Single photon detection circuit for suppressing post pulse in quantum communication

Abstract

The invention discloses a single photon detection circuit for suppressing post-pulse in quantum communication, a single photon avalanche diode (SPAD) and a resistor are connected in series to form a voltage division circuit, and the voltage division circuit and a resistor Ra are connected in parallel and then electrically connected with a transistor; if photons are received, signal pulses are output and fed back to the transistor, reverse bias voltage of SPAD is reduced through the transistor, avalanche is quenched, the number of carriers participating in the avalanche is reduced, the number of carriers captured by internal defects of diode materials is reduced, and then occurrence of post-pulses is restrained; the transistor works in a linear region, so that the response time of the circuit is shortened; by adjusting the static working point of the transistor, the bias voltage of the SPAD is adjusted, the circuit can be adjusted and calibrated conveniently when leaving a factory, and the bias voltage can be matched with a new SPAD quickly under the condition that the SPAD is replaced. In a strong interference environment in a power system, the single photon detection requirement in the quantum key distribution technology can be met.

Description

Technical Field

[0001] The invention relates to the field of quantum communication, and in particular to a single-photon detection circuit capable of suppressing after-pulses when an avalanche diode is used to detect a single photon. Background Art

[0002] Quantum communication is a communication technology based on the principles of quantum mechanics. It uses the characteristics of quantum states to provide higher security and confidentiality than traditional communications, and is therefore widely used in finance, government, and military fields. Quantum communication technology can be used for secure data transmission, and through quantum key distribution and encryption technology, the confidentiality and integrity of data can be ensured.

[0003] At present, the most widely used technology in quantum communication is quantum key distribution (QKD). In QKD, a single-photon detector is needed to detect a single photon. The most commonly used single-photon detector is the InGaAs / InP single-photon avalanche diode (SPAD). When detecting a single photon, the SPAD usually operates in Geiger mode, and the reverse bias applied to both ends of the diode is slightly greater than the threshold voltage. In this mode, if no photon is received, the SPAD does not break down; if a photon enters the SPAD, the single photon triggers the SPAD, generating electron-hole pairs inside the SPAD, which are then accelerated by the electric field and collide with the lattice to form more and more electron-hole pairs, resulting in a "self-sustaining avalanche" phenomenon, and the avalanche amplification gain can reach 10 6 The above, the signal can be amplified to a large enough level. After the "self-sustaining avalanche" occurs, in order to ensure that the SPAD can effectively detect the next photon, before the next photon reaches the SPAD, the previous avalanche must be quenched in time. Reducing the reverse bias of the SPAD to below its threshold voltage can effectively quench the avalanche and prepare for the measurement of the next photon.

[0004] In quantum communication, a gating signal is usually used to change the reverse bias on the SPAD. A square wave pulse is a common gating signal. During the high level of the square wave, the reverse bias of the SPAD is higher than the threshold voltage, and the SPAD is sensitive to photons. This period is the open state. During the low level of the square wave, the reverse bias of the SPAD is lower than the threshold voltage, and this period is the closed state.

[0005] When a SPAD avalanches, a large number of carriers flow through the diode. Due to a certain number of defects inside the diode material, a small number of carriers may be captured. These carriers will be released after the avalanche stops. If the voltage across the SPAD is higher than the threshold voltage at this time, these carriers will cause an avalanche again, forming an avalanche pulse that is no different from the previous one, called a post-pulse. At this time, the SPAD does not receive any photons but still outputs a signal. The post-pulse rate is a parameter to measure the post-pulse, which is defined as the probability of generating a post-pulse count within a certain period of time after a photon count. If the post-pulse rate is too large, the SPAD will not work properly, so it is necessary to suppress the post-pulse and reduce the post-pulse rate.

[0006] Although there are some methods to reduce the after-pulse rate, such as the public document with announcement number CN116608948A, publication date 2023-08-18, and patent name "Single Photon Detection Circuit and Single Photon Detector", which reduces material defects inside SPAD and ensures the promotion of QKD-based quantum communication technology, when quantum communication technology is introduced into the power system, the strong electromagnetic interference in the power system causes the after-pulse rate to increase significantly, thus affecting the application of quantum communication technology.

[0007] In the power grid, due to strong electromagnetic interference, the life of the SPAD is short and the SPAD needs to be replaced frequently. However, after each replacement, the bias voltage needs to be adjusted according to the characteristics of the SPAD, which is a lot of work. Therefore, a technical solution that can easily adjust the SPAD bias voltage is needed. Summary of the invention

[0008] The technical problem to be solved by the present invention is how to suppress after-pulses in power system quantum communication technology, reduce the after-pulse rate, and conveniently adjust the SPAD bias voltage.

