quantum encryption system has the problems of transmission distance, safety and transmission code rate: in transmission distance, the high efficiency and low dark count of the single photon detection system are the key points for expanding the key generation rate and the transmission distance of the quantum key distribution system. In terms of safety, the invention has a safer QKD scheme, and the preparation and measurement of the quantum state are easy to realize. In the aspect of transmission code rate, the method has the advantages of short dead time, low dark count and the like, and obtains more ideal time resolution by measuring the difference of the arrival time of two pulses by a time marking method.
Lexical interpretation:
1. photon: photons are carriers of electromagnetic radiation, a canonical boson, and in quantum theory photons are considered mediators of electromagnetic interaction. The photon resting mass is zero. Photons move at the speed of light and have energy, momentum, and mass.
2. Public key: the Public Key and the Private Key are a Key pair (i.e., a Public Key and a Private Key) obtained by an algorithm, the Public Key is a Public part of the Key pair, and the Private Key is an unpublished part. When using this key pair, if one of the keys is used to encrypt a piece of data, the other key must be used to decrypt the piece of data. For example, encrypting data with a public key necessitates decryption with the private key, and if encrypting with the private key, also must decrypt with the public key, otherwise decryption will not succeed.
An APD: avalanche photodiodes (Avalanche Photo Diode), a photovoltaic detector element used in the field of optical detection. After a reverse bias is applied to the P-N junction of a photodiode made of silicon or germanium, the incident light is absorbed by the P-N junction to form a photocurrent. Increasing the reverse bias voltage produces an "avalanche" (i.e., a multiple surge in photocurrent) phenomenon, and such diodes are referred to as "avalanche photodiodes".
4. Quantum cryptography: quantum cryptography is the use of quantum states primarily as keys for encryption and decryption of information. The most common encryption technique currently uses complex mathematical algorithms to alter the original information. Although the method has high safety, the method has the possibility of being decoded and is not absolutely reliable. While quantum cryptography is a distinct encryption method. Anyone who wants to measure and calculate and decipher the key can obtain meaningless information by changing the quantum state, and a receiver of the information can know that the key is intercepted from the change of the quantum state. Theoretically, the communication encrypted by the quantum cryptography cannot be intercepted, and the security degree is extremely high.
5. And (3) according with time: the time difference between the arrival of a gamma photon pair generated by positron annihilation at a pair of detectors is less than 2ns, the difference between the arrival times of two pulses measured by a time-stamping method, and the full width at Half Maximum (FWHM) of the histogram of this difference is the coincidence time.
QUENCH: the photon detector quenches the avalanche signal output interface.
Disclosure of Invention
The invention aims to provide a quantum encryption circuit system and a method based on coincidence time, wherein the quantum encryption circuit system utilizes Si-APD (Silicon-Avalanche-photo diode) photoelectric conversion, improves the precision of measuring time difference in a parallel detection mode, processes signals obtained by continuous experiments of parallel single-photon detectors through coincidence time windows, and effectively improves the detection of extremely weak target signals; because the emission of photons is based on time coincidence, two time values of coincidence events originate from one point in space, the time difference of photon pairs generated by a pulse laser to reach a pair of detectors is less than 2ns, and the difference value of the arrival times of two pulses is measured by a time marking method, so that a single photon detector obtains a proper coincidence counting time window, and therefore, the time measuring circuit is less than 40ns of the dead time of the single photon detector commonly adopted in the current experiment, and the dead time of the single photon detector is greatly reduced; meanwhile, an ideal random source is adopted, and randomness is good; because each photon pair disappears after being acted, the confidentiality of quantum encryption is effectively improved, and the method has the advantage of low power consumption.
In order to achieve the above object, the present invention provides a quantum encryption circuit system based on coincidence time, which comprises a single-event photon light source module, a Si-APD photon detector module, a global clock generation module, and an encryption and transmission module.
