Short answer: In a Rydberg blockade gate, exciting one neutral atom to a high-lying Rydberg state blocks the excitation of a neighbour, and that interaction can be used to entangle atoms. A 2010 experiment reported a Bell state fidelity of 0.58 between two atoms, and a 2023 Nature paper reports two-qubit gates with 99.5 percent fidelity on up to 60 atoms in parallel. Both are laboratory experiments from research groups, and the 2023 figure is the authors' own report.
The blockade idea
The 2018 Physical Review Applied paper on Deutsch, Toffoli and CNOT gates describes the blockade interaction in a three-atom configuration: when at least one control atom starts in state 0, the interaction blocks excitation, and when both controls are in the Rydberg state there is a residual interaction. The paper's author designs gates in which this interaction lets one atom's state condition the behaviour of another.
An early experiment, 2010
A 2010 Physical Review A rapid communication, "Deterministic entanglement of two neutral atoms via Rydberg blockade", reports the deterministic entanglement of two individually addressed neutral atoms using a blockade-mediated controlled-NOT gate. Parity oscillation measurements gave a Bell state fidelity of 0.58 plus or minus 0.04, above the entanglement threshold of 0.5 without any correction for atom loss, and 0.71 plus or minus 0.05 after correcting for background collisional losses. The authors say the fidelity results agree well with a detailed error model. The experiment used rubidium-87 atoms in optical traps.
Gates with more inputs, 2018
The 2018 paper designs a Deutsch gate, a three-qubit gate from which, it says, any feasible quantum computing can be built, using Rydberg blockade, with the rotation angle tunable by adjusting the strengths of external control fields. It shows that Toffoli and controlled-NOT gates can be achieved with only three laser pulses. This is a proposal in the form of a protocol design. The author says the possibility and speed of realizing these gates sheds light on quantum information with neutral atoms.
A 2023 result
A 2023 Nature paper says neutral-atom arrays have emerged as a promising platform, with coherent control over hundreds of qubits and any-to-any gate connectivity in a flexible, dynamically reconfigurable architecture. It says the main outstanding challenge was reducing errors in entangling operations mediated through Rydberg interactions. The authors report two-qubit entangling gates with 99.5 percent fidelity on up to 60 atoms in parallel, which they say surpasses the surface-code threshold. 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 three-qubit gates and demonstrate low-error three-qubit gates.
What this does not tell you
These papers show gate demonstrations on arrays of atoms. They do not show a complete error-corrected computation, and a gate fidelity above a threshold is one requirement among several for fault tolerance.
FAQ
What is Rydberg blockade?
An interaction in which one atom excited to a Rydberg state prevents a nearby atom from being excited, used to implement entangling gates.
How good have the gates been?
A 2010 experiment reported a 0.58 Bell state fidelity. A 2023 Nature paper reports 99.5 percent two-qubit gate fidelity on up to 60 atoms in parallel.
Can it do gates with three qubits?
A 2018 paper designs Deutsch, Toffoli and CNOT gates, and the 2023 paper demonstrates low-error three-qubit gates.
Does 99.5 percent mean error correction works?
The authors say it surpasses the surface-code threshold. That is not a demonstration of error-corrected computation.
What was the remaining challenge?
The 2023 paper says it was reducing errors in Rydberg-mediated entangling operations.
All field notes