David A. Long, Adam J. Fleisher, Szymon S. Wojtewicz, Joseph T. Hodges
Spectroscopic techniques with ultra-high sensitivity allow for the study of a wide variety of physical and chemical phenomena, ranging from the interrogation of individual atoms1 and molecules2,3 to the quantitative extraction of ensemble properties from detailed molecular line shape investigations.4,5 The fundamental limit to spectroscopic sensitivity is the quantum noise limit imposed by the photons themselves, often referred to as the shot noise limit. Cavity ring-down spectroscopy6 (CRDS) is a relatively simple technique for measuring absorption with high sensitivity, but despite its pervasiveness throughout research laboratories, no CRDS measurement has yet reached the sensitivity limit imposed by the quantum nature of light. Here we demonstrate quantum-noise-limited CRDS using a heterodyne-detected method based upon the work of Ye and Hall.7 In addition to reaching the quantum-noise limit, we also achieved a noise-equivalent absorption coefficient of ~6×10−14 cm−1 Hz−1/2, and demonstrate high frequency agility over a range of ~2 THz in the near-infrared.