Optical Homodyne Tomogyaphy Of Narrowband Paired Photons

Narrowband non-classical paired photons can bring great advantages to quantum information processing due to their unique properties.Entanglement for the paired photons can be encoded in various type of degrees of freedom,such as polarization,orbital angular momentum,time bin,frequency bin and so on.In this work we generate narrowband paired photons from a two-dimensional cold atomic ensemble.A pair of Stokes and Anti-Stokes photons is generated from four-wave mixing process assisted with electromagnetically-induced transparency.Using the Stokes photons as the trigger for the photo current captured by the balanced homodyne detection,we measure the quantum state of the Anti-Stokes photons.The photo current of narrowband photon source in homodyne detection is sensitive the beating noise.In order to improve the frequency mode-matching between the local field and the target photons,we split part of local beam and let this part of beam shifted the frequency by3.036 GHz by an electric-optical modulator to become the coupling beam in the four-wave mixing.By use of optical homodyne tomography,we reconstruct its Wigner function and obtain more than 29% for the single photon composition.Without other external filtering,we record their coherence time as more than 200 ns.