Cold-Atom Temporally Multiplexed Quantum Memory with Cavity-Enhanced Noise Suppression

Future quantum repeater architectures, capable of efficiently distributing information encoded in quantum states of light over large distances, will benefit from multiplexed photonic quantum memories. In this work we demonstrate a temporally multiplexed quantum repeater node in a laser-cooled cloud of ${}^{87}\mathrm{Rb}$ atoms. We employ the Duan-Lukin-Cirac-Zoller protocol where pairs of photons and single collective spin excitations (so-called spin waves) are created in several temporal modes using a train of write pulses. To make the spin waves created in different temporal modes distinguishable and enable selective readout, we control the dephasing and rephasing of the spin waves by a magnetic field gradient, which induces a controlled reversible inhomogeneous broadening of the involved atomic hyperfine levels. We demonstrate that by embedding the atomic ensemble inside a low finesse optical cavity, the additional noise generated in multimode operation is strongly suppressed. By employing feed forward readout, we demonstrate distinguishable retrieval of up to 10 temporal modes. For each mode, we prove nonclassical correlations between the first and second photon. Furthermore, an enhancement in rates of correlated photon pairs is observed as we increase the number of temporal modes stored in the memory. The reported capability is a key element of a quantum repeater architecture based on multiplexed quantum memories.

Figure 1

(a) Schematic overview of the experimental setup. $W$, write pulse; $R$, read pulse; $w$, write photon; $r$, read photon; $L$, cavity locking laser beam; PID, proportional-integral-derivative controller; FP cav, Fabry-Perot filtering optical cavity; $l$, atomic cloud length; QWP, quarter-wave plate; PBS, polarization beam splitter, AOM, acousto-optic modulator; Bg, magnetic field gradient. The polarizations indicated are in the atomic frame (cf. Ref. [40]). (b) Energy levels relevant for the photon generation process. The green color gradient bars illustrate the position dependent Zeeman level energy shift along the $z$ axis. (c) Time diagram of events occurring in the temporally multiplexed operation of the system. In this example, 4 write pulse modes are sent to the atomic cloud. A magnetic field gradient of amplitude $A_{\text{Bg}}$ is present which is reversed to $-A_{\text{Bg}}$ after the last write pulse mode. If a write photon is detected in the third mode, a feed forward instruction sends the read pulse at the time corresponding to the third-mode spin wave rephasing time.

Figure 2

(a) Write photon emission probability as a function of the cavity resonance frequency. Frequency zero corresponds to the center of the $|e⟩-|s⟩$ transition. The green solid line represents the emission without cavity enhancement. (b) Read photon detection probability from dephased spin waves and (c) write pulse power as a function of the spin wave excitation probability. The spin wave is read out after $1.2\mu \mathrm{s}$ of storage time. This time is much longer than the spin wave dephasing time set by the $B$ field gradient [35]. Blue (green) data are taken with (without) cavity enhancement.

Figure 3

(a) Probability to collect a heralded read photon into the read fiber as a function of the readout time for 6 different temporal modes. Time zero corresponds to time of writing of the last write mode (6). With cavity enhancement, intrinsic retrieval efficiency for the first mode is $p_{r|w}^{\mathrm{int}}\approx 26\%$. Blue (green) data are taken with (without) cavity enhancement. For both, single-mode excitation probability is $p_{1m}\approx 0.04$. Solid lines are a Gaussian fit of each retrieval peak. (b) Individual cross-correlation function between the different 6 write and read modes with cavity. The two bars in solid black lines at positions (1,1) and (1,6) represent the average for the diagonal and the off-diagonal values, respectively.

Figure 4

(a) Write and total write-read detection probability as a function of the number of temporal modes with cavity. (b) Averaged correlation function between write and read photons as a function of the number of modes. Average is computed based on the sum of coincidence and noise counts from all modes. The error bar represents one standard deviation, again based on the sum of counts in all modes. Blue (green) data are taken with (without) cavity enhancement. Gray dashed line shows the gain $(g_{w,r}^{(2),c}-1)/(g_{w,r}^{(2)}-1)$ in cross-correlation enabled by the cavity, as a function of the number of modes. Here, $g_{w,r}^{(2),c}$ ($g_{w,r}^{(2)}$) is the cross-correlation value with (without) cavity. Inset shows $g_{w,r}^{(2)}$ of each mode for the ten-mode data point. Single-mode excitation probability is $p_{1m}\approx 0.045$ for all measurements.