Quantum Interference of Electromagnetic Fields from Remote Quantum Memories

We observe quantum, Hong-Ou-Mandel, interference of fields produced by two remote atomic memories. High-visibility interference is obtained by utilizing the finite atomic memory time in four-photon delayed coincidence measurements. Interference of fields from remote atomic memories is a crucial element in protocols for scalable entanglement distribution.

Figure 1

Schematic showing Raman scattering of write pulses ($W$) at sites $A$ and $B$ with signal fields collected by polarizers $P_1$ and $P_2$ and optical fiber beam splitter and directed towards detectors D1, D2. A half-wave plate ($\lambda /2$) may be inserted at site $B$ to rotate light polarization. Raman scattering of delayed read pulses produces idler fields detected at D3, D4. The inset shows the atomic level structure and the write- and read-induced Raman processes.

Figure 2

Ratio of measured twofold coincidence rates for the $\perp$ and $\parallel$ configurations). The parameter $p_1\equiv (N_1+N_2)/N_T$ (averaged over the $\perp$ and $\parallel$ cases). Here $N_1(N_2)$ is the number of photoelectric detections in detector $D1(D2)$, $N_T$ is the number of experimental trials. Theoretically it can be expressed as $p_1={ϵ}_A{s}_A^2+{ϵ}_B{s}_B^2$, where ${ϵ}_A({ϵ}_B)\approx 0.05–0.07$ is the overall probability to detect a signal photon from site $A$ (site $B$) by either D1 or D2. The data acquisition time $5\min \le T_D\le 2\mathrm{h}$ over the range of $p_1$ values shown and $N_T\equiv f{T}_D$. Scatter beyond the estimated Poissonian level of uncertainty is consistent with systematic drifts in experimental conditions, in particular, the single count rates from each ensemble. The solid line is our theoretical prediction based on Eq. (6), for $R=T=1/2$ and ${ϵ}_A{s}_A^2={ϵ}_B{s}_B^2$.

Figure 3

Integrated fourfold coincidence rates ${\mathcal{R}}_{\parallel }^{(4)}/{\mathcal{W}}_{\perp }^{(4)}$ and ${\mathcal{R}}_{\perp }^{(4)}/{\mathcal{W}}_{\perp }^{(4)}$ as a function of $p_1$. Experiment, dots, theory, solid line, assuming identical signal mode wave packets from each ensemble. Uncertainties are based on the statistics of the photon counting events.