We investigate the quantum thermodynamical properties of localised
relativistic quantum fields, and how they can be used as quantum thermal
machines. We study the efficiency and power of energy transfer between the
classical gravitational degrees of freedom, such as the energy input due to the
motion of boundaries or an impinging gravitational wave, and the excitations of
a confined quantum field. We find that the efficiency of energy transfer
depends dramatically on the input initial state of the system. Furthermore, we
investigate the ability of the system to extract energy from a gravitational
wave and store it in a battery. This process is inefficient in optical cavities
but is significantly enhanced when employing trapped Bose Einstein condensates.
We also employ standard fluctuation results to obtain the work probability
distribution, which allows us to understand how the efficiency is related to
the dissipation of work. Finally, we apply our techniques to a setup where an
impinging gravitational wave excites the phononic modes of a Bose Einstein
condensate. We find that, in this case, the percentage of energy transferred to
the phonons approaches unity after a suitable amount of time. These results
give a quantitative insight into the thermodynamic behaviour of relativistic
quantum fields confined in cavities.Comment: 35 pages, 3 figures. Manuscript substantially updated. I. Fuentes
also published as I. Fuentes-Guridi and I. Fuentes-Schulle
