Hot quantum memories from nanoseconds to hours

We study coherent optical excitations of atoms in a hot gases. We develop new techniques for quantum memories that optimize bandwidth, storage time, and fidelity. 

Future photonic quantum networks and computers will require coherent optical memories for single photons, known as quantum memories. Quantum memories are needed for synchronizing photonic sources and gates, and for long-distance quantum communication. In our study, we develop coherent memories that map the optical field onto the state gaseous atoms in a controlled and reversible manner.

Warm atomic gases are among the simplest quantum systems, offering real-life applications in deployable millimeter-size devices. They strongly couple to optical fields and exhibit superb coherence properties at or above room temperature. In the context of quantum memories, we explore alkali vapors and noble gasses, where the optical field maps onto the superposition between electronic orbitals, between electronic spins, or between nuclear spins. These distinct degrees of freedom feature different coupling mechanisms to light and to the environment, which determine the bandwidth, efficiency, noise, and lifetimes of the memory.

On one extreme, we study fast memories for nanosecond-long single photons. We store photons on the electronic orbitals of rubidium vapor via stimulated two-photon absorption. Employing purely-orbital transitions enables high bandwidth and low noise. Such a memory can enable the efficient synchronization of probabilistic single photon sources and two-photon logic gates. We develop tools to elongate the memory lifetime by counteracting motional broadening, by that recovering the linewidths and absorption cross-sections of stationary atoms. We further consider the implementation of these memories via tapered fibers and their integration with Rydberg-level excitations for quantum nonlinear optics.

On the other extreme, we study memory systems based on atomic spins with extremely long lifetimes. Most hot-vapor memories use dense gases, where the random collisions between the atoms dominate the coherence times of the electronic spins. As a result, in most experiments, the memory lifetime is limited to a few milliseconds only. We develop storage schemes that are insensitive to spin-exchange collisions and can reach storage lifetimes longer than 100 milliseconds.

Noble gases with nonzero nuclear spins (and zero electronic spins) exhibit hour-long coherence times at room temperature. We rely on spin-exchange collisions with alkali spins to form a strong, coherent, externally-controllable interface with these optically-inaccessible noble-gas spins. This interface can reach the strong-coupling regime and be used to realize quantum memories with hour-long lifetimes.

To learn about the effect of spatial diffusion on stored light in atomic vapors, see our page on coherent diffusion of polaritons.

Mapping of (left) the polarization of the photons onto (right) the electronic spin of cesium atoms