  Quantum technology promises revolutionizing applications in information
processing, communications, sensing, and modelling. However, efficient
on-demand cooling of the functional quantum degrees of freedom remains a major
challenge in many solid-state implementations, such as superconducting
circuits. Here, we demonstrate direct cooling of a superconducting resonator
mode using voltage-controllable quantum tunneling of electrons in a nanoscale
refrigerator. This result is revealed by a decreased electron temperature at a
resonator-coupled probe resistor, even when the electrons in the refrigerator
itself are at an elevated temperature. Our conclusions are verified by control
experiments and by a good quantitative agreement between a detailed theoretical
model and the direct experimental observations in a broad range of operation
voltages and phonon bath temperatures. In the future, the introduced
refrigerator can be integrated with different quantum electric devices,
potentially enhancing their performance. For the superconducting quantum
computer, for example, it may provide an efficient way of initializing the
quantum bits.
