# Daemonic ergotropy: generalised measurements and multipartite settings

## TL;DR

This paper extends the concept of daemonic ergotropy by including generalised measurements and multipartite systems, showing how quantum correlations influence work extraction capabilities.

## Contribution

It introduces a see-saw algorithm for optimal measurements and generalizes daemonic ergotropy to multipartite states, highlighting the role of quantum correlations.

## Key findings

- Generalised measurements can increase daemonic ergotropy beyond projective measurements.
- Quantum correlations do not outperform classical correlations in work extraction with projective measurements.
- Multipartite quantum correlations influence the maximum achievable work extraction.

## Abstract

Recently, the concept of daemonic ergotropy has been introduced to quantify the maximum energy that can be obtained from a quantum system through an ancilla-assisted work extraction protocol based on information gain via projective measurements [G. Francica {\it et al.}, npj Quant. Inf. {\bf 3}, 12 (2018)]. We prove that quantum correlations are not advantageous over classical correlations if projective measurements are considered. We go beyond the limitations of the original definition to include generalised measurements and provide an example in which this allows for a higher daemonic ergotropy. Moreover, we propose a see-saw algorithm to find a measurement that attains the maximum work extraction. Finally, we provide a multipartite generalisation of daemonic ergotropy that pinpoints the influence of multipartite quantum correlations, and study it for multipartite entangled and classical~states.