The single-event photon light source module comprises a pulse laser module and an adjustable attenuator module. The pulse laser emits a pulse light source, the pulse light source is input to the optical attenuator, the attenuation multiple of the optical attenuator is flexibly controllable, laser is adjusted according to a certain attenuation multiple, the laser can reach a quasi-single photon light source with each pulse only containing 0.1 photon standard, and then photon signals are transmitted to the single photon detector through a channel. The photon signal output by the optical attenuator can then be detected by a single photon detector. Since the principle of light emitted by the laser is stimulated radiation, the statistical distribution law of photon numbers can be modeled by Poisson distribution. Therefore, the single photon light source of the quantum key distribution system is a quasi-single photon state light source, and there exist other states, namely, multi-photon states, which reduce the security of the quantum key distribution system, because the multi-photon states are vulnerable to photon splitting during the key distribution process, which needs to be avoided as much as possible in the actual communication operation. When a quantum key distribution system is built, laser emitted by a laser is generally connected to a large variable optical attenuator, so that a quasi-single photon light source is obtained. Used as a single photon source required by single photon detector experiments.
The Si-APD photon detector module comprises an avalanche amplifier, a photoelectron summation module, a pulse amplification module and a signal multiplexing module. The avalanche amplifier is used for generating avalanche breakdown and converting photons from a light source into photoelectrons, when the avalanche photodiode absorbs the photons to send an avalanche signal and transmits the avalanche signal to the coincidence time calculation and control module, the time sequence module quenches and quickly recovers the avalanche photodiode and waits for the arrival of the next photon, and the difference value of the arrival time of two pulses is measured by a time marking method to obtain a proper coincidence counting time window, so that the coincidence time is recorded; the photoelectron summation module sums all photoelectrons to form an electric pulse; the pulse amplification module amplifies the formed electric pulse; the signal multiplexing module multiplexes all channels, reduces the number of channels, and thus reduces the number of output signals. In principle, in order to obtain a signal after a single photon signal is incident on an APD to trigger avalanche, the avalanche photodiode must operate in Geiger mode. When the avalanche photodiode operates in geiger mode, the detector is in a metastable state, the multiplication factor can theoretically be infinite, and even if the absorbed signal is a weak signal of single photon magnitude, a measurable photocurrent pulse can be generated with a certain probability. When the avalanche multiplication factor M is large, avalanche will not stop as long as there are electron-hole pairs in the P-N junction in the APD, and the current flowing through the P-N junction of the APD needs to be limited in order to quench the avalanche. Since the current also follows a distribution over time, if the current is small enough that it is below a certain threshold, there must be a certain moment when no carriers are present in the junction and the avalanche can be quenched.
And the global clock generating module enables the clocks of all modules in the signal generator to be the same, and ensures that all signals can be kept synchronous.
And the encryption and transmission module comprises a transmission content storage module, a key generation module and an encryption module and is used for encrypting the data to be encrypted according to the BB84 protocol and packaging and transmitting the data to a receiving party through a classical public channel. The transmitting content storage module is used for storing and packaging data content to be subjected to quantum encryption into data packets and transmitting the data packets to the encryption module. And the key generation module generates a key through comparing the base vectors of the sender and the receiver. And the encryption module is used for coupling the content to be encrypted with the key to achieve the purpose of encryption.
And then the receiver publicly measures the used basis loss, Alice receives the basis vector information used by Bob to compare the basis vectors used by the locally prepared secret key, and after the comparison, shares the part with the same basis vector to the receiver, and finally the transceiver end keeps the part with the same basis vector.
The photon detector of the sender transmits the key to the photon detector of the receiver through the quantum channel, and transmits the base vector comparison information through the public channel, so that verification guarantee is provided for both communication parties. And a one-time pad encryption system is used for encrypting and transmitting the ciphertext to complete the secure communication.
As shown in fig. 5, a schematic diagram of original key formation is shown, and a transmitter and a receiver can screen out associated results by comparing basis vectors to achieve the purpose of reducing the bit error rate. The process is as follows: and the receiver publicly measures the used basis loss, Alice receives the basis vector information used by Bob to compare the basis vectors used by the locally prepared secret key, shares the part with the same basis vector to the receiver after the comparison is finished, and finally the transceiver end retains the part with the same basis vector, screens out the data close to one half, and the rest is the required original secret key.
