A reference-frame-independent quantum key distribution method with a light source monitoring function

Abstract

The present invention belongs to the field of quantum information technology, and discloses a reference-frame-independent quantum key distribution method with a light source monitoring function. It allows the light source to follow an unknown photon number distribution, only assuming that its light intensity fluctuates within a certain range. By using a beam splitter to estimate the parameters of four response events generated after splitting the light source, the passive light source monitoring function is completed. Compared with the RFI-QKD scheme using active light source monitoring, this method does not require an attenuator to actively modulate the light intensity attenuation coefficient, thus avoiding the introduction of additional modulation errors and improving the actual performance of the RFI-QKD system. Compared with the original RFI-QKD protocol, it has better anti-light source jitter ability.

Description

Technical Field

[0001] The present invention belongs to the fields of quantum communication, quantum information technology, etc., and specifically relates to a reference-frame-independent quantum key distribution method with a light source monitoring function. Background Art

[0002] In an actual quantum key distribution system (QKD), both communicating parties need to calibrate the reference frame in real time. For example, in a polarization-encoded QKD system based on a satellite and the ground, due to factors such as the rotation of the earth and the movement of the satellite around the earth, the satellite and the ground receiving station need to continuously adjust the polarization direction angle so that the QKD can operate normally; similarly, in phase encoding, due to factors such as environmental temperature changes and mechanical vibrations of the experimental platform, the arm length difference of the interferometers at the sending end Alice and the receiving end Bob will change, so they also need to actively compensate for the phase. The operations of Alice and Bob to actively adjust the polarization state and compensate for the phase are called calibrating the reference frame. Calibrating the reference frame consumes a large amount of time, reduces the efficiency of the entire communication process, increases the complexity and cost of the QKD system, and may even pose a security risk. Anthony Laing et al. published a paper "Reference-frame-independent quantum key distribution" in PHYSICAL REVIEW A, proposing a reference-frame-independent quantum key distribution protocol, which can effectively solve the problem of needing to calibrate the reference frame. The RFI-QKD protocol can be immune to the influence of slow reference frame drift, effectively estimate the amount of information stolen by Eve, and finally generate a secure key.

[0003] However, in previous RFI-QKD protocols and schemes, it is assumed that the light source is trusted and obeys a certain fixed photon number distribution, which is difficult to meet in an actual system; and the existing method is an RFI-QKD scheme using active light source monitoring, which needs to actively modulate the light intensity attenuation coefficient through an attenuator, is prone to introducing additional modulation errors, and has poor anti-light source jitter ability. Summary of the Invention

[0004] To solve the above technical problems, the present invention provides a reference-frame-independent quantum key distribution method with a light source monitoring function, which is applied to a quantum cryptography transmission system with an untrusted light source. This method allows the light source to obey an unknown photon number distribution, only assuming that its light intensity fluctuates within a certain range, and completes the passive light source monitoring function by estimating the parameters of four response events generated after splitting the light source using a beam splitter.

[0005] The steps of the reference-frame-independent quantum key distribution method with a light source monitoring function described in the present invention are as follows:

[0006] Step 1: The sender Alice sends N pulses through a non-ideal light source and randomly prepares and sends three intensity pulses through an intensity modulator IM, namely signal-state pulses, decoy-state pulses, and vacuum-state pulses;

[0007] Step 2: After passing through the fiber splitter BS1, the N pulses are divided into signal light and idler light. The signal light is used for encoding and sent to the receiver Bob, and the idler light is used to perform passive light source monitoring (PLSM); after passing through the passive light source monitoring module, the idler light is further split by the fiber splitter BS2 and finally detected by the sender detectors D1 and D2; therefore, 4 different detection events l∈{x, y, z, w} are obtained at the sender, where x represents that neither D1 nor D2 responds; y represents that only D1 responds; z represents that only D2 responds; w represents that both D1 and D2 respond;

[0008] Step 3: After the signal light passes through the encoding Encoding module, for the signal-state pulses and decoy-state pulses, Alice prepares quantum states in the Z A , X A and Y A bases with different probabilities;

