Synchronizing signal detection device in quantum key distribution system

Abstract

The invention discloses a synchronizing signal detection device in a quantum key distribution system, which is characterized in that the cathode of a PIN tube is connected with one input end of a voltage comparator through a first low-noise amplifier, a low-pass filter and a second low-noise amplifier, and the anode of the PIN tube is connected with the other input end of the voltage comparator through a logarithmic amplifier, an analog-to-digital converter, an MCU and a digital-to-analog converter. According to the synchronous signal detection device in the QKD system, accurate signal synchronization and normal parameter scanning of a receiving party and a transmitting party are realized through a scheme with lower design complexity and device index requirements.

Description

Technical Field

[0001] This invention relates to the fields of quantum key distribution and fiber optic time synchronization. Background Technology

[0002] Quantum key distribution (QKD) systems require real-time control of electro-optic devices to ensure that the control signal arrives at the device at exactly the same time as the optical pulse to be modulated. QKD systems also need to time-discriminate the detector output to determine the exact arrival time of the detected optical pulse, enabling the system to obtain the random number used by both the transmitter and receiver to modulate the corresponding optical pulse—the initial key. To meet these requirements, a precise synchronization mechanism is essential in a QKD system.

[0003] Before a QKD system can operate normally, it requires scanning and setting various parameters. The algorithm calculates the correct voltage and delay based on the detection results. If the system lacks the ability to detect channel attenuation, it cannot set a suitable scanning light intensity, leading to detector count saturation or very low counts, which affects the algorithm's scanning results. Therefore, a QKD system requires the synchronization optical detection module to simultaneously possess signal amplification and channel attenuation detection functions to achieve accurate signal synchronization and normal parameter scanning between the transmitter and receiver.

[0004] Therefore, how to achieve accurate QKD synchronization and channel attenuation detection without increasing system complexity has become a research hotspot.

[0005] Currently, synchronization mechanisms in QKD systems typically employ independent lasers for emission, with the receiver detecting the output as a synchronization signal between the transmitter and receiver. The light intensity reaching the receiver varies over different transmission distances, requiring the synchronization optical detection module to have a dynamic range in the pA to mA range. In high-speed QKD systems, the modulation precision of electro-optical devices is in the ps to ns range; therefore, the synchronization optical detection module must possess an operating bandwidth exceeding GHz, ps-level jitter, and a gain exceeding 40 dB.

[0006] Avalanche photodiodes (APDs) possess high gain and bandwidth, and are generally used for weak light detection. However, APDs require a high reverse bias voltage to operate, forming current by absorbing photon energy. This mode exhibits significant noise current. Furthermore, APD characteristics are greatly affected by ambient temperature; without temperature compensation, the APD may fail to detect properly. Therefore, APDs are required to possess high sensitivity, low dark current, good linearity, and good temperature stability.

[0007] Currently, there are also simplified control devices that utilize lasers, such as the patent application CN111010270A, published on April 14, 2020, entitled "A Synchronization Device and Method for an MDI-QKD System in an Optical-Electro-Optical Mode." The disclosed device includes a synchronization optical pulse generation module, a first photoelectric conversion module, an encoding module, a time delay adjustment module, an electro-optical conversion module, a second photoelectric conversion module, and a detection module. The method includes: transmitting synchronization optical pulses and signal optical pulses to Alice and Bob ends and converting them into synchronization electrical pulses; encoding the signal optical pulses; adjusting the time delay of the synchronization electrical pulses; converting the synchronization electrical pulses into synchronization optical pulses and transmitting them to the detection module for conversion into synchronization electrical pulses; and triggering the single-photon detector and the acquisition card to operate.

[0008] Although the structure and control have been simplified, there are still relatively stringent performance requirements, which increases the difficulty and complexity of implementation. Summary of the Invention

[0009] The technical problem to be solved by the present invention is to realize a synchronization signal detection device in a quantum key distribution system with lower complexity and device performance requirements.

[0010] To achieve the above objectives, the technical solution adopted by the present invention is as follows: a synchronization signal detection device in a quantum key distribution system, wherein the cathode of the PIN diode is connected to one input terminal of a voltage comparator via a first low-noise amplifier, a low-pass filter, and a second low-noise amplifier, and the anode of the PIN diode is connected to the other input terminal of the voltage comparator via a logarithmic amplifier, an analog-to-digital converter, an MCU, and a digital-to-analog converter.

