Quantum-Classical Equivalence for AND-Functions
Pith reviewed 2026-06-28 07:40 UTC · model grok-4.3
The pith
For every Boolean function f, the bounded-error quantum and classical deterministic communication complexities of f composed with AND are polynomially related up to polylog factors.
A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.
Core claim
For every Boolean function f, both the bounded-error quantum communication complexity and the classical deterministic communication complexity of F(x, y) = f(x1 ∧ y1, ..., xn ∧ yn) are polynomially related up to polylogarithmic factors in n. This is established by proving that each is characterized up to polynomial loss by the logarithm of the De Morgan sparsity of f. The result builds on structural characterizations of non-sparse Boolean functions.
What carries the argument
The logarithm of the De Morgan sparsity of the outer function f, which serves as a common characterization for both quantum and classical communication complexities up to polynomial factors.
If this is right
- The quantum and classical complexities are at most polynomially larger than the log De Morgan sparsity.
- They are at least the log De Morgan sparsity divided by a polynomial factor.
- This equivalence holds for all Boolean functions f, not just symmetric ones.
- It implies that there is no exponential quantum advantage for computing such AND-functions.
Where Pith is reading between the lines
- Similar polynomial relations might exist for other gate compositions like OR.
- De Morgan sparsity could be useful for analyzing communication complexity in other settings.
- The extension of the structural characterizations might apply to related problems in circuit complexity.
Load-bearing premise
The structural characterizations of non-sparse Boolean functions can be extended to resolve the conjecture for general AND-functions.
What would settle it
Identification of a Boolean function f for which the bounded-error quantum communication complexity of f ∘ AND₂ exceeds a polynomial in the logarithm of its De Morgan sparsity, or differs superpolynomially from the classical complexity.
Figures
read the original abstract
A major open problem in quantum communication complexity is whether quantum protocols can be exponentially more efficient than classical protocols for computing total Boolean functions; the prevailing conjecture is that they cannot be so. In a seminal work, Razborov (2002) resolved this question for AND-functions of the form $$ F(x,y) = f(x_1 \land y_1, \ldots, x_n \land y_n), $$ when the outer function $f$ is symmetric, by proving that their bounded-error quantum and classical communication complexities are polynomially related. Since then, extending this result to all AND-functions has remained open and has been posed by several authors. In this work, we settle this problem in a strong way. We show that for every Boolean function $f$, the bounded-error quantum and classical deterministic communication complexities of the function $f \circ \mathrm{AND}_2$ are polynomially related, up to polylogarithmic factors in $n$. We prove this by showing that both are characterized--up to polynomial loss--by the logarithm of the De Morgan sparsity of $f$. Our results build on the recent work of Chattopadhyay, Dahiya, and Lovett (2025) on structural characterizations of non-sparse Boolean functions, which we extend to resolve the conjecture for general AND-functions.
Editorial analysis
A structured set of objections, weighed in public.
Referee Report
Summary. The paper claims to resolve the open problem of polynomial equivalence between bounded-error quantum and classical deterministic communication complexity for all AND-functions of the form F(x,y) = f(x1 ∧ y1, ..., xn ∧ yn). It proves that both complexities are characterized up to polynomial loss by the logarithm of the De Morgan sparsity of f, by extending the structural characterizations of non-sparse Boolean functions from Chattopadhyay, Dahiya, and Lovett (2025).
Significance. If correct, this settles a major open question in quantum communication complexity posed since Razborov (2002), establishing that quantum protocols cannot be exponentially more efficient than classical deterministic ones for the entire class of AND-functions (up to polylog n factors). The sparsity-based characterization provides a strong structural tool and gives concrete evidence supporting the broader conjecture that no exponential quantum-classical separation exists for total Boolean functions.
major comments (2)
- [Abstract and §1] Abstract (final sentence) and §1: The central claim depends on extending the structural results of Chattopadhyay-Dahiya-Lovett (2025) to obtain the communication-complexity characterization for general f. The manuscript must explicitly delineate which parts of the 2025 characterization are used verbatim, which are extended, and how the extension controls the polylogarithmic factors in n; without this, the polynomial relation cannot be verified as load-bearing.
- The reduction from communication complexity to De Morgan sparsity (and vice versa) is stated to hold up to polynomial loss, but the precise loss factors and their dependence on the sparsity parameter are not shown to be independent of the particular extension steps from the 2025 paper.
Simulated Author's Rebuttal
We thank the referee for the detailed reading and for identifying areas where greater explicitness would strengthen the presentation. We agree that the manuscript would benefit from a clearer breakdown of the relationship to Chattopadhyay-Dahiya-Lovett (2025) and will revise accordingly. Below we respond point by point.
read point-by-point responses
-
Referee: [Abstract and §1] Abstract (final sentence) and §1: The central claim depends on extending the structural results of Chattopadhyay-Dahiya-Lovett (2025) to obtain the communication-complexity characterization for general f. The manuscript must explicitly delineate which parts of the 2025 characterization are used verbatim, which are extended, and how the extension controls the polylogarithmic factors in n; without this, the polynomial relation cannot be verified as load-bearing.
Authors: We accept this observation. The current draft states that we extend the 2025 structural results but does not provide a dedicated paragraph or subsection that separates verbatim citations, the precise technical extensions required for the AND-function communication setting, and the tracking of polylog(n) factors through those extensions. In the revised manuscript we will insert such a delineation (likely as a new subsection of §1 or an appendix table) that lists each 2025 lemma or theorem used, indicates whether it is applied directly or modified, and shows that the resulting polylogarithmic overhead remains independent of the particular extension steps and is absorbed into the overall polynomial loss. revision: yes
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Referee: [—] The reduction from communication complexity to De Morgan sparsity (and vice versa) is stated to hold up to polynomial loss, but the precise loss factors and their dependence on the sparsity parameter are not shown to be independent of the particular extension steps from the 2025 paper.
Authors: We agree that the current text does not explicitly verify independence of the loss factors from the extension steps. The revised version will contain an expanded analysis (in the section presenting the reductions) that isolates the loss terms arising from the 2025 structural results, shows that these terms depend only on the sparsity parameter s(f) and on n through polylog factors already bounded in the 2025 work, and confirms that no additional dependence is introduced by our extensions. This will make the claimed polynomial relation fully verifiable from the stated bounds. revision: yes
Circularity Check
No significant circularity identified
full rationale
The derivation relies on extending structural characterizations from the 2025 paper by overlapping authors (Chattopadhyay, Dahiya, Lovett), but this prior result is a separate publication providing independent support. The current work performs the extension to general AND-functions and derives the polynomial relation via the log De Morgan sparsity characterization. No step reduces by construction to a self-definition, fitted input renamed as prediction, or unverified self-citation chain; the central claim has new content from the extension. This matches the default expectation of no circularity for papers building on prior independent work.
Axiom & Free-Parameter Ledger
axioms (1)
- domain assumption Structural characterizations of non-sparse Boolean functions from Chattopadhyay, Dahiya, and Lovett (2025) hold and extend to general AND-functions.
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