Fig. 1: Schematics of TF-QKD setups.
From: Twin-field quantum key distribution without optical frequency dissemination
In TF-QKD, the users (Alice and Bob) communicate with each other by sending encoded quantum signals at the single-photon level to the intermediate node (Charlie), who measures the interference using two single photon detectors. The protocolʼs stringent requirement for phase stability has rendered all existing setups to adopt a closed interferometer configuration, which is resource-inefficient and inflexible. a Existing Mach–Zehnder interferometer setup28,31,32. Alice and Bob inherit a common optical frequency νR that is disseminated by Charlie via the long service fibres. b Open channel setup. Alice and Bob locally generate their own optical frequencies and their coherent side-bands of νA ± f0 and νB ± f0, with a nominally identical microwave frequency offset f0. One side-band is used to reconcile the laser frequency difference (Δν = νA − νB), while the other is allocated quantum signal encoding. The open scheme eliminates the need for the service fibre and the optical frequency locking hardware, and supports asymmetric links.