[0011] The PIN tube is a photosensitive device that operates in reverse bias mode.

[0012] The cathode of the PIN diode is used for signal acquisition and amplification, while the anode of the PIN diode is used for weak current measurement.

[0013] The synchronization pulse signal emitted by the PIN diode is output to the voltage comparator after passing through two stages of low-noise amplifiers. The cascaded gain of the first and second low-noise amplifiers exceeds 40dB.

[0014] The low-pass filter between the first low-noise amplifier and the second low-noise amplifier is used to filter out high-frequency noise and thermal noise from the amplifier.

[0015] A capacitor is connected in series between the cathode of the PIN diode and the first low-noise amplifier.

[0016] The cathode of the PIN diode is connected to the capacitor via a 1kΩ resistor for biasing.

[0017] One output of the voltage comparator serves as the comparison result output terminal.

[0018] The logarithmic amplifier obtains a linear relationship between the input optical power and the output current through logarithmic conversion, and sends the obtained linear relationship to the analog-to-digital converter, which converts the signal output by the logarithmic amplifier into a digital signal.

[0019] The MCU adjusts the output voltage of the digital-to-analog converter based on the channel attenuation sampled by the analog-to-digital converter, and inputs a dynamic threshold voltage to the voltage comparator.

[0020] This invention is a synchronization signal detection device in a QKD system, which achieves accurate signal synchronization and normal parameter scanning between the transmitter and receiver through a scheme with lower design complexity and device specification requirements. Attached Figure Description

[0021] The following is a brief description of the content represented by each figure in this specification: Figure 1 This is a schematic diagram of a synchronization signal detection device in a quantum key distribution system. Figure 2 The waveform diagram showing the width jitter of the output signal of an existing comparator; Figure 3 This is a waveform diagram showing the constant width of the comparator output signal in this invention. Detailed Implementation

[0022] The following description, with reference to the accompanying drawings, details the specific implementation of the present invention, including the shape and structure of each component, the relative positions and connections between the parts, the function and working principle of each part, the manufacturing process, and the operation and use methods, to help those skilled in the art to have a more complete, accurate, and in-depth understanding of the inventive concept and technical solution of the present invention.

[0023] like Figure 1 As shown, the synchronization signal detection device in the quantum key distribution system consists of a PIN diode, a low-noise amplifier, a low-pass filter, a voltage comparator, a logarithmic amplifier, an analog-to-digital converter (ADC), a digital-to-analog converter (DAC), and an MCU. Specifically, the cathode of the PIN diode is connected to one input of the voltage comparator via a first low-noise amplifier, a low-pass filter, and a second low-noise amplifier. The anode of the PIN diode is connected to the other input of the voltage comparator via a logarithmic amplifier, the ADC, the MCU, and the DAC. A capacitor is connected in series between the cathode of the PIN diode and the first low-noise amplifier. A 1kΩ resistor connects the cathode of the PIN diode to the capacitor for bias voltage. One output of the voltage comparator serves as the comparison result output.

[0024] The photosensitive device is a PIN diode. Compared to an APD, the photoresponsivity of a PIN diode does not change with temperature. PIN diodes also feature high saturation power, low junction capacitance, high bandwidth, and small size. Signal acquisition and amplification are performed at the cathode of the PIN diode, while weak current measurement is performed at the anode. This single PIN diode achieves both signal detection and channel attenuation detection functions.

[0025] The signal amplification module of the synchronization detection device consists of two stages of low-noise amplifiers, a low-pass filter, and a voltage comparator. The PIN diode operates in reverse bias mode; the reverse bias voltage improves the PIN diode's sensitivity, reduces junction capacitance, and increases response bandwidth. The synchronization pulse signal is amplified by the two stages of low-noise amplifiers and then output to the voltage comparator. The comparator's threshold voltage is controlled by the MCU and output to the DAC. The comparator converts the signal into a digital signal, which is then output to the receiver's main control unit as the synchronization signal between the transmitter and receiver. The low-noise amplifiers feature high gain, high bandwidth, and low noise figure. Higher amplifier bandwidth results in steeper signal edges, smaller voltage crossover time variations due to the same noise amplitude, and less output jitter. The cascaded two stages of low-noise amplifiers provide over 40dB of gain, improving detection sensitivity. An embedded low-pass filter between the two amplifier stages filters out high-frequency and thermal noise, improving the signal-to-noise ratio.

