We determine the exact worst-direction Fisher-information cost of using a parameter-independent readout to estimate an unknown qubit direction at known Bloch-vector length $η$. Every fixed-local architecture, including recorded classical randomization, heterogeneous single-copy measurements, and arbitrary outcome spaces, reduces exactly to a zero-barycenter probability measure on the Bloch ball. For every full-rank qubit and every trace-balanced spectral Fisher loss, the resulting minimax problem is rigid: the unique optimal aggregate design is the spin-coherent Haar positive-operator-valued measure (POVM). For $N$ copies, inverse-Fisher $A$ loss has the exact value $2/[Nf(η)]$, where $
f(η)=
\frac{2η-(1-η^2)\log[(1+η)/(1-η)]}{4η}$. This uniqueness has an immediate finite-readout consequence. No finite-support measurement attains the unrestricted mixed-state optimum, while at the smallest globally regular support the tetrahedral symmetric informationally complete (SIC) measurement is uniquely $A$- and $D$-minimax, with exact worst-direction values. Relaxing the fixed-readout constraint separates the asymptotic resources: one-way local operations and classical communication (LOCC), unrestricted LOCC, and separable measurements have $A$-loss coefficient $4/η^2$, whereas collective measurements attain $2(1+η)/η^2$. We further classify the rigidity conditions for unequal contrasts and show that Haar uniqueness survives at the nonregular pure-state endpoint.
We determine the exact worst-direction Fisher-information cost of using a parameter-independent readout to estimate an unknown qubit direction at known Bloch-vector length η. Every fixed-local architecture, including recorded classical randomization, heterogeneous single-copy measurements, and arbitrary outcome spaces, reduces exactly to a zero-barycenter probability measure on the Bloch ball. For every full-rank qubit and every trace-balanced spectral Fisher loss, the resulting minimax problem is rigid: the unique optimal aggregate design is the spin-coherent Haar positive-operator-valued measure (POVM). For N copies, inverse-Fisher A loss has the exact value 2/[Nf(η)], where $
f(η)=
\frac{2η-(1-η^2)\log[(1+η)/(1-η)]}{4η}.Thisuniquenesshasanimmediatefinite−readoutconsequence.Nofinite−supportmeasurementattainstheunrestrictedmixed−stateoptimum,whileatthesmallestgloballyregularsupportthetetrahedralsymmetricinformationallycomplete(SIC)measurementisuniquelyA−andD−minimax,withexactworst−directionvalues.Relaxingthefixed−readoutconstraintseparatestheasymptoticresources:one−waylocaloperationsandclassicalcommunication(LOCC),unrestrictedLOCC,andseparablemeasurementshaveA−losscoefficient4/η^2,whereascollectivemeasurementsattain2(1+η)/η^2$. We further classify the rigidity conditions for unequal contrasts and show that Haar uniqueness survives at the nonregular pure-state endpoint.
We determine the exact local precision limits for single-copy estimation of smooth multiparameter quantum statistical models contained in spin factors, a class of matrix Jordan algebras whose state spaces generalize the qubit Bloch ball. At any parameter point where the symmetric logarithmic derivative (SLD) Fisher information is positive definite, we characterize the entire attainable classical Fisher-information region over all finite-outcome positive-operator-valued measurements. After SLD normalization, this region consists exactly of the real symmetric positive semidefinite matrices with trace at most one, independently of the ambient Hilbert-space dimension. This yields a sharp weighted covariance bound for every positive definite weight, attained by an explicit locally unbiased estimator based on randomized spectral measurements of SLD directions. The proof combines a statistics-preserving positive projection onto the spin factor with the two-eigenvalue structure of its effects, revealing the Jordan-algebraic origin of the tradeoff. The result extends the qubit information tradeoff to models with more than three parameters. Since the optimal measurement depends on the unknown parameter, we simulate an adaptive scheme for a five-parameter model on a four-dimensional Hilbert space and observe performance close to the optimal local benchmark.
Abstract The central problem of quantum statistics is to devise measurement schemes for the estimation of an unknown state, given an ensemble of n independent identically prepared systems. For locally quadratic loss functions, the risk of standard procedures has the usual scaling of 1/ n . However, it has been noticed that for fidelity based metrics such as the Bures distance, the risk of conventional (non-adaptive) qubit tomography schemes scales as 1 / n for states close to the boundary of the Bloch sphere. Several proposed estimators appear to improve this scaling, and our goal is to analyse the problem from the perspective of the maximum risk over all states. We propose qubit estimation strategies based on separate adaptive measurements, and collective measurements, that achieve 1/ n scalings for the maximum Bures risk. The estimator involving local measurements uses a fixed fraction of the available resource n to estimate the Bloch vector direction; the length of the Bloch vector is then estimated from the remaining copies by measuring in the estimator eigenbasis. The estimator based on collective measurements uses local asymptotic normality techniques which allows us to derive upper and lower bounds to its maximum Bures risk. We also discuss how to construct a minimax optimal estimator in this setup. Finally, we consider quantum relative entropy and show that the risk of the estimator based on collective measurements achieves a rate O ( n − 1 log n ) under this loss function. Furthermore, we show that no estimator can achieve faster rates, in particular the ‘standard’ rate n −1 .
