Fig. 1: Setup and location of the experiment.
From: Passively stable distribution of polarisation entanglement over 192 km of deployed optical fibre
We used a fibre optic cable which links the Mediterranean islands of Malta and Sicily. A continuous-wave laser at 775 nm produces, via spontaneous parametric down-conversion, photon pairs which are entangled in polarisation due to the Sagnac geometry. Signal and idler photons are separated by frequency into two different fibres; one photon is detected immediately in Malta in a polarisation analysis module consisting of a half-wave plate in front of a PBS and one superconducting nanowire single-photon detector (SNSPD), and the other detected by a second detector in the same cryostat after transmission through the 192 km submarine fibre loop and a polarisation analysis module. (\(\lambda /4\), \(\lambda /2\): wave-plates; PBS, polarising beam-splitter; YVO\({}_{4}\), yttrium orthovanadate plate; DM, dichroic mirror; PPLN, MgO-doped periodically poled lithium niobate crystal (MgO:ppLN); WDM1: 0.6 nm band-pass filter (centre wavelength 1551.72 nm); WDM2: 0.6 nm band-pass filter (centre wavelength 1548.51 nm); PC, fibre polarisation controllers; LPF, 780 nm long-pass filter; SNSPD, superconducting nanowire single photon detectors; TTM, time-tagging module; FSB, free-space beam. Mirrors and fibre couplers not labelled, lenses omitted.) Image taken from NASA Worldview [https://earthdata.nasa.gov/worldview] which is free to use under public domain.
