Long-Distance Bell-State Analysis of Fully Independent Polarization Weak Coherent States

Here we report the experimental implementation of a linear-optics partial Bell-state analyzer following the propagation of single-photons through long telecommunication optical fibers. Polarization encoded weak coherent states were sent from independent remote continuous wave faint laser sources over two 8.5 km long optical fiber spools. Automatic polarization stabilization systems were used in each spool to compensate the random polarization drift. We demonstrate stable two-photon interference with a dip visibility of 47.8% over a 40 minute time-interval, close to the theoretical maximum of 50% for weak coherent states. We successfully show that it is possible to carry out remote long-distance Bell-state analysis of polarization weak coherent states in spite of random residual birefringence fluctuations in optical fibers. These results pave the way for important applications in quantum communications with polarization qubits such as measurement device-independent quantum key distribution and quantum repeaters.