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The key security of quantum key distribution (QKD) is guaranteed by the basic principle of quantum mechanics, which makes it possible to combine information theory security communication with one-time encryption. The key is usually encoded on the polarization dimension or phase dimension of a single-photon. It is considered that the birefringence effect of single-mode fiber leads to a random variation of polarization state, which would induce the bit error rate. So it is of great significance to keep the single-photon linear polarization state stable for both polarization encoding QKD system and phase encoding QKD system. By using the single-photon polarization modulation technology, the single-photon linear polarization state periodically varies with the external modulation signal. The flicker noise is suppressed effectively, and the signal-to-noise ratio (SNR) of single-photon counting is increased as indicated by the phase-sensitive detection with a lock-in amplifier (LIA). The error signal is generated by demodulating the modulated single photons and it is used to lock an arbitrary 1550 nm single-photon linear polarization state to the optical axis of in-line polarizer (ILP). The modulation frequency reaches up to 5 kHz, which can eliminate the influence of low frequency flicker noise. The LIA demodulates the single-photon pulses by using 78.1 Hz filter bandwidth, with a time constant of 1 ms and a filter slope of 24 dB. The error signal with a signal-to-noise ratio(SNR) of 20 is shown in
Fig. 3 of the main text. The zero-crossing point of error signal represents the single photon’s linear polarization state aligned to the optical axis of ILP. The linear slope around the zero-crossing point for the polarization state angle versus the error signal amplitude is 1.267 rad/V. When the negative feedback loop does not work, the polarization drift of single-photon pulses is 0.082 rad due to the random environmental noise. However, by using the single-photon polarization modulation technology and the precise and dynamic control of the polarization rotator, the polarization drift of stable single-photon pulse is limited to 0.0011 rad within 2000 s, 6.7×10–5 in an integration time of 128 ms. The advantages for the single-photon polarization modulation technology are as follows: i) the linear polarization state drift is compensated for real-time at the single-photon level; ii) single frequency polarization modulation can be extended to multiple frequency polarization modulation in order to achieve locking of multiple linear polarization states of single photons simultaneously; iii) these 1550 nm single-photon pulses with the 0.0011 rad linear polarization state stability can be directly used as the single-photon source in either polarization encoding or phase encoding QKD system. -
图 1 实时锁定1550 nm单光子线偏振态的实验装置(红色虚线代表光信号, 黑色实线代表电信号), 其中Iso为隔离开关; Att为衰减器; AM为强度调制器; PR为偏振旋转器; ILP为同轴检偏器; SPD为单光子探测器; FG为函数发生器; LIA为锁相放大器; Amp为放大器
Fig. 1. Experimental setup of real-time locking the 1550 nm single-photon linear polarization state (red dashed lines for the light signal, black solid lines for the electrical signal), where Iso represents isolator; Att represents attenuator; AM represents amplitude modulator; PR represents polarization rotator; ILP represents in-line polarizer; SPD represents single-photon detector; FG represents function generator; LIA represents lock-in amplifier; Amp represents amplifier.
图 4 (a) 单光子偏振态未锁定和锁定的测量结果; (b) 单光子偏振态未锁定时误差信号的统计结果; 单光子偏振态锁定时误差信号的统计结果(c)和对应的阿伦偏差分析结果(d)
Fig. 4. (a) Measurement results of unlocked and locked single-photon polarization states; (b) statistics results of error signal for unlocked single-photon polarization state; statistics results of error signal for locked single-photon polarization state (c) and corresponding analysis results of Allan deviation (d).
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