A novel time-domain balanced homodyne detector

Abstract

The application discloses a novel time-domain balanced homodyne detector, which can output a common-mode rejection ratio of 75 dB and a signal-to-noise ratio of 13.50 dB shot noise signal at a repetition rate of 1 MHz; meanwhile, a clock signal with the same frequency as the local oscillator light is generated, and a direct current voltage value with a good linear relationship with the power of the local oscillator light is output, and the gain is K=2.52*10 ‑7 V / photon, and the residual sum of squares is 2.51*10 ‑ 4 V 2 The detector can significantly reduce the complexity of a receiving end of a continuous variable quantum key distribution system, effectively avoid security vulnerabilities caused by changes in a beam splitter splitting ratio, and enhance the actual security of the system.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of continuous variable quantum key distribution technology, and particularly relates to a novel time-domain balanced homodyne detector. BACKGROUND

[0002] In the information age, information security is highly concerned by people. In the field of information security, the traditional encryption method is mainly based on the complexity of algorithm. With the continuous breakthroughs in the research of quantum computers, its security is facing severe challenges. Quantum key distribution (QKD) technology based on the basic principles of quantum mechanics can make the legitimate communication parties theoretically obtain unconditionally secure secret keys, which can ensure information security and has broad application prospects in the fields of national defense, finance, network and communication.

[0003] Continuous variable quantum key distribution has good compatibility with classical coherent communication and is one of the ideal schemes for building metropolitan high-speed networks. The time-domain balanced homodyne detector is a key device, which can directly detect the quadrature components of the optical field. In the process of quantum key distribution, in order to realize accurate measurement of the quadrature components and resist local light attacks, the power of the local light must be monitored in real time and its stability must be ensured. In the CVQKD system, the method of monitoring the power of the light field split by the beam splitter is usually adopted. Since the splitting ratio of the optical fiber beam splitter will slowly change with factors such as temperature, it will cause errors in monitoring and affect the power of the local light. Therefore, the method of monitoring the power of the light field split by the beam splitter to judge the power of the local light has certain deficiencies.

[0004] For the data acquisition device of the receiving end of the CVQKD system, the acquisition of the quadrature components of the signal light field needs the clock signal of the same frequency as the local light field. The usual way is to increase an optical detection module to convert part of the local light field into an electrical signal. After the electrical signal is externally connected to a commercial power amplifier and an adjustable delay, it is used as the clock signal of the data acquisition device of the receiving end. This method increases the complexity of the system receiving end, and at the same time, since the local light is split, the optical power of the local light is reduced.

[0005] In order to solve the above problems, we design a circuit to make one output of the detector have a good linear relationship with the power of the local light of the input balanced homodyne detector, and the output of the other output has the same frequency as the clock signal of the local light. In this way, the measurement of the quadrature components of the signal light field is realized, and the monitoring of the power of the local light and the generation of the clock signal are also realized. SUMMARY

[0006] In order to solve the defects and deficiencies of the prior art, a new time-domain balanced homodyne detector is provided, which is related to the detection end of a continuous variable quantum key distribution (CVQKD) system, can not only measure the orthogonal components of the signal light field, but also can monitor the local oscillator light power in real time and generate a clock signal with the same frequency as the local oscillator light. The detector makes full use of the electrical signals generated by each photodiode in the cascade photodiode, which can effectively reduce the complexity of the receiving end of the CVQKD system.

[0007] In order to achieve the purpose of the present application, a new time-domain balanced homodyne detector is provided, which comprises a 50 / 50 optical fiber coupler, signal light and local oscillator light are input from the signal light input end and the local oscillator light input end of the 50 / 50 optical fiber coupler, the optical field output from the 50 / 50 optical fiber coupler is divided into two paths, each entering a first high-sensitivity photodiode and a second high-sensitivity photodiode, one of the two paths passes through a high-precision optical fiber adjustable attenuator to make the detection efficiency of the two arms of the 50 / 50 optical fiber coupler consistent, the photocurrent generated by the first high-sensitivity photodiode and the second high-sensitivity photodiode is filtered by a first filter capacitor and a second filter capacitor respectively, and then subtracted through a first coupling circuit to enter a first charge amplifier, the output thereof is shaped by a shaping amplifier, and finally a Gaussian pulse is output through a first impedance matching resistor, realizing accurate measurement of the orthogonal components of the signal light field.

