A continuous variable quantum key distribution method based on an inner modulation pulse light source

Abstract

The application discloses a continuous variable quantum key distribution method based on an inner modulation pulse light source, which comprises the following steps: a sending end provides signal light and local light to a receiving end, or the sending end provides signal light to the receiving end, and the receiving end provides local light; the receiving end interferes the signal light and the local light, and extracts a component with channel noise and initial data from the interference result; and the receiving end processes the component with channel noise and the initial data to obtain amplitude modulation information related to the channel noise, namely, an initial key. The application makes full use of the random phase difference between the front and rear pulses generated by the inner modulation pulse light source, and makes the detection result independent of the phase difference, without any phase feedback and compensation, and the modulation mode is simpler; higher light pulse extinction ratio and light pulse intensity can be obtained, so that the transmission distance is farther, and the detection efficiency is higher; the high complexity double light source frequency locking technology can be avoided, and the system complexity is reduced.

Description

A continuous-variable quantum key distribution method based on an internally modulated pulsed light source Technical Field

[0001] This invention relates to the field of quantum key distribution technology, and in particular to a continuous variable quantum key distribution method based on an internally modulated pulsed light source. Background Technology

[0002] With the rapid development of quantum information technology, quantum key distribution (QKD), as an important branch, has achieved remarkable and significant results in the past three decades. Based on the different dimensions of the encoding space at the source end, QKD can be divided into discrete variable (DV) protocols and continuous variable (CV) protocols. Unlike DV-QKD, the Hilbert space of the quantum states used for encoding in CV-QKD systems is infinite-dimensional and continuous. The carrier of information is no longer the polarization or phase of a single photon, but rather the canonical components of the light field ("position" and "momentum" in phase space). Because the carrier of information is a continuous variable, the number of key bits that can be carried by each pulse increases. Although not every bit will become the final secure key, this still provides great potential for improving the final security code rate. Experimental studies have shown that continuous variable quantum key distribution systems have advantages such as high security code rate over short to medium distances and compatibility with classical optical communication systems, and have experienced rapid development in the past decade.

[0003] CV-QKD systems can be classified into Gaussian modulation schemes and discrete modulation schemes based on the number of coherent modulation states at the source end; and into two main categories based on the location of the system reference local oscillator at the transceiver end: in-circuit local oscillator CV-QKD systems and local local oscillator CV-QKD systems.

[0004] Among these, the CV-QKD system based on Gaussian modulation coherent state protocol with on-path local oscillator (LO) is the earliest developed and has the highest maturity. This type of scheme often employs time-division multiplexing plus polarization multiplexing. To avoid leakage light noise at the receiver affecting the quantum signal light, a local oscillator pulse with an extremely high extinction ratio needs to be prepared at the transmitter. Currently, such high extinction ratio pulses are mostly obtained by externally modulating continuous light using cascaded amplitude modulators. External modulation schemes have two problems: first, complex feedback control hardware and software are required to address the bias point drift of the amplitude modulator, resulting in a complex and costly system; second, the insertion loss of the two cascaded modulators is approximately 6–10 dB, leading to a weaker final pulse intensity, affecting the intensity of the local oscillator light at the receiver (transmission distance), and consequently affecting the detection capability of the system's detector.

[0005] Current mainstream system solutions all require amplitude and phase modulation of the signal light at the transmitting end to construct a two-dimensional Gaussian random variable in phase space, namely the x-component and p-component. Furthermore, since coherent detection is often used in these systems, phase drift is unavoidable when the modulated signal light, after transmission through the fiber optic channel, interferes with the local oscillator light at the receiving end. Therefore, phase drift compensation is necessary to ensure that the transmitting and receiving ends obtain stochastic data with a certain degree of correlation. In particular, in local oscillator systems, since the receiving and transmitting ends use two different lasers, obtaining stable interference results requires not only phase compensation but also frequency locking of the two lasers, resulting in high system complexity and implementation difficulty. Summary of the Invention

[0006] In view of this, the present invention provides a continuous variable quantum key distribution method based on an internally modulated pulsed light source to solve the above-mentioned technical problems.

