  Four-wave-mixing-based quantum cascade laser frequency combs (QCL-FC) are a
powerful photonic tool, driving a recent revolution in major molecular
fingerprint regions, i.e. mid- and far-infrared domains. Their compact and
frequency-agile design, together with their high optical power and spectral
purity, promise to deliver an all-in-one source for the most challenging
spectroscopic applications. Here, we demonstrate a metrological-grade hybrid
dual comb spectrometer, combining the advantages of a THz QCL-FC with the
accuracy and absolute frequency referencing provided by a free-standing,
optically-rectified THz frequency comb. A proof-of-principle application to
methanol molecular transitions is presented. The multi-heterodyne molecular
spectra retrieved provide state-of-the-art results in line-center
determination, achieving the same precision as currently available molecular
databases. The devised setup provides a solid platform for a new generation of
THz spectrometers, paving the way to more refined and sophisticated systems
exploiting full phase control of QCL-FCs, or Doppler-free spectroscopic
schemes.