[0008] Among them, part of the photocurrent signal generated by the first high-precision photodiode is input into a second charge amplifier through a second coupling circuit, the output thereof is filtered by a second high-pass filter and then output from a second impedance matching resistor, obtaining a clock signal with the same frequency as the local oscillator light field.

[0009] Part of the photocurrent signal generated by the second high-precision photodiode is input into a third charge amplifier through a third coupling circuit, the output thereof is amplified by a second voltage amplifier, and then shaped into a direct current signal by a one-way valve and a second low-pass filter, and finally output through a third impedance matching resistor, the amplitude of the direct current signal is in linear relationship with the optical power of the local oscillator light input into the 50 / 50 optical fiber coupler, and the real-time monitoring of the local oscillator light power is realized by monitoring the amplitude of the direct current signal.

[0010] The beneficial effects of the present application are:

[0011] Compared with the prior art, the new time-domain balanced homodyne detector provided by the present application mainly includes three functions:

[0012] (1) Measurement of the orthogonal components of the signal light field;

[0013] (2) Real-time monitoring of the power of the local oscillator light field;

[0014] (3) Generation of a clock signal with the same frequency as the local oscillator light field.

[0015] The present application extracts part of the photocurrent signal generated by the photodiode for detection. The detector can realize real-time monitoring of the local light field optical power and generate a clock signal with the same frequency as the local light field on the basis of accurate measurement of the signal light field orthogonal component. This approach not only makes full use of the electrical signal generated in the photodiode, but also reduces the complexity of the receiving end of the CVQKD system to some extent. BRIEF DESCRIPTION OF DRAWINGS

[0016] The specific embodiments of the present application are further described in detail below with reference to the accompanying drawings, in which:

[0017] Figure 1 is a specific scheme diagram of the present application;

[0018] Figure 2 is a structural schematic diagram of the present application;

[0019] Figure 3 is an example diagram of the present application;

[0020] Figure 4 is a schematic diagram of the output shot noise of the detector of the present application;

[0021] Figure 5 is a schematic diagram of the clock signal output by the detector of the present application and with the same frequency as the local light field;

[0022] Figure 6 is a curve graph of the change of the output voltage of the detector of the present application and the input diode optical power. DETAILED DESCRIPTION

[0023] As shown in Figure 1 , the specific scheme of the present application is as follows: the signal light and the local light are input from the signal light input end and the local light input end of the 50 / 50 optical fiber coupler 1, and the light field output by the 50 / 50 optical fiber coupler 1 is divided into two paths, each entering the first high-sensitivity photodiode 3 and the second high-sensitivity photodiode 4. One of the two paths passes through the adjustable high-precision optical fiber attenuator 2 to make the detection efficiencies of the two arms output by the 50 / 50 optical fiber coupler 1 consistent. The photocurrent generated by the first high-sensitivity photodiode 3 and the second high-sensitivity photodiode 4 respectively passes through the first filter capacitor 5 and the second filter capacitor 6 for filtering, and then subtracts through the first coupling circuit 7 to enter the first charge amplifier 8. The output thereof is shaped by the shaping amplifier 9, and finally outputs a Gaussian pulse through the first impedance matching resistor 13, thereby realizing accurate measurement of the signal light field orthogonal component.

[0024] Among them, part of the photocurrent signal generated by the first high-precision photodiode 3 is input into the second charge amplifier 15 through the second coupling circuit 14. The output thereof is filtered by the second high-pass filter 16 and then output from the second impedance matching resistor 17, thereby obtaining a clock signal with the same frequency as the local light field.

