REVIEW 3 major objections 3 minor 12 references
Lower Bounds on Relative Error Quantum Compression and Classical Shadows
T0 review · 3 major / 3 minor · reviewed 2026-08-06 · deepseek-v4-flash
Pith's one-line read Relative-error quantum observable estimation forces Ω(√(2^n) ε^{-2}) bits of one-way communication, even for Pauli observables.
desk verdict The claimed relative-error lower bounds are not established: the reduction's error propagation is off by a factor of about √C, a load-bearing gap that invalidates all five main theorems. read the letter →
The pith
A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.
The reading
What carries the argument
Central machinery is the reduction from Indexing to relative-error ⟨M⟩ through the Gap-Hamming Distance problem. The paper uses a modified version of Andoni et al.'s reduction (Theorem 2.3): from a random bitstring x and a random index i, shared random strings, one constructs binary strings a,b of length Cγ (γ=$ε^{{-2}}$) such that knowing Δ(a,b) to additive error d√(Cγ) recovers x_i with constant advantage. The link to expectation values is the polarization identity 2⟨a,b⟩ = ‖a+b‖² − ‖a‖² − ‖b‖², which converts a relative-error estimate of ‖a_j+b_i‖² into an additive estimate of the inner product and hence of Hamming distance. For the Pauli restriction, the additional object is the matrix Diag(P_n) whose rows are the diagonal entries of all tensor products of I and Z; it is an orthogonal basis scaled by 2^n, and Alice can pre-compute the vector of all ‖a_j+b_i‖² values, encode it (with extra 1s) into a state, and let Bob's Pauli observable P_l⊗X extract the desired entry. For the shadows connection, the reduction Theorem 4.1 converts a classical shadows learner into a communication protocol by simulating the measurement on a classical description of the state.
What would settle it
Compute the largest error in the Hamming distance Δ(a,b) induced by a (1±ε)-relative error in ‖a+b‖² for the constants C = 9/c² (0<c<√(2/π)) and d < 1/2 from Theorem 2.3; if it exceeds d√(Cγ) for γ=$ε^{{-2}}$, the reduction in Theorem 2.4 fails and the claimed lower bound lacks a proof.
Extended reading notes
Core claim
On the paper's own terms, the central discovery is that the relative-error ⟨M⟩ problem—estimate ⟨ψ|M|ψ⟩ to within a factor (1±ε) with high probability—has one-way randomized communication complexity Ω(√(2^n) $ε^{{-2}}$) for ε ∈ ($2^{{-n/4}}$, 1), for general Hermitian observables with operator norm at most 1 (Theorem 2.4) and also when M is restricted to Pauli observables (Theorem 3.1). The argument reduces from the Indexing problem via a Gap-Hamming Distance reduction (Theorem 2.3, adapted from [ACK+15]): Alice encodes indexing instances into a quantum state whose expectation value under Bob's observable encodes a Hamming distance, and a relative-error estimate of that expectation value is claimed to yield an additive estimate of the Hamming distance accurate enough to recover the index. The same reduction scheme gives Ω(2^n $ε^{{-2}}$) for observable compression by general observables, Ω(√n $ε^{{-2}}$) for Pauli observables, and Ω(√(2^n) $ε^{{-2}}$) for inner product estimation. The paper further shows (Theorem 4.1) that any relative-error classical shadows protocol yields a one-way communication protocol with the same cost, so the communication lower bound transfers directly to a lower bound on the size of compressed classical shadows.
Load-bearing premise
The chain of lower bounds rests on the step that converts a relative error in measuring ‖a+b‖² into a precise enough absolute error in counting the differing bits between two binary strings; if the conversion is too lossy, the reduction to the hard indexing problem fails.
Editorial extensions
If this is right
- Any ε-relative-error classical shadows protocol must produce a compressed classical description of size Ω(√(2^n) ε^{-2}) bits, no matter how many non-adaptive measurements are allowed.
- Relative-error Pauli shadow tomography is exponentially harder to compress than additive-error Pauli shadow tomography: polynomial-size shadow descriptions exist for additive error, while relative error requires 2^{n/2}-scale communication.
- The lower bound survives the restriction to Pauli observables, so the hardness comes from requiring multiplicative precision rather than from the size of the observable class.
- The same reduction scheme shows that observable compression requires Ω(2^n ε^{-2}) bits for general observables and Ω(√n ε^{-2}) bits for Pauli observables, and that inner product estimation requires Ω(√(2^n) ε^{-2}) bits.
- If the bounds are correct, relative error is strictly more informationally demanding than additive error in one-way settings, since the additive problem admits a nearly matching O~(√(2^n) ε^{-1}) upper bound.
Reading between the lines
- A patched error analysis for the Hamming-distance conversion would likely make the same framework apply to other relative-error overlap estimators, such as state fidelity or purity estimation.
