The Geometry of Qubit Weak Values

• Published in: arXiv.org 2015-12
• Main Authors: Farinholt, J M, Ghazarians, A, Troupe, J E
• Format: Article
• Language: English
• Subjects: Euclidean geometry; Operators (mathematics); Quantum mechanics; Quantum theory; Qubits (quantum computing)
• ISSN: 2331-8422

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.