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arxiv: 2604.21274 · v2 · pith:GRNOVVE6new · submitted 2026-04-23 · 🪐 quant-ph · cs.IT· math.IT

Random Access Codes: Explicit Constructions, Optimality, and Classical-Quantum Gaps

Pith reviewed 2026-05-21 00:00 UTC · model grok-4.3

classification 🪐 quant-ph cs.ITmath.IT
keywords random access codesquantum random access codesclassical-quantum separationgeometric characterizationbinary linear codesworst-case optimalityaverage-case optimality
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The pith

For the (2^k-1, k) family, an explicit quantum random access code exceeds the optimal classical worst-case success probability.

A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.

This paper introduces a geometric characterization of optimal classical random access codes that map an L-bit string to a k-bit message. The average-case version reduces to choosing 2^k representative strings from {0,1}^L, while the worst-case version reduces to placing 2^k points in the unit hypercube [0,1]^L to minimize a distance-like objective. With this reduction in hand, the authors prove that a classical construction based on binary linear codes is worst-case optimal for the family where L equals 2^k minus one. They then exhibit an explicit quantum version whose worst-case success probability is strictly higher, establishing a separation. The same framework also yields average-optimal classical codes for the (L, L-1) family and quantum codes that meet a previously conjectured performance bound.

Core claim

The paper shows that worst-case optimality for classical (L,k)-RACs is equivalent to selecting 2^k points in [0,1]^L that minimize a distance-like objective. For the parameter family (2^k-1, k), this characterization proves the optimality of a classical construction derived from binary linear codes, while an explicit quantum random access code achieves a strictly higher worst-case success probability and thereby establishes a classical-quantum gap. For the family (L, L-1), the same viewpoint identifies an average-optimal classical construction and recovers explicit quantum constructions that attain a conjectured upper bound.

What carries the argument

Geometric reduction of the worst-case RAC problem to selecting 2^k points in [0,1]^L that minimize a distance-like objective.

If this is right

  • Several (L,k) families now possess provably optimal classical RAC constructions realized by standard binary linear codes.
  • The (2^k-1, k) family has a classical worst-case optimum that is strictly exceeded by the given explicit QRAC.
  • The (L, L-1) family admits a classical construction optimal under the average success criterion.
  • Explicit (L, L-1) QRACs are recovered that attain the value of a previously conjectured tight upper bound.

Where Pith is reading between the lines

These are editorial extensions of the paper, not claims the author makes directly.

  • The same geometric lens may be used to search for classical-quantum gaps in other parameter regimes or for approximate versions of the RAC task.
  • The reduction to a distance-minimization problem in the hypercube suggests possible links to classical coding theory questions about covering radii or constant-weight codes.
  • For concrete small values of k the separation can be checked by exhaustive search over the classical side, providing a direct test of the claimed gap.

Load-bearing premise

The average-case and worst-case success probabilities of random access codes are exactly captured by the stated geometric selections of representatives and points.

What would settle it

Direct numerical computation, for any small fixed k such as k=2, of the worst-case success probability achieved by the paper's explicit QRAC versus the minimum value of the distance-like objective over all choices of 2^k points in [0,1]^L.

Figures

Figures reproduced from arXiv: 2604.21274 by Hiroshi Yano, Kosuke Ito, Naoki Yamamoto, Ruho Kondo, Yota Maeda, Yuki Sato.

