Mid-circuit measurement in neutral atom arrays

Short answer: Mid-circuit measurement means reading some qubits during a computation without disturbing the rest, which error correction needs. In 2023, two PRX papers on neutral atom arrays reported how to do it: one with a data-qubit process fidelity of 97.0% and ancilla measurement fidelities of 94.9% and 95.3%, the other with site-selective readout in ytterbium-171 that causes only percent-level errors on the remaining qubits. These are proof-of-principle experiments from research groups.

What the goal is

One of the 2023 abstracts explains that measurement-based quantum error correction relies on determining the state of a subset of qubits, the ancillas, within a processor without revealing or disturbing the state of the others. It says that, among neutral-atom platforms, a scalable, high-fidelity approach to mid-circuit measurement that leaves the ancilla qubits suitable for future operations had not yet been demonstrated.

Shelving data qubits

In the first paper, the authors demonstrate mid-circuit measurements in a neutral atom array by shelving data qubits in protected hyperfine-Zeeman substates while nondestructively measuring an ancilla qubit. They say measurement fidelity is enhanced by microwave repumping of the ancilla during the measurement. The shelved data qubits stay coherent during ancilla readout through dynamical decoupling pulses, after which they are returned to the computational basis states.

They report that the data qubits' quantum state is well preserved up to a constant phase shift, with a state-preparation-and-measurement-corrected process fidelity of 97.0% with an uncertainty of 0.5. The ancilla measurement fidelities, corrected for state preparation errors, are 94.9% with an uncertainty of 0.8 for the zero state and 95.3% with an uncertainty of 1.1 for the one state. They discuss extending the technique to repetitive error correction using quadrupole recooling and microwave-based state resetting, which is a proposed extension and not a demonstration.

Hiding atoms from the imaging light

In the second paper, the authors use a narrow-linewidth transition in an array of tweezer-confined ytterbium-171 atoms to demonstrate nondestructive state-selective and site-selective detection. By applying site-specific light shifts, selected atoms can be hidden from the imaging light, which allows a subset of qubits to be measured while causing only percent-level errors on the rest.

As a proof-of-principle of conditional operations, and of reusing ancilla qubits, they perform conditional refilling of ancilla sites to correct for occasional atom loss while maintaining the coherence of data qubits. Looking toward continuous operation, they also demonstrate loading a magneto-optical trap with a minimal degree of qubit decoherence.

A third paper

A third PRX paper from the same issue was listed as a source, but only its section headings were readable, so it is not described here.

Limits

The two experiments use different atoms and different techniques, and the abstracts do not compare them head to head. Fidelities near 95% to 97% are measured on small demonstrations, and the abstracts describe extension to repeated error correction as future work. Percent-level errors on the unmeasured qubits are a cost the authors report, not a negligible one.

FAQ

What is mid-circuit measurement?

Measuring some qubits during a computation without disturbing the others.

Why do atoms get lost?

Atom loss is a known occasional failure, which the second paper addresses with conditional refilling.

How accurate was the data qubit protection?

A corrected process fidelity of 97.0% in the first paper.

How good was the ancilla readout?

Corrected fidelities of 94.9% and 95.3% for the two states.

Is repeated error correction demonstrated?

No. Both abstracts discuss that as a goal.