[0025] Part of the photo current signal generated by the second high precision photodiode 4 enters the third charge amplifier 19 through the third coupling circuit 18, the output of which is amplified by the second voltage amplifier 20, then is shaped into a direct current signal by the one-way valve 21 and the second low pass filter 22, and finally is output through the third impedance matching resistor 23, the amplitude of which is linearly related to the optical power of the local oscillator light input to the 50 / 50 fiber coupler, and the real-time monitoring of the local oscillator light power is realized by monitoring the amplitude of the direct current signal.

[0026] The technical solution is further illustrated by the structural diagram of the present application:

[0027] 1. Measurement of vacuum optical field shot noise

[0028] As shown in Figure 2 , the signal light and the local oscillator light interfere on the 50 / 50 fiber coupler, and the output light fields 1 and 2 enter two reverse-biased and cascaded photodiodes PD1 and PD2 respectively. In practice, the 50 / 50 fiber coupler not only has a 3dB inherent loss η 3dB , but also has an additional loss η l , and the photodiodes PD1 and PD2 have actual quantum efficiencies η PD . The total transmission efficiency of the local oscillator light to one of the photodiodes PD1 is η 3dB ·η PD1 ·η l1 , and the total transmission efficiency of the local oscillator light to the other photodiode PD2 is η 3dB ·η PD2 ·η l2 . For convenience of description, we use η to replace η 3dB ·η PD ·η l . In order to make the optical powers entering the two photodiodes equal, an adjustable attenuator is introduced in the light path with high total transmission efficiency, and by adjusting the adjustable attenuator, η1 and η2 are made substantially equal, at which time the optical powers incident on the two photodiodes differ by less than one ten-thousandth. The photo current signals generated by the two cascaded photodiodes enter the charge amplifier through subtraction and a capacitor. In order to obtain a Gaussian-shaped pulse signal, the output signal of the charge amplifier is shaped and amplified by a shaping amplifier composed of a high pass filter, a voltage amplifier and a low pass filter, and finally a Gaussian-shaped electrical pulse signal V OUT1 is output through an impedance matching resistor.

[0029] 2. Generation of a clock signal with the same frequency as the local oscillator light field

[0030] Part of the AC signal generated by photodiode PD1 enters charge amplifier OPA2 through a coupling circuit composed of capacitors and resistors. The peak value of its output signal is related to the light power P1 incident on photodiode PD1. The output signal of the charge amplifier is filtered by an RC high-pass filter, and finally the output V is obtained through an impedance matching resistor. OUT2 Observe the local oscillator optical signal and the detector output signal V on the oscilloscope. OUT2 From the image, it is clear that the output signal V can be obtained. OUT2 Since the frequency of the photodiode PD1 is the same as the frequency of the local oscillator light, we can generate a clock signal V with the same frequency as the local oscillator light field by introducing a coupling circuit and a charge amplifier on the other side of the photodiode PD1. OUT2 .

[0031] 3. Real-time monitoring of the optical power of the local oscillator field

[0032] Part of the photocurrent signal generated on photodiode PD2 enters charge amplifier OPA3 through a coupling circuit. The peak value of its output signal is related to the light power P2 incident on photodiode PD2. A non-inverting amplifier circuit, composed of a voltage amplifier, amplifies the output signal of the charge amplifier before outputting it. To facilitate data acquisition, the signal output from the non-inverting amplifier is shaped by a rectifier circuit consisting of a unidirectional phase converter and an RC low-pass filter to obtain a smooth DC signal V. OUT3 The amplitude of this signal has a linear relationship with the optical power P2 of the input photodiode PD2, as expressed by the following expression:

[0033] V OUT3 =K·P2

[0034] In the experiment, the optical power P2 of the input photodiode PD2 was changed, and the voltage value V output by the detector corresponding to different optical powers was collected. OUT3 After performing linear fitting on the experimental data, the fitted value K of the slope of the straight line is obtained, and the linearity and magnitude of K are verified.

