The talk will describe experiments and theory showing the generation of counterpropagating paired photons with waveforms that are controllable by using EIT to vary the optical group velocity. Typical waveform durations are about 40 ns.
Controls of waveforms (pulse durations) of single photons are important tasks for effectively interconnecting disparate atomic memories in hybrid quantum networks. So far, the waveform control of a single photon that is entangled with an atomic memory remains unexplored. Here, we demonstrated control of waveform length of the photon that is entangled with an atomic spin-wave memory by varying light-atom interaction time in cold atoms. The Bell parameter S as a function of the duration of photon pulse is measured, which shows that violations of Bell inequality can be achieved for the photon pulse in the duration range from 40 ns to 50 µs, where, S = 2.64 ± 0.02 and S = 2.26 ± 0.05 for the 40-ns and 50-µs durations, respectively. The measured results show that S parameter decreases with the increase in the pulse duration. We confirm that the increase in photon noise probability per pulse with the pulse-duration is responsible for the S decrease.
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.