Fig. 3

Scheme of the experiment. A continuous wave laser at 1550 nm (100 GHz dense wavelength division multiplexed channel C33) is used for the quantum channel (QC). An intensity modulator (IM) is used for carving the optical pulses and to implement the decoy state method. A phase modulator (PM) is used to phase randomise the pulses. At the same time a different continuous wave laser at 1558 nm (100 GHz dense wavelength division multiplexed channel C23) followed by an intensity modulator, controlled by a bit pattern generator, allows a classical channel (CC) with on-off keying modulation at 10 Gbit s\({}^{-1}\). Through a beam combiner the quantum and classical signals are combined. Cascaded beam splitters (cBS) (two beam splitters 1 \(\times\) 5 and four beam splitters 1 \(\times\) 8) are used to divide the two signals into 37 cores. The multicore fibre is 7.9 km long and presents slightly different losses between cores. Each core is then received independently and routed to a wavelength division multiplexing (WDM) filter, used to separate the classical and the quantum signals. The classical signal is received by a photodetector (PD) and controlled by an error analyser. The quantum signal is then measured in two mutually unbiased bases (computational and Fourier bases). A 10 dB beam splitter (BS) is used to divide the two bases: in the computational we only require to measure the time of arrival with a single photon detector (SPD), while in the Fourier basis we use a delay line interferometer (DLI) to detect the phase difference between two pulses (a second SPD is connected to one output of the DLI). All the electrical outputs are collected by a time tagging unit and analysed by a computer. In the case of the transmission of only the quantum signals, the classical laser was removed in the transmitter and the WDM filter was removed from the receiver