[0009] In order to achieve the above object, the technical solution adopted by the present invention is: a single photon detection circuit for suppressing afterpulses in quantum communication:

[0010] The single photon avalanche diode SPAD1 and the resistor R1 are connected in series to form a first voltage divider circuit, and a first output terminal ST1 is led out between the single photon avalanche diode SPAD1 and the resistor R1;

[0011] The single photon avalanche diode SPAD2 and the resistor R2 are connected in series to form a second voltage divider circuit, and a second output terminal ST2 is led out between the single photon avalanche diode SPAD2 and the resistor R2;

[0012] The resistor Ra, the first voltage divider circuit, and the second voltage divider circuit are connected in parallel, one end of which is electrically connected to the transistor, and the other end is electrically connected to the high voltage source HV;

[0013] The signal output from the first output terminal ST1 passes through the capacitor C1 to obtain a signal S1;

[0014] The signal output from the second output terminal ST2 passes through the capacitor C2 to obtain a signal S2;

[0015] Only SPAD1 can receive photons.

[0016] The signal S1 and the signal S2 are both connected and output corresponding signals to a signal processing circuit, and the signal processing circuit outputs a signal pulse P1.

[0017] The signal pulse P1 output of the signal processing circuit is connected to the transistor controller circuit and serves as one output at the same time. The transistor controller circuit is connected to the gate signal output terminal.

[0018] During the door opening time, the difference △S between signals S1 and S2 is compared with the maximum value of the noise interference signal. If it is greater than the maximum value of the noise interference signal, a signal pulse P1 is output, indicating that a photon has been received.

[0019] The signal pulse P1 is fed back to the transistor through the transistor control circuit, and the reverse bias of SPAD1 and SPAD2 is reduced through the transistor, so as to quench the avalanche and reduce the number of electron-hole pairs involved in the avalanche.

[0020] After the signal pulse P1 disappears, SPAD1 continues to detect the next photon under the action of the gating signal.

[0021] The transistor works in the linear region, and the bias voltage of the SPAD is adjusted by adjusting the static operating point of the transistor.

[0022] Preferably, the transistor is an NPN-type triode, the E pole of the transistor is grounded, the B pole is connected to the transistor control circuit, and the C pole is connected to the single photon avalanche diode SPAD1 and the single photon avalanche diode SPAD2.

[0023] Preferably, the transistor is a PNP type triode, the E pole of the transistor is grounded, the B pole is connected to the transistor control circuit, and the C pole is connected to the single photon avalanche diode SPAD1 and the single photon avalanche diode SPAD2.

[0024] The advantages of the present invention are:

[0025] (1) Under the premise of ensuring that the avalanche signal can be accurately detected, the output signal pulse P1 is fed back to the transistor, and the reverse bias is reduced through the transistor to quench the avalanche, thereby reducing the number of electron-hole pairs participating in the avalanche and greatly reducing the number of carriers captured by the internal defects of the diode material, thereby greatly suppressing the occurrence of afterpulses;

[0026] (2) The voltage divider circuit is connected in parallel with the resistor Ra, and then in series with the transistor. The transistor in the circuit works in the linear region, there is no storage phenomenon of minority carriers, the response time is greatly shortened, the capacitance of the PN junction inside the transistor is greatly reduced, and the RC time constant is also reduced accordingly. The transmission delay time of the circuit is very short, which can improve the processing speed of the single photon detection circuit;

[0027] (3) The reverse bias voltage on the SPAD can be easily adjusted by adjusting the quiescent operating point of the transistor, which is convenient for adjusting and calibrating the circuit at the factory. When the SPAD is replaced, the bias voltage can also be matched with the new SPAD more quickly. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] The following is a brief description of the contents expressed in each figure in the specification of the present invention:

[0029] Figure 1 A schematic diagram of a single photon detection circuit for suppressing afterpulses in quantum communication according to an embodiment of the present invention;

[0030] Figure 2 A schematic diagram of a SPAD reverse bias voltage variation in the background art;

[0031] Figure 3 Schematic diagram of the change of the reverse bias voltage of the SPAD in an embodiment of the present invention;

[0032] Figure 4 A schematic diagram of a SPAD peripheral circuit in another embodiment of the present invention;

[0033] Figure 5 A circuit diagram of signal processing in an embodiment of the present invention;

[0034] Figure 6 Schematic diagram of a transistor control circuit in an embodiment of the present invention. DETAILED DESCRIPTION

[0035] The following is a further detailed description of the specific implementation methods of the present invention, such as the shape, structure, relative position and connection relationship between the various components involved, the function and working principle of each part, the manufacturing process and operation method, etc., through the description of the embodiments with reference to the accompanying drawings, so as 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.

