A matrix-discrepancy argument proves tight one-way quantum lower bounds for collision finding (Ω(N^{1/4})) and for streaming triangle finding (Ω(√Δ_V)) where Boolean-Hidden-Matching reductions fail.
Quantum communication complexity of symmetric predicates
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abstract
We completely (that is, up to a logarithmic factor) characterize the bounded-error quantum communication complexity of every predicate $f(x,y)$ depending only on $|x\cap y|$ ($x,y\subseteq [n]$). Namely, for a predicate $D$ on $\{0,1,...,n\}$ let $\ell_0(D)\df \max\{\ell : 1\leq\ell\leq n/2\land D(\ell)\not\equiv D(\ell-1)\}$ and $\ell_1(D)\df \max\{n-\ell : n/2\leq\ell < n\land D(\ell)\not\equiv D(\ell+1)\}$. Then the bounded-error quantum communication complexity of $f_D(x,y) = D(|x\cap y|)$ is equal (again, up to a logarithmic factor) to $\sqrt{n\ell_0(D)}+\ell_1(D)$. In particular, the complexity of the set disjointness predicate is $\Omega(\sqrt n)$. This result holds both in the model with prior entanglement and without it.
fields
quant-ph 2years
2026 2representative citing papers
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Quantum Communication Lower Bounds for Search Problems via Matrix Discrepancy
A matrix-discrepancy argument proves tight one-way quantum lower bounds for collision finding (Ω(N^{1/4})) and for streaming triangle finding (Ω(√Δ_V)) where Boolean-Hidden-Matching reductions fail.
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