Sample Complexity of Black Box Work Extraction

TL;DR

This paper investigates the sample complexity needed to estimate extractable work from unknown quantum states, revealing the limitations with single copies and providing scalable protocols for multiple copies in quantum thermodynamics.

Contribution

It introduces a sample-efficient protocol for estimating work extraction from unknown quantum states, addressing the challenge of quantum state unknowns in thermodynamic tasks.

Findings

Single-copy ergotropy approaches zero asymptotically.

Sample complexity scales with desired accuracy and success probability.

Protocol enables estimation of thermodynamic quantities with multiple copies.

Abstract

Extracting work from a physical system is one of the cornerstones of quantum thermodynamics. The extractable work, as quantified by ergotropy, necessitates a complete description of the quantum system. This is significantly more challenging when the state of the underlying system is unknown, as quantum tomography is extremely inefficient. In this article, we analyze the number of samples of the unknown state required to extract work. With only a single copy of an unknown state, we prove that ergotropy approaches zero in the asymptotic limit, rendering work extraction nearly impossible. In contrast, when multiple copies are available, we quantify the sample complexity required to estimate extractable work, establishing a scaling relationship that balances the desired accuracy with success probability. Our work develops a sample-efficient protocol to assess the utility of unknown states as quantum batteries and opens avenues for estimating thermodynamic quantities using near-term quantum computers.