[0035] Detector output signal V OUT3 The amplitude directly reflects the magnitude of the optical power P2 of the input photodiode PD2, therefore it can be monitored by the detector output signal V. OUT3 Real-time monitoring of the local oscillator power P is achieved.

[0036] To make the above-mentioned objectives, features, and advantages more apparent and understandable, the method will be further described in detail below with reference to the example figures of the present invention:

[0037] 1. Measurement of shot noise in vacuum optical field

[0038] like Figure 3As shown, vacuum optical field and local optical field are input from signal light input end and local light input end of 50 / 50 optical fiber coupler, and two optical fields 1 and 2 after 50 / 50 optical fiber coupler are respectively incident on two cascaded photodiodes PD1 and PD2 of the detector, and the quantum efficiencies of the two photodiodes are 81% and 80.2% respectively. By adjusting the adjustable attenuator in optical path 1, η1=η2, at this time, the photoelectric currents generated by the two photodiodes are substantially equal. The photoelectric current output by the two cascaded photodiodes is subtracted, and then input to the charge amplifier ADA4817 through the coupling circuit composed of C1 (100 pF) and R1 (10 MΩ) for amplification, and then the output of ADA4817 is shaped and amplified through the high-pass filter composed of C2, R3 and L1, the voltage amplifier and the low-pass filter composed of L2 and C3, and finally the Gaussian type electrical signal is output. Wherein C2 is 2.2 nF, R3 is 30 Ω, L1 is 5.6 uH, C3 is 50 pF, and L2 is 1.5 uH.

[0039] After the detector is powered on, when the signal light input end and the local light input end input the vacuum optical field at the same time, the output of the detector is collected by the high-speed data acquisition card to obtain the electronic noise variance of the detector as 2.37×10 -5 V 2 When measuring the shot noise of the vacuum optical field, the signal light input end inputs the vacuum optical field, and the local light input end inputs pulsed light with a repetition frequency of 1 MHz and a pulse width of 100 ns, and the optical power is 5.41 uw, and the number of photons contained in each optical pulse is about 4.22×10 7 V -4 2 At this time, the signal-to-noise ratio is 13.50 dB.

[0040] The output signal of the detector is shown in Figure 4 The horizontal coordinate in the figure is time, and the vertical coordinate is the voltage value V OUT1 of the output of the detector. From the figure, the vacuum noise fluctuation can be clearly observed, and the detector can realize accurate measurement of the shot noise of the vacuum optical field.

[0041] 2, generate a clock signal with the same frequency as the local optical field

[0042] ​Part of the photocurrent generated by photodiode PD1 is coupled into charge amplifier ADA4817 through C4 (100 pF). The amplitude of the output signal is related to the optical power P1 of the input photodiode PD1. The voltage signal output by the charge amplifier ADA4817 is filtered by an RC high-pass filter and then output through impedance matching resistor R9. The resistance R8 in the high-pass filter is 50 Ω, and the capacitance C5 is 2.2 nF. The electrical signal V OUT2 As shown in Figure 5 , the horizontal axis of the graph is time, and the vertical axis is the voltage value of the output of the detector, Figure 5 The lower part is the electrical pulse signal generated by the local oscillator light through the commercial photodetector, and the upper part is the output signal V OUT2 of the detector. It can be seen from the graph that the frequency of the clock signal V OUT2 output by the detector is consistent with the frequency of the local oscillator light, both being 1 MHz. Therefore, we can generate an electrical signal with the same frequency as the local oscillator light field by using the electrical signal generated by photodiode PD1 in the balanced zero beat detector. After delay and amplification by an external delay chip, this electrical signal can be directly used as the clock signal of the data acquisition device of the receiving end of the CVQKD system.

