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arXiv:2610.06835

Distributional Quantum Query Complexity

Shalev Ben-David, M. H. Ebtehaj

quant-phcs.CCSubmitted Oct 5, 2026• Announced Oct 6, 2026• 0 citations
v1 · Oct 5, 2026v2 · Oct 6, 2026
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Abstract

Quantum query complexity enjoys a variety of pleasing joint computation properties: for example, a composition theorem asserting $Q(f\circ g)=Θ(Q(f)Q(g))$ for all Boolean functions $f$ and $g$; a direct sum theorem asserting that computing $k$ copies of a function (or search problem) costs $Ω(k)$ times as much as the cost of computing one copy; and a direct product theorem asserting that for Boolean functions, even succeeding at the direct sum problem with exponentially small probability still requires $Ω(k)$ times the cost of computing one copy to bounded error. However, all of these results are strictly for worst-case quantum query complexity. For example, if we have a fixed distribution $μ$ over inputs, the direct sum theorem says nothing about the quantum query complexity of computing $k$ copies of $f$ when the input comes from the product distribution $μ^k$ instead of being worst-case. (Note that while a standard Yao-type minimax theorem guarantees a hard distribution for the direct sum problem, there's no guarantee that this hard distribution is a product distribution.) A similar problem occurs for the direct product theorem and the composition theorem: none of these results respect distributions. In this work, we give distributional joint computation lower bounds for the composition, direct sum, and direct product problems. Along the way, we introduce some new tools for handling quantum query lower bounds, including (a) a new ``multiplicative'' variant of the $γ_2$ norm (which we use in place of the multiplicative adversary method for proving the direct product theorem), and (b) a new ``Shaltiel-free'' measure of quantum query complexity, which we show characterizes the composition behavior of distributional quantum query complexity and satisfies pleasing properties.