[0009] Step 4: In the decoding Decoding module at the receiver, Bob prepares quantum states in the Z and bases with probabilities B , X B and Y B respectively, and records the corresponding measurement results;

[0010] Step 5: After the measurement, Alice and Bob announce their basis and intensity selection information through an authenticated classical channel; then Alice and Bob retain the data under the prepared measurement basis combinations Z A Z B , X A X B , X A Y B , Y A , X B , Y A , Y B and discard the data of other basis combinations; Alice and Bob randomly select some bits from the sifted key and estimate the bit error rate of the basis combinations Z A Z B , X A X B , X A , Y B , Y A , X B , Y A , YB The gain and total quantum bit error rate below;

[0011] Step 6: Combine the decoy state method to obtain the lower bound of the single - photon counting rate and the upper bound of the error rate, and then estimate the final code rate.

[0012] Furthermore, in step 1, the average photon number of the signal state pulse is u, the average photon number of the decoy state is v, and the average photon number of the vacuum state is 0, and u > v > 0; Alice sends the signal state with probability P u , sends the decoy state with probability P v , and sends the vacuum state with probability 1 - P u - P v .

[0013] Furthermore, in step 3, after the signal light passes through the encoding module Encoding, for the signal state pulse, Alice prepares quantum states in the Z and bases with probabilities A and A respectively; for the decoy state pulse, Alice prepares quantum states in the Z A bases with probabilities and respectively. A and A and A

[0014] Furthermore, due to the monitoring effect of the local detector at the sending end, the idle light is projected onto the quantum state , then the probability that an n - photon is projected onto the detection event l is

[0015]

[0016]

[0017]

[0018]

[0019] Wherein, respectively represent the probabilities that an n - photon is projected onto the x, y, z, w events, d s and η s are respectively the dark - count rate and detection efficiency of the detector at the sending end, t represents the transmittance of the beam splitter BS1; for further simplification, assume that the detectors D1 and D2 at the sending end have the same detection efficiency, i.e., η1 = η2 = η s ; similarly, the dark - count rates of the two detectors are also the same, denoted as d1 = d2 = d s; In addition, the photon number distributions under different events are defined. where P n (μ) represents the probability of the occurrence of n photons when the average photon number is μ, and it is assumed here that it follows a Poisson distribution.

[0020] Furthermore, by measuring the idler light, the gains of four events are obtained as

[0021]

[0022]

[0023]

[0024]

[0025] where Q x (μ), Q y (μ), Q z (μ), Q w (μ) respectively represent the gains of events x, y, z, and w at the μ intensity;

[0026] respectively represent the photon number distributions of events x, y, z, and w at the μ intensity;

[0027] According to the above formula, the upper and lower bounds of the probabilities of zero photons, single photons, and two photons in the light source are further estimated; specifically expressed as:

[0028]

[0029]

[0030]

[0031]

[0032]

[0033] where and are respectively the lower and upper bounds of the probability of zero photons at the average photon number of μ; and are respectively the lower and upper bounds of the probability of single photons at the average photon number of μ; and are respectively the lower and upper bounds of the probability of two photons at the average photon number of μ

[0034] Furthermore, the security key rate formula of the RFI-QKD protocol for passive light source monitoring is expressed as:

[0035]

[0036] Among them, and are the gain and bit error rate of the signal state pulse in the basis combination of Z A Z B respectively; and are the lower bound of the single - photon counting rate and the upper bound of the single - photon bit error rate in the basis combination of Z A Z B respectively, and their calculation formulas are:

[0037]

[0038]

