Room-temperature single-photon source with near-millisecond built-in memory

Abstract

Non-classical photon sources are a crucial resource for distributed quantum networks. Photons generated from matter systems with memory capability are particularly promising, as they can be integrated into a network where each source is used on-demand. Among all kinds of solid state and atomic quantum memories, room-temperature atomic vapours are especially attractive due to their robustness and potential scalability. To-date room-temperature photon sources have been limited either in their memory time or the purity of the photonic state. Here we demonstrate a single-photon source based on room-temperature memory. Following heralded loading of the memory, a single photon is retrieved from it after a variable storage time. The single-photon character of the retrieved field is validated by the strong suppression of the two-photon component with antibunching as low as [Formula: see text]. Non-classical correlations between the heralding and the retrieved photons are maintained for up to [Formula: see text], more than two orders of magnitude longer than previously demonstrated with other room-temperature systems. Correlations sufficient for violating Bell inequalities exist for up to τ_BI = (0.15 ± 0.03) ms.

Fig. 1. Experimental setup and excitation schemes.

a Write excitation scheme: π-polarized, far-detuned excitation light creates an atomic excitation via a Raman scattering process. Only relevant atomic levels shown. b Read excitation scheme: σ-polarized light used to retrieve stored excitation via Raman scattering and scattering desired deterministic single photon. Excess four-wave-mixing (FWM) noise is suppressed by choosing ${\mathrm{\Delta }}_{4^{\prime }}=924$ MHz . c Schematic of simplified experimental setup including paths for write and read scattered photons through polarization and spectral filtering.

Fig. 2. Experimental pulse sequence and temporal shape of detection events.

a Illustration of smoothened write, read and optical pumping pulses, variable delay between write and read, and optional repump for delayed readout. The solid areas represent integration windows used in the analysis. b Blue area—detected counts during heralding write pulses (31 μs, scaled 1/25). Solid curves—detected counts during read pulses (200 μs, delayed 10 to 710 μs) conditioned on the heralding write count (averaged over 7 μs bins). Dotted curves—the read noise level in the absence of write pulse (1 μs binning). Source data are provided as a Source Data file.

Fig. 3. Photon correlations and retrieval efficiency.

a 2nd-order cross-correlation function $g_{\mathrm{WR}}^{\left(2\right)}$, b retrieval efficiency η_R and c 2nd-order conditional auto-correlation function $g_{\mathrm{RR\mid W=1}}^{\left(2\right)}$ of the retrieved light field. $g_{\mathrm{RR\mid W=1}}^{\left(2\right)}$ = 1 (dotted line) is the classical limit and $g_{\mathrm{RR\mid W=1}}^{\left(2\right)}$ = 0.5 (dashed line) is the two-photon Fock state auto-correlation value. All functions are plotted against the mean number of detected write counts $⟨n_{\mathrm{W}}⟩$. Shown are measured data (circles) and the model (full lines). To improve the experimental uncertainty on $g_{\mathrm{RR\mid W=1}}^{\left(2\right)}$ we combine points for $⟨n_{\mathrm{W}}⟩<2\times 10^{-3}$ (green triangle). Error bars represent one standard deviation. Source data are provided as a Source Data file.

Fig. 4. Photon correlations for delayed readout.

Shown are $g_{\mathrm{WR}}^{\left(2\right)}$ (red) and the Cauchy-Schwarz parameter $\mathcal{R}$ (blue) versus various read pulse delays τ_D for an integrated read pulse duration of 40 μs together with the fit to $g_{\mathrm{WR}}^{\left(2\right)}$ (red line) and the resulting $\mathcal{R}$ (blue line). The black line marks the Bell-inequality limit $g_{\mathrm{WR}}^{\left(2\right)}$ ≥ 5.7 and the dashed line marks the typical non-classicallity signature $g_{\mathrm{WR}}^{\left(2\right)}$ > 2. The dash-dotted line is the formal non-classicallity criterion $\mathcal{R}$ > 1. Error bars represent one standard deviation. Source data are provided as a Source Data file.