[Paper Review] A Mølmer-Sørensen Gate with Rydberg-Dressed Atoms

Michael Martin, Yuan‐Yu Jau+4 more|arXiv (Cornell University)|Nov 29, 2021

Cold Atom Physics and Bose-Einstein Condensates4 citations

TL;DR

This paper demonstrates a Mølmer-Sørensen entangling gate between two neutral atoms using single-photon Rydberg dressing, achieving high-fidelity entanglement by leveraging adiabatic dressing to suppress decoherence. The gate is robust against atomic motion, laser noise, and light shifts, with experimental fidelity limited primarily by laser frequency noise and spontaneous decay, suggesting potential for >95% fidelity with further optimization.

ABSTRACT

Neutral atoms are building blocks of ground-up quantum many-body systems. Well-controlled and high-fidelity entangling gates are an essential component for realizing complex neutral atom architectures for quantum computing, quantum simulation, and measurement with precision better than the standard quantum limit. In this Letter we report the realization of a Mølmer-Sørensen unitary between two neutral atoms, based on adiabatic single-photon dressing to Rydberg levels. We show that this technique is highly robust to noise sources and experimental imperfections that have limited the fidelity of other approaches to neutral atom gates.

Motivation & Objective

• To develop a high-fidelity entangling gate for neutral atoms that overcomes limitations of existing Rydberg-based approaches.
• To address decoherence from atomic motion, laser noise, and light shifts that degrade gate fidelity in prior schemes.
• To implement a Mølmer-Sørensen gate via adiabatic single-photon dressing to Rydberg states, enabling robust two-qubit operations.
• To demonstrate a spin-echo topology that mitigates dephasing from static and dynamic laser detuning errors.
• To enable scalable quantum architectures by reducing sensitivity to experimental imperfections such as beam pointing and intensity gradients.

Proposed method

• The gate is implemented using adiabatic single-photon dressing of ground-state atoms to Rydberg states, creating effective spin-spin interactions via dressed-state energy shifts.
• The interaction Hamiltonian is engineered such that the dressed-state energy splitting depends on the collective spin state, enabling a controlled-phase operation.
• A spin-echo sequence is applied to cancel single-atom light shifts and suppress dephasing from laser frequency noise.
• The gate unitary is realized as $\hat{U}_{\mathrm{MS}} = \exp[-i\phi_{J}\hat{S}_{y}^{2}]$, with $\phi_J$ accumulated via time-dependent Rabi coupling $\Omega$ and detuning $\Delta$.
• The interaction strength $J^\pm$ is extracted from a dressed Ramsey measurement, yielding $c_6 = 25(1)\;\mathrm{GHz} \cdot \mu\mathrm{m}^6$.
• The protocol uses a two-pulse sequence with symmetric turn-on times to ensure echo cancellation of non-adiabatic and static phase errors.

Experimental results

Research questions

• RQ1 Can a Mølmer-Sørensen gate be implemented in neutral atoms using single-photon Rydberg dressing instead of two-photon transitions?
• RQ2 How does Rydberg dressing improve robustness against atomic motion and laser noise compared to conventional Rydberg blockade gates?
• RQ3 To what extent can a spin-echo topology suppress dephasing from laser frequency fluctuations in a dressed-atom system?
• RQ4 What is the dominant source of infidelity in this gate protocol, and can it be mitigated with improved laser stability or shortcuts to adiabaticity?
• RQ5 Can this approach achieve high-fidelity entanglement comparable to the best existing neutral-atom gates?

Key findings

• The experiment realizes a Mølmer-Sørensen gate via single-photon Rydberg dressing, with a measured interaction strength $J^+ = 2\pi \times 2.95(5)$ MHz and $c_6 = 25(1)\;\mathrm{GHz} \cdot \mu\mathrm{m}^6$.
• The gate fidelity is limited primarily by laser frequency noise, which is estimated to reduce fidelity by 5–10% due to non-stationary detuning and $J(\Delta,\Omega)$ fluctuations.
• Spontaneous decay from the Rydberg state contributes approximately 0.25% error probability for a 170 μs lifetime at 300 K, which can be reduced by increasing Rabi frequency or using shortcuts to adiabaticity.
• Measurement-error-corrected parity measurements confirm coherent superposition with $|\rho_{00,11}|$ extracted via sinusoidal fit, indicating high coherence of the entangled state.
• The protocol shows strong immunity to atomic motion and intensity gradients due to the use of a single-photon, adiabatic dressing scheme.
• Theoretical analysis suggests that with improved laser stability and faster ramping, gate fidelities exceeding 95% are achievable.