[0039] Among them, and represent the sum of the gain of the decoy state pulse in the basis combination of Z A Z B and the upper bound of the gain of the signal state pulse in the basis combination of Z A Z B respectively; S 0,U and S 0,L represent the upper and lower bounds of the gain of the vacuum state pulse; and are the lower and upper bounds of the probability of two - photons in the signal state pulse; is the lower bound of the probability of two - photons in the signal state pulse; and are the lower bound of the single - photon probability in the signal state pulse and the upper bound of the single - photon probability in the decoy state pulse respectively; and represent the lower bound of the zero - photon probability in the signal state pulse and the upper bound of the zero - photon in the decoy state pulse respectively; and are the lower bounds of the probabilities of zero - photon and single - photon in the decoy state pulse respectively; and are the upper bound of the bit error rate and the upper bound of the gain of the decoy state pulse in the basis combination of Z A Z B respectively; Estimate the upper bound of the single - photon bit error rate of the basis combinations X A X B , X A Y B , Y A X B , Y A Y B in the same way; Using the above parameters, the intermediate parameter C is obtained:

[0040]

[0041] The amount of information I stolen by the untrusted third party Eve E is expressed as:

[0042]

[0043] wherein,

[0044]

[0045]

[0046] H(x) = -x log2(x) - (1 - x) log2(1 - x) is the binary Shannon entropy function, ψ and φ are intermediate parameters, and f represents the negotiation efficiency of the key negotiation algorithm.

[0047] The beneficial effects of the present invention are as follows: A reference-frame-independent quantum key distribution method with a light source monitoring function according to the present invention estimates the upper and lower bounds of the photon number distribution of zero photons, single photons, and double photons of a light source by using four different counting events, and then obtains the secure key generation rate under an untrusted light source; it does not require adjusting the detection efficiency of the local detector, reduces the experimental difficulty, and avoids the performance degradation caused by modulation errors. The simulation results show that the method according to the present invention has better anti-light source jitter ability compared with the existing RFI-QKD protocol. BRIEF DESCRIPTION OF THE DRAWINGS

[0048] Figure 1 is the schematic diagram of passive light source monitoring of the solution of the present invention;

[0049] Figure 2 is the comparison diagram of the key generation rate between the solution of the present invention and the original solution under different pulse numbers;

[0050] Figure 3 is the comparison diagram of the key generation rate between the solution of the present invention and the original solution when considering different deviation angles under a fixed light source fluctuation;

[0051] Figure 4 is the comparison diagram of the key generation rate between the solution of the present invention and the original solution when considering different light source fluctuations under a fixed deviation angle. DETAILED DESCRIPTION OF THE INVENTION

[0052] In order to make the content of the present invention be more clearly understood, the following further describes the present invention in detail according to specific embodiments and in conjunction with the accompanying drawings.

[0053] The present invention proposes a reference-frame-independent quantum key distribution method with a light source monitoring function, which is applicable to a reference-frame-independent quantum key transmission system. In this transmission system, there are two users, Alice and Bob. Alice serves as the sender, and Bob serves as the receiver.

[0054] Step 1: Alice sends N pulses through a non-ideal light source and randomly prepares and sends three-intensity pulses through an intensity modulator (IM), namely the signal state (average photon number is u), the decoy state (average photon number is v), and the vacuum state (average photon number is 0), and satisfies u > v > 0. Alice sends the signal state with probability P u , the decoy state with probability P v , and the vacuum state with probability 1 - P u - P v ;

[0055] Step 2: After passing through the fiber optic splitter (BS1), the N pulses are divided into signal light and idler light. The signal light is used for encoding and sent to Bob, and the idler light is used to perform passive light source monitoring (PLSM). After passing through the passive light source monitoring module, it is further split by BS2 and finally detected by two local detectors. Therefore, four different detection events l ∈ {x, y, z, w} can be obtained at the local end: x represents that neither D1 nor D2 responds; y represents that only D1 responds; z represents that only D2 responds; w represents that both D1 and D2 respond;

[0056] Step 3: For the signal light, after passing through the encoding Encoding module, for the signal state pulses, Alice prepares quantum states in the Z and bases with probabilities A and A respectively; for the decoy state pulses, Alice prepares quantum states in the Z A bases with probabilities and respectively; A and A and A ;

