Rydberg two-qubit gates in neutral-atom arrays

Short answer: In neutral-atom quantum computers, a two-qubit gate works by exciting atoms to a Rydberg state, where a strong interaction blocks the second atom from being excited alongside the first. A 2023 Nature paper reports two-qubit gates with 99.5% fidelity on up to 60 atoms in parallel, and a 2023 Physical Review Research paper designs robust gates in simulation that keep a Bell-state fidelity above 0.999 despite control deviations. These are a laboratory experiment and a theory study, both reporting results from their authors' own methods.

The challenge

The Nature abstract says neutral-atom arrays have emerged as a promising platform, featuring coherent control over hundreds of qubits and any-to-any gate connectivity in a flexible, reconfigurable architecture. It calls the main outstanding challenge reducing errors in entangling operations mediated through Rydberg interactions.

2023 Nature: the experiment

The authors report two-qubit entangling gates with 99.5% fidelity on up to 60 atoms in parallel, which they say surpasses the surface-code threshold for error correction. Their method uses fast single-pulse gates based on optimal control, atomic dark states to reduce scattering, and improvements to Rydberg excitation and atom cooling. They benchmark fidelity with several methods based on repeated gate applications, characterize the physical error sources and outline future improvements. They also generalize the method to design gates on more qubits, and demonstrate low-error three-qubit gates. They say the advances lay the groundwork for large-scale implementation of algorithms, error-corrected circuits and digital simulation, which is a statement about direction.

2023 Physical Review Research: robust pulses

The theory paper studies how robust two-qubit gates are to deviations of experimental controls on a neutral atom platform using Rydberg states. The authors construct robust controlled-Z gates with quantum optimal control that retain a Bell-state fidelity above 0.999 when the coupling strength to the Rydberg state deviates significantly, for example from laser intensity noise or atomic motion in an inhomogeneous coupling field. They also discuss methods to mitigate errors from laser detuning deviations. The pulses are short compared with the decay timescale from spontaneous emission and blackbody radiation, and the authors include the finite Rydberg lifetime in both the optimization and the fidelity calculation. They compute lifetimes, polarizabilities and blockade strengths for strontium-88 atoms to identify optimal Rydberg states for their protocols.

Limits

The 99.5% figure comes from one experiment and is the authors' benchmark. The 0.999 figure in the second paper is from calculations, not a measured gate. The first paper's comparison with the surface-code threshold is the authors' framing and refers to a threshold that depends on the noise model.

FAQ

What is the Rydberg blockade?

In these papers, the use of strongly interacting Rydberg states of atoms to create entangling two-qubit gates.

What fidelity was measured?

99.5% for two-qubit gates on up to 60 atoms in parallel, per the Nature authors.

What did the theory paper add?

Robust controlled-Z pulses that keep a simulated Bell-state fidelity above 0.999 under deviations in coupling strength.

Did the experiment do more than two qubits?

The authors report low-error three-qubit gates.

Is a large error-corrected machine shown?

No. Both papers are about gates, and the error-correction statements are future directions.