  We demonstrated microwave-induced cooling in a superconducting flux qubit.
The thermal population in the first-excited state of the qubit is driven to a
higher-excited state by way of a sideband transition. Subsequent relaxation
into the ground state results in cooling. Effective temperatures as low as
Teff~ 3 millikelvin are achieved for bath temperatures Tbath = 30 - 400
millikelvin, a cooling factor between 10 and 100. This demonstration provides
an analog to optical cooling of trapped ions and atoms and is generalizable to
other solid-state quantum systems. Active cooling of qubits, applied to quantum
information science, provides a means for qubit-state preparation with improved
fidelity and for suppressing decoherence in multi-qubit systems.
