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arXiv · 2507.06232

Error Exponents for Quantum Packing Problems via An Operator Layer Cake Theorem

Hao-Chung Cheng·Po-Chieh Liu
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Abstract

In this work, we prove a one-shot random coding bound for classical-quantum channel coding, a problem conjectured by Burnashev and Holevo in 1998. By choosing the optimal input distribution, the bound implies the optimal error exponent (i.e., the reliability function) of classical-quantum channels for rates above the critical rate, even in infinite-dimensional Hilbert spaces. Our result extends to various quantum packing-type problems, including classical communication over any fully quantum channel with or without entanglement-assistance, constant composition codes, and classical data compression with quantum side information via fixed-length or variable-length coding. Our technical ingredient is to establish an operator layer cake theorem - the directional derivative of an operator logarithm admits an integral representation of certain projections. This shows that a kind of pretty-good measurement is equivalent to a randomized Holevo-Helstrom measurement, which provides an operational explanation of why the pretty-good measurement is pretty good.

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Hao-Chung Cheng, Po-Chieh Liu. 2026-06-04. Error Exponents for Quantum Packing Problems via An Operator Layer Cake Theorem. https://arxiv.org/abs/2507.06232

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