Then both parties randomly disclose part of the original key, and the error rate is estimated to find out whether an eavesdropper exists or not. If the error rate can be lower than a specific threshold, error correction related technology can be adopted to carry out error correction processing on the secret key, then information leakage caused in the error correction process or the communication transmission process is eliminated through privacy amplification, and finally a safety secret key is formed. And if the error rate exceeds a specific threshold value, abandoning the communication and carrying out the communication again.
The method comprises the following specific working steps:
and S1, starting the global clock module to ensure that the clocks of all modules in the signal generator are the same, and ensuring that all signals can be kept synchronous. The pulse laser emits a pulse light source, the pulse light source is input to the optical attenuator, the attenuation multiple of the optical attenuator is flexibly controllable, laser is adjusted according to a certain attenuation multiple, the laser can reach a quasi-single photon light source with each pulse only containing 0.1 photon standard, and then photon signals are transmitted to the single photon detector through a channel.
S2, by injecting the light pulse emitted by the laser into an optical attenuator with controllable attenuation factor, we can find the point of the average photon number we need by slowly adjusting the attenuation factor. The photon signal output by the optical attenuator can then be detected by a single photon detector.
And S3, adding a detection stage of the photon detector, wherein the loading voltage of the APD cathode is higher than the avalanche voltage, when a photon enters, the avalanche photodiode generates avalanche, the current gradually increases, and the voltage is increased, the voltage is amplified and then an avalanche signal is extracted through a hysteresis comparator U3, wherein the amplification factor of the amplifier is 10 times, the reference level is 1.3V, and the output standard TT L level of the hysteresis comparator is sent to the FPGA time sequence control unit.
S4: and then adding a quenching stage of the photon detector, wherein a coincidence time calculation and control module outputs a signal QUENCH (the effective width of high level is 20ns) with a certain width after receiving the hysteresis comparator signal and adds the signal QUENCH to a Q3 triode, so that the Q3 is conducted, the voltage of a collector rapidly drops, the Q1 is conducted, the voltage transformation of an APD anode rapidly drops at the moment, the Q1 is conducted, the voltage of the APD anode becomes Vq, and the aim of quenching avalanche is achieved after a period of time delay because Va-Vq < Vbr.
S5: and then, a recovery stage of the photon detector is added, after avalanche quenching, the FPGA outputs RESET, and the switches of the invention all adopt fast switch tubes but have transmission delay, wherein the opening time of the PNP tube is 12ns, the closing time of the PNP tube is 18ns, the opening time of the NPN tube is 15ns, and the closing time of the NPN tube is 20 ns. Therefore, the timing sequence needs to be accurately adjusted when the FPGA outputs the RESET, and the PNP tube and the NPN tube are prevented from being conducted simultaneously, so that a VQ power supply is prevented from being short-circuited, and a circuit board is prevented from being burnt out. Therefore, the FPGA pulls up the RESET signal (the effective width of the high level of Q5 is 20ns) 40ns after outputting the QUENCH pulse to conduct Q5, the anode of the APD is grounded, the APD is rapidly charged, and the APD enters a detection stage after being rapidly recovered to wait for receiving the photons to arrive.
And S6, the receiver opens the basis loss used by the measurement, Alice receives the basis vector information used by Bob to compare the basis vectors used by the local preparation key, shares the part with the same basis vector to the receiver after the comparison, and finally the transceiver end keeps the part with the same basis vector.
And S7, the photon detector of the sender transmits the key to the photon detector of the receiver through the quantum channel, and simultaneously transmits the basic vector comparison information through the public channel, thereby providing key security verification guarantee for both communication parties, encrypting and transmitting the ciphertext by using a one-time pad encryption system, and completing secure communication.
Drawings
FIG. 1 is a block diagram of the overall structure of a coincidence temporal photon detector according to the present invention;
FIG. 2 is a schematic diagram of a quantum cryptography embodiment according to the present invention;
FIG. 3 is a quantum cryptography communication flow diagram according to one embodiment of the invention;
fig. 4 is a BB84 protocol quantum cryptography system signaling diagram;
FIG. 5 is a schematic diagram of original key formation according to the present invention;
FIG. 6 is a statistical distribution of p (n) at different values of μ according to the present invention.