[0007] This invention discloses a continuous-variable quantum key distribution method based on an internally modulated pulsed light source, which includes the following steps:

[0008] The transmitting end provides signal light and local oscillator light to the receiving end, or the transmitting end provides signal light to the receiving end and the receiving end provides local oscillator light;

[0009] The receiver interferes with the signal light and the local oscillator light, and extracts the component with channel noise and its initial data from the interference result;

[0010] The receiving end processes the component with channel noise and its initial data to obtain amplitude modulation information related to the channel noise, i.e., the initial key.

[0011] Furthermore, the transmitting end provides signal light and local oscillator light to the receiving end, including:

[0012] The pulse light output by the modulated pulse light source in the transmitting end is split into signal light and local oscillator light by the first coupler. The signal light is combined with the local oscillator light after passing through the amplitude modulator and attenuator and then enters the second coupler to complete the beam combination.

[0013] Furthermore, the receiving end interferes with the signal light and the local oscillator light, including:

[0014] After the combined signal enters the receiving end, it is split by the third coupler. The local oscillator light obtained after splitting is split into two beams after passing through the second delay line and the fourth coupler. One beam directly enters the sixth coupler, and the other beam enters the seventh coupler after passing through a 90-degree phase shifter.

[0015] After beam splitting, the signal light is split into two beams after passing through the fifth coupler. One beam directly enters the sixth coupler, and the other beam enters the seventh coupler after passing through a 90-degree phase shifter.

[0016] The local oscillator light and the signal light interfere in pairs at the sixth coupler and the seventh coupler, respectively. The signal after interference in the sixth coupler enters the first differential amplifier through the first detector and the second detector, respectively. The signal after interference in the seventh coupler enters the second differential amplifier through the third detector and the fourth detector, respectively.

[0017] Furthermore, the second delay line is a polarization-maintaining fiber, which ensures that when the local oscillator light and the signal light after being split by the third coupler reach the sixth and seventh couplers, there is a delay difference. This ensures that the delay difference between the local oscillator light and the signal light achieved by combining the first delay line at the transmitting end is a multiple of the reciprocal of the system repetition frequency, that is, the delay difference between the local oscillator light and the signal light is equal to an integer multiple of the period.

[0018] Furthermore, the transmitting end not only does not perform phase modulation, but the local oscillator light does not interfere with the signal light from the same light source at the same moment, but interferes with the signal light of the next cycle.

[0019] Furthermore, the transmitting end provides signal light to the receiving end and the receiving end provides local oscillator light, including:

[0020] The first internally modulated pulsed laser at the transmitting end outputs pulsed light, which is then converted into an optical signal after passing through an amplitude modulator and an attenuator.

[0021] When the optical signal reaches the receiving end, it first passes through the second delay line, and then is split into two beams of equal intensity by the fourth coupler. One beam directly enters the sixth coupler, and the other beam reaches the seventh coupler after passing through a 90-degree phase shifter. At this time, the second internally modulated pulsed laser inside the receiving end outputs pulsed light, which becomes the local oscillator light. Its repetition frequency is the same as that of the first internally modulated pulsed laser. After passing through the fifth coupler, it is split into two beams of equal intensity and enters the sixth coupler and the seventh coupler, respectively.

[0022] Furthermore, the second delay line is a polarization-maintaining fiber, which allows the signal light and the local oscillator light to reach the fourth coupler and the fifth coupler simultaneously.

[0023] Furthermore, the receiving end interferes with the signal light and the local oscillator light, including:

[0024] At the sixth and seventh couplers, after the signal light and the local oscillator light interfere with each other, the signal output from the sixth coupler enters the first differential amplifier through the first detector and the second detector, respectively, and the signal output from the seventh coupler enters the second differential amplifier through the third detector and the fourth detector, respectively.

