A fault-tolerant quantum computer needs a fast decoder and a steady supply of high-quality |0⟩ states for syndrome extraction. A recent paper introduces a novel way to reduce errors in the preparation of these |0⟩ states, which could enable demonstrations of quantum error correction on early-stage hardware.
The conventional approach is to prepare two |0⟩ states, then compare them using a CNOT gate and a measurement. If the measurement is zero, suggesting that the qubits were prepared identically, we have increased confidence that the unmeasured qubit is in the |0⟩ state.
We can repeat this process until we reach the level of quality desired.
Waiting for repeated attempts when we measure 1 instead of 0 greatly complicates the scheduling of processes such as syndrome extraction. Our paper presents an alternative.
We introduce a family of low depth CNOT and Toffoli circuits which permute the computational basis to produce high-quality |0⟩ states on the first attempt, without the repeats that can be required by post-selection.
Our models (available to everyone on GitHub here) suggest meaningful performance enhancements once two-qubit gate fidelities go below 0.2%. We would very much like to see data on the practical effect of running our circuits on real quantum computing hardware.
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