As in fig. 1, the sender: a is a single-event photon light source module, A1 is a pulse laser module, A2 is an adjustable attenuator module, B is an SiPM photon detector module, B1 is an avalanche amplification module, B2 is a coincidence time calculation and control module, B3 is a photoelectron summation module, B4 is a pulse amplification module, B5 is a signal multiplexing module, T is a global clock generation module, C is an encryption and transmission module, C1 is a transmission content storage module, C2 is an encryption module, C3 is a key generation module, a is a single-event photon light source module, a1 is a pulse laser module, and a2 is an adjustable attenuator module; the receiving side: b is an SiPM photon detector module, B1 is an avalanche amplification module, B2 is a coincidence time calculation and control module, B3 is an optoelectronic addition module, B4 is a pulse amplification module, B5 is a signal multiplexing module, D is a decryption and receiving module, D1 is a received content storage module, D2 is a decryption module, and D3 is a key generation module (wherein A and a are the same single-event photon light source, B and B are the same SiPM photon detector module, and a receiver T and a sender T use a synchronous global clock generation module).
As shown in fig. 2, α is a scintillation crystal connected to the sender, β is a scintillation crystal connected to the receiver, and the sender photon detector module 100, the first photon detection module 110, the first timer module 120, the first coincidence time window temporary storage module 121, the first sending module 122, and the first parallel detection data determination module 130 are all connected to the receiver.
A receiver photon detector module 200, a second photon detection module 210, a second timer module 220, a second coincidence time window temporary storage module 221, a second sending module 222, and a second parallel detection data determination module 230
A sender memory module 300, a first data receiving module 310, a first array conversion module 320, a first register module 321, a first conversion module 322, a first data sending module 330, a sending content storage module 340
A receiving-side memory module 400, a second data receiving module 410, a second group conversion module 420, a second register module 421, a second conversion module 422, and a second data transmitting module 430
Processing and encryption unit module 500, first processing module 510 and encryption module 520
A decryption unit module 600, a second processing module 610, a receiving module 620, and a decryption module 630. (the sender photon detector module 100 is embodied as a function of B, the receiver photon detector module 200 is embodied as a function of B, the sender memory module 300 is embodied as a function of C1, the receiver memory module 400 is embodied as a function of D1, the processing and encryption unit module 500 is embodied as a function of C, and the decryption unit module 600 is embodied as a function of D)
Detailed Description
In order to make the objects, technical solutions and advantages of the present invention more apparent, the present invention is described in further detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative of the invention and are not intended to limit the invention.
According to an embodiment of the present invention, as shown in fig. 2, the functional module includes: a sender photon detector module 100, a receiver photon detector module 200, a sender memory module 300, a receiver memory module 400, and a processing and encryption unit module 500, wherein: the photon emitter module 100 at the sender side, the photon emitter 1 generates a pair of photons to move towards two directions (the photon detector 3 at the sender side and the photon detector 2 at the receiver side) through electron annihilation;
the sender photon detector module 100 is configured to receive a pair of gamma photon pairs generated by the photon emitter 1 through positron annihilation; the sender photon detector module 100 includes a first photon detection module 110, a first timer module 120, and a first parallel detection data determination module 130; where the sender photon detector module 100 outputs to the sender memory module 300. A first photon detection module 110, configured to detect whether a photon is received, where the first photon detection module 110 outputs to the first timer module 120 and the first parallel detection data determination module 130; the first timer module 120 is configured to record a photon arrival time, and includes a first coincidence time window temporary storage module 121 and a first sending module 122, which are subordinate to the sending-side photon detector module 100, wherein the first coincidence time window temporary storage module 121 outputs to the first sending module 122, and then sends to the first data receiving module 310.
