Spatially Modulated Spontaneous Four-Wave Mixing: Parallel-to-Series Quantum Conversion
Shengwang Du
Biphotons are entangled photon pairs that cannot be described as a product state of two individual photons. In this talk we summarize our recent progress in generating narrowband biphotons from spontaneous four-wave mixing (SFWM) in cold atoms with electromagnetically induced transparency (EIT). Here we are particularly interested in the group delay regime, where the biphoton time-frequency entanglement and joint wave function can be manipulated by controlling the group-delay-induced phase matching condition of the paired photons. The biphoton coherence time, thus its bandwidth of the joint spectrum, is determined by the relative group delay between the paired photons. The longest biphoton coherence time on record is more than 3 µs. We find that for a fixed optical depth, the twophoton waveform is independent of the atomic density distribution. However, the spatial profiles of the two driving laser beams have significant effects on the biphoton temporal wavefunction. We demonstrate a technique of shaping the biphoton temporal wavefunction by directly mapping the spatial profile of the pump laser beam onto the biphoton temporal waveform. This spatial-to-temporal mapping, or series-toparallel conversion, allows for coding the spatial information of the pump spatial profile to the sequential temporal modes of the biphtoon quantum state. Using a computer-controlled spatial light modulator, we produced biphotons with temporal waveforms of Gaussian, exponential, and double-peak shapes. As compared to temporally modulating the driving laser fields that reduces the frequency entanglement, this spatial-to-temporal biphoton shaping does not scarify the time-frequency entanglement and can be used to produce heralded narrowband single photons with controllable waveforms.
•Jan 1, 2016•0 citations