Entanglement and the Jaynes-Cummings model with Rydberg-dressed atoms

Phu Nguyen

Grant W. Biedermann

We demonstrate a Rydberg-dressed ground-state blockade that provides a strong tunable interaction energy (~1 MHz), nearly 4 orders of magnitude larger than that in ultra cold collisions, between spins of individually trapped cesium atoms.With this interaction we directly produce Bell-state entanglement between two atoms with a fidelity ≥ 81(2)%, excluding atom loss events, and ≥ 60(3)% when loss is included [1].Our method is based on a Rydberg-dressed spin-flip blockade which allows single-step, direct entanglement of long-lived ground (clock) states.For the Bell-state entanglement generation, the probability remains entirely in the Rydbergdressed ground state, freeing the approach from the experimental burden of coherently exciting optical transitions to and from the Rydberg states.In principle Rydberg-dressed atoms are more insensitive to various decoherence mechanisms and lead to a better quantum control fidelity [2].In addition, we observed Jaynes-Cummings (JC) ladder and its nonlinearity of a Rydbergdressed atomic system.This nonlinearity induced by the Rydberg-dressed spin-flip blockade and the symmetric coupling between a Rydberg atom and atomic ensembles in the ground state, and this differentiates single and two microwave photon excitations from Rydberg-dressed state atoms.

G. F. Zhang, X. C. Xie

Jongmin Lee, Michael J. Martin, Yuan‐Yu Jau +3 more

The Jaynes-Cummings model, a widely employed theoretical framework in cavity quantum electrodynamics, is experimentally tested on a platform involving Rydberg-blockaded atomic ensembles. The work opens the way to a richer exploration of protocols for quantum control or, more broadly, quantum computing.

Samina Masood, Allen H. Miller

Tyler Keating

In any physical platform, two ingredients are essential for quantum information processing: single-qubit control, and entangling interactions between qubits. Neutral atoms can be individually controlled with high fidelity and are resilient to environmental noise, making them attractive candidates for implementing quantum information protocols. However, achieving strong interactions remains a major obstacle. One way to increase the interaction strength between neutral atoms is to excite them into high-lying Rydberg states, which exhibit large electric dipole moments (and by extension, strong electric dipole-dipole interactions). By slowly ramping up the Rydberg level coupling in a system, one can "dress'' the atomic ground states with some Rydberg character; this maps the Rydberg dipole interaction to an effective interaction between ground states. Such Rydberg-dressed interaction is the focus of this dissertation. After describing the physics of the Rydberg-dressed interaction, we propose three protocols that demonstrate its versatility and provide a framework for considering some of the details of realistic implementation. In all three cases, Rydberg dressing --- along with some form of single-atom control --- is used to generate highly entangled states of interest. Our first proposal relates to the adiabatic model of quantum computing, in which solutions to problems are encoded in the ground states of carefully engineered Hamiltonians. The Rydberg-dressed interaction can provide nonlinear Hamiltonian terms, allowing us to encode NP-hard and other interesting problems. We model this protocol in the presence of decoherence, and find that computational fidelities of ~0.98 for four atoms should be possible with currently realistic experimental parameters. Our second proposal is also related to quantum computing, this time in the circuit model. The Rydberg-dressed interaction can be used to generate a controlled-NOT logic gate which, when interwoven with single-qubit gates, can perform universal quantum computation. Experimentally, noise due to atomic thermal motion has been a primary limitation on the fidelities of these gates. We show that a Doppler-free setup, with counterpropagating lasers, effectively suppresses this type of noise, allowing simulated fidelities of up to ~0.998 per gate. Such strong suppression is only possible because the Doppler-free configuration can harness the natural robustness of adiabatic dressing; other gate schemes using, e.g., resonant pulses, do not exhibit the same degree of improvement. Finally, we consider exploiting the many-body character of the Rydberg-dressed interaction to generate collective entanglement in mesoscopic ensembles of neutral atoms. An atomic ensemble uniformly illuminated by a single Rydberg-exciting laser is isomorphic to the well-known Jaynes-Cummings model. In addition to adapting generic Jaynes-Cummings entanglement protocols developed in other platforms, one can apply microwaves to drive entanglement in a way that is unique to the atomic platform. We prove that by allowing the microwave phase to vary in time, one can generate arbitrary symmetric states of the ensemble. While this method compares favorably with other entanglement protocols in many ways, the required frequency of phase switching presents a fundamental limitation on its effectiveness. To mitigate this, we propose a variant scheme in which parameters are chosen to only allow excitations within the system's dressed-ground subspace; this effectively cuts phase switching demands in half. All three protocols serve to illustrate the power of the Rydberg-dressed interaction and suggest directions for future study.