  We present a method for measuring the internal state of a superconducting
qubit inside an on-chip microwave resonator. We show that one qubit state can
be associated with the generation of an increasingly large cavity coherent
field, while the other remains associated with the vacuum. By measuring the
outgoing resonator field with conventional devices, an efficient single-shot
QND-like qubit readout can be achieved, enabling a high-fidelity measurement in
the spirit of the electron-shelving technique for trapped ions. We expect that
the proposed ideas can be adapted to different superconducting qubit designs
and contribute to the further improvement of qubit readout fidelity.