[0036] refer to Figure 1, a single-photon detection circuit for suppressing post-pulses in quantum communication, wherein a single-photon avalanche diode SPAD1 and a resistor R1 are connected in series to form a first voltage divider circuit, and a first output terminal ST1 is led out between the single-photon avalanche diode SPAD1 and the resistor R1; a single-photon avalanche diode SPAD2 and a resistor R2 are connected in series to form a second voltage divider circuit, and a second output terminal ST2 is led out between the single-photon avalanche diode SPAD2 and the resistor R2; the first voltage divider circuit is connected in parallel with the resistor Ra, and the first voltage divider circuit is connected in parallel with the second voltage divider circuit, and one end of the parallel connection is electrically connected to a transistor, and the other end is electrically connected to a high-voltage source HV; a signal output from the first output terminal ST1 passes through a capacitor C1 to obtain a signal S1;

[0037] The signal output from the second output terminal ST2 passes through the capacitor C2 to obtain the signal S2; only SPAD1 can receive photons; during the door opening time, the signal processing module compares the difference △S between the signals S1 and S2 with the maximum value of the noise interference signal. If it is greater than the maximum value of the noise interference signal, a signal pulse P1 is output, indicating that a photon has been received. Figure 1 The signal processing module receives signals S1 and S2, calculates the difference △S between them, and compares △S with the maximum value of noise and interference signals. If SPAD1 receives a photon and avalanches occur in SPAD1, the amplitude of S1 is significantly greater than that of S2, △S is greater than the maximum value of the noise interference signal, and the signal processing circuit outputs a signal pulse P1.

[0038] The signal pulse P1 is fed back to the transistor through the transistor control circuit, and the reverse bias of SPAD1 and SPAD2 is reduced through the transistor, so as to quench the avalanche and reduce the number of electron-hole pairs involved in the avalanche.

[0039] After the signal pulse P1 disappears, SPAD1 continues to detect the next photon under the action of the gating signal.

[0040] refer to Figure 2 , a schematic diagram of the change of the reverse bias voltage of SPAD in the background technology, at time t1, the reverse bias voltage of SPAD1 rises to V2, which is higher than the threshold voltage and is in the open state. SPAD1 is sensitive to a single photon. If no photon is received, the reverse bias voltage remains at V2 until time t2, when the reverse bias voltage of SPAD1 drops to V1, which is lower than the threshold voltage and is insensitive to photons and is in the closed state. At time t3, SPAD1 re-enters the open state. Figure 2 In the example, if SPAD1 receives a photon at time ta, SPAD1 avalanches. Since SPAD1 is connected in series with a resistor, the reverse bias voltage on SPAD1 decreases and follows Figure 2The dotted line in the figure drops, and avalanche occurs from time ta to t2. The avalanche lasts for a long time, and there are many electron-hole pairs involved in the avalanche. The probability of defects in the semiconductor material capturing carriers is relatively large, and the probability of a post-pulse is relatively high.

[0041] refer to Figure 3 , a schematic diagram of the change of the reverse bias voltage of SPAD in an embodiment of the present invention, using the improved single-photon detection circuit of the present invention, in the open state, at time ta, SPAD1 receives a photon and an avalanche occurs, and the subsequent circuit detects the avalanche signal and feeds it back to the transistor, reducing the reverse bias voltage of SPAD1 to V1, pulling it down to below the threshold voltage, quickly quenching the avalanche, and being in the closed state. Such operation ensures that the avalanche signal can be accurately detected and the electron-hole pairs involved in the avalanche can be reduced in time, reducing the probability of defects in the semiconductor material capturing carriers, thereby effectively reducing the after-pulse rate. In an embodiment of the present invention, under the same experimental conditions, the after-pulse rate of the single-photon detection circuit improved by the present invention is reduced by 21.5% compared with the circuit in the background technology.

[0042] exist Figure 2 In the figure, if a photon is received at time ta, an avalanche occurs, and an avalanche occurs from ta to t2. The avalanche lasts for a relatively long time, and more carriers participate in the avalanche, so the avalanche signal is relatively strong. If a photon is received at time tb, an avalanche occurs, and an avalanche occurs from tb to t2. The avalanche lasts for a relatively short time, and fewer carriers participate in the avalanche, so the avalanche signal is relatively weak. It can be seen that in the circuit disclosed in the background technology, the strength of the avalanche signal varies greatly, whether it is the transient signal of the circuit or the intensity of the interference to other components, and the subsequent circuit development is difficult.

