Single photon states are the building blocks for quantum information processors and quantum network. Information is encoded in polarization, frequency or timing of arrival of the photons and are manipulated with linear optical elements like beam-splitters and wave-plates. Interfacing such states with atomic ensemble to store and process information is an active field of research and there is therefore an increasing volume of work on generating bright sources of single photons to implement a chain of sequential tasks. Application of such states are in sub-wavelength imaging and optical-lithography, precision measurement, spectroscopy and imaging, and to push the limits of signal-to-noise beyond the classically allowed limits.

To generate quantum states of light, one requires non-linear photonic interactions in the medium. Such multi-photon processes mix and correlate the emitted photons in time, frequency and/or spatial modes. A most common method employed is parametric down-conversion (PDC) in birefringent crystals which inherently generated correlated photons, such that detection of one of the photons ensures the presence of the other photon in a well defined spatial mode. Such sources have been used to generate broad-band triggered single-photons. PDC process is however extremely inefficient owing to the fact that the generation of photon pair is completely random in time. Alternatively, atomic vapors with on or near-resonant interactions provide significantly larger non-linearities. Low-power diode-lasers in this case are used to trigger a four-wave mixing process (FWM), such that two fields mix in the medium to produce correlated photon pairs. In addition, atomic-medium also acts as a memory for the generated or incident photons and therefore the time of arrival of photons can be tuned. More recently, laser-cooling and trapping of alkali atoms down to sub-Kelvin temperatures has opened up several new possibilities in preparing, manipulating and storing such correlated photonic states. A single photon, scattered collectively by an ensemble of N laser-cooled atoms, projects the atomic ensemble into a collective superposition state. Such a super-atom or a magnon stores the scattering information. The stored information can be retrieved optically in the form of a highly directed (phase-matched) emission of a photon. Furthermore, the emission-rate of the photon gets enhanced by a factor of N, a phenomenon known as Dicke's super-radiance.

A specific quantum state of great interest is a heralded source of photon pairs. Because of its very nature, such a source is also analogous to on-demand source of entanglement. It can therefore serve as a quantum gate forming the most fundamental building block for protocols in quantum information and computing, together with major applications in quantum metrology and spectroscopy. Demonstration of a practical high brightness heralded two-photon source, which a precise control on the generation timing, therefore, remains one of the most important of open problems in the field of quantum optics. We work in this direction using a cascaded four-wave mixing as a viable route for generation of heralded photon-pairs or conditional entanglement. At the heart of the proposal is a laser-cooled ensemble of alkali atoms, coupled to two low finesse optically cavity. The ensemble of atoms, all initially prepared in a well-defined energy eigenstate is pumped with external fields to produce bright correlated photon pairs together with a timed photon, announcing the arrival of the pair.

We have already obtained correlated photon pairs using four wave mixing (FWM) processes in a laser-cooled ensemble of Rubidium atoms. The major advantage of this process is that emission of photons is highly directional. This is a result of phase-matching process which precisely cancels the momentum of photons. FWM process together with sensitive phase matching was established and we were able to obtain highly quantum photon source with a quantum-correlation factor(g2) upto 40.

References

1. Heralded single-magnon quantum memory for photon polarization state. H Tanji, S Ghosh, J Simon, B Bloom, V Vuletić. Physical review letters 103 (4), 043601 (2009) (pdf)

2. Single-photon bus connecting spin-wave quantum memories. J Simon, H Tanji, S Ghosh, V Vuletić. Nature Physics 3 (11), 765 (2007) (pdf)