[0057] Step 4: In the decoding Decoding module at the receiving end, Bob prepares quantum states in the Z and bases with probabilities B and B respectively, and records the corresponding measurement results; B ; [[ID=�3]]

[0058] Step 5: After the signal transmission stage is completed, Alice and Bob announce their basis and intensity selection information through the authenticated classical channel; then they retain the data under the prepared measurement basis combination Z A Z B ,X A X B ,X A Y B ,Y A X B ,Y A Y B and discard the data under other basis combinations; Alice and Bob randomly select some bits from the sifted key and estimate the gain and total quantum bit error rate under the basis combination Z A Z B ,X A X B ,X A Y B ,Y A X B ,Y A Y B .

[0059]

[0060]

[0061] where P n (μ) represents the average photon number μ ∈ {u, v, 0}, Y n and e n represent the n-photon state counting rate and error rate, ξ A ξ B represents the basis selected by Alice when preparing the quantum state, ξ A ={X A ,Y A ,Z A}, and the basis selected by Bob when measuring the quantum state is ξ B ={X B ,Y B ,Z B}. Considering that the pulses sent by Alice are finite, the influence of statistical fluctuations on parameter estimation needs to be considered. Here, we use the Gaussian analysis method for statistical fluctuation analysis. At this time, the lower bounds of the signal state gain the decoy state gain and the vacuum state gain S 0 , and the upper bounds of the decoy state bit error rate and the vacuum state gain S 0 can be expressed as:

[0062]

[0063]

[0064]

[0065]

[0066]

[0067] where P μ represents the probability that Alice selects the intensity μ when preparing the quantum state, represents the conditional probability that Alice selects the ξ A basis under the condition of intensity μ when preparing the quantum state, represents the probability that Bob selects the ξ B basis when measuring the quantum state, γ represents the standard deviation selected for statistical fluctuation analysis, and N represents the total number of pulses emitted by Alice.

[0068] Step 6, finally, combining the decoy state method can obtain the lower bound of the single-photon counting rate and the upper bound of the bit error rate, and further estimate the final code rate.

[0069] In the passive light source monitoring scheme, the idle light is projected onto the quantum state , then the probability that n photons are projected onto event l is

[0070]

[0071]

[0072]

[0073]

[0074] In the above formula, respectively represent the probabilities that n photons are projected onto events x, y, z, and w, d s and η s are respectively the dark count rate and detection efficiency of the detector at the Alice end, and t represents the transmittance of the beam splitter BS1. To further simplify the present invention, we assume that the detectors at the Alice end (D1 and D2) in the structural diagram of this scheme have the same detection efficiency, that is, η1 = η2 = η s , similarly, the dark count rates of the two detectors are also the same, denoted as d1 = d2 = d s . In addition, we define the photon number distribution under different events as where P n (μ) represents the probability that n photons appear when the average photon number is μ, and it is assumed here that it follows a Poisson distribution.

[0075] By measuring the idle light, we can obtain the gains of the four events as

[0076]

[0077]

[0078]

[0079]

[0080] where Q x (μ), Q y (μ), Q z (μ), Q w (μ) represent the gains of the x, y, z, and w events at the μ intensity respectively; respectively represent the photon number distributions of the x, y, z, and w events at the μ intensity; according to the above formula, the upper and lower bounds of the probabilities of zero photons, single photons, and two photons in the light source are further estimated; specifically expressed as:

[0081]

[0082]

[0083]

[0084]

[0085]

[0086] where and are the lower and upper bounds of the probability of zero photons with an average photon number of μ respectively; and are the lower and upper bounds of the probability of single photons with an average photon number of μ respectively; and are the lower and upper bounds of the probability of two photons with an average photon number of μ respectively;

[0087] Combining the above analysis, the security key rate formula of the passive light source monitoring RFI-QKD protocol is expressed as:

[0088]

[0089] and are the lower bound of the single photon counting rate and the upper bound of the single photon bit error rate with the basis combination of Z A Z B respectively, and their calculation formulas are:

[0090]

[0091]

