Accurate Shot-Noise-Limited Calibration of a Time-Domain Balanced Homodyne Detector for Continuous-Variable Quantum Key Distribution

The time-domain balanced homodyne detector (TBHD) is critical component in continuous-variable quantum key distribution (CVQKD). The shot-noise-limited attribute of a TBHD depends on the whole frequency spectrum of the detector, in contrast to a frequency-domain balanced homodyne detector that focuses only on part of the frequency spectrum. We propose a method for accurate calibration of the shot-noise-limited attribute of the TBHD by overlapping the gain lines of the TBHD and an integral photodetector which is realized by blocking one photodiode of the TBHD. The overlapping of two lines indicates that the detector is indeed shot noise limited, which is crucial to the security of CVQKD. Experimentally, we use this method to obtain a calibration error of
$\pm 1$
‰. Theoretical analysis showed that the CVQKD security is very sensitive to calibration errors of shot noise, and proper countermeasure was proposed to eliminate this issue. The presented methods are expected to contribute to CVQKD security and TBHD standardization.