- The Ω(√n ε^{-2}) Pauli-observable compression bound suggests the trivial O(n)-bit encoding of a Pauli is essentially optimal under relative error, up to the ε^{-2} factor.
- An information-theoretic proof of the lower bound, avoiding the Gap-Hamming reduction constants, could extend the result to multi-round or interactive communication protocols.
- If the claimed separation is correct, it implies that relative-error guarantees are the wrong target for near-term classical-shadows pipelines that aim for polynomial-size classical memory.
Editorial analysis
A structured set of objections, weighed in public.
Referee Report
Summary. The manuscript claims one-way randomized communication lower bounds for relative-error expectation value estimation from compressed classical descriptions of quantum states, covering arbitrary observables (Theorem 2.4), Pauli observables (Theorem 3.1), observable-side communication variants (Theorems 5.1 and 6.1), and inner product estimation (Theorem 7.1), together with a reduction to relative-error classical shadows (Theorem 4.1). The common proof template reduces from Indexing via the Gap-Hamming Distance problem: Alice encodes many binary strings into a quantum state, Bob chooses an observable whose expectation equals a scaled squared norm ||a_j + b_i||^2, and a relative-error estimate of that expectation is meant to yield an additive estimate of the Hamming distance Delta(a_j,b_i).
Significance. If the theorems were correct, they would improve the additive-error bound of Gosset and Smolin by an epsilon^{-2} factor and establish a sharp separation between relative and additive error settings for Pauli classical shadows. The paper has useful expository features: the reduction template is explicit, and Appendix A gives concrete constants for the Gap-Hamming-to-Indexing reduction. However, the central error-propagation step is invalid and is reused in every main theorem; consequently the claimed lower bounds are not established and the shadow-compression separation does not follow from the arguments given.
major comments (3)
- [Section 2, proof of Theorem 2.4, after the display 2<â_j,b_i> = ||â_j+b_i||^2 - ||â_j||^2 - ||b_i||^2] The conversion from a (1±epsilon)-relative-error estimate of S = ||â_j+b_i||^2 into a d√(Cγ)-additive estimate of Delta(a_j,b_i) is invalid. A relative-error guarantee on S gives an absolute error of at most epsilon*S on S, and since the subsequent identities only subtract exactly known quantities ||â_j||^2 and ||b_i||^2, the same additive error epsilon*S propagates to 2<â_j,b_i> and to Delta(a_j,b_i). On the Gap-Hamming input distribution from Theorem 2.3, S = Θ(Cγ) = Θ(C epsilon^{-2}) with high probability, so the resulting additive error on Delta is Θ(C epsilon^{-1}). Theorem 2.3 permits only d√(Cγ) = d√C epsilon^{-1} with d < 1/2. With the paper's constant C = 9/c^2 ≥ 9π/2 ≈ 14.1, the achieved error exceeds the allowed error by a factor Θ(√C) > 3.7. Therefore the reduction does not solve Indexing, and Theorem 2.4 is not established.
- [Sections 3, 5, and 6] The same flawed conversion is reused in the proofs of Theorems 3.1, 5.1, and 6.1. In Theorem 3.1 the proof says to proceed exactly as in the previous proof; in Theorem 5.1 the proof invokes the same argument as before; and in Theorem 6.1 the relative-error guarantee on the Pauli expectation -Delta(a_j,b_i)/(Cγ) yields, after rescaling, an absolute error of epsilon*Delta(a_j,b_i) = Θ(C/epsilon) on the Hamming distance, again exceeding the allowed d√(Cγ) = Θ(√C/epsilon). Since Theorem 4.1 derives its shadow lower bound from Theorems 2.4 and 3.1, its main conclusion is unsupported as well. Although the reduction in Theorem 4.1 is logically sound in isolation, it cannot compensate for the failure of the lower bounds on which it relies.
- [Section 7, proof of Theorem 7.1] The claimed error bound c'*epsilon*|<ψ|φ>| ≤ c'*epsilon*γ ≤ c'/epsilon does not bridge to the required d√(Cγ)-additive estimate of Delta. Even if the inner-product estimate had absolute error O(epsilon*|<ψ|φ>|), rescaling by √(DD') to recover <â_j,b_i> multiplies that error by a factor of size Θ(2^{n/4}√(Cγ)), giving again Θ(C/epsilon) absolute error on Delta, rather than the required d√C/epsilon. The displayed inequality is also not a valid measure of the final Hamming error, since |<ψ|φ>| ≤ 1 makes c'*epsilon*γ = c'/epsilon bound a quantity much larger than the actual inner-product error. Thus Theorem 7.1 is not established by the given argument.
minor comments (3)
- [Appendix A, proof of Theorem 2.3] In the displayed algebra, the term involving 6·√γ/c^2 is inconsistent: since C = 9/c^2 and N = Cγ, the computation should give N/2 - 6√γ/c = N/2 - 2√N. The current display appears to have a typo in the exponent of c.
