  Quantum microwave photonics aims at generating, routing, and manipulating
propagating quantum microwave fields in the spirit of optical photonics. To
this end, the strong nonlinearities of superconducting quantum circuits can be
used to either improve or move beyond the implementation of concepts from the
optical domain. In this context, the design of a well-controlled broadband
environment for the superconducting quantum circuits is a central task. In this
work, we place a superconducting transmon qubit in one arm of an on-chip
Mach-Zehnder interferometer composed of two superconducting microwave beam
splitters. By measuring its relaxation and dephasing rates we use the qubit as
a sensitive spectrometer at the quantum level to probe the broadband
electromagnetic environment. At high frequencies, this environment can be well
described by an ensemble of harmonic oscillators coupled to the transmon qubit.
At low frequencies, we find experimental evidence for colored quasi-static
Gaussian noise with a high spectral weight, as it is typical for ensembles of
two-level fluctuators. Our work paves the way towards possible applications of
propagating microwave photons, such as emulating quantum impurity models or a
novel architecture for quantum information processing.