The receiver photon detector module 200 is configured to receive photons emitted by the photon emitter 1; the receiver photon detector module 200 includes a second photon detection module 210, a second timer module 220, and a second parallel detection data decision module 230; where the recipient photon detector module 200 outputs to recipient memory module 400. The second photon detection module 210 is configured to detect whether a photon is received, where the second photon detection module 210 outputs the photon to the second timer module 220 and determines whether data is valid; the second timer module 220 is configured to record the arrival time of the photon, and includes a second coincidence time window temporary storage module 221 and a second sending module 222, and belongs to the receiver-side photon detector module 200, wherein the second coincidence time window temporary storage module 221 outputs to the second sending module 222, and then sends to the receiver-side memory 400.
The sender memory module 300 is configured to store related data content; the sender memory module 300 includes a first data receiving module 310, a first array conversion module 320, a first data sending module 330, and a sending content storage module 340; wherein the sender memory module 300 outputs to the processing and encryption unit module 500. The first data receiving module 310 is configured to receive the time information sent by the photon detector 3 of the sending party, where the first data receiving module 310 outputs the time information to the first array converting module 320; the first array conversion module 320 is used for converting the time information into an array, and includes a first register module 321, a first conversion module 322 belonging to the sender memory module 300, wherein the first conversion module 322 outputs to the first data sending module 330; a sending content storage module 340, configured to store data content to be sent.
The receiver memory module 400 is used for storing related data contents; the receiver memory module 400 includes a second data receiving module 410, a second group converting module 420, and a second data transmitting module 430; where the recipient memory module 400 outputs to the decryption module 600. The second data receiving module 410 is configured to receive the time information sent by the receiving-side photon detector 2, where the second data receiving module 410 outputs the time information to the second group converting module 420; the second group conversion module 420 is configured to convert the time information into an array, and includes a second register module 421 and a second conversion module 422, and belongs to the receiving-side memory module 400, where the second conversion module 422 outputs the time information to the second data sending module 430.
The processing and encryption unit module 500 is configured to generate a key and encrypt content to be sent; the processing and encryption unit module 500 comprises a first processing module 510 and an encryption module 520; wherein the processing and encryption unit module 500 outputs to the decryption unit module 600. The first processing module 510 is used for receiving data and converting the time group into a key, and the second processing module 610 outputs the key to the encryption module 520; and an encryption module 520, configured to couple the content to be encrypted with the key for encryption.
The decryption unit module 600 receives the content sent by the sender and decrypts and restores the content to the initial correct information; the decryption unit module 600 comprises a second processing module 610, a receiving module 620 and a decryption module 630; the second processing module 610 is configured to receive data and convert the time group into a key, and the second processing module 610 outputs the key to the decryption module 630; a receiving module 620, configured to receive the content sent by the sender. And a decryption module 630, configured to decrypt the received information to restore to the original correct information.
As shown in fig. 3, which is a quantum communication flow chart of the present invention, a sender first generates a string of random binary data streams, selects a basis vector to be prepared according to information of the data streams, prepares a corresponding qubit, and transmits the qubit to a receiver through a quantum channel. Information transmission can in principle be sent out all at once. The receiver also randomly selects the received quantum-state measurement for the basis vector pair | H >/| V > or | + > | - > measurement. Since the base vectors chosen by the sender and receiver are completely random and independent of each other, there is only a 50% chance that the same base will be chosen. In fact, receiver measurements include two possibilities: receiver measurement basis selection error; the receiver measurement basis selection is correct. The former case measurement should be discarded. In addition, when the receiver has a probability of 50% and selects different bases, the result obtained by the measurement of different basis vectors also has a probability of 50% and is the same as the sender, and the result obtained by the measurement of 50% is different from the sender. The data measured by the receiver will have a 25% probability of yielding an error.
As shown in fig. 4, a schematic diagram of quantum cryptography signal transmission according to the present invention shows that 1 signal generating source can randomly generate two sets of basis vectors, each set including two mutually perpendicular quantum states; signals can be transmitted through quantum channels; the received quantum signals can be effectively measured, the basis vector comparison information needs to be transmitted through a public channel, and verification guarantee is provided for both communication parties. The classical common channel requires that any eavesdropper can obtain the classical information by eavesdropping but cannot modify the information.
