Engineering polarisation entanglement for quantum network applications

International audienceEntanglement is a key resource for quantum infor- mation protocols. Low-loss optical fibers and reliable guided-wave components make it possible to create and distribute photonic entanglement in the telecom C-band of wavelengths (1530-1565 nm). Entanglement is easily produced using spontaneous parametric down-conversion (SPDC) in nonlinear crystals. Usual observables are time-bin and polarisation, the latter being easier to analyze, thanks to simple experimental apparatus, real-time fiber birefringence compensation, and the natural design of heralded quantum memories for this observable. In quantum networks, where telecom photons are used to distribute, and quantum memories to store, entanglement, the photonic spectral and memory acceptance bandwidths have to be matched. The latter ranges from a few MHz (cold atoms) to a few GHz (solid- state). In the following, we discuss a versatile polar- isation entangling scheme based on a birefringent delay line (BDL) that can be applied to photon bandwidths down to 25 MHz. We demonstrate such versatility by violating, by large amounts, the Bell inequalities for 3 different bandwidths: 25 MHz, 500 MHz, and 100 GHz