[0025] Furthermore, the extraction of the channel noise component and its initial data from the interferometric result includes:

[0026] The signal output from the first differential amplifier is converted to an analog-to-digital converter, from which the x-component is extracted. Its expression is: Among them, E L For the local oscillator amplitude, E s The amplitude of the signal light, where, Let be the phase of the local oscillator light at time i, which is the same as the phase of the signal light at time i, i.e. The phase of the signal light at time i+1. It follows a random uniform distribution;

[0027] The signal from the second differential amplifier is converted to a digital signal by an analog-to-digital converter, from which the p-component is extracted. Its expression is:

[0028] Further, the receiving end processes the component with channel noise and its initial data to obtain amplitude modulation information related to the channel noise, i.e., the initial key, including:

[0029] The data for the x-component and p-component are named D1 and D2 respectively. Numerically, the sum of the squares of D1 and D2 is calculated, i.e. Taking the square root of the result will give E, which is related to the modulation information at the transmitting end. s That is, the transmitting and receiving ends share an initial key containing channel noise that does not require phase compensation.

[0030] Furthermore, the signal light provided by the transmitting end to the receiving end and the local oscillator light provided by the receiving end are both pulsed light generated by internal modulation, or one is pulsed light generated by internal modulation and the other is continuous light.

[0031] Because of the adoption of the above technical solution, the present invention has the following advantages:

[0032] 1. Compared to the narrow linewidth lasers commonly used in existing continuous-variable quantum key distribution systems, internally modulated pulsed lasers are easier to integrate onto chips, resulting in higher system integration.

[0033] 2. The continuous variable quantum key distribution system can achieve random phase modulation without the need for an additional phase modulator at the transmitter, and can achieve high extinction ratio optical pulse output without the need for two amplitude modulators to be cascaded. Therefore, it has low requirements for the performance of the quantum random number generator, and the system structure is simpler and the cost is lower.

[0034] 3. This scheme has a wide range of applications. In continuous variable protocols, it can be extended to quasi-continuous modulation, and it can also be applied to discrete modulation protocols. It can be applied to in-circuit local oscillator systems, as well as local local oscillator systems.

[0035] 4. Applying this solution to a local oscillator system eliminates the need for frequency locking between the local oscillator light and the signal light, effectively reducing the difficulty and complexity of system implementation;

[0036] 5. Immune to phase drift, eliminating the need for complex active phase compensation. Attached Figure Description

[0037] To more clearly illustrate the technical solutions in the embodiments of the present invention, the accompanying drawings used in the description of the embodiments will be briefly introduced below. Obviously, the accompanying drawings described below are only some embodiments recorded in the embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings.

[0038] Figure 1 is a schematic diagram of the in-circuit local oscillator CV-QKD system according to an embodiment of the present invention;

[0039] Figure 2 is a schematic diagram of the local oscillator CV-QKD system according to an embodiment of the present invention. Detailed Implementation

[0040] The present invention will be further described in conjunction with the accompanying drawings and embodiments. Obviously, the described embodiments are only a part of the embodiments of the present invention, and not all of them. All other embodiments obtained by those skilled in the art should fall within the protection scope of the present invention.

[0041] The working principle of an internally modulated light source is to control the driving current of a laser diode by using an external modulation voltage to control the intensity of the laser output. Specifically, when the driving current switches between below and above a threshold, alternating states of light emission and emission are obtained, thus generating pulsed light. Due to the threshold operating characteristics of the laser diode, the internally modulated pulsed light source has a naturally high extinction ratio. Furthermore, since no external modulator is required, the total intensity of its output light pulses is higher. Most importantly, the phase difference between two adjacent pulses output by the internally modulated pulsed light source follows a uniform distribution in the range [0, 2π]. This invention utilizes this random characteristic of the phase difference to design a novel and efficient continuous-variable quantum key distribution scheme.

[0042] Example 1

[0043] As shown in Figure 1, the pulsed light output from the modulated pulsed light source in the transmitting end is split into two branches by the first coupler, named the signal light and the local oscillator light, respectively. The upper branch, after passing through the amplitude modulator and attenuator, merges with the lower branch, which passes through the first delay line, into the second coupler and then enters the optical fiber channel. After the combined optical signal reaches the receiving end, it is split by the third coupler. The local oscillator light is split into two beams after passing through the second delay line and the fourth coupler, and the signal light is split into two beams after passing through the fifth coupler. The split local oscillator light and the signal light interfere pairwise at the sixth and seventh couplers. The interference signal without passing through the 90-degree phase shifter enters the first differential amplifier through the first and second detectors, while the interference signal after passing through the 90-degree phase shifter enters the second differential amplifier through the third and fourth detectors, and each signal is acquired by an analog-to-digital converter (ADC).