As shown in fig. 5, a schematic diagram of original key formation is shown, and a transmitter and a receiver can screen out associated results by comparing basis vectors to achieve the purpose of reducing the bit error rate. The process is as follows: and the receiver publicly measures the used basis loss, Alice receives the basis vector information used by Bob to compare the basis vectors used by the locally prepared secret key, shares the part with the same basis vector to the receiver after the comparison is finished, and finally the transceiver end retains the part with the same basis vector, screens out the data close to one half, and the rest is the required original secret key.
Then both parties randomly disclose part of the original key, and the error rate is estimated to find out whether an eavesdropper exists or not. If the error rate can be lower than a specific threshold, error correction related technology can be adopted to carry out error correction processing on the secret key, then information leakage caused in the error correction process or the communication transmission process is eliminated through privacy amplification, and finally a safety secret key is formed. And if the error rate exceeds a specific threshold value, abandoning the communication and carrying out the communication again.
The working process of the time-coincident single photon detector circuit is divided into a detection stage, a quenching stage and a recovery stage
And in the detection stage, the loading voltage of the APD cathode is higher than the avalanche voltage, when photons enter, the avalanche photodiode generates avalanche, the current is gradually increased, and the voltage is increased accordingly, the voltage is amplified and then an avalanche signal is extracted through a hysteresis comparator U3, wherein the amplification factor of an amplifier is 10 times, the reference level is 1.3V, and the output standard TT L level of the hysteresis comparator is sent to an FPGA time sequence control unit.
A quenching stage: the coincidence time calculation and control module receives the hysteresis comparator signal and then outputs a signal QUENCH (the effective width of high level is 20ns) with a certain width to be added to a Q3 triode, so that Q3 is conducted, the voltage of a collector rapidly drops, Q1 is conducted, the voltage transformation of an APD anode rapidly drops, Q1 is conducted, the voltage of the APD anode becomes Vq, and the purpose of quenching avalanche is achieved after a period of time delay because Va-Vq < Vbr.
And (3) a recovery stage: after avalanche quenching, the FPGA outputs RESET, and the switches in the device all adopt fast switching tubes but have transmission delay, wherein the opening time of a PNP tube is 12ns, the closing time of the PNP tube is 18ns, the opening time of an NPN tube is 15ns, and the closing time of the NPN tube is 20 ns. Therefore, the timing sequence needs to be accurately adjusted when the FPGA outputs the RESET, and the PNP tube and the NPN tube are prevented from being conducted simultaneously, so that a VQ power supply is prevented from being short-circuited, and a circuit board is prevented from being burnt out. Therefore, the FPGA pulls up the RESET signal (the effective width of the high level of Q5 is 20ns) 40ns after outputting the QUENCH pulse to conduct Q5, the anode of the APD is grounded, the APD is rapidly charged, and the APD enters a detection stage after being rapidly recovered to wait for receiving the photons to arrive.
The coincidence time photon detector has higher quantum efficiency and lower dark count. If the detection efficiency of the single photon detector is to be measured, a standard and stable single photon signal light source is needed. By single photon signal source is meant a signal source that triggers emission of only one photon at a time. In experiments, laser light can be attenuated to single photon magnitude by a coherent laser with an adjustable attenuator. The number of photons n contained in each pulsed light satisfies a poisson distribution:
p(n)＝μne-μ/n！
where p (n) is the probability that the pulsed light contains n photons, μ is the average photon number of the pulsed light, and n is the number of photons in the pulsed light. The statistical distribution of p (n) is different for different values of μ, and fig. 6 shows the statistical distribution curve of p (n) for different values of μ.
Depending on the value of n, p (n) can be divided into three categories:
n is 0, and p (0) is e-μAt the moment, no photon exists in the light pulse, and the signal at the moment can be determined as a null pulse;
2.n＝1,p(1)＝μetime of muAt the moment, the light pulse only contains one photon, namely the single photon signal required in the single photon detector experiment;
n.gtoreq.2, i.e. pmulti1-p (0) -p (1), the optical pulse contains two or more photons, and is a multiphoton state.
