Generation of narrowband, entangled photon pairs in birefringent fibre

An in-line source of polarization entangled photons based on a commercial birefringent fibre is demonstrated. When two similar sections of this fibre are spliced together, incorporating a 90° rotation between them, and pumped using a linearly polarised pump beam at 45° to the principle polarization axes, the total generated state resulting from the two fibre sections can be entangled. Classical transmission measurements confirm that the splice loss was low (<; 0.5 dB), and the polarisation extinction of the device was more than 20:1. With 90 mW of average pump power from a 1064 nm, picosecond pulsed fibre laser launched into the device, correlated signal and idler photons were detected at a rate of 1000 s^-1 (limited by the available pump power). The phase-matched idler wavelength from this device is suitable for long distance transmission through fibre and the corresponding signal wavelength lies within the operating range of efficient silicon-based single photon detectors. Furthermore, group velocity matching of the idler and pump is expected to occur close to this pump wavelength, resulting in the generation of signal photons with naturally narrow bandwidth.