The gravitational quantum Otto refrigeration cycle

We take advantage of the gravitational redshift experienced by a photon propagating in curved spacetime in order to construct a quantum Otto refrigeration cycle. Deriving a lower bound for the relative temperature between the cold and hot reservoirs at which the engine operates, we provide examples of refrigeration cycles in the presence of a uniform gravitational field as well as in the case of a gravitational field produced by a non-rotating spherically symmetric charged body, and the one obtained from the vacuum solution of Einstein's field equations in an expanding universe, as a function of the parameters which describe each spacetime.