  We discuss a quantum refrigerator to increase the ground state probability of
a target qubit whose energy difference between the ground and excited states is
less than the thermal energy of the environment. We consider two types of
quantum refrigerators: (1) one extra qubit with frequent pulse operations and
(2) two extra qubits without them. These two types of refrigerators are
evaluated from the viewpoint of quantum thermodynamics. More specifically, we
calculate the heat removed from the target qubit, the work done for the system,
and the coefficient of performance (COP), the ratio between the heat ant the
work. We show that the COP of the second type outperforms that of the first
type. Our results are useful to design a high-performance quantum refrigerator
cooling down a qubit.
