Narrowband polarization-entangled photon pairs distributed over a WDM link for qubit networks

A brighter, more narrowband and portable source using two 50 mm long orthogonally oriented, quasi-phase-matched crystals is built. The new source generates polarization-entangled photon pairs at 809 nm and 1555 nm at a maximum rate of 1.2 x 10⁶s^-1THz^-1mW^-1 after coupling to single-mode fiber. For transmission experiments, the quantum and synchronization channels are multiplexed in a WDM environment, and with only 0.8 nm spacing between them, a detected rate of 1.1 x 103 s^-1 photon pairs is achieved with a raw visibility of 85 % in all measurement bases after transmission over 27 km of single mode fiber. With the current isolation of about 100 dB achieved between channels, the quantum channel could even be multiplexed with a classical data channel.