Narrowband Source of Correlated Photon Pairs via Four-Wave Mixing in Atomic Vapour

Many quantum communication protocols require entangled states of distant qubits which can be implemented using photons. To efficiently transfer entanglement from photons to stationary qubits such as atoms, one requires entangled photons with a frequency bandwidth matching the absorption profile of the atoms. In our setup, a cold $Rb^{87}$ atomic ensemble is pumped by two laser beams (780nm and 776nm) resonant with the $5S_{1/2}\rightarrow 5P_{3/2} \rightarrow 5D_{3/2}$ transition. This generates time-correlated photon pairs (776nm and 795nm) by nondegenerate four-wave mixing via the decay path $5D_{3/2}\rightarrow 5P_{1/2} \rightarrow 5S_{1/2}$. Coupling the photon pairs into single mode fibres and using silicon APDs, we observe $g^(2)$ of about $2000$ and pairs to singles ratio of 11.2\% (2800 photon pairs per second) with an optical bandwidth $<$ $30/(2\pi)$ MHz.