Deterministic and Storable Single-Photon Source Based on a Quantum Memory

A single-photon source is realized with a cold atomic ensemble (${}^{87}\mathrm{Rb}$ atoms). A single excitation, written in an atomic quantum memory by Raman scattering of a laser pulse, is retrieved deterministically as a single photon at a predetermined time. It is shown that the production rate of single photons can be enhanced considerably by a feedback circuit while the single-photon quality is conserved. Such a single-photon source is well suited for future large-scale realization of quantum communication and linear optical quantum computation.

Figure 1

(a) Illustration of the experimental setup and (b) the time sequence with the feedback circuit for the write and read process. The atomic ensemble is first prepared in the initial state $|a⟩$ by applying a pump beam resonant with the transition $|b⟩$ to $|e^{\prime }⟩$. A write pulse with the Rabi frequency ${\Omega }_W$ is applied to generate the spin excitation and an accompanying photon of the mode ${\widehat{a}}_{\mathrm{AS}}$. Waiting for a duration $\Delta t$, a read pulse is applied with orthogonal polarization and spatially overlap with the write beam in PBS1. The photons, whose polarization is orthogonal to that of the write beam, in the mode ${\widehat{a}}_{\mathrm{AS}}$ are spatially extracted from the write beam by PBS2 and detected by detector D1. Similarly, the field ${\widehat{a}}_{\mathrm{S}}$ is spatially extracted from the read beam and detected by detector D2 (or D3). Here, FC1 and FC2 are two filter cells, BS is a $50/50$ beam splitter, and AOM1 and AOM2 are two acousto-optic modulators.

Figure 2

Intensity correlation function $g_{\mathrm{AS},\mathrm{S}}^{(2)}$ along the excitation probability $p_{\mathrm{AS}}$ with $\delta t=500\mathrm{ns}$ (a) and along the time delay $\delta t$ between read and write pulses with $p_{\mathrm{AS}}=3\times {10}^{-3}$ (b). The black symbols are obtained from the current experiment, and the curves correspond to a least-square fit procedure according to Eqs. (2, 3). The observed lifetime is ${\tau }_c=12.5\pm 2.6\mu \mathrm{s}$.

Figure 3

The anticorrelation parameter as a function of (a) $p_{\mathrm{AS}}$ and (b) $\Delta t$. In panel (a), the data in black correspond to the experiment without feedback circuit, in which each write sequence is followed by one read pulse. The data in red (gray) correspond to the experiment with feedback circuit, in which 12 successive write sequences are followed by one read pulse. The red (gray) curve is the theoretical evaluation taking into account the fitted background of the black symbols. In panel (b), 12 write sequences were applied in each trial while measuring.