  Qubits are physical, a quantum gate thus not only acts on the information
carried by the qubit but also on its energy. What is then the corresponding
flow of energy between the qubit and the controller that implements the gate?
Here we exploit a superconducting platform to answer this question in the case
of a quantum gate realized by a resonant drive field. During the gate, the
superconducting qubit becomes entangled with the microwave drive pulse so that
there is a quantum superposition between energy flows. We measure the energy
change in the drive field conditioned on the outcome of a projective qubit
measurement. We demonstrate that the drive's energy change associated with the
measurement backaction can exceed by far the energy that can be extracted by
the qubit. This can be understood by considering the qubit as a weak
measurement apparatus of the driving field.