[0043] exist Figure 3 In the present invention, when the circuit disclosed in the present application is used and a photon is received at time ta or time tb, the duration of the avalanche is basically the same, the number of carriers participating in the avalanche is roughly the same, the intensity of the avalanche signal is also basically the same, and the intensity of both the circuit transient signal and the interference to other components is basically the same, so the subsequent circuit development is less difficult.

[0044] exist Figure 1 In the embodiment, HV is a positive high voltage, the potential of the resistor R1 is higher than the potential of the SPAD1, Figure 1The middle transistor adopts an NPN transistor, and SPAD1 is connected in series with resistor R1 to form a first voltage divider circuit; when the current flowing through the collector and emitter of the transistor is very small, the voltage between the collector and the emitter is high, the reverse bias voltage borne by SPAD1 is small and lower than the threshold voltage, SPAD1 is insensitive to photons and is in a closed state; when the current flowing through the collector and emitter of the transistor is large, the voltage across the collector and emitter of the transistor is low, the reverse bias voltage borne by SPAD1 is large and higher than the threshold voltage, SPAD1 is sensitive to photons and is in an open state.

[0045] Figure 1 SPAD1 and SPAD2 use PGA-314-50 single-photon avalanche photodiodes produced by a domestic company. Capacitors C1 and C2 can isolate high voltage to prevent it from damaging subsequent circuits. Transistors use NPN transistors, V CBO can reach 120V, with a gain bandwidth product of 300MHz, V BE The typical value of is 1.2V. In other embodiments, in order to increase the processing speed of the circuit, it is necessary to use transistors with higher operating frequencies. Some field effect transistors (Field Effect Transistor) can meet higher speed requirements, such as JFET, HEMT, etc. By controlling the gate voltage, the working state of the field effect transistor can be easily controlled, thereby changing Figure 1 In another embodiment, a microwave-band transistor BFU730LX is used. The transistor can work stably at a very high frequency. At a working frequency of 6 GHz, the noise coefficient value is only 0.75 dB, and the maximum power gain reaches 15.8 dB.

[0046] exist Figure 1 In the figure, the dark current of SPAD1 and SPAD2 is usually only 0.3nA. It is difficult to control the bias voltage on the SPAD using a transistor alone. In order to solve this problem, Ra is connected in parallel with the first voltage divider circuit and the second voltage divider circuit. When the transistor is working, there is a small current between the collector and the emitter of the transistor, but the current is much larger than the sum of the dark currents on the SPAD. Therefore, Ra is used for shunting. By reasonably selecting the size of Ra, the voltage of the first voltage divider circuit can be controlled so that the voltage on the SPAD is less than the threshold voltage. At this time, the SPAD is insensitive to photons and is in a closed state; increasing the current of the transistor increases the voltage of the resistor Ra, and the SPAD bias voltage is greater than the threshold voltage. At this time, the SPAD is sensitive to photons and is in an open state. Since the voltage divider circuit is connected in parallel with the resistor Ra, the reverse bias voltage on the SPAD can be easily adjusted through the resistor Ra and the transistor, which is convenient for adjusting and calibrating the circuit at the factory. In the case of replacing the SPAD, the bias voltage can also be matched with the new SPAD quickly.

[0047] Because the resistor Ra is connected in parallel with the first voltage divider circuit, the transistor in the circuit only works in the linear region, there is no storage phenomenon of minority carriers, and the response time is greatly shortened; the collector junction capacitance is greatly reduced, the RC time constant is also correspondingly reduced, and the transmission delay time of the circuit is very short, which can improve the processing speed of the single-photon detection circuit. In the embodiment of the present invention, under the same experimental conditions, the processing speed of the improved single-photon detection circuit using the present invention is 16.1% higher than that of the circuit in the background technology.

[0048] exist Figure 4 In the embodiment, HV uses negative high voltage, the potential of the resistor R1 is lower than the potential of SPAD1, and a PNP transistor is used.

