Otto refrigerator based on a superconducting qubit: Classical and quantum performance

We analyze a quantum Otto refrigerator based on a superconducting qubit coupled to two LC resonators, each including a resistor acting as a reservoir. We find various operation regimes: nearly adiabatic (low driving frequency), ideal Otto cycle (intermediate frequency), and nonadiabatic coherent regime (high frequency). In the nearly adiabatic regime, the cooling power is quadratic in frequency, and we find a substantially enhanced coefficient of performance epsilon, as compared to that of an ideal Otto cycle. Quantum coherent effects lead invariably to a decrease in both cooling power and epsilon as compared to purely classical dynamics. In the nonadiabatic regime we observe strong coherent oscillations of the cooling power as a function of frequency. We investigate various driving wave forms: Compared to the standard sinusoidal drive, a truncated trapezoidal drive with optimized rise and dwell times yields higher cooling power and efficiency.