Simultaneous generation of two spin-wave–photon entangled states in an atomic ensemble

Spontaneous Raman scattering (SRS) in atomic ensembles provides a promising method to generate spin-wave–photon entangled states. In the past experiments, a spin-wave–photon entangled state was produced via SRS from an atomic ensemble. Here, we report a scheme of simultaneously generating two spin-wave–photon (atom-photon) entangled states in a cold Rb ensemble via SRS. Based on joint Bell-state measurements on the two photons coming from the two atom-photon entangled sources, respectively, we projected the two stored spin waves into a Bell state and then mapped the quantum memory into a polarization-entangled photon pair. Such a polarization-entangled photon pair can be released on demand and thus the presented scheme has potential application in the preparation of large-size photonic entangled states.

Figure 1

Overview of the experiment. (a) Experimental setup of two atom-photon entangled states. The magnetic field $B=200\mathrm{mG}$ is applied on the cold atoms along the $z$ axis. The $z$-polarized write beam goes through the atoms along the $x$ direction, while the $z$-polarized read beam counterpropagates with respect to the write beam. (b, c) Relevant atomic levels without and with considering Zeeman sublevels. (d) The time sequence of an experimental cycle. (e) Experimental setup for generating either polarization-entangled photon pairs or three-photon GHZ entanglement.

Figure 2

Real and imaginary parts of the reconstructed density matrices of the polarization-entangled two-photon states for three values of storage time τ: (a) 30, (b) 230, and 430 ns.