Quantum key distribution system information extraction method based on width discrimination photon detection

Abstract

A quantum key distribution system information extraction device based on a low-pass filtering and width identification avalanche single-photon detector comprises an information extraction end and a QKD system receiving end, and the QKD system receiving end comprises a second beam splitter, a first polarization decoding module, a second polarization decoding module and the avalanche single-photon detector. The quantum key distribution information extraction end comprises a first beam splitter and a laser emitting injection light pulses. A first output channel of a first beam splitter of the information extraction end is connected with a quantum channel and is connected with a receiving end of the QKD system, and a second output channel of the first beam splitter is connected with the information extraction end; an injection light pulse emitted by a pulse laser is transmitted to a receiving end through a first beam splitter, so that the injection light pulse and a signal light pulse of a QKD system simultaneously reach an avalanche single photon detector; and two paths of split beams of the second beam splitter at the receiving end of the QKD system are respectively connected with the first polarization decoding module and then connected with the avalanche single-photon detector, and the second polarization decoding module and then connected with the avalanche single-photon detector.

Description

Technical Field

[0001] This invention belongs to the field of quantum key distribution (QKD) technology, and relates to a method and apparatus for extracting information based on fluorescence reflection characteristics at the receiver of a polarization-coded QKD system. Background Technology

[0002] Quantum key distribution (QKD) is a key transmission method based on the laws of quantum physics, theoretically possessing unconditional security. "Unconditional security" means that its security can be theoretically proven from an information theory perspective without imposing any restrictions on the computing resources or operational techniques available at the information extraction end. In the development of QKD technology, improving the key generation rate remains a primary focus for researchers. Currently, QKD systems with repetition frequencies exceeding GHz are achievable. To adapt to high-speed QKD systems, researchers have invented many effective techniques to improve the frequency and other performance characteristics of avalanche single-photon detectors.

[0003] CN202110276787.4 provides a quantum key distribution (QKD) device and method. An optical signal processing module generates and sends a laser beam to a QKD processing module. The QKD processing module modulates the received laser beam based on a first-level drive signal sent by an IC chip, obtaining an optical signal. This optical signal is then transmitted via optical fiber to an optical fiber communication module, which outputs the optical signal. The IC chip also sends random numbers to an external control device via a connector, enabling the external control device to perform QKD post-processing operations based on the random numbers. By using a QKD processing module and an IC chip, the device is smaller in size. Compared to using independent optical components, this reduces the size and number of devices in the quantum key distribution device, thereby improving the stability and power consumption of the devices and increasing the reliability of the quantum key distribution device.

[0004] An avalanche single-photon detector (SPAD) is a high-sensitivity semiconductor device that achieves single-photon level detection through the avalanche multiplication effect. Its core principle is an avalanche photodiode operating in Geiger mode. The mechanism is that when the reverse bias voltage exceeds the breakdown voltage, a single photon excites charge carriers, triggering the avalanche effect and generating a detectable current pulse. This process is similar to a "microscopic domino effect": photons impacting the avalanche layer trigger a chain reaction of electrons, amplifying the optical signal into an electrical signal. The operating voltage needs to be precisely controlled to keep the device in a critical avalanche state, where a single photon can induce a macroscopic current pulse, which is then quickly reset by an active quenching circuit.

[0005] Currently, the fastest shutter frequency avalanche single-photon detectors used in receivers employ low-pass filtering and width discrimination schemes, achieving a gating rate of 2.5 GHz. When avalanche single-photon detectors using low-pass filtering experience consecutive avalanches, the avalanche signal becomes distorted, forming a wider avalanche signal. This distorted avalanche signal significantly increases the afterpulse probability. Researchers have found that using a width discriminator can filter out the wider avalanche signal, thereby reducing the afterpulse probability. However, we have discovered a critical security vulnerability in such avalanche single-photon detectors. The injected light pulse emitted from the information extraction end allows the receiver detector to sense photons and trigger a strong avalanche, but it cannot output a counting response signal to the user. Summary of the Invention

[0006] The purpose of this invention is to provide a method and apparatus for information extraction in a quantum key distribution system based on low-pass filtering and width-discriminatory avalanche single-photon detectors, thereby enabling the acquisition of key information in the QKD system.

[0007] To solve the above-mentioned technical problems, the technical solution adopted by the present invention is: an information extraction device for a quantum key distribution system based on low-pass filtering and width discrimination avalanche single-photon detector, including an information extraction end and a QKD system receiving end. The QKD system receiving end includes a second beam splitter, first and second polarization decoding modules, and an avalanche single-photon detector.

