Spectroscopic detection of radiocarbon dioxide at parts-per-quadrillion sensitivity

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 mu m, 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.