Coherent-State Overcompleteness, Path Integrals, and Weak Values

Abstract

In the Hilbert space of a quantum particle the standard coherent-state resolution of unity is written in terms of a phase-space integration of the outer product $|z\rangle \langle z|$. Because no pair of coherent states is orthogonal, one can represent the closure relation in non-standard ways, in terms of a single phase-space integration of the "unlike" outer product $|z'\rangle \langle z|$, $z'\ne z$. We show that all known representations of this kind have a common ground, and that our reasoning extends to spin coherent states. These unlike identities make it possible to write formal expressions for a phase-space path integral, where the role of the Hamiltonian ${\cal H}$ is played by a weak energy value ${\cal H}_{weak}$. Therefore, in this context, we can speak of weak values without any mention to measurements. The quantity ${\cal H}_{weak}$ appears as the ruler of the phase-space dynamics in the semiclassical limit.