[0008] The avalanche single-photon detector employs low-pass filtering technology to reduce capacitive noise and width discrimination technology to eliminate the influence of avalanche signal distortion present in the low-pass filtering technology, thereby reducing the afterpulse probability of the detector.

[0009] The quantum key distribution information extraction end includes a first beam splitter and a laser that emits injected light pulses.

[0010] The first output channel of the first beam splitter at the information extraction end is connected to the quantum channel, i.e., to the QKD system receiver; the second output channel is connected to the information extraction end; the characteristics of the injected light pulse include: the maximum period is equal to the sum of the dead time D of the avalanche single-photon detector and the injected light pulse width W; the frequency of the injected light pulse is... Integer multiples of; each pulse contains multiple photons and carries specific polarization or phase encoding information;

[0011] The injected light pulse emitted by the pulsed laser is transmitted to the receiving end through the first beam splitter, so that the injected light pulse and the signal light pulse of the QKD system arrive at the avalanche single photon detector at the same time.

[0012] The second beam splitter at the receiver of the QKD system connects to the first polarization decoding module and then to the first and second avalanche single-photon detectors, and the second polarization decoding module and then to the third and fourth avalanche single-photon detectors. The injected light pulse has multiphoton components and its specific quantum state, and the injected light pulse will reach the three specific avalanche single-photon detectors. Due to the distortion of the avalanche signal caused by the low-pass filter, the three detectors that receive the injected light pulse generate avalanche signals that exceed the width of the width discriminator, and thus these avalanche signals are filtered out by the width discriminator. The three avalanche single-photon detectors that receive the injected light pulse cannot respond to the signal light pulse, while the fourth avalanche single-photon detector that does not receive the injected light pulse can respond normally to the signal light pulse.

[0013] The method for information extraction in a quantum key distribution system based on low-pass filtering and width-discrimination avalanche single-photon detectors is described. Its features include the following steps:

[0014] Step 1: Turn on the laser at the information extraction end, and adjust the injection light period and duty cycle to make the frequency of the injection light pulses as follows: It is an integer multiple of D. Where D is the dead time of the avalanche single-photon detector and W is the pulse width of the injected light.

[0015] Step 2: Encode each injected light pulse by randomly selecting one of the four quantum states (|H〉, |V〉, |+〉, |-〉) and encoding it onto each pulse. Quantum states |H〉 and |V〉 belong to the Z basis, and quantum states |+〉 and |-〉 belong to the X basis.

[0016] Step 3: The injected optical pulse is transmitted to the receiver via a beam splitter, ensuring that it arrives at the avalanche single-photon detector simultaneously with or before the signal pulse from the QKD system. Due to its multiphoton component and specific quantum state, the injected optical pulse reaches three specific avalanche single-photon detectors. The three detectors receiving the injected pulse generate avalanche signals exceeding the width of the width discriminator due to distortion caused by the low-pass filter. These avalanche signals are then filtered out by the width discriminator. Therefore, the three avalanche single-photon detectors that receive the injected optical pulse cannot respond to the signal pulse, while the one avalanche single-photon detector that does not receive the injected optical pulse can respond normally. After responding to the signal pulse, the receiver transmits its fundamental information to the transmitter via a classical channel for fundamental comparison.

[0017] Step 4: The two communicating parties perform a base comparison on the classical channel, and the information extraction end obtains the base selection information of the receiving end through the classical channel.

[0018] Step 5: The information extraction end obtains the time information corresponding to the basis selection information of the receiver and compares it with the quantum state |φ〉 of the light pulse injected by the information extraction end each time the receiver generates a counting response. From this, the information extraction end can deduce that the quantum state orthogonal to |φ〉 under the same basis is the quantum state of the signal pulse decoded by the receiver. Based on the bit information corresponding to this quantum state, the key information shared by both communicating parties is extracted.

[0019] The information extraction method for a quantum key distribution system based on low-pass filtering and variable-width discrimination avalanche single-photon detector is characterized in that the time when the injected light pulse arrives at the detector in step 3 must be consistent with the time when the signal light pulse arrives.

[0020] The information extraction method for a quantum key distribution system based on low-pass filtering and a variable-width discriminative avalanche single-photon detector is characterized in that the number of injected photons in step 3 is related to the performance of the detector itself. When the number of injected photons reaches C, the count rate of the avalanche single-photon detector reaches the dark count level. In a passively selected QKD system, the number of injected photons should be 4C, and in an actively selected QKD system, the number of injected photons should be 2C.

