  Detection sensitivity is one of the most important attributes to consider
during selection of spectroscopic techniques. However, high sensitivity alone
is insufficient for spectroscopic measurements in spectrally congested regions.
Two-color cavity ringdown spectroscopy (2C-CRDS), based on intra-cavity
pump-probe detection, simultaneously achieves high detection sensitivity and
selectivity. The technique enables mid-infrared detection of radiocarbon
dioxide ($^{14}$CO$_2$) molecules in room-temperature CO$_2$ samples, with
better than 10 parts-per-quadrillion (ppq, 10$^{15}$) quantitation accuracy (4
ppq on average). These $highly$-$reproducible$ measurements, which are the most
$\it{sensitive}$ and $\it{quantitatively}$ $\it{accurate}$ in the mid-infrared,
are accomplished despite the presence of
$\it{orders}$-$\it{of}$-$\it{magnitude}$ stronger, one-photon signals from
other CO$_2$ isotopologues. This is a major achievement in laser spectroscopy.
A room-temperature-operated, compact, and low-cost 2C-CRDS sensor for
$^{14}$CO$_2$ benefits a wide range of scientific fields that utilize $^{14}$C
for dating and isotope tracing, most notably atmospheric $^{14}$CO$_2$
monitoring to track CO$_2$ emissions from fossil fuels. The 2C-CRDS technique
significantly enhances the general utility of high-resolution mid-infrared
detection for analytical measurements and fundamental chemical dynamics
studies.
