  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.