[0021] This invention proposes an information extraction method based on vulnerabilities in such avalanche single-photon detectors. The information extraction end does not require intercept-retransmission operations on the quantum channel, nor does it require injecting strong light to put the detector into linear mode. This information extraction method bypasses the basis selection phase between the communicating parties, making the shared key information between them completely unreliable. Therefore, this information extraction method can be applied to high-speed quantum key distribution systems and is very simple to implement.

[0022] The basic operation of a polarization-encoded QKD system is as follows: The transmitter prepares a single-photon laser pulse and randomly encodes it into one of four polarization states. These four polarization states are |H〉, |V〉, |+〉, and |-〉. The horizontal polarization state |H〉 and the vertical polarization state |V〉 form one set of bases, one representing 0 and the other 1. The π / 4 polarization states |+〉 and |-π / 4 polarization states |-〉 form another set of bases, representing 0 and 1 respectively. The receiver randomly uses one set of bases to measure the quantum state. If an incorrect base is used, there is a 1 / 2 probability of obtaining an incorrect result. The communicating parties compare the base selection information through a classical channel; information measured using the same base is retained.

[0023] During information extraction, the extraction end injects multiphoton pulses into the receiving end. Each transmitting end prepares a single-photon laser pulse with one of four random quantum states (|H〉, |V〉, |+〉, |-〉). The injected pulse is split into two beams by the beam splitter at the receiving end, each beam assigned to two polarization decoding modules connected to two avalanche single-photon detectors. Each polarization decoding module corresponds to a set of bases. When the base of the injected pulse matches the base of the decoding module at the receiving end, the photons in the injected pulse are focused into a single avalanche single-photon detector; when they do not match, the photons are evenly distributed across the two detectors. Therefore, in a passively base-selective QKD system, each transmission of the injected pulse results in the three detectors at the receiving end being in a controlled state with no counting response. Even if the signal pulse and the injected pulse arrive at a detector simultaneously, the detector in the controlled state will not generate any counts. Only when the signal pulse reaches the unique uncontrolled detector (the photons in the injected pulse are focused onto a matched single avalanche single-photon detector) will it normally receive the photons of the signal pulse and generate a count. Subsequently, the receiver publishes the basis information of this counting response on a classical channel. Since each injected light pulse only allows a specific avalanche single-photon detector to respond normally to the signal pulse, when the information extraction end learns that the receiver has generated a counting response at a certain point in time, it can deduce which detector received the signal pulse and generated the count. When the two communicating parties compare the basis information, the information extraction end can deduce which detector was not affected by the attack pulse and normally received the signal pulse based on the receiver's basis selection time information, thus obtaining the key information. Therefore, under ideal conditions, the information extraction end can obtain almost all filtered keys. This injected light is also applicable to quantum key distribution schemes based on active basis selection.

[0024] This invention proposes an information extraction method for quantum key distribution systems based on low-pass filtering and width-discriminatory avalanche single-photon detectors. The optimized information extraction method includes injecting pulsed laser light to control the response of the receiver's avalanche single-photon detector, allowing key information to be inferred from basis selection information. Specific light pulses are injected into the receiver to control whether the detector responds normally. The key information can then be extracted based on the basis selection information published by both communicating parties on the classical channel. In BB84 protocol-based quantum key distribution systems, this invention allows for the acquisition of almost all key information without interception and retransmission operations on the quantum channel. Furthermore, this information extraction method is applicable to high-speed QKD systems with both active and passive basis selection.

[0025] The advantages of this invention are: the information extraction end can control the receiver detector response without injecting high-intensity laser to put the detector into linear mode; the information extraction end can obtain the key information of both communicating parties without interception and retransmission operations; and the laser pulse frequency used by the information extraction end can be much lower than the system repetition frequency, making information extraction convenient and easy to operate. Attached Figure Description

[0026] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0027] Figure 1 This is a schematic diagram of the information extraction end and the QKD system receiving end of the present invention. Detailed Implementation

[0028] The present invention will be further illustrated below with reference to the accompanying drawings and specific embodiments. It should be understood that these examples are for illustrative purposes only and are not intended to limit the scope of the invention. After reading this invention, any modifications of the invention in various equivalent forms by those skilled in the art fall within the scope defined by the appended claims. To facilitate better use of the information extraction device by users, the actual usage process is described here with reference to the schematic diagram of the information extraction device and specific diagrams of each module.

[0029] As shown in the figure, the QKD system transmitter transmits the signal light pulse to the receiver via a quantum channel. The receiver's beam splitter 2 has a 50:50 fractional ratio, enabling random basis selection from the two sets of bases. The first and second polarization decoding modules decode the quantum state using the two sets of bases. An avalanche single-photon detector detects single photons. The information extraction end combines the prepared injection light pulse and the signal light pulse through a beam splitter before transmitting them to the receiver to control the avalanche single-photon detector. After receiving the injection light pulse, the avalanche single-photon detector forms a wide avalanche signal. The width discriminator filters out the wide avalanche signal, and after pulse shaping, there is no counting response output.

