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arXiv · 2508.10664

Deciding Whether a C-Q Channel Preserves a Bit is QCMA-Complete

Kiera Hutton·Arthur Mehta·Andrej Vukovic
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Abstract

We prove that deciding whether a classical-quantum (C-Q) channel can exactly preserve a single classical bit is QCMA-complete. This "bit-preservation" problem is a special case of orthogonality-constrained optimization tasks over C-Q channels, in which one seeks orthogonal input states whose outputs have small or large Hilbert-Schmidt overlap after passing through the channel. Both problems can be cast as biquadratic optimization with orthogonality constraints. Our main technical contribution uses tools from matrix analysis to give a complete characterization of the optimal witnesses: computational basis states for the minimum, and |+>, |-> over a single basis pair for the maximum. Using this characterization, we give concise proofs of QCMA-completeness for both problems.

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Kiera Hutton, Arthur Mehta, Andrej Vukovic. 2025-08-14. Deciding Whether a C-Q Channel Preserves a Bit is QCMA-Complete. https://arxiv.org/abs/2508.10664

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