REVIEW 4 major objections 4 minor 1 cited by
Optimal quantum measurements for additive information and disturbance measures
T0 review · 4 major / 4 minor · reviewed 2026-08-06 · deepseek-v4-flash
Pith's one-line read Logarithmic information and disturbance measures change which quantum measurement is optimal.
desk verdict Solid extension of Terashima's own geometric framework; the I-DR result is likely right, but the paper hands the load-bearing convex-hull and curvature arguments to earlier papers and needs to close that gap for DR unbounded. 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
The load-bearing machinery is the physically allowed region for a single outcome together with the center-of-mass picture for averaging. A measurement is a set of operators, each represented by a point on the information–disturbance plane with weight equal to its outcome probability; the average values are the convex hull of those points, and the lower boundary of that hull is the information–disturbance tradeoff. Optimal measurements are configurations whose center of mass lies on the lower boundary. The second derivative of the single-outcome lower boundary $(1,d-1)$ decides whether that center of mass sits at one point on a convex curve or at the ends of a straight hull segment, and this curvature sign is the mechanism behind all the paper's optimality results.
What would settle it
Compute the average $I$ and $D_R$ directly from Eqs. (7) and (21) for a concrete two-outcome measurement in $d=4$ whose single-outcome points lie far into the unbounded $D_R$ region, and check whether the average point lies on or above the claimed lower boundary of the blend region; a point below that boundary would falsify the center-of-mass step.
Extended reading notes
Core claim
On the information–disturbance plane, each single-outcome measurement operator lands on a point, and averaging over outcomes is represented by the convex hull of those points. The paper's discovery is that the lower boundary $(1,d-1)$ of this region has a curvature whose sign—$+$, $-$, $0$, or $\mp$—completely determines the optimal measurement form for the pair. For $I_G$–$D_F$ and $I_G$–$D_R$ the boundary is convex, so an optimal measurement must consist of measurement operators all corresponding to the same point on $(1,d-1)$; the explicit $d$-outcome family in Eq. (32) saturates both tradeoffs. For $I$–$D_F$ and $I$–$D_R$ the boundary is inverted S-shaped, so below a threshold information $I_T$ the optimum is a mixture of rank-one operators at the tangency point $T$ and the identity, the $d+1$-outcome measurement in Eq. (34). The paper concludes that the optimal measurements for $I_G$–$D_R$ coincide with those for $G$–$F$ rather than $G$–$R$, while the optimal measurements for $I$–$D_R$ are completely different from those for $I$–$R$ yet are always optimal for $I$–$D_F$.
Load-bearing premise
The load-bearing premise is that averaging over outcomes is exactly the center-of-mass blend of single-outcome points, including when $D_R(m)$ is infinite and many operators sit at infinite disturbance; if that step fails, the claimed optimal measurement forms in Eqs. (32) and (34) do not follow.
Editorial extensions
If this is right
- The $d$-outcome measurement family in Eq. (32) simultaneously saturates the $I_G$–$D_F$ and $I_G$–$D_R$ tradeoffs for any chosen information value, so a single physical construction achieves both additive tradeoffs.
- For entropy-reduction information below $I_T$, the $d+1$-outcome mixture in Eq. (34) lowers the disturbance compared with the $d$-outcome family; the paper reports reductions of about 0.8% for $D_F$ and 30% for $D_R$ at $d=4$ and $I=0.05$.
- The curvature sign acts as a classification: pairs with the same sign have optimal measurements of the same form, and the four signs $+$, $-$, $0$, and $\mp$ organize all additive and original information–disturbance pairs.
- The known failure of $G$–$R$ optima to be $G$–$F$ optimal does not extend to the additive measures: every $I$–$D_R$ optimal measurement is also $I$–$D_F$ optimal, even though $I$–$D_R$ and $I$–$R$ optima are completely different.
- Because the method only requires single-outcome multiplicative measures, it carries over to any other multiplicative information or disturbance quantity and to triplewise tradeoffs via two curvature signs.
Reading between the lines
- A natural testable extension, not reported in the paper, is to realize Eq. (34) for $d=4$ and check whether the measured $D_R$ drop below $I_T$ follows the predicted roughly 30% reduction.