[0092] Among them, and respectively represent the sum-signal state pulse of the gain of the decoy-state pulse in the basis combination of Z A Z B and the upper bound of the gain of the sum-signal state pulse in the basis combination of Z A Z B ; S 0,U and S 0,L represent the upper and lower bounds of the gain of the vacuum-state pulse; and are the lower and upper bounds of the probability of two photons in the signal-state pulse; is the lower bound of the probability of two photons in the signal-state pulse; and are respectively the lower bound of the probability of a single photon in the signal-state pulse and the upper bound of the probability of a single photon in the decoy-state pulse; and respectively represent the lower bound of the probability of zero photons in the signal-state pulse and the upper bound of the probability of zero photons in the decoy-state pulse; and are respectively the lower bounds of the probabilities of zero photons and single photons in the decoy-state pulse; and are respectively the upper bound of the bit error rate and the upper bound of the gain of the decoy-state pulse in the basis combination of Z A Z B ; The upper bound of the single-photon bit error rate of the basis combinations X A X B , X A Y B , Y A X B , Y A Y B is estimated by the same method; Using the above parameters, the intermediate parameter C is obtained:

[0093]

[0094] And the amount of information I stolen by the untrusted third party Eve is E expressed as:

[0095]

[0096] Among them,

[0097]

[0098]

[0099] H(x) = -x log2(x) - (1 - x) log2(1 - x) is the binary Shannon entropy function, ψ and φ are intermediate parameters, and f represents the negotiation efficiency of the key negotiation algorithm.

[0100] The following defines the parameters for measuring the optical intensity fluctuation. Assume that the photon number distribution of the sending state no longer follows the Poisson distribution (ideal light source), but follows an unknown distribution, and its average optical intensity μ follows a Gaussian distribution. Then the average optical intensity distribution can be expressed as:

[0101]

[0102] where μ0 and σ μ represent the mean and standard deviation of μ respectively. μ belongs to a confidence interval μ ∈ [μ L , μ U , and the confidence level is Let ε = 1 - 10 -10 . In the present invention, σ = σ μ / μ0 is used to measure the degree of optical intensity fluctuation.

[0103] To better elaborate the purpose, technical solution, and advantages of the present invention, the present invention will be further described in detail in the following part in combination with specific embodiments with reference to the description.

[0104] Att Figure 1 is the schematic diagram of the passive light source monitoring of the present invention's solution. The present invention is based on the RFI-QKD protocol. The sending end includes Alice, and the receiving end is Bob. The sending end pulse passes through the intensity modulator IM, beam splitters BS1 and BS2 to obtain the idler light and the signal light. The idler light is sent to the passive light source monitoring module at the local end for estimating the photon number distribution in the signal light; the signal light selects the decoy state window and the signal state window. If the signal light selects the decoy state window, the signal light that conforms to the RFI-QKD protocol is used to estimate the lower bound of the single photon counting rate and the upper bound of the single photon bit error rate; if the signal light selects the signal state window, the signal light that conforms to the RFI-QKD protocol is used to extract the final secure key after error detection.

[0105] Att Figure 2 is the comparison diagram of the key generation rate of the present invention's method and the original scheme under different pulses in the case of an ideal light source. The detection efficiency of the detector at the sending end in the present invention is set to η s = 0.9. To simulate the actual situation, the parameters are unified as shown in Table I during the simulation.

[0106] α (dB / km) <![CDATA[d s > <![CDATA[η d > <![CDATA[e d > f γ 0.2 <![CDATA[2.5×10 -6 > 70％ 1.5％ 1.16 5.3

[0107] Table I

[0108] Among them, α represents the transmission loss of the optical fiber, Y0 represents the dark count rate of the detector, and η D represents the detection efficiency of the detector at Charlie's end, and e d is the background error rate of the QKD system, f represents the negotiation efficiency of the key negotiation algorithm, and γ represents the standard deviation selected for statistical fluctuation analysis.