[0044] The internally modulated pulsed laser outputs pulses with a repetition frequency of 100MHz and a pulse width of 5ns.

[0045] The first coupler has a beam splitting ratio of 90:10, with 10% of the intensity transmitted through the upper branch and 90% of the intensity transmitted through the lower branch. The two branches are named the signal light and the local oscillator light, respectively.

[0046] The amplitude modulator is an electro-optic intensity modulator with a bandwidth of not less than 100MHz; the modulation signal applied to it has a frequency of 100MHz, a width of not less than 5ns, and a random modulation amplitude value, and the amplitude modulation result follows a Rayleigh distribution.

[0047] The attenuator is a knob-type adjustable mechanical attenuator with an attenuation range greater than 30dB.

[0048] The first delay line is a polarization-maintaining fiber, which allows the signal light and the local oscillator light to reach the second coupler with a time delay difference of 5ns, with the signal light arriving later and the local oscillator light arriving earlier.

[0049] The second coupler is a polarization coupler, which enables polarization multiplexing of the signal light and the local oscillator light;

[0050] The third coupler is a polarization coupler, which can realize the polarization demultiplexing of signal light and local oscillator light;

[0051] The second delay line is a polarization-maintaining fiber, which ensures that the delay difference between the local oscillator light and the signal light after being split by the third coupler is 5ns when they reach the sixth and seventh couplers. This ensures that the delay difference between the local oscillator light and the signal light achieved by combining the first delay line at the transmitting end is the reciprocal of the system repetition frequency, that is, the delay difference between the local oscillator light and the signal light is equal to the period.

[0052] The beam splitting ratio of the fourth and fifth couplers is 50:50.

[0053] The 90-degree phase shifter can apply a 90-degree phase modulation to the input.

[0054] The sixth and seventh couplers are 2x2 couplers with a split ratio of 50:50.

[0055] The first differential detector and the second differential detector can achieve differential amplification of the input signal;

[0056] In a conventional Gaussian modulation coherent state protocol-assisted local oscillator (CV-QKD) system, the transmitter needs to perform Rayleigh distribution modulation on the amplitude modulator and uniform distribution modulation on the phase modulator. The expression for the homodyne detection result inside the receiver is as follows:

[0057]

[0058] When the phase difference between the local oscillator light and the signal light is θ = π / 2, the detection result is the p component; when θ = 0, the detection result is the x component. In phase space, the x component corresponds to the amplitude E of the local oscillator light. L , quantum signal light amplitude E s and its phase difference relative to the local oscillator light. The product of cosines, i.e. Where E s It follows a Rayleigh distribution in the range [0,1]. It follows a uniform distribution in the range [0, 2π].

[0059] In the proposed solution, the transmitting end does not perform phase modulation, and the local oscillator light does not interfere with the signal light at the same moment, but interferes with the signal light of the next cycle.

[0060] In the interference structure where the local oscillator light does not pass through a 90-degree phase shifter, the expression for the x-component extracted by the analog-to-digital converter after the differential amplifier is still: However, since the local oscillator light and the signal light interfere with each other at intervals, the expression for the phase difference between the local oscillator light and the signal light, which includes time, is: Since the local oscillator light and the signal light both originate from the same source, therefore

[0061] Therefore, in this scheme, the x component extracted by the detector is make but It follows a uniform distribution in the range [0, 2π]. Combined with heterodyne detection, in the interference result of the local oscillator light passing through a 90-degree phase shifter, the expression for the p-component extracted by the analog-to-digital converter after the differential amplifier is: The data for the x-component and p-component are named D1 and D2 respectively. Numerically, the sum of the squares of D1 and D2 is calculated, i.e. Taking the square root of the result will give E, which is related to the modulation information at the transmitting end. s At this point, due to the data results and This is irrelevant, meaning the transmitting and receiving ends share an initial key containing channel noise that does not require phase compensation. Finally, after data post-processing to complete steps such as parameter estimation, data error correction, and private key amplification, the transmitting and receiving ends achieve completely consistent sharing of random bits.

