  The resolution limits of classical spectroscopy can be surpassed by
quantum-inspired methods leveraging the information contained in the phase of
the complex electromagnetic field. Their counterpart in spatial imaging has
been widely discussed and demonstrated; however, the spectral-domain
implementations are few and scarce. We experimentally demonstrate a
spectroscopic super-resolution method aimed at broadband light (10s to 100s of
GHz), and based on the spectral-domain analog of image inversion
interferometry. In a proof-of-principle experiment, we study the paradigmatic
problem of estimating a small separation between two incoherent spectral
features of equal brightness, with a small number of photons per coherence
time. On the grounds of asymptotic estimation theory, more than a $2$-fold
improvement over the spectral direct imaging is demonstrated in terms of
required resources (photons) for a given estimator variance. The setup is based
on an actively stabilized Mach-Zehnder-type interferometer with electro-optic
time lenses and passive spectral dispersers implementing the inversion. As
such, the method promises on-chip integration, good scalability, and further
applications e.g. for mode sorting.
