Entangled Photons

We report the observation of symmetry protected two-photon coherence time of biphotons generated from backward spontaneous four-wave mixing in laser-cooled 87 Rb atoms. When biphotons are nondegenerate, nonsymmetric photonic absorption loss results in exponential decay of the temporal waveform of the two-photon joint probability amplitude, leading to shortened coherence time. In contrast, in the case of degenerate biphotons, when both paired photons propagate with the same group velocity and absorption coefficient, the two-photon coherence time, protected by space-time symmetry, remains unaffected by medium absorptive losses. Our experimental results validate these theoretical predictions. This outcome highlights the pivotal role of symmetry in manipulating and controlling photonic quantum states.

We produce energy-time entangled narrow-band biphotons from spontaneous four-wave mixing in a cloud of 85Rb atoms prepared in a dark-line two-dimensional magnetooptical trap. With the continuous-wave pump and coupling laser fields, the spontaneously generated phase-matched Stokes and anti-Stokes photon pairs are frequency-time entangled because of the energy originating from time translation symmetry. We first determine the joint temporal uncertainty by measuring the two-photon temporal correlation using two single-photon counters and confirm its nonclassical property and quantum nature by correlation function measurement. The two-photon joint spectrum is then determined by an ultra-narrowband transmission optical cavity, showing the Stokes and anti-Stokes photon frequencies are anticorrelated and hence indicating the energy-time entanglement. Moreover, the joint frequency-time uncertainty product also satisfies the steering inequality, which is sufficient for raising the EPR paradox. 

Since the quantum storage efficiency is limited by the atom-light interaction strength, i.e. the optical depth (OD). We design and build up a new apparatus by loading our two-dimension cold atomic ensemble into an optical cavity to enhance the photon-atom interaction. Photons in cavity mode can be bounced back and forth by cavity mirrors, so that they pass through the atomic ensemble many times before leaking out. In this apparatus, we get the cavity-enhanced atomic optical depth up to 7600 by measuring vacuum Rabi splitting, with peak splitting up to 627 MHz. The cavity-enhanced OD is 190 times higher than that without enhancement. This apparatus is an ideal platform for future photon-atom interaction studies, such as high-efficiency quantum memory or high-fidelity photon gate generation.