[0062] Example 2

[0063] As shown in Figure 2, the first internally modulated pulsed laser at the transmitting end outputs pulsed light, which enters the optical fiber channel after passing through an amplitude modulator and an attenuator. This signal is then called a quantum optical signal. When the quantum optical signal reaches the receiving end, it first passes through a time-delay fiber and is then split into two beams of equal intensity by a fourth coupler. One of these beams passes through a 90-degree phase shifter and reaches the seventh coupler. At this point, the second internally modulated pulsed laser inside the receiving end outputs pulsed light, which becomes the local oscillator light. Its repetition frequency is the same as that of the first internally modulated pulsed laser, and it is split into two beams of equal intensity after passing through a fifth coupler. At the sixth and seventh couplers, the quantum signal light and the local oscillator light interfere with each other. The interference signal that has not passed through the 90-degree phase shifter enters the first differential amplifier through the first and second detectors, while the interference signal that has passed through the 90-degree phase shifter enters the second differential amplifier through the third and fourth detectors. Each of these signals is then acquired by an analog-to-digital converter (ADC).

[0064] The first and second internally modulated pulsed lasers have an output pulse repetition frequency of 100MHz and a pulse width of 5ns.

[0065] The amplitude modulator is an electro-optic intensity modulator with a bandwidth of not less than 100MHz; the modulation signal applied to it has a frequency of 100MHz, a width of not less than 5ns, and a random modulation amplitude value, and the amplitude modulation result follows a Rayleigh distribution.

[0066] The attenuator is a knob-type adjustable mechanical attenuator with an attenuation range greater than 30dB.

[0067] The second delay line is a polarization-maintaining fiber, which allows the signal light and the local oscillator light to reach the fourth coupler and the fifth coupler simultaneously.

[0068] The beam splitting ratio of the fourth and fifth couplers is 50:50.

[0069] The 90-degree phase shifter can apply a 90-degree phase modulation to the input.

[0070] The sixth and seventh couplers are 2x2 couplers with a split ratio of 50:50.

[0071] The first differential detector and the second differential detector can achieve differential amplification of the input signal;

[0072] In a conventional Gaussian modulation coherent state protocol local oscillator CV-QKD system, the transmitter needs to perform Rayleigh distribution modulation on the amplitude modulator and uniform distribution modulation on the phase modulator. The expression for this modulation under zero-difference detection results is:

[0073]

[0074] When the phase of the local oscillator light is θ = π / 2, the detection result is the p component; when θ = 0, the detection result is the x component. In phase space, the x component corresponds to the amplitude E of the local oscillator light. L , quantum signal light amplitude E s and its phase difference relative to the local oscillator light. The product of cosines, i.e. Where E s It follows a Rayleigh distribution in the range [0,1]. It follows a uniform distribution in the range [0, 2π].

[0075] In the interference structure where the local oscillator light does not pass through a 90-degree phase shifter, the expression for the x-component extracted by the analog-to-digital converter after the differential amplifier is still: However, since the local oscillator light and the signal light come from two different lasers, their phase difference is completely random, making... but It follows a uniform distribution in the range [0, 2π].

[0076] Combining heterodyne detection, in the interference result of the local oscillator light passing through a 90-degree phase shifter, the expression for the p-component extracted by the analog-to-digital converter after the differential amplifier is: Name the data of the x-component and p-component D1 and D2 respectively, calculate the sum of the squares of D1 and D2, and take the square root of the result to obtain E, which is related to the modulation information at the transmitting end. s This means that the transmitting and receiving ends share an initial key with channel noise. Finally, after data post-processing to complete steps such as parameter estimation, data error correction, and private key amplification, the transmitting and receiving ends achieve completely consistent sharing of random bits.