[0030] During the operation of the QKD system based on BB84 protocol polarization coding, the transmitting end transmits signal photons to the receiving end. At the same time, the information extraction end performs the following steps:

[0031] Step 1: Turn on the laser at the information extraction end, and adjust the injection light period and duty cycle to make the frequency of the injection light pulses as follows: It is an integer multiple of D. Where D is the dead time of the avalanche single-photon detector and W is the pulse width of the injected light.

[0032] Step 2: Encode each injected light pulse by randomly selecting one of the four quantum states (|H〉, |V〉, |+〉, |-〉) and encoding it onto each pulse. Quantum states |H〉 and |V〉 belong to the Z basis, and quantum states |+〉 and |-〉 belong to the X basis.

[0033] Step 3: The injected optical pulse is combined with the signal optical pulse through beam splitter 1 and transmitted to the receiving end. The information extraction end must ensure that the injected optical pulse arrives at the avalanche single-photon detector simultaneously with or before the signal optical pulse of the QKD system. When the injected optical pulse passes through beam splitter 2, half of the injected photons enter the first polarization decoding module, and the other half enter the second polarization decoding module. When the polarization state of the injected photons matches the basis of the polarization decoding module, the injected photons enter the same avalanche single-photon detector in a concentrated manner. When the polarization state of the injected photons does not match the basis of the polarization decoding module, the injected photons enter the two avalanche single-photon detectors evenly. Due to the distortion of the avalanche signal caused by the low-pass filter, the three detectors receiving the injected optical pulse generate avalanche signals that exceed the width of the width discriminator, and these avalanche signals are filtered out by the width discriminator. Even if a violent avalanche occurs within the avalanche diode, there is no counting response output after pulse shaping. Therefore, the three avalanche single-photon detectors that received the injected light pulse could not respond to the signal light pulse, while the one avalanche single-photon detector that did not receive the injected light pulse could respond normally to the signal light pulse. After the receiver responds to the signal light pulse, it transmits its fundamental information to the transmitter through a classical channel for fundamental comparison.

[0034] Step 4: The two communicating parties perform a base comparison on the classical channel, and the information extraction end obtains the base selection information of the receiving end through the classical channel.

[0035] Step 5: The information extraction end obtains the time information corresponding to the basis selection information of the receiver and compares it with the quantum state |φ〉 of the light pulse injected by the information extraction end each time the receiver generates a counting response. From this, the information extraction end can deduce that the quantum state orthogonal to |φ〉 under the same basis is the quantum state of the signal pulse decoded by the receiver. Based on the bit information corresponding to this quantum state, the key information shared by both communicating parties is extracted.

[0036] Therefore, this invention addresses a significant security vulnerability inherent in low-pass filtering and width-discrimination avalanche single-photon detectors. We have developed an information extraction method based on low-pass filtering and width-discrimination avalanche single-photon detectors. This method can obtain almost all key information without interception and retransmission. It also eliminates the need to consider the small attack window of high-speed quantum key distribution systems. For the information extraction end, the minimum technical requirement is matching the optical pulse period to the dead time of the single-photon avalanche diode, significantly reducing the difficulty of information extraction.

[0037] It should be noted that, in this document, relational terms such as "first" and "second" are used only to distinguish one entity or operation from another, and do not necessarily require or imply any such actual relationship or order between these entities or operations. Furthermore, the terms "comprising," "including," or any other variations thereof are intended to cover non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements includes not only those elements but also other elements not expressly listed, or elements inherent to such a process, method, article, or apparatus. Without further limitations, an element defined by the phrase "comprising one..." does not exclude the presence of other identical elements in the process, method, article, or apparatus that includes said element.

[0038] The various embodiments in this specification are described in a related manner. The same or similar parts between the various embodiments can be referred to each other. Each embodiment focuses on describing the differences from other embodiments.

[0039] The above description is merely a preferred embodiment of the present invention and is not intended to limit the scope of protection of the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention are included within the scope of protection of the present invention.

