Figure 1

(a) Schematic view of the quad-mode holographic quantum memory. The control field is shined into the atomic cloud horizontally, and the signal modes are incident from different directions in the same plane, with angles $({\theta }_1,{\theta }_2,{\theta }_3,{\theta }_4)=(-1°,-0.6°,0.6°,1°)$ relative to the control field. (b) Typical $\Lambda$-type energy levels, with two ground states $|g⟩$ and $|s⟩$ and an excited state $|e⟩$. The $|e⟩\mathrm{\text{−}}|g⟩$ and the $|e⟩\mathrm{\text{−}}|s⟩$ transition are coupled to the signal and control fields, respectively. (c) Illustration of the wave vectors of the spin waves. The input signal field with a wave vector of ${\mathbf{k}}_{s,i}$ ($i=1$, 2, 3, 4) is mapped to a spin wave with the wave vector ${\mathbf{q}}_i={\mathbf{k}}_{s,i}-{\mathbf{k}}_c$, with ${\mathbf{k}}_c$ the wave vector of the control field.Reuse & Permissions

Figure 2

Illustration of the experimental setup. The two polarization-entangled photons produced by a narrowband SPDC source are directed to the memory lab through 20 m fibers. Different polarization components are separated by the polarization beam splitter (PBS) 1, and are coupled to the quad-mode holographic quantum memory. All the light fields are turned into right-hand circular polarization (${\sigma }^{+}$) by wave plates. The entangled photons are stored into the four quantum registers by adiabatically switching off the control light. After a controllable delay, the photons are retrieved, transferred back to their original polarization states by wave plates, and combined on ${\mathrm{PBS}}_2$. Then the retrieved entangled photons are guided into filters, containing a Fabry-Perot cavity and a hot atomic cell to filter out the leakages from the control light, and detected by single-photon detectors. The path length difference between ${\mathrm{PBS}}_1$ and ${\mathrm{PBS}}_2$ is actively stabilized by an additional phase beam (dashed line). Inset: The experimental laser lights and atomic levels.Reuse & Permissions

Figure 3

Average visibility (red, left axis) and overall storage efficiency (blue, right axis) of the retrieved biphoton state versus storage time. An exponential fitting (blue [dark gray] solid line) of the storage efficiency yields a lifetime of $\tau =2.8\pm 0.2\mu \mathrm{s}$. The retrieved biphoton state is measured under $|H/V⟩$, $|\pm ⟩$, and $|R/L⟩$ bases with $|\pm ⟩=(|H⟩\pm |V⟩)/\sqrt{2}$, and $|R/L⟩=(|H⟩\pm i|V⟩)/\sqrt{2}$. The average visibility is fitted using $V=1/(a+b{e}^{2t/\tau })$ (red [medium gray] solid line) with $a$ and $b$ the fitting parameters. The result shows that within about $1.6\mu \mathrm{s}$ the visibility is above the threshold of 0.71 to violate the CHSH-Bell inequality. Error bars represent $\pm$ standard deviation.Reuse & Permissions

Figure 4

Density matrix of the input state [(a), (b)] and of the output state after $1\mu \mathrm{s}$ storage [(c), (d)] obtained from quantum state tomography. (a) and (c) are for the real parts, and (b) and (d) are for the imaginary parts.Reuse & Permissions