[0077] In Examples 1 and 2: Amplitude modulation can be achieved simultaneously by internal modulation modes, or it can be achieved by various optoelectronic components and combinations of elements with different modulation rates and modulation principles, such as electro-optic intensity modulators, acousto-optic intensity modulators, and magneto-optic modulators; the modulation data distribution of amplitude modulation can be a Rayleigh distribution or a Gaussian distribution of continuous modulation, or it can be a four-state, eight-state, or 256-state distribution of discrete modulation; when the quantum signal light interferes with the local oscillator light, the interference delay difference Δt is controlled according to the following principles:

[0078] When the quantum signal light and the local oscillator light are from the same source and are internally modulated pulses, the interference delay difference between them is an integer multiple of the pulse period (Δt = nT, n ≥ 1 and is a positive integer, where T is the period of the pulse).

[0079] When the quantum signal light and the local oscillator light pulse are not from the same source: a. If the quantum signal light is a pulse with the same pulse width and repetition frequency as the local oscillator light, then the timing of the local oscillator light pulse and the quantum signal light pulse is aligned, that is, the lit parts are aligned with the lit parts, and the unlit parts are aligned with the unlit parts; b. If the quantum signal light is continuous light (although it is continuous light, its modulation frequency is the same as the local oscillator light), then the lit position of the local oscillator light pulse and the timing position of the modulated signal light are aligned.

[0080] Internally modulated pulsed light, including quantum signal light and local oscillator light, is generated by using an electrical signal to control the internal drive current of a laser diode. This allows for switching of current intensity when it is above or below a threshold, thus achieving alternating output with and without light. The drive signal provided to the laser diode can be an integrated chip controlled by software, or it can be generated by external function generators, arbitrary waveform generators, or other devices.

[0081] Heterodyne detection refers to a process where, at the receiving end, the local oscillator light is split into a first local oscillator light and a second local oscillator light, and the quantum signal light is split into a first signal light and a second signal light. The first local oscillator light and the first signal light interfere through a 2×2 coupler, and the interference result is fed into two photodiodes. After the optical signal is converted into an electric current, a canonical component, called the x-component, is output through a differential amplifier. The second local oscillator light passes through a 90-degree phase shifter or phase modulator and then interferes with the second signal light through a 2×2 coupler. The interference result is fed into two photodiodes, and the optical signal is converted into an electric current. After the differential amplifier outputs a canonical component, called the p-component.

[0082] This method employs an internally modulated pulsed light source, utilizing the characteristic that the phase difference between consecutive pulses follows a random and uniform distribution. By controlling the delay difference between the reference light and the signal light, periodic interference is achieved. This fully leverages the random distribution of the phase difference between consecutive pulses while ensuring that the detection results are independent of this phase difference, eliminating the need for any phase feedback or compensation, and simplifying the modulation mode. In a local oscillator system, this scheme can achieve a higher optical pulse extinction ratio and intensity, resulting in longer transmission distances and higher detection efficiency. In a local oscillator system, this scheme avoids the complex dual-source frequency-locking technique, reducing system complexity.

[0083] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and not to limit it. Although the present invention has been described in detail with reference to the above embodiments, those skilled in the art should understand that modifications or equivalent substitutions can still be made to the specific implementation of the present invention. Any modifications or equivalent substitutions that do not depart from the spirit and scope of the present invention should be covered within the scope of protection of the claims of the present invention.