[0049] refer to Figure 5 In the embodiment of the present invention, the signal processing circuit realizes the functions of signal subtraction and voltage comparison. The signals S1 and S2 enter the inverting input terminal and the non-inverting input terminal of the operational amplifier U1 after passing through the resistors R3 and R4. The resistor Rf is connected between the inverting input terminal and the output terminal. According to the characteristics of the operational amplifier, when R3=R4=R, Rf=R5, the voltage V of the signal △S output by the operational amplifier U1 is △S It can be expressed by the following formula: V △S =Rf / R(V S2 -V S1 ), where V S1 and V S2 is the voltage value of signal S1 and S2. The operational amplifier U1 in the figure is a current feedback operational amplifier developed by a domestic research institute. The chip has excellent signal fidelity and a high slew rate of 1800V / us. These characteristics make it suitable for a variety of high-speed applications. Figure 5 In the embodiment, U2 is a high-speed comparator, and the selected one is HMC675LP3E. The transmission delay of this chip is 100ps, and the output signal is P1 and

[0050] Figure 6 is a transistor control circuit in an embodiment of the present invention, which converts the signal The gate signal is input into the logic AND gate together, and the signal obtained after AND is Q. Figure 6In the embodiment, AND is a high-speed logic AND gate, and SY10EP05V is selected. The transmission delay of this chip is 180ps, and the signal Q enters the gate of the transistor through the resistor R6. In the transistor control circuit, by adjusting the static operating point of the transistor, the voltage across Ra can be easily adjusted, thereby changing the bias voltage of the SPAD. In the embodiment, the static operating point is adjusted by adjusting the size of R7 or R8, thereby changing the bias voltage of the SPAD, which is convenient for adjusting and calibrating the circuit at the factory. When the SPAD is replaced, the bias voltage can also be matched with the new SPAD more quickly.

[0051] The present invention is described above by way of example in conjunction with the accompanying drawings. It is obvious that the specific implementation of the present invention is not limited to the above-mentioned method. As long as various non-substantial improvements are made using the method concept and technical solution of the present invention, or the concept and technical solution of the present invention are directly applied to other occasions without improvement, they are all within the protection scope of the present invention.

Claims

1. A single-photon detection circuit for suppressing afterpulses in quantum communication, characterized in that: The single photon avalanche diode SPAD1 and the resistor R1 are connected in series to form a first voltage divider circuit, and a first output terminal ST1 is led out between the single photon avalanche diode SPAD1 and the resistor R1; The single photon avalanche diode SPAD2 and the resistor R2 are connected in series to form a second voltage divider circuit, and a second output terminal ST2 is led out between the single photon avalanche diode SPAD2 and the resistor R2; The resistor Ra, the first voltage divider circuit, and the second voltage divider circuit are connected in parallel, one end of which is electrically connected to the transistor, and the other end is electrically connected to the high voltage source HV; The signal output from the first output terminal ST1 passes through the capacitor C1 to obtain a signal S1; The signal output from the second output terminal ST2 passes through the capacitor C2 to obtain a signal S2; Only SPAD1 can receive photons.

2. The single photon detection circuit according to claim 1, characterized in that: The signal S1 and the signal S2 are both connected and output corresponding signals to a signal processing circuit, and the signal processing circuit outputs a signal pulse P1.

3. The single photon detection circuit according to claim 2, characterized in that: The signal pulse P1 output of the signal processing circuit is connected to the transistor controller circuit and serves as one output at the same time. The transistor controller circuit is connected to the gate signal output terminal.

4. The single photon detection circuit according to claim 3, characterized in that: During the door opening time, the difference △S between signals S1 and S2 is compared with the maximum value of the noise interference signal. If it is greater than the maximum value of the noise interference signal, a signal pulse P1 is output, indicating that a photon has been received.

5. The single photon detection circuit according to claim 4, characterized in that: The signal pulse P1 is fed back to the transistor through the transistor control circuit, and the reverse bias of SPAD1 and SPAD2 is reduced through the transistor, so as to quench the avalanche and reduce the number of electron-hole pairs involved in the avalanche.

6. The single photon detection circuit according to claim 5, characterized in that: After the signal pulse P1 disappears, SPAD1 continues to detect the next photon under the action of the gating signal.

7. The single photon detection circuit according to claim 3, 4, 5 or 6, characterized in that: The transistor works in the linear region, and the bias voltage of the SPAD is adjusted by adjusting the static operating point of the transistor.

8. The single photon detection circuit according to claim 7, characterized in that: The transistor is an NPN type triode, the E pole of the transistor is grounded, the B pole is connected to the transistor control circuit, and the C pole is connected to the single photon avalanche diode SPAD1 and the single photon avalanche diode SPAD2.

9. The single photon detection circuit according to claim 8, characterized in that: The transistor is a PNP type triode, the E pole of the transistor is grounded, the B pole is connected to the transistor control circuit, and the C pole is connected to the single photon avalanche diode SPAD1 and the single photon avalanche diode SPAD2.

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