- The curvature-sign rule suggests a general screening principle: applying any monotone function to an information or disturbance measure changes optimal measurements only if it changes the sign of the second derivative of the boundary; logarithms are one such case.
- Because $D_R(m)$ is unbounded, the center-of-mass step could be stress-tested numerically by sampling measurements with outcomes that have very large $D_R(m)$ and checking whether the average ever falls below the claimed blend region; if it does, $I$–$D_R$ optimality needs a separate argument.
Editorial analysis
A structured set of objections, weighed in public.
Referee Report
Summary. This paper defines additive information and disturbance measures IG, DF, and DR as per-outcome logarithms of estimation fidelity, operation fidelity, and physical reversibility, and pairs them with the inherently additive entropy reduction I. Using the physically allowed single-outcome region from the author's earlier work, the paper argues that averaging over outcomes corresponds to taking the convex hull of that region, and then reads off optimal measurements from the curvature of the lower boundary. The central claims are that for IG–DR the optimal measurements coincide with those for G–F rather than G–R, while for I–DR the optimal measurements form a new family obtained by mixing the identity operation with the tangent point T.
Significance. If the results are correct, the paper establishes an interesting qualitative phenomenon: switching from multiplicative to logarithmic measures changes which measurements achieve the optimal information–disturbance tradeoff, even though the separate measures are monotonically related. The classification by curvature signs is simple and potentially useful, and the explicit constructions in Eqs. (32) and (34) are concrete and falsifiable. The paper is not fully self-contained, relying on Refs. [32,33,36] for the allowed regions, formulas, and curvature signs; however, those are published parameter-free derivations, so the main risk is not circularity but missing rigor in the convex-hull step for the unbounded DR region.
major comments (4)
- [Section 4] Eq. (23) and Fig. 1(b),(d): the DR-based single-outcome region is unbounded, with DR(m)=∞ for rank-deficient operators, but the convex-hull step for averaging over outcomes is asserted rather than proved. The center-of-mass analogy requires a precise treatment for unbounded sets: one must show that any measurement with finite average DR uses only finite-DR outcomes, that the average pair lies in the convex hull of the finite-DR part of the single-outcome region, and that the lower boundary of that convex hull is the curve (1,d−1) for IG–DR or the tangent line from P_d to T for I–DR. Since Eqs. (32) and (34) are derived from this lower boundary, this gap is load-bearing; the manuscript currently offers no argument that the point P_1, which lies at infinite DR, cannot affect the finite-DR boundary.
- [Section 5 / Table 1] The curvature signs in Table 1 and in Eqs. (26)–(27) are the entire basis for the classification of optimal measurements, but they are only cited to Ref. [36] and stated locally 'near P_d.' Please derive the second derivatives of D_F(m) and D_R(m) with respect to I(m) along the boundary (1,d−1), or quote the exact formulas from Ref. [36] and show explicitly that they imply the signs in Table 1 for all relevant d. In particular, reconcile the local negative sign in Eq. (27) with the ∓ entry for I–DR in Table 1, and state where the inflection point occurs.
- [Section 5] The argument that 'for the center of mass to be at a point on a convex boundary, all particles must be at that point' presupposes strict convexity and excludes endpoints. The paper does not prove that (1,d−1) is strictly convex on the relevant interval for IG–DF and IG–DR, and it does not separately treat the endpoint cases P_d and P_1. Since Eq. (32) follows directly from this step, a rigorous proof should state the strictness condition and handle zero-curvature or endpoint cases.
- [Section 5, after Eq. (31)] The statement 'the optimal measurements for I–DR are always optimal for I–DF' appears to contradict the earlier statement in the same section that the two inequalities are not necessarily saturated at the same time because I_T is not equal between I–DF and I–DR. For I smaller than both threshold values, the optimal I–DR measurement is a mixture of P_d and T_DR, whereas the optimal I–DF measurement is a mixture of P_d and T_DF; unless T_DR = T_DF, these are different physical measurements. Please clarify whether 'always optimal' means 'has the same form' or 'belongs to the optimal set,' and correct the claim if it is meant as a statement about the same measurement.
minor comments (4)
- [Equations (28)–(29)] Please use explicit superscript notation such as 2^{I_G}, 2^{D_F}, and 2^{D_R} instead of the ambiguous '2IG', '1/2DF', and '1/2DR' in these inequalities.
