Fig. 3: Schematic of overall experimental setup for entangled photon transmission from network deployed entangled light emitting diode.

Optical signals from three sub-systems are sent over the network. Single photons are shown as ‘red’ (XX) and ‘violet’ (X) dots, laser pulses for polarisation stabilisation are shown as short ‘red’ lines and classical data traffic is shown as continuous ‘blue’ line. Entangled photon pairs are generated from the tuneable entangled light emitting diode at West Cambridge. X and XX photons are spatially separated with a spectral filter module. X photons pass through a polarisation analyser consisting of a half waveplate (HWP), a quarter waveplate (QWP) and a linear polariser (LP) before being sent over one of the two installed fibres. Entangled XX photons are sent over the other field fibre which is stabilised for polarisation drifts using a polarisation stabilisation sub-system (Stabilisation 1 and 2). At the Cambridge Research Laboratory (CRL), their quantum state is measured using an electronic polarisation controller (EPC), a polarising beam splitter (PBS) and two superconducting single-photon detectors (SSPDs). For remote control of the deployed source and system components, a classical data connection in the telecom C-band is established over the same fibre using a classical-quantum multiplexing system (labelled as Multiplexing). Arrival time of the X photons in the first fibre is measured with a third SSPD at CRL. Map: Imagery ©2019 Google, Getmapping plc, Infoterra Ltd and Bluesky, The GeoInformation Group, Maxar Technologies, Data SIO, NOAA, U.S. Navy, NGA, GEBCO, Maxar Technologies, Landsat/Copernicus, Map data ©2019.