Hamiltonians generated by Parseval frames

TL;DR

This paper explores how Hamiltonians can be expressed using Parseval frames instead of orthonormal bases, analyzing the spectral implications and introducing the concept of E-connection for observables in quantum systems.

Contribution

It extends the spectral decomposition of Hamiltonians from orthonormal bases and Riesz bases to Parseval frames, with physical motivation and new theoretical insights.

Findings

Spectral properties change when using Parseval frames instead of orthonormal bases.

Introduction of the E-connection concept for observables.

Several examples illustrating the theoretical developments.

Abstract

It is known that self-adjoint Hamiltonians with purely discrete eigenvalues can be written as (infinite) linear combination of mutually orthogonal projectors with eigenvalues as coefficients of the expansion. The projectors are defined by the eigenvectors of the Hamiltonians. In some recent papers, this expansion has been extended to the case in which these eigenvectors form a Riesz basis or, more recently, a $\D$-quasi basis, \cite{bell,bit}, rather than an orthonormal basis. Here we discuss what can be done when these sets are replaced by Parseval frames. This interest is motivated by physical reasons, and in particular by the fact that the {\em mathematical } Hilbert space where the physical system is originally defined, contains sometimes also states which cannot really be occupied by the {\em physical} system itself. In particular, we show what changes in the spectrum of the observables, when going from orthonormal bases to Parseval frames. In this perspective we propose the notion of $E$-connection for observables. Several examples are discussed.