[0026] The channel attenuation detection function of the synchronous detection device consists of a logarithmic amplifier and an ADC. If a traditional proportional amplifier is used to achieve a detection range of pA to mA, the amplification factor starts from 10... 7 For input optical power and output current of ~10, not only is resistance matching difficult, but it also introduces significant errors. Using a logarithmic amplifier can solve this problem, providing a large dynamic range and allowing for a linear relationship between input optical power and output current through logarithmic conversion. The ADC acquires the output signal of the logarithmic amplifier and converts it into a digital signal, enabling real-time detection of channel attenuation in the QKD system. This design scheme is highly integrated, reliable, and easy to debug and calibrate.

[0027] The QKD deployment environment and fiber quality affect the magnitude of channel attenuation. Changes in channel attenuation also alter the optical power input to the synchronous optical detection device. For example... Figure 2 As shown, the amplifier output amplitude changes with the input optical power. When a fixed threshold is used for discrimination, the width of the comparator output signal will jitter, affecting the time synchronization accuracy of the transmitter and receiver.

[0028] To address the issue of jitter in the output width of the synchronization probe signal under different channel attenuation conditions, we employed a dynamic threshold discrimination method. For example... Figure 3 As shown, the MCU adjusts the DAC output voltage in real time according to the channel attenuation sampled by the ADC to achieve dynamic threshold discrimination and ensure that the comparator output signal width remains constant.

[0029] The present invention has been described above with reference to the accompanying drawings. Obviously, the specific implementation of the present invention is not limited to the above-described manner. Any non-substantial improvements made using the inventive concept and technical solution of the present invention, or the direct application of the inventive concept and technical solution to other situations without modification, are all within the protection scope of the present invention.

Claims

1. A synchronization signal detection device in a quantum key distribution system, characterized in that: The cathode of the PIN diode is connected to one input terminal of the voltage comparator via a first low-noise amplifier, a low-pass filter, and a second low-noise amplifier. The anode of the PIN diode is connected to the other input terminal of the voltage comparator via a logarithmic amplifier, an analog-to-digital converter, an MCU, and a digital-to-analog converter.

2. The synchronization signal detection device in the quantum key distribution system according to claim 1, characterized in that: The PIN tube is a photosensitive device that operates in reverse bias mode.

3. The synchronization signal detection device in the quantum key distribution system according to claim 2, characterized in that: The cathode of the PIN diode is used for signal acquisition and amplification, while the anode of the PIN diode is used for weak current measurement.

4. The synchronization signal detection device in the quantum key distribution system according to claim 3, characterized in that: The synchronization pulse signal emitted by the PIN diode is output to the voltage comparator after passing through two stages of low-noise amplifiers. The cascaded gain of the first and second low-noise amplifiers exceeds 40dB.

5. The synchronization signal detection device in the quantum key distribution system according to claim 4, characterized in that: The low-pass filter between the first low-noise amplifier and the second low-noise amplifier is used to filter out high-frequency noise and thermal noise from the amplifier.

6. The synchronization signal detection device in the quantum key distribution system according to any one of claims 1-5, characterized in that: A capacitor is connected in series between the cathode of the PIN diode and the first low-noise amplifier.

7. The synchronization signal detection device in the quantum key distribution system according to claim 6, characterized in that: The cathode of the PIN diode is connected to the capacitor via a 1kΩ resistor for biasing.

8. The synchronization signal detection device in the quantum key distribution system according to claim 7, characterized in that: One output of the voltage comparator serves as the comparison result output terminal.

9. The synchronization signal detection device in the quantum key distribution system according to any one of claims 1-5, 7, and 8, characterized in that: The logarithmic amplifier obtains a linear relationship between the input optical power and the output current through logarithmic conversion, and sends the obtained linear relationship to the analog-to-digital converter, which converts the signal output by the logarithmic amplifier into a digital signal.

10. The synchronization signal detection device in the quantum key distribution system according to claim 9, characterized in that: The MCU adjusts the output voltage of the digital-to-analog converter based on the channel attenuation sampled by the analog-to-digital converter, and inputs a dynamic threshold voltage to the voltage comparator.

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