The Geometry of Qubit Weak Values

TL;DR

This paper explores the mathematical and geometric structure of quantum weak values, especially for qubits, revealing new insights into their properties and representations in quantum mechanics.

Contribution

It introduces a linear map framework for weak values of any Hermitian operator and provides a complete geometric characterization for qubit systems.

Findings

Weak values can be represented as linear maps between Hermitian operators.

A geometric decomposition of weak values is possible using Euclidean structure.

Complete geometric characterization of weak values for qubits is achieved.

Abstract

The concept of a \emph{weak value} of a quantum observable was developed in the late 1980s by Aharonov and colleagues to characterize the value of an observable for a quantum system in the time interval between two projective measurements. Curiously, these values often lie outside the eigenspectrum of the observable, and can even be complex-valued. Nevertheless, the weak value of a quantum observable has been shown to be a valuable resource in quantum metrology, and has received recent attention in foundational aspects of quantum mechanics. This paper is driven by a desire to more fully understand the underlying mathematical structure of weak values. In order to do this, we allow an observable to be \emph{any} Hermitian operator, and use the pre- and post-selected states to develop well-defined linear maps between the Hermitian operators and their corresponding weak values. We may then use the inherent Euclidean structure on Hermitian space to geometrically decompose a weak value of an observable. In the case in which the quantum systems are qubits, we provide a full geometric characterization of weak values.