[Paper Review] High-fidelity parallel entangling gates on a neutral atom quantum computer

Simon J. Evered, Dolev Bluvstein+13 more|arXiv (Cornell University)|Apr 11, 2023

Quantum Computing Algorithms and Architecture88 references12 citations

TL;DR

The paper demonstrates 99.5% fidelity two-qubit CZ gates performed in parallel on up to 60 neutral-atom qubits using Rydberg blockade and optimal-control single-pulse gates, and also realizes high-fidelity three-qubit CCZ gates.

ABSTRACT

The ability to perform entangling quantum operations with low error rates in a scalable fashion is a central element of useful quantum information processing. Neutral atom arrays have recently emerged as a promising quantum computing platform, featuring coherent control over hundreds of qubits and any-to-any gate connectivity in a flexible, dynamically reconfigurable architecture. The major outstanding challenge has been to reduce errors in entangling operations mediated through Rydberg interactions. Here we report the realization of two-qubit entangling gates with 99.5% fidelity on up to 60 atoms in parallel, surpassing the surface code threshold for error correction. Our method employs fast single-pulse gates based on optimal control, atomic dark states to reduce scattering, and improvements to Rydberg excitation and atom cooling. We benchmark fidelity using several methods based on repeated gate applications, characterize the physical error sources, and outline future improvements. Finally, we generalize our method to design entangling gates involving a higher number of qubits, which we demonstrate by realizing low-error three-qubit gates. By enabling high-fidelity operation in a scalable, highly connected system, these advances lay the groundwork for large-scale implementation of quantum algorithms, error-corrected circuits, and digital simulations.

Motivation & Objective

• Motivate high-fidelity entangling operations as a prerequisite for scalable quantum computation and error correction on neutral-atom platforms.
• Achieve parallel, scalable two-qubit entangling gates with fidelities surpassing fault-tolerance thresholds.
• Investigate and mitigate dominant error sources in Rydberg-mediated gates to enable larger-scale quantum processors.
• Generalize the approach to higher-qubit entangling operations (e.g., CCZ) and assess their performance.

Proposed method

• Use a family of single-pulse, Rydberg-blockade–based CZ gates with optimally controlled phase profiles.
• Employ atomic dark and bright states to reduce intermediate-state scattering while maintaining large two-photon Rabi frequency.
• Implement Lambda-enhanced gray molasses cooling and improved optical pumping to reduce motional and dephasing errors.
• Calibrate gates with global pulse parameters, enabling robust operation across large, defect-free qubit arrays.
• Benchmark gate performance via Bell-state tomography, odd-number gate trains, and global randomized benchmarking.
• Extend to multi-qubit gates by designing time-optimal phase profiles for CCZ and testing on triangular triplet gate zones.

Experimental results

Research questions

• RQ1 What fidelity can be achieved for parallel two-qubit entangling gates in a neutral-atom array under practical experimental conditions?
• RQ2 What are the dominant physical error sources in Rydberg-mediated gates as system size increases, and how can they be mitigated?
• RQ3 Can the same methodology realize high-fidelity multi-qubit (three- and higher-qubit) gates in parallel?
• RQ4 How uniform is gate performance across large arrays, and what is the scaling behavior when increasing qubit count to 60 or more?
• RQ5 How do different gate-pulse implementations compare in fidelity and practicality for scalable quantum circuits?

Key findings

• Two-qubit CZ gates with 99.5% fidelity achieved while operating on up to 60 neutral-atom qubits in parallel.
• Bell-state raw fidelity reached 98.0(2)% with SPAM-corrected fidelity ~99.4%(not plotted) for a single CZ gate.
• Global randomized benchmarking yields CZ gate fidelities of 99.54(2)% and 99.55(3)% for parameterized time-optimal and smooth-amplitude gates, respectively.
• In a 60-qubit parallel implementation, the CZ gate fidelity remains 99.48(2)% with good homogeneity across the array.
• The method generalizes to three-qubit CCZ gates, achieving ~97.9(2)% fidelity across 21 qubits in parallel in GHZ-type tests, and enabling faster than decomposed multi-qubit gate implementations.
• Analysis pinpoints dominant errors as Rydberg decay, coupling to adjacent Rydberg mJ levels, intermediate-state scattering, and ground-Rydberg dephasing (T2* ≈ 3 μs).