[0043] 3. Real-time monitoring of the power of the local oscillator light field

[0044] Part of the photocurrent generated by photodiode PD2 is coupled into charge amplifier ADA4817 through a coupling circuit composed of capacitor C6 (100 pF) and R 10 (10 MΩ). The peak value of the output pulse signal of the charge amplifier ADA4817 is related to the optical power P2 of the input photodiode PD2, and the two are in a linear relationship. The output signal of the charge amplifier ADA4817 is amplified by a same-phase amplification circuit composed of a voltage amplifier and then output. The amplified voltage signal is shaped by a shaping network composed of a unidirectional diode germanium tube and an RC filter circuit and then output through impedance matching resistor R 14 , outputting a smooth DC signal. The amplitude V OUT3 of the output signal is in good linear relationship with the optical power P2 of the input photodiode PD2. The turn-on voltage of the unidirectional diode in the shaping circuit is 0.25 V, the resistance R 13 in the RC filter circuit is 110 kΩ, and the capacitance C7 is 22 uF.

[0045] In the experiment, the average number of photons input into photodiode PD2 was set to 2 x 10 6 -1.2 x 10 7 , and the output voltage V OUT3 of the detector was collected by a high-speed data acquisition card, obtaining Figure 6 . The horizontal axis of the graph is the average number of photons contained in each pulse of the input photodiode PD2, and the vertical axis is the voltage value VOUT3 The black dots are experimental data, and the straight line is a linear fitting of the experimental data. The slope K of the straight line obtained by linear fitting of the experimental data is K = 2.52 x 10 -7 V / Photon, and the residual sum of squares is only 2.51 x 10 -4 V 2 It is proved that the experimental data and the fitted straight line are in high degree of conformity, that is, the voltage value V OUT3 and the light power P2 input into the photodiode PD2 have a good linear relationship, that is, the voltage value V OUT3 output by the detector can accurately reflect the size of the input local oscillator light power P, so the input local oscillator light power P can be monitored by monitoring the voltage value V OUT3 output by the detector.

[0046] The above embodiments are not limited to the technical solutions of the embodiments themselves, and the embodiments can be combined with each other to form new embodiments. The above embodiments are only used to illustrate the technical solutions of the present application and not to limit them, and any modification or equivalent replacement without departing from the spirit and scope of the present application should be covered in the scope of the technical solutions of the present application.

Claims

1. A novel time-domain balanced homodyne detector characterized by: The application relates to a high-precision optical power monitor, which comprises a 50 / 50 optical fiber coupler (1), signal light and local oscillator light are input from the signal light input end and the local oscillator light input end of the 50 / 50 optical fiber coupler (1), the light field output via the 50 / 50 optical fiber coupler (1) is divided into two paths, each of which enters a first high-sensitivity photodiode (3) and a second high-sensitivity photodiode (4), one of the two paths passes through a high-precision optical fiber adjustable attenuator (2) to make the detection efficiency of the two arms of the 50 / 50 optical fiber coupler (1) consistent, the photoelectric current generated by the first high-sensitivity photodiode (3) and the second high-sensitivity photodiode (4) is filtered by a first filter capacitor (5) and a second filter capacitor (6) respectively, and then is subtracted through a first coupling circuit (7) to enter a first charge amplifier (8), the output of the first charge amplifier (8) is shaped by a shaping amplifier (9), and finally a Gaussian pulse is output through a first impedance matching resistor (13), so that the accurate measurement of the orthogonal components of the signal light field is realized; Part of the photoelectric current signal generated by the first high-precision photodiode (3) is input into a second charge amplifier (15) through a second coupling circuit (14), the output of the second charge amplifier (15) is filtered by a second high-pass filter (16) and then is output from a second impedance matching resistor (17), so that a clock signal with the same frequency as the local oscillator light field is obtained; Part of the photoelectric current signal generated by the second high-precision photodiode (4) is input into a third charge amplifier (19) through a third coupling circuit (18), the output of the third charge amplifier (19) is amplified by a second voltage amplifier (20) and then is shaped into a direct current signal by a one-way valve (21) and a second low-pass filter (22), and finally the direct current signal is output through a third impedance matching resistor (23), the amplitude of the direct current signal and the optical power of the local oscillator light input into the 50 / 50 optical fiber coupler are in linear relationship, and the real-time monitoring of the local oscillator power is realized by monitoring the amplitude of the direct current signal.

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