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.

Experimental work on cold, trapped metastable noble gases is reviewed. The aspects which distinguish work with these atoms from the large body of work on cold, trapped atoms in general is emphasized. These aspects include detection techniques and collision processes unique to metastable atoms. Several experiments exploiting these unique features in fields including atom optics and statistical physics are described. Precision measurements on these atoms including fine structure splittings, isotope shifts, and atomic lifetimes are also discussed.

We measure the absolute frequency of seven out of the nine allowed transitions between the $2{}^{3}S$ and $2{}^{3}P$ hyperfine manifolds in a metastable ${}^3\mathrm{He}$ beam by using an optical frequency comb synthesizer-assisted spectrometer. The relative uncertainty of our measurements ranges from $1\times {10}^{-11}$ to $5\times {10}^{-12}$, which is, to our knowledge, the most precise result for any optical ${}^3\mathrm{He}$ transition to date. The resulting $2{}^{3}P–2{}^{3}S$ centroid frequency is 276 702 827 204.8(2.4) kHz. Comparing this value with the known result for the ${}^4\mathrm{He}$ centroid and performing ab initio QED calculations of the ${}^4\mathrm{He}\mathrm{\text{−}}{}^3\mathrm{He}$ isotope shift, we extract the difference of the squared nuclear charge radii $\delta r^2$ of ${}^3\mathrm{He}$ and ${}^4\mathrm{He}$. Our result for $\delta r^2=1.074(3)\mathrm{f}{\mathrm{m}}^2$ disagrees by about $4\sigma$ with the recent determination [R. van Rooij et al., Science 333, 196 (2011)].

By measuring the frequency-noise power spectral density of a cryogenically-cooled mid-infrared quantum cascade laser, we investigate the different contributions to the noise spectrum and identify the main differences with respect to standard bipolar semiconductor devices. In particular, the existence of a thermal cut-off on the 1/f noise allows to identify the current fluctuations through the heterostructure as the physical mechanism, intrinsic to the device, at the basis of the measured flicker noise. This result, marking the difference with bipolar semiconductor devices, is confirmed analyzing the laser frequency response to a modulation of the driving current.

The search for small violations of the validity of the symmetrization postulate and of the spin-statistics connection (SSC) has been addressed in the last four decades by experimental tests performed in different physical systems of identical fermions or bosons. In parallel and consequently, theories extending the quantum mechanics to a more general level have been proposed to explain such possible violations. In this paper, we present the most stringent test to a possible violation of the SSC under permutation of the bosonic ${}^{16}\mathrm{O}$ nuclei in the ${}^{12}\mathrm{C}{}^{16}\mathrm{O}_2$ molecule. An upper limit of $3.8\times {10}^{-12}$ for an SSC-anomalous ${\mathrm{CO}}_2$ molecule is obtained using saturated-absorption cavity ring-down spectroscopy in the SSC-forbidden $\left({00}^{0}1-{00}^{0}0\right) R$(25) rovibrational transition of ${}^{12}\mathrm{C}{}^{16}\mathrm{O}_2$ at a $4.25\text{−}\mu \mathrm{m}$ wavelength. Quantum mechanics implications of this result are discussed in the frame of the $q$-mutator theory. Finally, the perspective of stringent experimental tests of the symmetrization postulate in molecules that contain three or more identical nuclei is discussed.

Accurate frequency measurements of molecular transitions around $2 \mu \mathrm{m}$ are performed by using a direct-frequency-comb spectroscopy approach that combines an ${\mathrm{Er}}^{+}$ frequency-comb oscillator at $1.5 \mu \mathrm{m}$, a Tm-Ho fiber amplifier, and a Fabry-Perot-filter, high-resolution dispersive spectrometer optical multiplex-detection system. This apparatus has unique performances in terms of a wide dynamic range to integrate the intensity per comb mode, which allows one to measure molecular absorption profiles with high precision. Spectroscopic information about transition frequencies and linewidths is very accurately determined. Relative frequency uncertainties of the order of a few parts in ${10}^{-9}$ are achieved for rovibrational transitions of the ${\mathrm{CO}}_2$ molecule around $5100 {\mathrm{cm}}^{-1}$. Moreover, tiny frequency shifts due to molecular collisions and interacting laser power using direct comb spectroscopy are investigated in a systematic way.