Mid-Circuit Measurement
Reading out some qubits partway through a computation while their neighbors stay coherent, then acting on the result. The capability that unlocks error correction.
Most early quantum computers measured everything once, at the end, like developing a roll of film. Mid-circuit measurement means reading out selected qubits in the middle of a computation while their neighbors stay coherent, and acting on the result in real time.
It's harder than it sounds: measurement is a violent act in quantum mechanics, and doing it next door to fragile superpositions without collateral damage takes serious engineering. It's also non-negotiable for error correction, which is nothing but a loop of measure-decode-correct running continuously under the algorithm.
Quantinuum's H-series machines made mid-circuit measurement a signature capability early, which is a large part of why Broomfield keeps showing up in error-correction milestone papers.