Rydberg Quantum Gate Performance: Simulated Infidelity and Experimental Parameters

The study reports on two-photon (TO) and single-photon (AR) entangling gates realized with neutral-atom qubits excited to Rydberg states. A linearly polarized laser, oriented perpendicular to the quantization axis, provides equal ?+ and ?? components that address Zeeman-split Rydberg transitions. For the local single-qubit phase gate the beam is tuned to the midpoint between the two transitions, yielding an off-resonant detuning of ? 8 MHz (?r/2). This off-resonant coupling generates a differential AC Stark shift that implements RZ rotations. Simulations incorporate spontaneous decay (Rydberg lifetime ?_Ryd = 88 µs) and intensity-dependent light shifts to predict gate infidelity across a range of experimental parameters.

Nature

Neutral-atom arrays exploit strong, controllable interactions between atoms excited to high-lying Rydberg states. The ability to execute high-fidelity two-qubit gates underpins scalable quantum processors. Prior work has demonstrated entangling operations using either single-photon or two-photon excitation pathways; the present work extends these efforts by systematically evaluating how laser intensity errors, beam geometry, and Rabi frequency affect gate performance.

Nature

Simulated infidelity curves were generated for nominal Rabi frequencies ?/2? = 2, 4, 8, 12, 16 MHz. The AR gate, driven by a single-photon transition, shows negligible intensity-dependent light shift, whereas the TO gate exhibits additional differential light-shift contributions in the two-photon case. Gate infidelity was also examined as a function of the elliptical beam waist along the gate zone (targeted sizes 30 µm, 50 µm, 70 µm) while keeping total laser power constant at 75 W and the perpendicular waist fixed at 12 µm (1/e² radius). Optimal fidelities for each gate type and zone size are indicated by star and circle markers on the simulated curves. Randomized circuit characterization of a controlled-Z (CZ) gate revealed fidelity trends as circuit depth d increased, following the protocol of Ref. 40.

Nature

The analysis identifies laser-intensity stability and beam-waist engineering as critical levers for minimizing gate error. The negligible intensity-dependent shift of the AR gate suggests a pathway to robust entangling operations, while the TO gate’s sensitivity to two-photon light shifts highlights the trade-off between excitation flexibility and error sources. Demonstrated fidelity scaling with circuit depth provides a benchmark for error-correction thresholds in larger neutral-atom processors.

Nature

• “(a) In our experiment, the Rydberg laser is linearly polarized perpendicular to the quantization axis, resulting in equal ?+ and ?– components.” — Authors, Nature Physics

Nature

• “3,F=1/2, mm_F=1/2 angle\) with a detuning ? r from the Zeeman splitting of the Rydberg state.” — Authors, Nature Physics

Nature

• “The off-resonant coupling produces a differential AC Stark shift that implements R Z rotations.” — Authors, Nature Physics

Nature

• “The AR gate is driven via a single-photon Rydberg transition and therefore has negligible intensity-dependent light shift.” — Authors, Nature Physics

Nature

• “(d) Simulated gate infidelity (including spontaneous decay), averaged over the targeted gate-zone, is plotted against the waist of an elliptical laser beam along the gate-zone direction.” — Authors, Nature Physics

Nature

The authors plan to experimentally validate the simulated optimal fidelities by varying beam waists and laser powers in situ. Extending the randomized circuit analysis to deeper circuits and multi-qubit configurations will further clarify error accumulation mechanisms. Continued refinement of laser-stability techniques and exploration of alternative Rydberg states are anticipated to push gate fidelities toward fault-tolerant thresholds.

Nature