High-Fidelity Universal Quantum Gates in a Cryogenic Atom Array

Abstract

Neutral atoms in optical tweezer arrays have emerged as a leading platform for quantum information processing. The fidelity of entangling gates mediated by Rydberg interactions has improved rapidly and is increasingly constrained by blackbody-radiation-induced transitions, which shorten Rydberg lifetimes and create contaminant states that can cause correlated errors. Cryogenic environments can suppress these transition rates by more than an order of magnitude. However, realizing a universal gate set in such an environment has remained an outstanding challenge. Here, we demonstrate a universal quantum gate set in a cryogenic $^{133}$Cs atom array, with a single-qubit gate fidelity of 99.970(2)% and a raw two-qubit entangling-gate fidelity of 99.76(8)%, within the range of leading room-temperature implementations. The entangling gate uses direct single-photon coupling between the electronic ground state and a Rydberg $P$ state, avoiding scattering and light shifts associated with two-photon coupling. The gate operates at a modest Rabi frequency of $2π\times4.3$ MHz. The measured fidelity agrees with our error model dominated by atomic motion, indicating a straightforward route to higher fidelity through improved cooling, while the fundamental error contribution from Rydberg decay is below $3\times10^{-4}$. These results establish a path to entangling-gate fidelities substantially beyond the current state of the art through cryogenic suppression of Rydberg decay, reducing the overhead for fault-tolerant computation.

The paper addresses whether cryogenic neutral-atom arrays can support a universal gate set while suppressing blackbody-radiation-induced Rydberg decay. It demonstrates single-qubit control and Rydberg-mediated entangling gates in a $^{133}$Cs array with radiation shields at 35 K and 4 K, using seven atom pairs. Single-qubit rotations between hyperfine clock states use Raman excitation, BB1 composite pulses and virtual $Z$ rotations. Randomized benchmarking gives an average Clifford fidelity of 99.970(2)%. A 316 ns CZ gate uses direct single-photon excitation at 319 nm to $55P_{3/2}$, with Rabi frequency $2\pi\times4.3$ MHz. Electric-field compensation and pair orientation address the sensitivity and anisotropic interactions of the $P$ state. Echo randomized benchmarking measures a raw CZ fidelity of 99.76(8)%; symmetric stabilizer benchmarking gives 99.78(10)%. A model incorporating thermal motion, finite interactions, Rydberg decay and measured laser noise predicts 99.80% fidelity. Doppler shifts dominate the modeled infidelity at $1.26\times10^{-3}$, while the approximately $406\,\mu\mathrm{s}$ Rydberg lifetime contributes $2.7\times10^{-4}$. These results demonstrate cryogenic gates rather than a fidelity exceeding room-temperature records. Modeled fidelities of 99.93% with Raman sideband cooling and 99.97% with additional excitation power and cavity filtering remain projections. Reduced contaminant-induced correlated errors are motivated but not measured. Current implementation constraints include restricted optical access, approximately 99.2% state preparation, and an 80 $\mu$m usable array diameter.