Storage and Control of Optical Photons Using Rydberg Polaritons

We use a microwave field to control the quantum state of optical photons stored in a cold atomic cloud. The photons are stored in highly excited collective states (Rydberg polaritons) enabling both fast qubit rotations and control of photon-photon interactions. Through the collective read-out of these pseudospin rotations it is shown that the microwave field modifies the long-range interactions between polaritons. This technique provides a powerful interface between the microwave and optical domains, with applications in quantum simulations of spin liquids, quantum metrology and quantum networks.

Figure 1

Schematic of the experiment. (a) The atomic level scheme used to interface optical photons to the microwave field. (b) Optical photons are stored as Rydberg polaritons in a cold atomic ensemble. Subsequently, the internal states of the polaritons and their interactions are controlled using a microwave field. Finally, the modified optical field is read out and detected using time-resolved single photon counters arranged in a Hanbury Brown-Twiss configuration.

Figure 2

Photon storage and retrieval: (a) The photon storage process begins at $t=0$ when the control field (blue) is turned off. The signal pulse (red), which has a total duration (not shown) of $1.1\mu \mathrm{s}$, turns off approximately 200 ns later. After a storage time of roughly 900 ns the control field is turned back on to read out the polariton field. During the storage interval a microwave pulse can be used to couple the polariton to a neighboring Rydberg state. The retrieved signal (closed circles) appears as a peak with a FWHM of $120\pm 20\mathrm{ns}$. The background signal without atoms (open circles) is shown for reference. A black band highlights the time window taken as the retrieved signal. The relative heights of pulses are not to scale. (b) The normalized second-order correlation function $g^{(2)}$ of the retrieved signal, binned over the entire retrieved pulse, as a function of the time delay $\tau$ between the two detectors. The suppression at $\tau =0$ is a signature of dipole blockade during the polariton write process. Note that no microwave coupling has been applied in this case.

Figure 3

Controlling the interaction between Rydberg polaritons. (a) The retrieved signal, normalized to the case where no microwave coupling is applied, is plotted as a function of the microwave Rabi frequency, ${\Omega }_{\mu }$. The microwave pulse duration is fixed at 300 ns. The dynamics depend on the ratio between the Rabi coupling and the dipole-dipole interaction ${\Omega }_{\mu }/V_{\mathrm{dd}}$—the condition ${\Omega }_{\mu }=V_{\mathrm{dd}}$ is indicated by the dashed line. For ${\Omega }_{\mu }<V_{\mathrm{dd}}$ (left-hand side), resonant energy exchange between polaritons dominates over Rabi oscillations. For ${\Omega }_{\mu }>V_{\mathrm{dd}}$, Rabi oscillations dominate and the exchange process is suppressed as the strong driving lifts the degeneracy between the dipole–dipole coupled states. The solid line is a phenomenological fit using the characteristic form for $\mathcal{N}$-particle Rabi oscillations coupled to a single optical read out mode, $P=[{cos}^2({\Omega }_{\mu }t/2){]}^{\mathcal{N}}$. This function is combined with a tanh envelope, and an exponential decay at low microwave Rabi frequencies. From the fit we obtain $\mathcal{N}=2.70\pm 0.16$. Inset: Spin model of the dynamics. The dipole-dipole interaction (circles between atoms) favors excitation exchange between out-of-phase atomic spins (straight arrows) whereas strong microwave driving (circles around atoms) favors in-phase oscillations and suppresses the exchange process. (b) Higher resolution data of Rabi oscillations in the strong driving regime. The line is a similar fit to Fig. 3a, with $\mathcal{N}=3.0\pm 0.2$. The microwave pulse duration is 150 ns.