Tomography of a high-purity narrowband photon from four-wave mixing in atomic vapour

Much work has been done in recent years to develop interesting quantum states in the optical regime. A natural step forward would be to extend our quantum state engineering abilities to collective spin excitations (CSE’s) in hot atomic vapour. Such an extension could find applications in quantum memory, long distance quantum communication, quantum logic gates, and quantum metrology. We demonstrate a setup that serves as a high quality single photon source and a first step towards quantum state engineering of arbitrary states in CSE’s. We use four-wave mixing in a hot atomic vapour cell to create a two-mode squeezed state, similar to parametric down conversion. Heralding on a single photon event in one channel yields a high purity narrow-bandwidth single photon in the other channel. Employing optical homodyne tomography, we reconstruct the density matrix of the generated photon and observe a Wigner function reaching the zero value without correcting for inefficiencies. The intrinsic narrow bandwidth and high production rate of our system result in a high spectral brightness source. Since these photons are also naturally resonant to atomic transitions, our source is attractive for applications in light-atom interfacing. Moving to the pulsed regime would allow for time separated events of CSE creation and optical readout, akin to the well-known DLCZ scheme. Conditional measurements during the write phase prepare the CSE in an arbitrary superposition state, which can then be read out optically and measured via homodyne tomography.