  We present a method for detecting electromagnetic (EM) modes that couple to a
superconducting qubit in a circuit-QED architecture. Based on
measurement-induced dephasing, this technique allows the measurement of modes
that have a high quality factor (Q) and may be difficult to detect through
standard transmission and reflection measurements at the device ports. In this
scheme the qubit itself acts as a sensitive phase meter, revealing modes that
couple to it through measurements of its coherence time. Such modes are
indistinguishable from EM modes that do not couple to the qubit using a vector
network analyzer. Moreover, this technique provides useful characterization
parameters including the quality factor and the coupling strength of the
unwanted resonances. We demonstrate the method for detecting both high-Q
coupling resonators in planar devices as well as spurious modes produced by a
3D cavity.
