Rydberg Polaritons Gain Coherence Via Velocity-Memory Scheme

Researchers are addressing a key limitation in the development of quantum technologies by devising a method to enhance the coherence of Rydberg polaritons, hybrid light-matter quasiparticles created using states designated |r₁⟩ and |r₂⟩ linked by a low-lying intermediate state |f⟩. A major obstacle to utilizing these polaritons, the team reports, was that atomic motion caused rapid dephasing; however, they have now demonstrated a scheme to significantly improve coherence by essentially allowing the atoms to “remember” their velocities. This enhancement is achieved by inducing transitions between the Rydberg states |r₁⟩ and |r₂⟩ with two laser fields, while maintaining a large detuning from state |f⟩. The work, by Xiao-Feng Shi of Hainan University and colleagues, allows Rydberg polaritons to function as building blocks for devices like single-photon transistors and switches.

This isn’t about physically halting atomic motion, but rather adjusting the internal state of each Rydberg atom to compensate for its external movement. According to the researchers, this approach can nearly completely eliminate motional dephasing, leaving Rydberg-state decay as the only fundamental channel of decoherence. They find that protocols like the 2πℕ protocol, the π-wait-π protocol, and the wait-π protocol, along with an appropriate choice of |f⟩, can lead to a phase-coherent Rydberg polariton upon its retrieval.

Rydberg polaritons are increasingly investigated as fundamental components for advanced quantum devices, offering a pathway toward building practical single-photon transistors and switches. While Rydberg polaritons can induce quantum nonlinear optical effects, researchers are not necessarily leveraging them as a crucial step for quantum technologies. A major hindrance in this study is that atomic motion generates a rapid dephasing and wipes out the quantum nature of Rydberg polaritons.

Researchers are increasingly focused on harnessing Rydberg polaritons, hybrid light-matter quasiparticles, as building blocks for advanced quantum devices. The fast dephasing originates from the motion of atoms with unpredictable velocities within the polariton ensemble. The creation operator of the prepared polariton, as described by the researchers, includes each atom’s initial location and velocity. These fields induce transitions between Rydberg states |r₁⟩ and |r₂⟩ via a low-lying intermediate state |f⟩, which is largely detuned. Specifically, the team explored protocols, including a 2πℕ protocol, a π-wait-π protocol, and a wait-π protocol, which can lead to a phase-coherent Rydberg polariton.

A significant challenge hindering the widespread adoption of Rydberg polaritons is that atomic motion causes a rapid dephasing and wipes out the quantum nature of Rydberg polaritons. The team’s approach centers on manipulating the internal states of Rydberg atoms during the storage of the polariton, allowing atoms to “remember” their velocities. This isn’t about physically halting atomic motion, but rather adjusting the internal state of each Rydberg atom to compensate for its external movement.

While seemingly counterintuitive, the very motion that creates these polaritons also disrupts their quantum state. Researchers at Hainan University and collaborating institutions have identified that the fast dephasing originates from unpredictable atomic velocities, leading to inhomogeneous broadening. The creation of Rydberg polaritons relies on a superposition of atomic states, where the velocity of each atom introduces a phase shift. This random variation is characterized by timescales related to the equation T₂^∗ = 1/( k v). Alternative methods included trapping atoms in lattices or using Bose-Einstein condensates, but this new approach focuses on letting the atoms “remember” their velocities, manipulating the internal states of Rydberg atoms during the storage of the polariton.

Researchers are actively pursuing methods to extend the coherence of Rydberg polaritons, essential for building quantum devices like single-photon transistors and switches. Currently, a major hindrance remains that atomic motion generates a rapid dephasing and wipes out the quantum nature of Rydberg polaritons. This approach centers on manipulating the internal states of Rydberg atoms during the storage of the polariton, allowing the atoms to “remember” their velocities. Simulations show that this theory can nearly completely eliminate the motional dephasing, ensuring that Rydberg-state decay is the only fundamental channel of decoherence.

While Rydberg polaritons hold promise for components like single-photon transistors and switches, their utility has been limited by the fast motional decoherence inherent in atomic motion. The fast dephasing of Rydberg polaritons originates from the motion of atoms with unpredictable velocities in real space. The team’s approach centers on manipulating the internal states of Rydberg atoms during the storage of the polariton, allowing the atoms to effectively “remember” their velocities.