Overview This preprint develops a finite-history framework for quantifying how coherently stored environment records reveal logical quantum information. Instead of considering only which error occurred, the analysis treats the full complementary environment channel associated with a labeled family of finite quantum-error histories. For two selected history labels, the leakage is expressed exactly through the numerical-range diameter of the compressed relative operator. This yields global finite-history compression bounds, a zero-leakage characterization equivalent to Knill–Laflamme scalarity, and lower and upper bounds for recovery by a single common channel. Main results For a balanced independent two-step dephasing model on one logical qubit encoded into two physical qubits, the paper solves the worst-case optimization over the full rank-two code Grassmannian: infV∈Gr(2,4)supϕ,ψLhist(V)=21. The transverse leakage is determined exactly by Ltr(η,t)=max{21,2η,41−η21+t}. All global minimizers are classified. Besides the parity-isoclinic orthogonal-axis family, the minimizer set contains a genuine non-isoclinic region characterized by 0<η≤41,ez=oz=0,∣e⋅o∣≤1−4η24η2. A universal quartic inequality needed for the all-code theorem is reduced analytically to an exact polynomial-positivity problem and closed using rational Bernstein-basis verification. Distributional stability On a fixed labeled history-operator family, the complementary channel, worst-parameter leakage, and all-code minimax value are Lipschitz-stable under changes of the history probability distribution. With dA(p,q)=[∑h(ph−qh)2]1/2, the minimax value satisfies ∣Λ(p)−Λ(q)∣≤2dA(p,q). Consequently, key quantitative implications of the balanced complete solution—including the minimax value, principal near-minimizer coordinate constraints, and a universal operational recovery lower bound—remain stable under nearby biased or correlated perturbations.
Informational completeness (IC) guarantees that an inverse exists, not that it is statistically well conditioned. For minimal rank-one Weyl--Heisenberg (WH) measurements, covariance makes the nonidentity projector-Gram spectrum proportional to the fiducial's ambiguity intensities, with eigenvalues d∣χφ(u)∣2, turning stability into an explicit worst-direction design problem; write λ for its smallest nonidentity eigenvalue. Haar fiducials are IC almost surely while E[λ−1]=∞, and an explicit geometric family used to establish balanced informationally complete measurements in every dimension has a normalized spectral floor bounded above by an exponentially decaying envelope. We then construct a hierarchy of minimal measurements. A cyclic family with exactly d2 outcomes in every integer dimension has floors Θ(d−3) for odd d and Θ(d−5) for even d; a finite-field family for q=2m obeys the uniform bound λ≥4/9. Our main result treats every prime-power dimension of characteristic p≥5. A balanced one-coordinate perturbation repairs the zero ambiguity axis of a cubic Alltop state, gives an attained floor uniformly bounded below by a positive constant, and confines the entire nonidentity spectrum to [Lq,Uq] with Uq/Lq→1. Its SIC-normalized minimum tends to one, and λ(φq)/Λq⋆→1 for the global finite-field WH max--min optimum Λq⋆, without assuming SIC existence. The complete spectrum determines the exact finite-sample Hilbert--Schmidt error of canonical linear inversion at I/d, while its lower edge controls local Fisher efficiency and canonical-shadow bounds.
Abstract We study the one-shot reconstruction of a quantum state when a single copy of the source is measured, the outcome is stored in a classical record of fixed finite dimension, and a state is prepared at the destination from the record. This acquisition-limited problem is distinguished from a visible benchmark in which the source label is available to the encoder. For a finite ensemble we derive finite-record bounds on the visible distortion in terms of the cumulative prior mass covered by the record and the optimal discrimination probability, identify the regimes in which they are exact, and solve the two-state visible problem at every record dimension. For pure-state ensembles, exact acquisition-limited reconstruction is possible if and only if the states are perfectly distinguishable, so nonorthogonal targets retain a positive error at every record dimension. For two equiprobable nonorthogonal pure states we derive the optimal trace-distance distortion within the reflection-symmetric two-outcome class in closed variational form and find that unrestricted numerical optimization with up to six outcomes attains the same value; the resulting symmetric value lies strictly below the guess-and-prepare, unambiguous-discrimination, fidelity-optimal, and Haar-random strategies and is realized by one classical bit and one feed-forward rotation. Every classical-record reconstruction is shown to be entanglement breaking, with a quantitative bound on the loss of entanglement with an inaccessible reference, and the record dimension is related to the spectral rank of an energy-constrained memory. These results quantify the separation between a finite classical description and the information obtainable from a single quantum system. MSC Classification: 81P15 , 81P17 , 81P45 , 81P47
How much information about an unknown parameter can be assigned to one realized outcome, and how is that information constrained by the sensitivity of the same outcome? We derive upper bounds on the pointwise mutual information in terms of the stochastic Fisher information, both for parameters in a bounded interval and for sufficiently regular differentiable priors, and state the corresponding saturation conditions. Averaging the pointwise inequality and applying Jensen's inequality twice yields the known mutual-information--Fisher-information bound, while making explicit the outcome dependence discarded by the ensemble description. We illustrate the classical result using the instantaneous position of an overdamped Langevin particle as the observed outcome. For a fixed, parameter-independent quantum measurement, the stochastic Fisher information is further bounded by the conditional quantum Fisher information (CQFI), whose probability-weighted mean is the quantum Fisher information. A thermal-qubit field-sensing example evaluates this finite-prior bound deterministically from exact 2×2 matrices and, separately, decomposes the projected SLD second moment into population, coherent, and interference contributions. The interference contribution can be negative for an individual probe direction and has zero unweighted trace, but it is not itself an operational measure of readout quality. Neither the CQFI nor its mean selects a measurement basis: the latter is measurement-independent, whereas the relevant basis-dependent local objective is the classical Fisher information of the chosen measurement.