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Quantum query complexity enjoys a variety of pleasing joint computation properties: for example, a composition theorem asserting Q(f∘g)=Θ(Q(f)Q(g))Q(f\circ g)=Θ(Q(f)Q(g))Q(f∘g)=Θ(Q(f)Q(g)) for all Boolean functions fff and ggg; a direct sum theorem asserting that computing kkk copies of a function (or search problem) costs Ω(k)Ω(k)Ω(k) times as much as the cost of computing one copy; and a direct product theorem asserting that for Boolean functions, even succeeding at the direct sum problem with exponentially small probability still requires Ω(k)Ω(k)Ω(k) times the cost of computing one copy to bounded error.
However, all of these results are strictly for worst-case quantum query complexity. For example, if we have a fixed distribution μμμ over inputs, the direct sum theorem says nothing about the quantum query complexity of computing kkk copies of fff when the input comes from the product distribution μkμ^kμk instead of being worst-case. (Note that while a standard Yao-type minimax theorem guarantees a hard distribution for the direct sum problem, there's no guarantee that this hard distribution is a product distribution.) A similar problem occurs for the direct product theorem and the composition theorem: none of these results respect distributions.
In this work, we give distributional joint computation lower bounds for the composition, direct sum, and direct product problems. Along the way, we introduce some new tools for handling quantum query lower bounds, including (a) a new multiplicative'' variant of the $γ_2$ norm (which we use in place of the multiplicative adversary method for proving the direct product theorem), and (b) a new Shaltiel-free'' measure of quantum query complexity, which we show characterizes the composition behavior of distributional quantum query complexity and satisfies pleasing properties.
We establish two new direct product theorems for the randomized query complexity of Boolean functions. The first shows that computing nnn copies of a function fff, even with a small success probability of γnγ^nγn, requires Θ(n)Θ(n)Θ(n) times the "maximum distributional" query complexity of fff with success parameter γγγ. This result holds for all success parameters γγγ, even when γγγ is very close to 1/21/21/2 or to 111. As a result, it unifies and generalizes Drucker's direct product theorem (2012) for γγγ bounded away from 12\frac1221​ and 111 as well as the strong direct sum theorem of Blais and Brody (2019) for γ≈1−1/nγ\approx 1-1/nγ≈1−1/n. The second establishes a general "list decoding" direct product theorem that captures many different variants of partial computation tasks related to the function fnf^nfn consisting of nnn copies of fff. Notably, our list decoding direct product theorem yields a new threshold direct product theorem and other new variants such as the labelled-threshold direct product theorem. Both of these direct product theorems are obtained by taking a new approach. Instead of directly analyzing the query complexity of algorithms, we introduce a new measure of complexity of functions that we call "discounted score". We show that this measure satisfies a number of useful structural properties, including tensorization, that make it particularly suitable for the study of direct product questions.
Quantum strong direct-product theorems for specific functions have been known for nearly two decades. These have been extended to general results for function computation and state generation. The proofs of these results use a version of the multiplicative adversary method that does not naturally extend to relations. Standard strong direct-product theorems apply when algorithms must correctly answer every given question. Prior work extended them to equivalent threshold direct-product theorems, which require answers to all questions but only require that most answers are correct. We focus on two further generalizations. Strong selective direct-products apply to algorithms that adaptively choose, based on what they learn from queries, which questions from a large list to answer. This generalization is relational and useful for proving time-space tradeoffs. We prove a quantum strong selective direct-product theorem for all functions using a relational formulation of the multiplicative adversary method by Jeffery and Zur which we prove, via an equivalent formulation, satisfies a strong selective direct product property and captures any query lower bound for functions proven by negative-weights adversaries. The second generalization is list-decoding direct product problems introduced by Ben-David and Blais for classical query complexity. These allow an algorithm to produce a large list of possible output vectors such that one of them is fully correct. They proved that such theorems hold for randomized complexity of all Boolean functions. We prove a quantum analogue of this theorem for all partial Boolean-valued functions. We show that strong list-decoding direct-product theorems are implied by a special case of multiplicative adversaries which we show, via a new reduction, can be obtained from negative-weights adversaries for any Boolean-valued function.
Consider the expected query complexity of computing the k-fold direct product f⊗kf^{\otimes k}f⊗k of a function f to error ε\varepsilon ε with respect to a distribution μk\mu ^kμk . One strategy is to sequentially compute each of the k copies to error ε/k\varepsilon /kε/k with respect to μ\mu μ and apply the union bound. We prove a strong direct sum theorem showing that this naive strategy is essentially optimal. In particular, computing a direct product necessitates a blowup in both query complexity and error. Strong direct sum theorems contrast with results that only show a blowup in query complexity or error but not both. There has been a long line of such results for distributional query complexity, dating back to (Impagliazzo, Raz, Wigderson 1994) and (Nisan, Rudich, Saks 1994), but a strong direct sum theorem had been elusive. A key idea in our work is the first use of the Hardcore Theorem (Impagliazzo 1995) in the context of query complexity. We prove a new “resilience lemma" that accompanies it, showing that the hardcore of f⊗kf^{\otimes k}f⊗k is likely to remain dense under arbitrary partitions of the input space.
We establish two new direct product theorems for the randomized query complexity of Boolean functions. The first shows that computing n copies of a function f, even with a small success probability of γn\gamma^{n}γn, requires Θ(n)\Theta(n)Θ(n) times the maximum distributional query complexity of f with success parameter γ\gammaγ. This result holds for all success parameters γ\gammaγ, even when γ\gammaγ is very close to 1/2 or to 1. As a result, it unifies and generalizes Drucker’s direct product theorem (2012) for γ\gammaγ bounded away from 12\frac{1}{2}21​ and 1 as well as the strong direct sum theorem of Blais and Brody (2019) for γ≈1−1/n\gamma \approx 1-1 / nγ≈1−1/n. The second establishes a general list decoding direct product theorem that captures many different variants of “partial computation” tasks related to the function fnf^{n}fn consisting of n copies of f. Notably, our list decoding direct product theorem yields a new threshold direct product theorem and other new variants such as the labelled-threshold direct product theorem. Both of these direct product theorems are obtained by taking a new approach. Instead of directly analyzing the query complexity of algorithms, we introduce a new measure of complexity of functions that we call discounted score. We show that this measure satisfies a number of useful structural properties, including tensorization, that make it particularly suitable for the study of direct product questions.
We initiate a systematic study of pseudo-deterministic quantum algorithms. These are quantum algorithms that, for any input, output a canonical solution with high probability. Focusing on the query complexity model, our main contributions include the following complexity separations, which require new lower bound techniques specifically tailored to pseudo-determinism: - We exhibit a problem, Avoid One Encrypted String (AOES), whose classical randomized query complexity is O(1)O(1)O(1) but is maximally hard for pseudo-deterministic quantum algorithms (Ω(N)Ω(N)Ω(N) query complexity). - We exhibit a problem, Quantum-Locked Estimation (QL-Estimation), for which pseudo-deterministic quantum algorithms admit an exponential speed-up over classical pseudo-deterministic algorithms (O(log⁡(N))O(\log(N))O(log(N)) vs. Θ(N)Θ(\sqrt{N})Θ(N​)), while the randomized query complexity is O(1)O(1)O(1). Complementing these separations, we show that for any total problem RRR, pseudo-deterministic quantum algorithms admit at most a quintic advantage over deterministic algorithms, i.e., D(R)=O~(psQ(R)5)D(R) = \tilde O(psQ(R)^5)D(R)=O~(psQ(R)5). On the algorithmic side, we identify a class of quantum search problems that can be made pseudo-deterministic with small overhead, including Grover search, element distinctness, triangle finding, kkk-sum, and graph collision.
A fundamental question in computer science is: Is it harder to solve nnn instances independently than to solve them simultaneously? This question, known as the direct sum question or direct sum theorem, has been paid much attention in several research fields. Despite its importance, however, little has been discovered in many other research fields. In this paper, we introduce a novel framework that extends to classical/quantum query complexity, PAC-learning for machine learning, statistical estimation theory, and more. Within this framework, we establish several fundamental direct sum theorems. The main contributions of this paper include: (i) establishing a complete characterization of the amortized query/oracle complexities, and (ii) proving tight direct sum theorems when the error is small. Note that in our framework, every oracle access needs to be performed \emph{classically} even in the quantum setting. This can be thought of one limitation of this work. As a direct consequence of our results, we obtain the following: (A) The first known asymptotic separation of the randomized query complexity. Specifically, we show that there is a function f:{0,1}k→{0,1}f: \{0, 1\}^k \to \{0, 1\}f:{0,1}k→{0,1} and small error ε>0\varepsilon > 0ε>0 such that solving nnn instances simultaneously requires the query complexity O~(nk)\tilde{O}(n\sqrt{k})O~(nk​) but solving one instance with the same error has the complexity Ω~(k)\tildeΩ(k)Ω~(k). In communication complexity this type of separation was previously given in~Feder, Kushilevitz, Naor and Nisan (1995). (B) The query complexity counterpart of the ``information = amortized communication" relation, one of the most influential results in communication complexity shown by Braverman and Rao (2011) and further investigated by Braverman (2015). We hope that our results will provide further interesting applications in the future.