Role of energy-invariant assistants in energy extraction from quantum batteries

TL;DR

This paper explores how energy-invariant assistants can enable complete energy extraction from quantum batteries, establishing conditions for activation and inactivity.

Contribution

It introduces the concept that energy-invariant assistants can fully extract stored energy from quantum batteries and provides criteria for battery inactivity.

Findings

All stored energy can be extracted with an energy-invariant assistant of equal dimension.

A necessary and sufficient condition for a battery to be inactive is established.

Energy-invariant assistants can transform batteries into their ground state.

Abstract

We investigate the role of energy-invariant assistants in energy extraction from quantum batteries. To this end, for energy extraction, we restrict ourselves to unitaries that jointly act on the battery and the assistant but preserve the energy of the assistant. We demonstrate that, in the presence of an energy-invariant assistant having the same dimension as the battery, all stored energy of the battery can always be extracted, transforming the battery into its ground state when an appropriate joint unitary and assistant state are employed. Additionally, we provide a necessary and sufficient condition for a battery to be unable to provide any energy, i.e., to be inactive, even when an energy-invariant assistant is present and prepared in an arbitrary but fixed state.