Claims

1. An information extraction device for a quantum key distribution system based on low-pass filtering and width-discriminatory avalanche single-photon detector, characterized in that, It includes an information extraction end and a QKD system receiver. The QKD system receiver includes a second beam splitter, first and second polarization decoding modules, and an avalanche single-photon detector. The avalanche single-photon detector employs low-pass filtering technology to reduce capacitive noise and width discrimination technology to eliminate the influence of avalanche signal distortion present in the low-pass filtering technology, thereby reducing the afterpulse probability of the detector. The quantum key distribution information extraction end includes a first beam splitter and a laser that emits injected light pulses; The first output channel of the first beam splitter at the information extraction end is connected to the quantum channel, i.e., to the QKD system receiver; the second output channel is connected to the information extraction end. The injected optical pulse is characterized by a maximum period equal to the sum of the dead time D of the avalanche single-photon detector and the injected optical pulse width W; the frequency of the injected optical pulse is... Integer multiples of; each pulse contains multiple photons and carries specific polarization or phase encoding information; The injected light pulse emitted by the pulsed laser is transmitted to the receiving end through the first beam splitter, so that the injected light pulse and the signal light pulse of the QKD system arrive at the avalanche single photon detector at the same time. The second beam splitter at the receiver of the QKD system connects to the first polarization decoding module and then to the first and second avalanche single-photon detectors, and the second polarization decoding module and then to the third and fourth avalanche single-photon detectors. The injected light pulse has multiphoton components and its specific quantum state, and the injected light pulse will reach the three specific avalanche single-photon detectors. Due to the distortion of the avalanche signal caused by the low-pass filter, the three detectors that receive the injected light pulse generate avalanche signals that exceed the width of the width discriminator, and thus these avalanche signals are filtered out by the width discriminator. The three avalanche single-photon detectors that receive the injected light pulse cannot respond to the signal light pulse, while the fourth avalanche single-photon detector that does not receive the injected light pulse can respond normally to the signal light pulse.

2. An information extraction method for a quantum key distribution device based on a low-pass filter and width-discriminatory avalanche single-photon detector according to claim 1; characterized in that, Includes the following steps: Step 1: Turn on the pulsed laser at the information extraction end, and adjust the injection light period and duty cycle to make the frequency of the injection light pulses [missing information]. Integer multiples of; where D is the dead time of the avalanche single-photon detector and W is the pulse width of the injected light; Step 2: Encode each injected light pulse by randomly selecting one of the four quantum states (|H〉, |V〉, |+〉, |-〉) and encoding it on each pulse; quantum states |H〉 and |V〉 belong to the Z basis, and quantum states |+〉 and |-〉 belong to the X basis; Step 3: The injected optical pulse is transmitted to the receiver via a beam splitter, ensuring that the injected optical pulse arrives at the avalanche single-photon detector simultaneously with or before the signal pulse of the QKD system. Due to its multiphoton component and specific quantum state, the injected optical pulse will reach three specific avalanche single-photon detectors. Because of the distortion of the avalanche signal caused by the low-pass filter, the three detectors receiving the injected optical pulse generate avalanche signals exceeding the width of the width discriminator, which then filters out these avalanche signals. Therefore, the three avalanche single-photon detectors that receive the injected optical pulse cannot respond to the signal pulse, while the fourth avalanche single-photon detector, which has not received the injected optical pulse, can respond normally to the signal pulse. After responding to the signal pulse, the receiver transmits its fundamental information to the transmitter via a classical channel for fundamental comparison. Step 4: The two communicating parties perform a base comparison on the classical channel, and the information extraction end obtains the base selection information of the receiving end through the classical channel; Step 5: The information extraction end obtains the time information corresponding to the basis selection information of the receiver and compares it with the quantum state |φ〉 of the light pulse injected by the information extraction end each time the receiver generates a counting response; from this, the information extraction end can deduce that the quantum state orthogonal to |φ〉 under the same basis is the quantum state of the signal pulse decoded by the receiver; based on the bit information corresponding to this quantum state, the key information shared by both communicating parties is extracted.

3. The information extraction method for a quantum key distribution system based on low-pass filtering and a variable-width discriminative avalanche single-photon detector according to claim 2, is characterized in that, In step 3, the time when the injected light pulse arrives at the detector must be consistent with the time when the signal light pulse arrives.

4. The information extraction method for a quantum key distribution system based on low-pass filtering and a variable-width discriminative avalanche single-photon detector according to claim 2, characterized in that, In step 3, the number of injected photons is related to the performance of the detector itself. As the number of injected photons increases from 0.1 photons per pulse, the count of the avalanche single-photon detector first increases and then decreases. When the count rate of the avalanche single-photon detector increases and then decreases to the dark count level, the number of photons C contained in each injected pulse is recorded at this time. In a passively selected QKD system, the number of injected photons should be 4C per pulse, and in an actively selected QKD system, the number of injected photons should be 2C per pulse.

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