The availability of controllable macroscopic devices, which maintain quantum coherence over relatively long time intervals, for the first time allows an experimental realization of many effects previously considered only as Gedankenexperiments, such as the operation of quantum heat engines. The theoretical efficiency η of quantum heat engines is restricted by the same Carnot boundary η_C as for the classical ones: any deviations from quasistatic evolution suppressing η below η_C. Here we investigate an implementation of an analog of the Otto cycle in a tunable quantum coherent circuit and show that the specific source of inefficiency is the quantum squeezing of the thermal state due to the finite speed of compression/expansion of the system. ; Comment: 17 pages, 5 figures