Quantum Storage of Heralded Single Photons in a Praseodymium-Doped Crystal

We report on experiments demonstrating the reversible mapping of heralded single photons to long-lived collective optical atomic excitations stored in a ${\mathrm{Pr}}^{3+}:{\mathrm{Y}}_2{\mathrm{SiO}}_5$ crystal. A cavity-enhanced spontaneous down-conversion source is employed to produce widely nondegenerate narrow-band ($\approx 2\mathrm{MHz}$) photon pairs. The idler photons, whose frequency is compatible with telecommunication optical fibers, are used to herald the creation of the signal photons, compatible with the ${\mathrm{Pr}}^{3+}$ transition. The signal photons are stored and retrieved using the atomic frequency comb protocol. We demonstrate storage times up to $4.5\mu \text{s}$ while preserving nonclassical correlations between the heralding and the retrieved photon. This is more than 20 times longer than in previous realizations in solid state devices, and implemented in a system ideally suited for the extension to spin-wave storage.

Figure 1

(a) Experimental setup. AOM, acousto-optic modulator (2x when double pass); FC, filter cavity; ETA, etalon; SMF, single-mode fiber; MS, mechanical shutter; SPD, single photon detector; $\lambda /2$, half-wave plate. (b) Energy level scheme of ${\mathrm{Pr}}^{3+}$ in ${\mathrm{Y}}_2{\mathrm{SiO}}_5$ restricted to the fundamental levels of the ground ${}^3{\mathrm{H}}_4$ and the excited ${}^1{\mathrm{D}}_2$ manifolds. (c) Example of AFC with $\mathrm{\Delta }=500\mathrm{kHz}$.

Figure 2

(a) $G_{s,i}^{(2)}(t)$ histogram without (blue) and with (orange) AFC. The preprogrammed storage time is $2\mu \text{s}$ and the power of the 426.2 nm pump is 2 mW. The time-bin size is 5 ns. The dashed lines and shaded area define the detection windows for the coincidences ($\mathrm{\Delta }{t}_d=400\mathrm{ns}$) and noise, respectively, used to compute $g_{s,i}^{(2)}$. (b) The $g_{s,i}^{(2)}$ values as a function of the pump power for the AFC echo (empty squares) are compared to those for the input photons (empty circles). The heralding efficiencies ${\eta }_H$ are also reported (filled triangles).The integration times range between 30 and 60 minutes. The error bars are evaluated from the raw number of counts assuming Poissonian statistics. The dotted line corresponds to the classical limit $g_{s,i}^{(2)}=2$ for two-mode squeezed states.

Figure 3

(a) AFC echoes at different predetermined storage times observed in coincidence histograms measured at 2 mW pump power (integration time is 40 minutes and the time-bin size is 5 ns). The curves are vertically shifted for clarity. (b) and (c), AFC efficiency and $g_{s,i}^{(2)}$ (calculated in a 400 ns window) of the AFC echo as a function of the storage time. Diamonds: filter cavity in the heralding photon path; squares: filter cavity removed from the heralding photon path. The error bars are evaluated from the raw counts assuming Poissonian statistics. The dotted line corresponds to the classical limit $g_{s,i}^{(2)}=2$ for two-mode squeezed states.