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Exploiting all ancilla outcomes in linear combinations of unitaries: low-rank recovery and quantum trapdoor functions

Year: 2026
Type: preprint
Abstract: The linear combination of unitaries (LCU) is a fundamental quantum computing primitive used to embed non-unitary operators into larger systems via ancilla post-selection. In standard LCU implementations, only the zero-state ancilla outcome is retained, while all other measurement branches are discarded as unusable overhead. We study these discarded parts. Every computational-basis outcome of the ancilla projects the system onto a different linear combination of the target unitaries; collecting these outcome states in a matrix gives a factorization $Φ=CX$, where $C$ is determined by the ancilla preparation and the rows of $X$ are the states $U_t|ψ\rangle$, so that $rank(Φ)\le K$. Although this holds for any LCU circuit; to make the analysis easier, we use a simplified model circuit with Hadamard gates on the index register and a single rotation qubit in place of the coefficient preparation unitary, in which $C$ is explicit. The structure suggests two complementary applications: (i) in an explicit amplitude-access model, classical low-rank matrix completion can reconstruct the full output (including the target) from a subset of its amplitude entries, while computational basis counts provide intensities and generally require additional phase information for full complex recovery; (ii) treating $C$ as a secret key suggests a candidate quantum trapdoor function, whose security remains informal and depends on the difficulty of structured phase retrieval. The scheme thus turns the "junk" ancilla outcomes into a structured resource, possibly opening paths for further applications.
DOI: 10.48550/arxiv.2605.02986
Language: English
Open Access status: green
OpenAlex ID: W7160419056
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