- [Theorem 2.3 statement and proof] The theorem states an additive approximation of d√(Cγ), but the proof later refers to a d√(Δ(a,b))-additive approximation; this should be d√(Cγ) (or d√N) to match the statement and the subsequent application.
- [Section 4, Theorem 4.1] The reduction from classical shadows to the ⟨M⟩ problem is stated for success probability 1 - δ, but the communication lower bounds are quoted at constant failure probability 1/3; a sentence explaining how the parameters are matched would improve readability, although this is not a load-bearing issue.
Circularity Check
No significant circularity: the lower bounds are reductions from the standard Indexing problem via external Gap-Hamming and majority-bias results.
full rationale
The paper derives its lower bounds by reduction from the Indexing problem (Theorem 2.1, cited from [JKS08]), using a Gap-Hamming-to-Indexing reduction (Theorem 2.3, credited to [ACK+15] with a proof in Appendix A based on [JKS08], [Rou15], and [Woo07]). No parameter in these reductions is fitted to the claimed conclusion; the states and observables are explicitly constructed from the Indexing instance, and the relative-error oracle is used only as a subroutine. Theorems 3.1, 5.1, 6.1, and 7.1 follow the same reduction pattern rather than assuming what they prove. Theorem 4.1 is a reduction from the classical shadows task to the communication problem and is logically independent of the lower bounds. There is no load-bearing self-citation: the cited lower bounds and reductions are external standard results. The suspected relative-error-to-additive-error conversion issue is a quantitative error-bound gap, not a step that is equal to its input by construction, so it does not amount to circularity.
Assumptions & free parameters
free parameters (1)
- C =
C = 9/c^2, 0 < c < sqrt(2/pi)
assumptions (5)
- standard math Indexing problem requires Ω(n) one-way randomized communication (Theorem 2.1).
- standard math The Gap-Hamming to Indexing reduction (Theorem 2.3) from [ACK+15, Lemma 5.3] is valid.
- standard math Majority bias lemma (Lemma A.1) from [Woo07] holds, providing the constant c.
- domain assumption Relative error can be converted to additive error via the identity 2<a,b> = ||a+b||^2 - ||a||^2 - ||b||^2.
- domain assumption Quantum state descriptions can be represented with O(n) bit precision.
Cite this review
Pith. "Pith review of Lower Bounds on Relative Error Quantum Compression and Classical Shadows." pith.science (2026). https://pith.science/paper/2IXR7LWB
@misc{pith2026250621345,
author = {Pith},
title = {Pith review of: Lower Bounds on Relative Error Quantum Compression and Classical Shadows},
year = {2026},
howpublished = {\url{https://pith.science/paper/2IXR7LWB}},
note = {Machine review of arXiv:2506.21345}
}
abstract
We study the question of how much classical communication is needed when Alice is given a classical description of a quantum state $|\psi\rangle$ for Bob to recover any expectation value $\langle \psi | M |\psi\rangle$ given an observable $M$ with $M$ Hermitian and $||M||_{\text{op}} \leq 1$. This task, whose study was initiated by Raz (ACM 1999) and more recently investigated by Gosset and Smolin (TQC 2019), can be thought of as a fully classical version of the pure state case of the well-known classical shadows problem in quantum learning theory. We show how the hardness of these two seemingly distinct problems are connected. We first consider the relative error version of the communication question and prove a lower bound of $\Omega(\sqrt{2^{n}}\epsilon^{-2})$ on the one-way randomized classical communication, improving upon an additive error lower bound of $\Omega(\sqrt{2^{n}})$ as shown by Gosset and Smolin. Notably, we show that this lower bound holds not only for the set of all observables but also when restricted to just the class of Pauli observables. This fact implies a $\Omega(\sqrt{2^{n}})$ versus $O(\text{poly}(n))$ separation in the compression size between the relative and additive error settings for non-adaptive Pauli classical shadows with classical memory. Extending this framework, we prove randomized communication lower bounds for other relative error one-way classical communication tasks: an $\Omega(2^{n}\epsilon^{-2})$ lower bound when instead Alice is given an observable and Bob is given a quantum state and they are asked to estimate the expectation value, an $\Omega(\sqrt{n}\epsilon^{-2})$ lower bound when restricted to Paulis, and an $\Omega(\sqrt{2^{n}}\epsilon^{-2})$ lower bound when Alice and Bob are both given quantum states and asked to estimate the inner product.
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Reviewed August 6, 2026 · model on record in the stance chip above.
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