Universal Bounds on Quantum Mechanics through Energy Conservation and the Bootstrap Method

TL;DR

This paper demonstrates how the bootstrap method can derive universal bounds on quantum expectation values, generalizing uncertainty relations, but faces challenges with multi-particle systems and thermal states.

Contribution

It introduces the bootstrap method as a way to find state-independent bounds in quantum mechanics, extending the concept of uncertainty relations.

Findings

Bootstrap method derives universal bounds on expectation values.

Challenges exist in applying the method to multi-particle systems.

Some results are obtainable for micro-canonical ensembles in integrable systems.

Abstract

The range of motion of a particle with certain energy $E$ confined in a potential is determined from the energy conservation law in classical mechanics. The counterpart of this question in quantum mechanics can be regarded as what the possible range of the expectation values of the position operator $ \langle x \rangle$ of a particle, which satisfies $E= \langle H \rangle$. This range depends on the state of the particle, but the universal upper and lower bounds, which is independent of the state, must exist. In this study, we show that these bounds can be derived by using the bootstrap method. We also point out that the bootstrap method can be regarded as a generalization of the uncertainty relations, and it means that the bounds are determined by the uncertainty relations in a broad sense. Furthermore, the bounds on possible expectation values of various quantities other than position can be determined in the same way. However, in the case of multiple identical particles (bosons and fermions), we find some difficulty in the bootstrap method. Because of this issue, the predictive power of the bootstrap method in multi-particle systems is limited in the derivation of observables including energy eigenstates. In addition, we argue an application of the bootstrap method to thermal equilibrium states. We find serious issues that temperature and entropy cannot be handled. Although we have these issues, we can derive some quantities in micro-canonical ensembles of integrable systems governed by generalized Gibbs ensembles.