Controlled Rephasing of Single Collective Spin Excitations in a Cold Atomic Quantum Memory

We demonstrate active control of inhomogeneous dephasing and rephasing for single collective atomic spin excitations (spin waves) created by spontaneous Raman scattering in a quantum memory based on cold ${}^{87}\mathrm{Rb}$ atoms. The control is provided by a reversible external magnetic field gradient inducing an inhomogeneous broadening of the atomic hyperfine levels. We demonstrate experimentally that active rephasing preserves the single photon nature of the retrieved photons. Finally, we show that the control of the inhomogeneous dephasing enables the creation of time-separated spin waves in a single ensemble followed by a selective read-out in time. This is an important step towards the implementation of a functional temporally multiplexed quantum repeater node.

Figure 1

(a) Schematic of the experimental setup. HWP, half-wave plate; QWP, quarter-wave plate; PBS, polarizing beam splitter; SPD, single photon detector. (b) Energy level structure. (c) Experimental sequence timeline for the active rephasing experiment. W, write pulse; C, cleaning stage; R, read pulse; $T_s$, storage time; $T_i$, interrogation time.

Figure 2

Retrieval efficiency vs read-out time for $p_w=1\%$. Gray open circles: Standard DLCZ experiment. The oscillations are due to imperfect optical pumping. Gray line: Fit of the experimental data. Blue dots: Active rephasing experiment. Green line: Fit of the experimental data. The insets display magnified regions around the initial fast dephasing (left) and the rephasing peak (right).

Figure 3

Antibunching parameter as a function of write photon detection probability. Open circles: Experimental data. The dashed line represents the limit for classical states ($\alpha \ge 1$).

Figure 4

Creation and selective read-out of a spin wave in two temporal modes. (a) Single rephasing case for $p_w=1\%$: first write pulse in green, second write pulse in blue. (b) Both pulses sent in black. Here, write photons from both time slots can initiate a coincidence detection. The histograms display the relative weight of each peak at the circled points. (c) Selectivity as a function of $p_w$ for each rephasing.