- [Figure 1] For panels (b) and (d), state in the caption that the DR axis is truncated and that P_1 and other rank-deficient points with DR=∞ are not drawn, so the blue shaded region is only a finite portion of the true single-outcome region.
- [Table 1] The symbols +, −, 0, and ∓ should be defined explicitly in the caption or in the text; in particular, ∓ should be described as a sign change along the boundary, with the first symbol referring to the behavior near P_d.
- [Section 3] The sentence 'the net amount of information should be G − (1/d)' is not used in the derivation of I_G; either connect it to Eq. (11) or remove it to avoid confusion.
Circularity Check
No circular reduction: the additive-measure optimality claims are new consequences of parameter-free prior derivations; the unbounded-region convex-hull step is an omitted proof (correctness risk), not circularity.
full rationale
The derivation chain is: (1) single-outcome formulas for G(m), F(m), R(m) from Ref. [32]; (2) new additive measures IG, DF, DR defined in Eqs. (11)-(21); (3) single-outcome allowed regions plus the convex-hull/center-of-mass step for averaged values (Sec. 4, Ref. [33]); (4) curvature signs of (1,d-1) in Table 1, computed from derivative formulas of Ref. [36], with Eqs. (26)-(27) computed in-text; (5) optimal forms (32) and (34) read off from the convex-hull lower boundary. No step equates a prediction with an input by construction. The central claims ('IG-DR optimal measurements are those of G-F, not G-R' and 'I-DR optimal measurements are completely different from those for I-R') are new consequences of the computed curvature signs; they are not assumed by the definitions, and no parameter is fitted to the target result. The tangency point T, lambda_T, and c = sqrt(I/I_T) are determined by geometry and normalization, not by data. Self-citations [32], [33], [36] are load-bearing, but they are published parameter-free derivations whose assumptions (singular-value parametrization of measurement operators; outcome averaging as convex combination) do not include the additive-measure conclusions, so under the hard rules they are real evidence and do not raise the circularity score. Per the reviewing rule I flag one omitted proof, weighed in the verdict but not itself circular: Sec. 4 asserts 'the region for the average values is given by the convex hull of the region of a single outcome' without proof for the unbounded case, since Eq. (23) allows DR(m) = infinity and points Pr (r != d) and (k,l) (k+l != d) lie at DR = infinity; if the convex-hull identification fails on unbounded sets, the lower-boundary lines behind Eqs. (32) and (34) would not follow. That is a correctness/rigor risk, not a self-referential reduction, and is placed under correctness per the rules.
Assumptions & free parameters
assumptions (5)
- standard math Standard quantum measurement formalism: measurement operators satisfy the completeness relation, with Born-rule probabilities and state update.
- domain assumption The system is initially in a completely unknown pure state with a uniform prior.
- domain assumption Averaging over outcomes can be represented as the convex hull (center of mass) of the single-outcome allowed region.
- domain assumption The curvature signs of the lower boundary (1,d-1) summarized in Table 1 are correct for both original and additive measures.
- ad hoc to paper Information and disturbance should be additive under independent measurements on separable systems.
Cite this review
Pith. "Pith review of Optimal quantum measurements for additive information and disturbance measures." pith.science (2026). https://pith.science/paper/UCTZNPWU
@misc{pith2026250703346,
author = {Pith},
title = {Pith review of: Optimal quantum measurements for additive information and disturbance measures},
year = {2026},
howpublished = {\url{https://pith.science/paper/UCTZNPWU}},
note = {Machine review of arXiv:2507.03346}
}
read the original abstract
Additive measures for information and disturbance in quantum measurements of a system are defined from well-known multiplicative measures such as estimation and operation fidelities using a logarithm. This is motivated by the fact that information and disturbance are naturally assumed to be additive while performing independent measurements on separable systems. Although the additivity makes no remarkable difference when information and disturbance are separately considered, it can change measurements that only introduce minimal disturbance relative to the amount of information. Such optimal measurements are shown for additive information and disturbance measures with a tradeoff relationship.
Figures
Forward citations
Cited by 1 Pith paper
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The statistical disturbance bound of quantum measurements
A quantum measurement's statistics alone determine the minimum disturbance any implementation of it must cause, and this minimum is computable and experimentally estimable.
Reference graph
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