Fig. 2: Experimental setup.
From: Twin-field quantum key distribution without optical frequency dissemination
Alice (Bob) owns an independent ultrastable laser, the signal of which is modulated by a phase modulator (PM) to produce a frequency comb of 25 GHz spacing. Two comb lines separated by 100 GHz are chosen for quantum signal encoding (λq) and channel stabilisation (λc), respectively. Charlie contains a receiving 50/50 beam splitter to interfere the incoming signals. The λq photons are registered by D0 and D1 and the λc photons by Dc. Dcʼs count rate is used as error signal to the fast PID controller that cancels the twin-field phase fluctuation of the λc signals. a Optical frequency comb measured after the PM; b Encoder box. DWDM dense wavelength-division multiplexing; EPC electrically driven polarisation controller; FS fibre stretcher; IM intensity modulator; VOA variable optical attenuator.
