Researchers have used extra qubits, the units of quantum information, to check a measurement before it determines the next step of a computation. A test on IBM's Brussels processor found that taking a majority vote across the measured group reduced error in the output. The method acts while the computation is running.
In one-way quantum computing, measurements of an entangled group of qubits drive the calculation. Entanglement links the qubits' states, and each measurement tells the machine which correction to apply to the qubits that remain. A wrong result can select the wrong correction; adjusting the final statistics after many runs cannot undo that earlier decision.
The proposed scheme adds verification qubits to the qubit about to be measured. Controlled-NOT gates, quantum logic operations used here to link their states, prepare the group before each qubit is measured. The majority result then selects the correction. The scheme is designed to address both an apparatus reporting the wrong bit and errors caused by the measurement's effect on the quantum state.
The hardware experiment tested one elementary computation step, using eight randomly selected rotation settings and 10,000 repetitions per circuit. Its error measure describes how far the aggregate output departed from an ideal 50:50 split. Average relative error fell from 8.04% with no verification qubits to 4.50% with two and 2.66% with four. This compares three versions of the same voting scheme, without a matched test against competing methods.
Verification has a cost: it needs more qubits and extra operations to link them. The authors' simulations showed that errors in those added operations can reduce or erase the benefit. Their theoretical improvement also assumes independent measurement errors; correlated errors can invalidate that calculation. The experiment has not established the accuracy or delay of a complete multi-step algorithm, although it demonstrates a way to check the intermediate decision directly.
Sources
- Scientific Reports paper: Real-time measurement error mitigation for one-way quantum computation
- Full manuscript and version history on arXiv
- Archived simulation and IBM hardware data
- Lead image: IBM Quantum System One at the Fraunhofer campus in Ehningen, Germany. Credit: IBM Research via Wikimedia Commons, CC BY 2.0. The photograph is illustrative; the study ran on the ibm_brussels cloud processor.
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