Quantum Memory with Optically Trapped Atoms

We report the experimental demonstration of quantum memory for collective atomic states in a far-detuned optical dipole trap. Generation of the collective atomic state is heralded by the detection of a Raman scattered photon and accompanied by storage in the ensemble of atoms. The optical dipole trap provides confinement for the atoms during the quantum storage while retaining the atomic coherence. We probe the quantum storage by cross correlation of the photon pair arising from the Raman scattering and the retrieval of the atomic state stored in the memory. Nonclassical correlations are observed for storage times up to $60\mu \mathrm{s}$.

Figure 1

A schematic of our apparatus. (a) The atomic ensemble is confined in an optical trap formed by a red-detuned, focused laser beam. The optical-trapping beam is overlapped with the write and read beams (counterpropagating to each other) on the dichroic mirrors (DM). Stokes and anti-Stokes photons are detected by single-photon detectors D1 and D2, respectively, at an angle of $\sim 2°$ with respect to the optical-trapping beam. (b) An absorption image of the optically trapped atoms along the radial direction after a time-of-flight of 0.3 ms.

Figure 2

Time sequence and relevant atomic transitions of the experiment. (a) After the MOT is switched off, atoms are transferred into an optical trap. Subsequently, 10 000 alternating write and read pulses, with pulse lengths of 100 ns and 500 ns, respectively, and a controllable time delay of $t$, illuminate the atomic ensemble. Before the first write pulse, a read pulse (not shown) is applied to ensure that no atoms are in the metastable state $|s⟩$. (b) and (c) illustrate the relevant atomic levels involved in the write and read processes, respectively, where $|g⟩=|5S_{1/2},F=1⟩$, $|s⟩=|5S_{1/2},F=2⟩$, and $|e⟩=|5P_{1/2},F=2⟩$. Zeeman splitting is not shown in this figure.

Figure 3

Normalized cross correlation of the Stokes and anti-Stokes fields as a function of the time delays between the write and read pulses. Nonclassical correlation is observed for storage times up to $60\mu \mathrm{s}$. The curve is a Gaussian fit with two time constants, ${\tau }_{\mathrm{nc}}=16(3)\mu \mathrm{s}$ and ${\tau }_c=45(5)\mu \mathrm{s}$, and its asymptotic value of 1 as time delay $t\gg {\tau }_c$ corresponds to the probabilistic coincidence events, $p_{S,\mathrm{AS}}=p_S{p}_{\mathrm{AS}}$. The dotted line is a decay curve with only one time constant ${\tau }_c$, assuming the absence of atoms in the nonclock state. The horizontal line at $g_{S,\mathrm{AS}}=2$ indicates the onset of quantum correlation. The error bars indicate the statistical errors.