Figure 1

Illustration of the relevant energy levels of the atoms and arrangement of laser beams (a) and the experimental setup (b). (a) ${}^{87}\mathrm{Rb}$ atoms are prepared in the initial state $|a⟩$. A write pulse ${\Omega }_W$ with the detuning of $\Delta =10\mathrm{MHz}$ and a beam diameter about $400\mu \mathrm{m}$ is applied to generate the spin excitation and an accompanying photon of the anti-Stokes field ${\widehat{a}}_{\mathrm{AS}}$ with a beam diameter about $100\mu \mathrm{m}$. The mode ${\widehat{a}}_{\mathrm{AS}}$, tilted 3° from the direction of the write beam, is coupled in a single-mode fiber (SMF) and guided to a single-photon detector. Waiting for a duration $\delta t_R$, a read pulse is applied with orthogonal polarization and spatially mode-matched with the write beam from the opposite direction. The spin excitation in the atomic ensemble will be retrieved into a single photon of the Stokes field ${\widehat{a}}_S$, which propagates to the opposite direction of the field ${\widehat{a}}_{\mathrm{AS}}$ and is also coupled in SMF. (b) Alice and Bob each keeps a single-photon source at two remote locations. As elucidated in Ref. 14, Alice applies write pulses continuously until an anti-Stokes photon is registered by detector D1. Then she stops the write pulse, holds the spin excitations, and meanwhile sends a synchronization signal to Bob and waits for his response. (This is realized by the feedback circuit and the acousto-optic modulators, AOM.) In parallel, Bob prepares a single excitation in the same way as Alice. After they both agree that each has a spin excitation, each of them will apply a read pulse simultaneously to retrieve the spin excitation into a light field ${\widehat{a}}_S$. The two Stokes photons propagate to the place for entanglement generation and Bell measurement. They overlap at a $50:50$ beam splitter (BS) and then will be analyzed by latter half wave plates ($\lambda /2$), polarized beam splitters (PBS) and single-photon detectors Da, Db, Dc, and Dd.Reuse & Permissions

Figure 2

Hong-Ou-Mandel dips in time domain (left panel) and frequency domain (right panel). The circle in the right panel was obtained by setting the polarization of the two photons perpendicular to each other and zero detuning between two read lasers. The Gaussian curves that roughly connect the data points are only shown to guide the eye. The dashed line shows the plateau of the dip. Error bars represent statistical errors, which are $\pm$ 1 standard deviation.Reuse & Permissions