  Quantum feedback is a technique for measuring a qubit and applying
appropriate feedback depending on the measurement results. Here, we propose a
new on-chip quantum feedback method where the measurement-result information is
not taken from the chip to the outside of a dilution refrigerator. This can be
done by using a selective qubit-energy shift induced by measurement apparatus.
We demonstrate on-chip quantum feedback and succeed in the rapid initialization
of a qubit by flipping the qubit state only when we detect the ground state of
the qubit. The feedback loop of our quantum feedback method closed on a chip,
and so the operating time needed to control a qubit is of the order of 10 ns.
This operating time is shorter than with the convectional off-chip feedback
method. Our on-chip quantum feedback technique opens many possibilities such as
an application to quantum information processing and providing an understanding
of the foundation of thermodynamics for quantum systems.
