Coherence enhancement of Rydberg polaritons

Quantum nonlinear optics using Rydberg polaritons is a promising platform for single-photon transistors, switches, and quantum networking. However, a major obstacle is the rapid dephasing caused by the thermal motion of atoms, which limits the coherence time of the stored Rydberg polariton to the microsecond scale. This paper investigates how to suppress this motional dephasing to extend the lifetime of these quantum states.

Instead of attempting to physically trap atoms or reverse their motion, the authors propose a method to adjust the internal state of each Rydberg atom based on its external velocity. By employing a two-photon Raman transition between two nearby Rydberg states (|r1⟩ and |r2⟩) via a detuned intermediate state, the researchers can imprint a velocity-dependent phase on the Rydberg polariton. They introduce three specific protocols—a 2πN protocol, a π-wait-π protocol, and a wait-π protocol—to synchronize the phase of the Rydberg excitation with the atoms' motion during the storage time.

The study demonstrates that by choosing an appropriate intermediate state and pulse sequence, the motional dephasing can be nearly completely eliminated. Numerical simulations indicate that with a moderate, experimentally feasible Rabi frequency of 2 MHz, the coherence time can be extended to hundreds of microseconds. The protocols ensure that the Rydberg polariton retains its ability to block the excitation of nearby atoms, which is essential for its function in quantum nonlinear optical applications. The theory is compatible with various neutral-atom platforms, including cesium and rubidium.

By overcoming the fundamental limitation of motional dephasing, this work provides a practical pathway for implementing long-lived Rydberg-mediated quantum devices. Extending the coherence time of Rydberg polaritons is a critical step toward realizing scalable all-optical quantum information processing and robust quantum networks, where the ability to store and retrieve single photons with high fidelity is paramount.