4. Is represented by the formula p (n) ═ mune-μA/n! The probability of multiple photons (i.e., more than two photons) occurring in the light pulse can be derived:
when μ is 0.1, the pulse contains 0.1 photons per pulse. This is calculated from equation 4-2, where the probability that a light pulse contains no photons is p (0) ═ 90.5%, the probability that a light pulse contains one photon is p (1) ═ 9%, and the probability that a light pulse contains two or more photons is p (0) — 90.5%, and the probability that a light pulse contains two or more photons is p ═ 9%multi0.5%. Then, when μ is 0.1, it is considered herein as an ideal quasi-single photon light source for experiment. By directing the light pulses emitted by the laser into an optical attenuator with a controllable attenuation factor, we can find the point of the average number of photons we need by slowly adjusting the attenuation factor.
According to one embodiment of the invention, a series of data 1867 is encrypted.
S1: starting a photon detector, starting a photon emitter, and annihilating positrons to generate gamma photon pairs;
s2: the difference value of the arrival time of the two pulses is measured by a time marking method, so that the single photon detector obtains a proper coincidence counting time window, when the sender and the receiver detect the photons, the time recorded by the photon detector is considered to be effective and stored, and is respectively recorded as T1N，T2N(the time difference between the arrival of a pair of gamma photons produced by positron annihilation at a pair of detectors is less than 2 ns);
s3: by T1NSequence calculation event difference Δ T1a＝T1(a+1)-T1aWhere a is a natural number from 1 to 4, then a sequence Δ T is obtained1aKey of (1 [ Delta T ]11，ΔT12，ΔT13，ΔT14]；
S4: the information to be transmitted is multiplied by the key to obtain an encrypted information sequence [ Delta T ]11*1，ΔT12*8，ΔT13*6，ΔT14*7]And sending to the receiver;
s5: t for receiving party2NSequence calculation event difference Δ T2a＝T2(a+1)-T2aWhere a is a natural number from 1 to 4, then a sequence Δ T is obtained2aKey of (1 [ Delta T ]21，ΔT22，ΔT23，ΔT24]；
S6: the receiver decrypts the encrypted information by using the key to obtain the initial information.
The above embodiments are merely preferred embodiments of the present invention, which are not intended to limit the scope of the present invention, and various changes may be made in the above embodiments of the present invention. All simple and equivalent changes and modifications made according to the claims and the content of the specification of the present application fall within the scope of the claims of the present patent application. The invention has not been described in detail in order to avoid obscuring the invention.
Claims (10)
1. A coincidence-time based quantum cryptography circuit system, the system comprising:
the global clock generation module enables the clocks of all modules in the signal generator to be the same, and ensures that all signals can be kept synchronous;
the single-event photon light source module outputs photon signals with adjustable attenuation times to the single-photon detector;
the Si-APD photon detector module is used for collecting the changed photon signals and converting the changed photon signals into synchronously changed electric pulse signals;
and the encryption and transmission module generates a key according to the generated electric pulse signal, encrypts data to be encrypted, and packages and transmits the data to a receiving party through a classical public channel.
2. The single-event photon light source module according to claim 1, wherein the single-event photon light source module comprises an adjustable attenuation module, receives a pulse light source, adjusts the laser according to a preset attenuation multiple, enables the laser to reach a quasi-single photon light source with a standard of only 0.1 photon per pulse, and then transmits a photon signal to a single photon detector through a channel.
3. The Si-APD photon detector module of claim 1 which includes an avalanche amplification module which avalanche breaks down photons from the light source into photoelectrons which emit avalanche signals when absorbed by the avalanche photodiode and transmitted to the coincidence time calculation and control module.
4. The Si-APD photon detector module of claim 1 which contains a coincidence time calculation and control module to perform quenching and fast recovery of the avalanche photodiode and wait for the arrival of the next photon, measuring the difference in arrival times of the two pulses by time-stamping method to obtain a suitable coincidence counting time window to record the coincidence time.
5. The Si-APD photon detector module of claim 1, comprising an optoelectronic summing module that sums all of the photoelectrons to form electrical pulses.
