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arXiv:quant-ph/0602086

Optimal Trade-Off: Information Gain vs Distortion Loss In Finite-Dimensional Quantum Systems

Thijs van der Valk

Published Feb 9, 2006• 0 citations
arXivHTMLPDF

Abstract

In this thesis, I reflect on quantum instruments that measure the state of pure finite dimensional quantum systems. As the Heisenberg principle dictates, there exists a joint restriction to the information gain and distortion by measurement of a quantum system. I minimize the distortion of measurement and maximize the quality of the outcome by finding the optimal (covariant) instrument. In order to optimize this joint restriction, the family of covariant instruments is classified. This is an important side result.

DOI
10.48550/arxiv.quant-ph/0602086

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In this thesis, I reflect on quantum instruments that measure the state of pure finite dimensional quantum systems. As the Heisenberg principle dictates, there exists a joint restriction to the information gain and distortion by measurement of a quantum system. I minimize the distortion of measurement and maximize the quality of the outcome by finding the optimal (covariant) instrument. In order to optimize this joint restriction, the family of covariant instruments is classified. This is an important side result.
Abstract We discuss symmetric quantum measurements and the associated covariant observables modelled, respectively, as instruments and positive-operator-valued measures. The emphasis of this work are the optimality properties of the measurements, namely, extremality, informational completeness, and the rank-1 property which contrast the complementary class of (rank-1) projection-valued measures. The first half of this work concentrates solely on finite-outcome measurements symmetric w.r.t. finite groups where we derive exhaustive characterizations for the pointwise Kraus-operators of covariant instruments and necessary and sufficient extremality conditions using these Kraus-operators. We motivate the use of covariance methods by showing that observables covariant with respect to symmetric groups contain a family of representatives from both of the complementary optimality classes of observables and show that even a slight deviation from a rank-1 projection-valued measure can yield an extreme informationally complete rank-1 observable. The latter half of this work derives similar results for continuous measurements in (possibly) infinite dimensions. As an example we study covariant phase space instruments, their structure, and extremality properties.
This paper investigates the optimization of quantum state measurement using the principles of information entropy. The core idea is to treat quantum state measurement as an information entropy problem, aiming to maximize the information gained from the measurement process. Traditional quantum measurement techniques often rely on probabilistic outcomes, lacking control and inherent optimization. This work proposes a novel approach that leverages information entropy to design and optimize measurement schemes. The theoretical framework centers around quantifying uncertainty associated with quantum states and formulating measurement strategies to minimize this uncertainty, thereby maximizing the information obtained. Specifically, we explore the relationship between measurement outcomes, entropy, and the achievable precision. The proposed methodology offers a pathway to more efficient and accurate quantum state determination, with potential applications in quantum sensing, quantum communication, and fundamental quantum information theory. The key contributions of this research lie in the systematic application of information entropy to guide measurement selection and the development of a quantifiable metric for evaluating the effectiveness of measurement strategies. The final result is a framework that allows for controlled optimization of quantum measurements, moving beyond purely probabilistic approaches.
Optimization of quantum measurement processes has a pivotal role in carrying out better, more accurate or less disrupting, measurements and experiments on a quantum system. Especially, convex optimization, i.e., identifying the extreme points of the convex sets and subsets of quantum measuring devices plays an important part in quantum optimization since the typical figures of merit for measuring processes are affine functionals. In this thesis, we discuss results determining the extreme quantum devices and their relevance, e.g., in quantum-compatibility-related questions. Especially, we see that a compatible device pair where one device is extreme can be joined into a single apparatus essentially in a unique way. Moreover, we show that the question whether a pair of quantum observables can be measured jointly can often be formulated in a weaker form when some of the observables involved are extreme. Another major line of research treated in this thesis deals with convex analysis of special restricted quantum device sets, covariance structures or, in particular, generalized imprimitivity systems. Some results on the structure ofcovariant observables and instruments are listed as well as results identifying the extreme points of covariance structures in quantum theory. As a special case study, not published anywhere before, we study the structure of Euclidean-covariant localization observables for spin-0-particles. We also discuss the general form of Weyl-covariant phase-space instruments. Finally, certain optimality measures originating from convex geometry are introduced for quantum devices, namely, boundariness measuring how ‘close’ to the algebraic boundary of the device set a quantum apparatus is and the robustness of incompatibility quantifying the level of incompatibility for a quantum device pair by measuring the highest amount of noise the pair tolerates without becoming compatible. Boundariness is further associated to minimum-error discrimination of quantum devices, and robustness of incompatibility is shown to behave monotonically under certain compatibility-non-decreasing operations. Moreover, the value of robustness of incompatibility is given for a few special device pairs.
This thesis addresses the problem of developing a quantum counter-part of the well established classical theory of control. We dwell on the fundamental fact that quantum states are generally not perfectly distinguishable, and quantum measurements typically introduce noise in the system being measured. Because of these, it is generally not clear whether the central concept of the classical control theory -- that of observing the system and then applying feedback -- is always useful in the quantum setting. We center our investigations around the problem of transforming the state of a quantum system into a given target state, when the system can be prepared in different ways, and the target state depends on the choice of preparation. We call this the "quantum tracking problem" and show how it can be formulated as an optimization problem that can be approached both numerically and analytically. This problem provides a simple route to the characterization of the quantum trade-off between information gain and disturbance, and is seen to have several applications in quantum information.