Radiocarbon (${}^{14}\mathrm{C}$) concentrations at a 43 parts-per-quadrillion level are measured by using saturated-absorption cavity ringdown spectroscopy by exciting radiocarbon-dioxide (${}^{14}\mathrm{C}{}^{16}\mathrm{O}_2$) molecules at the $4.5\mu \mathrm{m}$ wavelength. The ultimate sensitivity limits of molecular trace gas sensing are pushed down to attobar pressures using a comb-assisted absorption spectroscopy setup. Such a result represents the lowest pressure ever detected for a gas of simple molecules. The unique sensitivity, the wide dynamic range, the compactness, and the relatively low cost of this table-top setup open new perspectives for ${}^{14}\mathrm{C}$-tracing applications, such as radiocarbon dating, biomedicine, or environmental and earth sciences. The detection of other very rare molecules can be pursued as well thanks to the wide and continuous mid-IR spectral coverage of the described setup.

A comprehensive investigation of the frequency-noise spectral density of a free-running midinfrared quantum-cascade laser is presented for the first time. It provides direct evidence of the leveling of this noise down to a white-noise plateau, corresponding to an intrinsic linewidth of a few hundred hertz. The experiment is in agreement with the most recent theory on the fundamental mechanism of line broadening in quantum-cascade lasers, which provides a new insight into the Schawlow-Townes formula and predicts a narrowing beyond the limit set by the radiative lifetime of the upper level.

We report on a novel approach to cavity ring-down spectroscopy with the sample gas in saturated-absorption regime. This technique allows us to decouple and simultaneously retrieve the empty-cavity background and absorption signal, by means of a theoretical model that we developed and tested. The high sensitivity and frequency precision for spectroscopic applications are exploited to measure, for the first time, the hyperfine structure of an excited vibrational state of O17C12O16 in natural abundance with an accuracy of a few parts in 10-11.

High-sensitivity radiocarbon detection has been, until now, a prerogative of accelerator mass spectrometry (AMS). Here we present a compact and simple spectroscopic apparatus, based on saturated-absorption cavity ring-down, approaching the ultimate AMS sensitivity. We measure radiocarbon dioxide concentration down to a few parts per quadrillion by use of a heterodyne-spectroscopy system with two quantum cascade lasers at 4.5 μm, a reference cell, and a high-finesse cavity with the sample gas cooled down to 170 K. Our results pave the way to a paradigm change in radiocarbon measurement, with a number of significant applications in areas such as environment, nuclear security, pharmacology, and cultural heritage.

Miniaturized frequency comb sources across hard-to-access spectral regions, i.e. mid- and far-infrared, have long been sought. Four-wave-mixing based Quantum Cascade Laser combs (QCL-combs) are ideal candidates, in this respect, due to the unique possibility to tailor their spectral emission by proper nanoscale design of the quantum wells. We demonstrate full-phase-stabilization of a QCL-comb against the primary frequency standard, proving independent and simultaneous control of the two comb degrees of freedom (modes spacing and frequency offset) at a metrological level. Each emitted mode exhibits a sub-Hz relative frequency stability, while a correlation analysis on the modal phases confirms the high degree of coherence in the device emission, over different power-cycles and over different days. The achievement of fully controlled, phase-stabilized QCL-comb emitters proves that this technology is mature for metrological-grade uses, as well as for an increasing number of scientific and technological applications.