Sisyphus cooling and amplification by a superconducting qubit

A superconducting qubit—a mesoscopic structure that behaves like a quantum two-level system—has been used to change the temperature of a resonant circuit, in close analogy to the so-called Sisyphus cooling and amplification protocols used in laser cooling of atoms. Laser cooling of atoms paved the way for remarkable achievements in quantum optics, including Bose–Einstein condensation and trapping in optical lattices. Recently, superconducting qubits—micrometre-size superconducting circuits—were shown to act as artificial atoms, exhibiting quantum effects such as Rabi oscillations and Ramsey fringes 1 , 2 , 3 . Coupling superconducting circuits to resonators brought them into the realm of quantum electrodynamics 4 , 5 , 6 , 7 and opened up perspectives for using them as micro-coolers or to create a population inversion inducing lasing behaviour 8 , 9 , 10 , 11 , 12 . Here, we demonstrate so-called Sisyphus cooling 13 and amplification of an LC resonator, which consists of an inductor L and a capacitor C, by a superconducting qubit, furthering the analogies between optical and circuit quantum electrodynamics. In quantum optics, the motion of the atom is cooled or amplified by a laser driving its electronic degrees of freedom. In our system, the roles of the two degrees of freedom are played by the levels of the resonator and the qubit. Red-detuned high-frequency driving of the qubit produces cooling, because the low-frequency LC circuit carries out work in the forward and backward oscillation cycle, always increasing the energy of the qubit. For blue-detuning, the same mechanism leads to Sisyphus amplification and a precursor of lasing. Parallel to the experimental demonstration, we analyse these processes theoretically, quantitatively confirming our interpretation.