Figure 1
Figure 1. Figure 1: Decoding success probability of RACs and QRACs for L ≤ 7 and k = 3. Conjectural upper bound of QRAC (Eq. (4)) is included. References [1] Wiesner, S. Conjugate coding. ACM Sigact News, 15 (1):78–88, 1983. doi: 10.1145/1008908.1008920. [2] Ambainis, A., Nayak, A., Ta-Shma, A. and Vazirani, U. Dense quantum coding and a lower bound for 1- way quantum automata. In Proceedings of the thirty￾first annual ACM sy… view at source ↗
Figure 2
Figure 2. Figure 2: Achievable (conjecturally maximum) success probability of (L, L − 1)-RACs and (L, L − 1)-QRACs. Note that the maximum average success probability and the maximum worst case success probability are the same for (L, L − 1)-QRAC. For clarity, markers are shown only for L ≤ 10. [7] Sharma, M., Jin, Y., Lau, H.C. and Raymond, R. Quantum relaxation for solving multiple knapsack problems. In 2024 IEEE Internation… view at source ↗
read the original abstract

A random access code (RAC) encodes an $L$-bit string into a $k$-bit message, where $L>k$, such that any requested bit can be decoded with high probability; a quantum RAC (QRAC) replaces the message with $k$ qubits. This paper provides a geometric characterization of optimal classical $(L,k)$-RACs under both average and worst-case success criteria. We show that the average problem reduces to selecting $2^k$ representatives in $\{0,1\}^L$, whereas the worst-case problem reduces to selecting $2^k$ points in $[0,1]^L$ that minimize a distance-like objective. This framework establishes optimality for several parameter families $(L,k)$, with optimal constructions in many cases realized by standard infinite families of binary linear codes. For the parameter family $(2^k-1,k)$, we prove the worst-case optimality of a classical construction and present an explicit QRAC whose worst-case success probability is strictly higher than the classical optimum, thereby establishing a classical--quantum separation for this family. For the parameter family $(L,L-1)$, the framework identifies a classical RAC construction that is optimal under the average criterion and, assuming a stated conjecture, also optimal under the worst-case criterion. As a by-product, the same geometric viewpoint recovers explicit $(L,L-1)$-QRACs similar to existing constructions that attain the value of an upper bound conjectured in prior work to be tight.

Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, simulated authors' rebuttal, and a circularity audit. Tearing a paper down is the easy half of reading it; the pith above is the substance, this is the friction.

Referee Report

2 major / 3 minor

Summary. The manuscript develops a geometric characterization of classical (L,k)-random access codes under both average-case and worst-case success criteria. The average-case problem reduces to selecting 2^k representatives in {0,1}^L; the worst-case problem reduces to selecting 2^k points in [0,1]^L that minimize a distance-like objective. This framework is applied to prove worst-case optimality of classical constructions (realized by binary linear codes) for the family (2^k-1,k), to exhibit an explicit QRAC whose worst-case success probability strictly exceeds the classical optimum (establishing a separation), and to identify optimal classical constructions for (L,L-1) under the average criterion (and under the worst-case criterion assuming a stated conjecture). The same viewpoint recovers explicit (L,L-1)-QRACs attaining a previously conjectured upper bound.

Significance. If the central claims hold, the geometric reduction supplies a clean, unified method for proving optimality of RAC constructions and for certifying classical-quantum separations. The explicit separation for the infinite family (2^k-1,k) and the recovery of QRAC constructions matching a prior upper-bound conjecture are concrete advances. The connection to standard families of binary linear codes for optimal classical RACs is a further strength.

major comments (2)
  1. [Geometric characterization section] The section presenting the geometric characterization of the worst-case problem: the claimed exact equivalence between the max-min success probability over deterministic classical encodings and the continuous minimization of the distance-like objective over [0,1]^L must be shown to be tight; if the relaxation admits values unattainable by any function of the input bits, the optimality proof for the classical construction on (2^k-1,k) and the resulting separation would not be established.
  2. [(L,L-1) optimality subsection] The subsection on the (L,L-1) family: worst-case optimality is asserted only under a stated conjecture; the manuscript should either prove the conjecture, supply supporting numerical evidence for small L, or clearly delineate which results remain conditional.
minor comments (3)
  1. [Geometric characterization section] Clarify the precise definition of the distance-like objective (including any normalization) and verify that the decoder that achieves the minimum is explicitly constructible from the chosen points.
  2. [QRAC construction for (2^k-1,k)] In the explicit QRAC construction for (2^k-1,k), state the worst-case success probability achieved and compare it numerically to the classical optimum for at least one small k (e.g., k=2 or 3).
  3. [Related work and (L,L-1) QRAC] Ensure all references to prior RAC and QRAC literature are complete; the recovery of the (L,L-1)-QRAC should cite the original conjecture it matches.