[0109] From Figure 2 it can be seen that at high pulse numbers (N = 10 12 and N = 10 11 ), the performance of the passive scheme proposed by the present invention is superior to that of the original RFI-QKD under light source fluctuations.

[0110] Appendix Figure 3 is a comparison chart of the key rates of the method described in the present invention with those of other original schemes when considering light source fluctuations with σ = 2%.

[0111] From Appendix Figure 3 it can be seen that when the deviation angles are β = 0, β = π / 18, and β = π / 9, the maximum transmission code distance differences between the passive scheme proposed by the present invention and the original RFI-QKD at high pulse numbers (N = 10 12 ) are 32 km, 35 km, and 37 km respectively. This shows that under light source fluctuations, our protocol can have better performance than the original protocol even in the case of a deflection angle, and as the deflection angle increases, the advantage of our protocol will be further increased.

[0112] Appendix Figure 4 is a comparison chart of the key rates of the scheme of the present invention with those of other schemes under different light intensity fluctuation conditions (σ = 0.01, σ = 0.02). From Appendix Figure 4 it can be seen that in the case of a non-ideal light source and the presence of a deviation angle, our scheme has an obvious advantage in the code rate over the original scheme. When the fluctuation coefficient σ increases by 0.01, the maximum transmission code distance of the original protocol decreases by 35 km, while for our protocol, it can almost maintain the original performance, indicating that our protocol has good robustness under light source fluctuations.

[0113] The specific embodiments described above further elaborate on the purpose, technical solutions, and beneficial effects of the present invention. It should be understood that the current patent specification only introduces a reference frame-independent quantum key distribution scheme with a light source monitoring function. For example, the method used in the specific embodiments of the present invention is equally applicable to quantum key distribution systems based on other protocols and does not limit the present invention. Any modifications, equivalent replacements, improvements, etc. made within the spirit and principles of the present invention shall be included within the protection scope of the present invention.

Claims

1. A reference-frame-independent quantum key distribution method with a light source monitoring function, characterized in that The steps of the method are as follows: Step 1: The sender Alice sends N pulses through a non-ideal light source and randomly prepares and sends three types of intensity pulses through an intensity modulator IM, namely signal-state pulses, decoy-state pulses, and vacuum-state pulses. Step 2: The N pulses are divided into signal light and idler light after passing through an optical fiber splitter BS1. The signal light is used for encoding and sent to the receiver Bob, and the idler light is used to perform passive light source monitoring PLSM. After passing through the passive light source monitoring module, the idler light is further split by the optical fiber splitter BS2 and finally detected by the sender detectors D1 and D2. Therefore, four different detection events l∈{x, y, z, w} are obtained at the sender. x represents that neither D1 nor D2 responds; y represents that only D1 responds; z represents that only D2 responds; w represents that both D1 and D2 respond. Step 3: After the signal light passes through the Encoding module, for the signal state pulses and decoy state pulses, Alice prepares quantum states in the Z A , X A , and Y A bases with different probabilities; Step 4. In the decoding module at the receiving end, Bob prepares quantum states in the Z , X basis with probabilities B and B respectively, and records the corresponding measurement results; B ​ Step 5. After the measurement, Alice and Bob announce their basis and intensity selection information through the authenticated classical channel; then Alice and Bob retain the data under the prepared measurement basis combination Z A Z B ,X A X B ,X A Y B ,Y A X B ,Y A Y B and discard the data of other basis combinations; Alice and Bob randomly select some bits from the sifted key to estimate the basis combination Z A Z B ,X A X B ,X A Y B ,Y A X B ,Y A Y B under the gain and total quantum bit error rate; Step 6: Combine the decoy-state method to obtain the lower bound of the single-photon counting rate and the upper bound of the bit error rate, and then estimate the final code rate. Among them, the security key rate formula of the RFI-QKD protocol with passive light source monitoring is expressed as: Among them, and are the gain and bit error rate of the signal state pulse in the basis combination of Z A Z B respectively; Pu is the probability that Alice sends the signal state; is the probability that Alice prepares the quantum state in the Z A basis; and are the lower bound of the single - photon counting rate and the upper bound of the single - photon bit error rate in the basis combination of Z A Z B respectively, and their calculation formulas are as follows: Among them, and respectively represent the sum signal state pulse of the gain of the decoy state pulse in the basis combination of Z A Z B and the upper bound of the gain of the signal state pulse in the basis combination of Z A Z B ; S 0,U and S 0,L represent the upper and lower bounds of the gain of the vacuum state pulse; and are the lower and upper bounds of the probability of two photons in the signal state pulse; is the lower bound of the probability of two photons in the signal state pulse; and are respectively the lower bound of the probability of a single photon in the signal state pulse and the upper bound of the probability of a single photon in the decoy state pulse; and respectively represent the lower bound of the probability of zero photons in the signal state pulse and the upper bound of the probability of zero photons in the decoy state pulse; and are respectively the lower bounds of the probabilities of zero photons and single photons in the decoy state pulse; and are respectively the upper bound of the bit error rate and the upper bound of the gain of the decoy state pulse in the basis combination of Z A Z B ; Using the same method to estimate the upper bound of the single - photon bit error rate of the basis combinations X A X B , X A Y B , Y A X B , Y A Y B Using the above parameters, the intermediate parameter C is obtained: The amount of information I stolen by the untrusted third party Eve E is expressed as: Among them, H(x)=-xlog2(x)-(1 - x)log2(1 - x) is the binary Shannon entropy function, ψ and φ are intermediate parameters, and f represents the negotiation efficiency of the key negotiation algorithm.

