# Single atom energy-conversion device with a quantum load

## TL;DR

This paper demonstrates a fully quantum single-atom energy-conversion device functioning as an engine or refrigerator, transferring energy cyclically to a quantum load, with performance aligning with theoretical predictions.

## Contribution

It presents the first experimental realization of a fully quantum 4-stroke energy-conversion device coupled to a quantum load.

## Key findings

- Ergotropy increases with engine cycles despite rising load entropy.
- Performance matches theoretical models across various parameters.
- Achieved cyclic energy transfer between a single ion's optical and vibrational modes.

## Abstract

This work reports the functioning of a single atom energy-conversion device, operating either as a quantum engine or a refrigerator, coupled to a quantum load. The "working fluid" is comprised of two optical levels of a single ion, and the load is one vibrational mode of the same ion cooled down to the quantum regime. The energy scales of these two modes differ by 9 orders of magnitude. We realize cyclic energy transfers between the working fluid and the quantum load, either increasing or decreasing the population of the vibrational mode. This is achieved albeit the interaction between the load and the working fluid leads to a significant population redistribution and quantum correlations between them. The performance of the engine cycles as a function of several parameters is examined, and found to be in agreement with theory. We specifically look at the ergotropy of the load, which indicates the amount of energy stored in the load that can be extracted with a unitary process. We show that ergotropy rises with the number of engine cycles despite an increase in the entropy of the load. Our experiment represents the first fully quantum 4-stroke energy-conversion device operating with a generic coupling to a quantum load.