Simulated Author's Rebuttal

2 responses · 0 unresolved

We thank the referee for the positive evaluation and constructive comments. We address each major comment below and will update the manuscript to strengthen the presentation.

read point-by-point responses
  1. Referee: [Geometric characterization section] The section presenting the geometric characterization of the worst-case problem: the claimed exact equivalence between the max-min success probability over deterministic classical encodings and the continuous minimization of the distance-like objective over [0,1]^L must be shown to be tight; if the relaxation admits values unattainable by any function of the input bits, the optimality proof for the classical construction on (2^k-1,k) and the resulting separation would not be established.

    Authors: We thank the referee for highlighting the need to rigorously establish tightness. The geometric reduction in the manuscript interprets the worst-case objective as a continuous optimization over points in [0,1]^L, where each coordinate is the marginal probability of a 1 in that position under a (possibly randomized) encoding. Because the objective is a convex combination of success probabilities and the feasible set is the convex hull of the deterministic 0-1 encodings, the minimum is attained at a vertex. We will add an explicit lemma in the revised Section 3 proving that any interior point can be replaced by a convex combination of deterministic encodings without increasing the objective value, confirming that the continuous minimum equals the discrete maximum-minimum success probability. This closes the gap and validates both the optimality claim for (2^k-1,k) and the separation. revision: yes

  2. Referee: [(L,L-1) optimality subsection] The subsection on the (L,L-1) family: worst-case optimality is asserted only under a stated conjecture; the manuscript should either prove the conjecture, supply supporting numerical evidence for small L, or clearly delineate which results remain conditional.

    Authors: We agree that the worst-case optimality statement for the (L,L-1) family is conditional on the conjecture. In the revision we will (i) explicitly mark all claims that depend on the conjecture, (ii) add a short paragraph delineating the conditional results, and (iii) include numerical verification for L=3 to L=7 obtained by exhaustive enumeration of small encodings, showing that the conjectured construction matches the computed optimum in every checked case. While we do not prove the conjecture here, the added evidence and clearer delineation address the referee's concern without overstating the result. revision: yes

Circularity Check

0 steps flagged

No significant circularity; geometric reductions and code constructions are independent

full rationale

The paper derives its optimality claims from explicit geometric reductions of the average-case RAC to selecting 2^k representatives in {0,1}^L and the worst-case to minimizing a distance-like objective over 2^k points in [0,1]^L, then invokes standard infinite families of binary linear codes for constructions. The classical-quantum separation for (2^k-1,k) follows from proving the classical optimum via this framework and exhibiting an explicit QRAC exceeding it. No step reduces by definition to its inputs, no fitted parameters are relabeled as predictions, and no load-bearing premise rests solely on self-citation; the framework is self-contained against external coding-theory benchmarks. The stated conjecture for (L,L-1) is noted but does not collapse the central derivation.

Axiom & Free-Parameter Ledger

0 free parameters · 1 axioms · 0 invented entities

The central claims rest on the validity of the stated geometric reductions from RAC success probabilities to point-selection problems in the hypercube and unit cube; these reductions are presented as equivalences but their justification is not visible in the abstract.

axioms (1)
  • domain assumption The average-case and worst-case success probabilities of an (L,k)-RAC are exactly captured by the minimum-distance objectives over representatives in {0,1}^L and [0,1]^L respectively.
    This equivalence is invoked to reduce the optimality question to a geometric selection problem.

pith-pipeline@v0.9.0 · 5809 in / 1325 out tokens · 62935 ms · 2026-05-21T00:00:41.149988+00:00 · methodology

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Works this paper leans on

28 extracted references · 28 canonical work pages · 2 internal anchors

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