2. A reference-frame-independent quantum key distribution method with a light source monitoring function according to claim 1, characterized in that In Step 1, the average photon number of the signal state pulse is u, the average photon number of the decoy state is v, and the average photon number of the vacuum state is 0, and u > v > 0 is satisfied; Alice sends the signal state with probability P u , P v sends the decoy state, and 1 - P u - P v sends the vacuum state.

3. A reference-frame independent quantum key distribution method with a light source monitoring function according to claim 2, characterized in that, In step 3, after the signal light passes through the encoding module Encoding, for the signal state pulses, Alice prepares quantum states in the Z , basis, X A , A basis, and Y A basis, respectively; for the decoy state pulses, Alice prepares quantum states in the Z , basis, X A , A basis, and Y A basis, respectively.

4. A reference-frame-independent quantum key distribution method with a light source monitoring function according to claim 3, characterized in that, Due to the monitoring effect of the local detector at the sending end, the idle light is projected onto the quantum state , and the probability that the n photons are projected onto the detection event l is Among them, respectively represent the probabilities of n photons being projected onto events x, y, z, and w, d s and η s are respectively the dark count rate and detection efficiency of the transmitter detector, and t represents the transmittance of the beam splitter BS1; for further simplification, assume that the transmitter detectors D1 and D2 have the same detection efficiency, that is, η1 = η2 = η s ; similarly, the dark count rates of the two detectors are also the same, denoted as d1 = d2 = d s ; in addition, define the photon number distribution under different events where P n (μ) represents the probability of n photons occurring when the average photon number is μ, and here it is assumed to follow a Poisson distribution.

5. A reference-frame-independent quantum key distribution method with a light source monitoring function according to claim 4, characterized in that, By measuring the idler light, the gains of the four events are obtained as Among which Q x (μ), Q y (μ), Q z (μ), Q w (μ) represent the gains of events x, y, z, w under the intensity of μ respectively; Respectively represent the photon number distributions of the x, y, z, and w events under the μ intensity; According to the above formula, the upper and lower bounds of the probabilities of zero photons, single photons, and two photons in the light source are further estimated. Specifically, it is expressed as: where and are the lower and upper bounds of the probability of zero photons with an average photon number of μ, respectively; and are the lower and upper bounds of the probability of single photons with an average photon number of μ, respectively; and are the lower and upper bounds of the probability of two photons with an average photon number of μ, respectively.

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