Observable (quantum computation)

Introduction

In quantum mechanics, an observable is a property of the system, whose value may be determined by performing physical operations on the system. To every observable of the system, there is a corresponding self-adjoint operator, that is to say one whose matrix is Hermitian. Upon measurement, the value of the observable must become sharp. This means that the observable takes a value which is one of the eigenvalues of the Hermitian matrix. This set of values is the observable's spectrum. It is the case that for certain observables the expectation value of it (notated by angular brackets) may not be one of the eigenvalues of the matrix.