Claims

1. A continuous-variable quantum key distribution method based on an internally modulated pulsed light source, characterized in that, Includes the following steps: The transmitting end provides signal light and local oscillator light to the receiving end, or the transmitting end provides signal light to the receiving end and the receiving end provides local oscillator light; the receiving end interferes with the signal light and local oscillator light, and extracts the component with channel noise and its initial data from the interference result; the receiving end processes the component with channel noise and its initial data to obtain amplitude modulation information related to channel noise, i.e., the initial key; The transmitting end provides signal light and local oscillator light to the receiving end, including: the transmitting end outputs pulsed light from a modulated pulse light source, which is split into signal light and local oscillator light by a first coupler; the signal light, after passing through an amplitude modulator and an attenuator, is combined with the local oscillator light after passing through a first delay line by a second coupler; the receiving end interferes with the signal light and local oscillator light, including: the combined signal light enters the receiving end and is split by a third coupler; the resulting local oscillator light, after being split, is split into two beams by a second delay line and a fourth coupler, one beam directly entering a sixth coupler, and the other beam entering a seventh coupler after passing through a 90-degree phase shifter; the split signal light, after passing through a fifth coupler, is split into two beams, one beam directly entering the sixth coupler, and the other beam entering the seventh coupler after passing through a 90-degree phase shifter; the local oscillator light and the signal light are respectively coupled at the sixth and seventh couplers. Interference occurs in the following manner: the signal after interference in the sixth coupler passes through the first detector and the second detector, respectively, and enters the first differential amplifier; the signal after interference in the seventh coupler passes through the third detector and the fourth detector, respectively, and enters the second differential amplifier; the second delay line is a polarization-maintaining fiber, which ensures that the local oscillator light and the signal light, after being split by the third coupler, have a delay difference when they reach the sixth and seventh couplers, thus guaranteeing that the delay difference between the local oscillator light and the signal light achieved by the first delay line at the transmitting end is a multiple of the reciprocal of the system repetition frequency, i.e., the delay difference between the local oscillator light and the signal light is an integer multiple of the period; the transmitting end not only does not actively perform phase modulation, but the local oscillator light does not interfere with the signal light from the same light source at the same moment, but interferes with the signal light of the next period; the phase difference between two adjacent pulses output by the internally modulated pulse light source follows a certain order. The uniform distribution; the extraction of the channel noise component and its initial data from the interference result includes: extracting the signal output from the first differential amplifier after passing through an analog-to-digital converter. Components, whose expressions are as follows ;in, For the amplitude of the local oscillator light, The amplitude of the signal light, where, ， Let be the phase of the local oscillator light at time i, which is the same as the phase of the signal light at time i, i.e. ， The phase of the signal light at time i+1. It follows a random uniform distribution; the signal from the second differential amplifier is then processed by an analog-to-digital converter, from which the signal is extracted. Components, whose expressions are as follows The receiving end processes the component with channel noise and its initial data to obtain amplitude modulation information related to the channel noise, i.e., the initial key, including: respectively... Components and The data components are named D1 and D2, where the numerical values ​​are calculated. and The sum of squares, i.e. Taking the square root of the result will give the result related to the modulation information at the transmitting end. That is, the transmitting and receiving ends share an initial key containing channel noise that does not require phase compensation.

2. The method according to claim 1, characterized in that, The transmitting end provides signal light to the receiving end and the receiving end provides local oscillator light, including: the first internal modulation pulse laser of the transmitting end outputs pulse light, which is then output as an optical signal after passing through an amplitude modulator and an attenuator; when the optical signal reaches the receiving end, it first passes through a second delay line, and then is split into two beams of equal intensity by a fourth coupler, with one beam directly entering the sixth coupler and the other beam reaching the seventh coupler after passing through a 90-degree phase shifter; at this time, the second internal modulation pulse laser inside the receiving end outputs pulse light, which becomes local oscillator light, with the same repetition frequency as the first internal modulation pulse laser, and after passing through a fifth coupler, it is split into two beams of equal intensity, which then enter the sixth coupler and the seventh coupler respectively.

3. The method according to claim 2, characterized in that, The second delay line is a polarization-maintaining fiber, which allows the signal light and the local oscillator light to reach the fourth coupler and the fifth coupler simultaneously.

4. The method according to claim 2, characterized in that, The receiving end interferes with the signal light and the local oscillator light, including: at the sixth coupler and the seventh coupler, after the signal light and the local oscillator light interfere with each other, the signal output from the sixth coupler enters the first differential amplifier through the first detector and the second detector, respectively, and the signal output from the seventh coupler enters the second differential amplifier through the third detector and the fourth detector, respectively.

5. The method according to claim 1, characterized in that, The signal light provided by the transmitter to the receiver and the local oscillator light provided by the receiver are both pulsed light generated by internal modulation, or one is pulsed light generated by internal modulation and the other is continuous light.

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