Fig. 1: Overview of the intercity QKD experiments on the ‘Niedersachsen quantum link’ using single photons from a semiconductor quantum dot (QD).
From: High-rate intercity quantum key distribution with a semiconductor single-photon source
a Distribution of quantum bits between Hannover (Alice) and Braunschweig (Bob) over 79 km of deployed fibre with a total loss of 25.49 dB. Map data from Google (©2023 Google). b Sketch of the experimental setup. The QD-based SPS of the transmitter is mounted in a cryostat and excited by a pulsed laser at different clock rates (CRs) (76 MHz, 228 MHz, 608 MHz, and 1063 MHz). The emitted single photons are collected by an aspherical lens with a numerical aperture of 0.7. State encoding is performed by a polarisation control module (P. Cont.) comprising a polariser, a half-wave plate (HWP) and a quarter-wave plate (QWP). The single-photon and excitation laser signals are then together coupled into either a sequence of fibre spools or the deployed fibre. In the receiver module, a fibre Bragg grating (FBG) demultiplexes the single photon and laser signals by wavelength. An electronically controlled polarisation compensation (P. Comp.) module with QWP and HWP counteracts polarisation fluctuations in the quantum channels by monitoring and minimising the quantum bit error ratio (QBER). A non-polarising 50:50 beam splitter (BS) then acts as a random selector of the decoding basis, with rectilinear projection in the transmitted path using a polarisation beam splitter (PBS), and diagonal projection in the reflected path using a HWP at an angle of 22.5° followed by a PBS. The four single-photon signals and the laser signal are detected at superconducting nanowire single-photon detectors (SNSPDs) and the timing events recorded with a time-correlated single-photon counting (TCSPC) unit