6. The Si-APD photon detector module of claim 1, which contains a signal multiplexing module that multiplexes all channels, reducing the number of channels and thereby reducing the number of output signals.
7. The encryption and transmission module according to claim 1, wherein the encryption and transmission module comprises a key generation module that generates the key by a basis vector ratio provided to the transmission side and the reception side.
8. A quantum cryptography method based on coincidence time, the method comprising the steps of:
s1, starting a global clock module to enable clocks of all modules in a signal generator to be the same, ensuring that all signals can be kept synchronous, enabling a pulse laser to emit a pulse light source and input the pulse light source to an optical attenuator, enabling the attenuation multiple of the optical attenuator to be flexible and controllable, adjusting laser according to a preset attenuation multiple, enabling the laser to reach a quasi-single photon light source with each pulse only containing 0.1 photon standard, and then transmitting photon signals to a single photon detector through a channel;
s2, injecting the light pulse emitted by the laser into an optical attenuator with controllable attenuation multiple, finding out the point of the average photon number required by people through slowly adjusting the attenuation multiple, and detecting the photon signal output by the optical attenuator by using a single photon detector;
s3, adding a detection stage of the photon detector, wherein the loading voltage of the APD cathode is higher than avalanche voltage, when photons are incident, the avalanche photodiode generates avalanche, the current gradually increases, and the voltage is increased accordingly, the voltage is amplified and then an avalanche signal is extracted by a hysteresis comparator U3, and the output standard TT L level of the hysteresis comparator is sent to the FPGA time sequence control unit;
s4: then adding a quenching stage of a photon detector, outputting a signal QUENCH with a certain width to a Q3 triode after a time calculation and control module receives a hysteresis comparator signal, enabling Q3 to be conducted, enabling the voltage of a collector to rapidly drop, conducting Q1, rapidly dropping the voltage transformation of an APD anode at the moment, conducting Q1, enabling the voltage of the APD anode to be Vq, and achieving the purpose of quenching avalanche after a period of time delay (the effective width of the high level of the signal QUENCH is 20ns) because Va-Vq < Vbr);
s5: then, a recovery stage of the photon detector is added, after avalanche quenching, the FPGA outputs RESET, and because the switches adopt quick switching tubes to have transmission delay, the time sequence is required to be accurately adjusted when the FPGA outputs the RESET, so that the PNP tube and the NPN tube are prevented from being conducted at the same time, the VQ power supply is prevented from being short-circuited, and a circuit board is burnt out; the FPGA pulls up a RESET signal 40ns after outputting a QUENCH pulse to enable Q5 to be conducted, the anode of the APD is grounded, the APD is rapidly charged, and the APD enters a detection stage after being rapidly recovered to wait for receiving photons to arrive;
s6, the receiver opens the basis loss used by the measurement, Alice receives the basis vector information used by Bob to compare the basis vector used by the local preparation key, shares the part with the same basis vector to the receiver after the comparison, and finally the transceiver end keeps the part with the same basis vector;
and S7, the photon detector of the sender transmits the key to the photon detector of the receiver through the quantum channel, and simultaneously transmits the basic vector comparison information through the public channel, so that verification guarantee is provided for both communication parties, and a one-time pad encryption system is used for encrypting and transmitting the ciphertext to complete safe communication.
9. The quantum cryptography method according to claim 8, wherein in the step S3, the amplification factor of the voltage amplification is 10 times, and when the voltage is used to extract the avalanche signal by the hysteresis comparator U3, the reference level is 1.3V.
10. The quantum cryptography method of claim 8, wherein in the step S5, the propagation delay of the fast switching transistor is set, wherein the PNP transistor has an on time of 12ns, the off time of 18ns, the NPN transistor has an on time of 15ns, the off time of 20ns, and the high-level effective width of Q5 is 20 ns.
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| Publication number | Priority date | Publication date | Assignee | Title |
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| CN116032376A (en) * | 2023-03-30 | 2023-04-28 | 国开启科量子技术(北京)有限公司 | Optimizing method, device, medium and equipment for quantum communication system |
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