Identification and quantification of trace gas sources is a major challenge for understanding and regulating air quality and greenhouse gas emissions. Current approaches either provide continuous but localized monitoring, or quasi-instantaneous "snapshot-in-time" regional monitoring. There is a need for emissions detection that provides both continuous and regional coverage, because sources and sinks can be episodic and spatially variable. We field deploy a dual frequency comb laser spectrometer for the first time, enabling an observing system that provides continuous detection of trace gas sources over multiple-square-kilometer regions. Field tests simulating methane emissions from oil and gas production demonstrate detection and quantification of a 1.6 g min -1 source (approximate emissions from a small pneumatic valve) from a distance of 1 km, and the ability to discern two leaks among a field of many potential sources. The technology achieves the goal of detecting, quantifying, and attributing emissions sources continuously through time, over large areas, and at emissions rates ~1000x lower than current regional approaches. It therefore provides a useful tool for monitoring and mitigating undesirable sources and closes a major information gap in the atmospheric sciences.

Spectroscopy has an illustrious history delivering serendipitous discoveries and providing a stringent testbed for new physical predictions, including applications from trace materials detection, to understanding the atmospheres of stars and planets, and even constraining cosmological models. Reaching fundamental-noise limits permits optimal extraction of spectroscopic information from an absorption measurement. Here, we demonstrate a quantum-limited spectrometer that delivers high-precision measurements of the absorption lineshape. These measurements yield a very accurate measurement of the excited-state (6P1/2) hyperfine splitting in Cs, and reveals a breakdown in the well-known Voigt spectral profile. We develop a theoretical model that accounts for this breakdown, explaining the observations to within the shot-noise limit. Our model enables us to infer the thermal velocity dispersion of the Cs vapour with an uncertainty of 35 p.p.m. within an hour. This allows us to determine a value for Boltzmann's constant with a precision of 6 p.p.m., and an uncertainty of 71 p.p.m.

Challenging applications in trace gas measurements require low uncertainty and high acquisition rates [1][2][3][4] . Many cavityenhanced spectroscopies exhibit significant sensitivity and potential 5,6 , but their scanning rates are limited by reliance on either mechanical or thermal frequency tuning 7 . Here, we present frequency-agile, rapid scanning spectroscopy (FARS) in which a high-bandwidth electro-optic modulator steps a selected laser sideband to successive optical cavity modes. This approach involves no mechanical motion and allows for a scanning rate of 8 kHz per cavity mode, a rate that is limited only by the cavity response time itself. Unlike rapidly frequency-swept techniques 8-11 , FARS does not reduce the measurement duty cycle, degrade the spectrum's frequency axis or require an unusual cavity configuration. FARS allows for a sensitivity of ∼2 3 10 212 cm 21 Hz 21/2 and a tuning range exceeding 70 GHz. This technique shows promise for fast and sensitive trace gas measurements and studies of chemical kinetics.A multitude of applications have emerged that require rapid sensing of trace gas species, encompassing areas as varied as greenhouse gas monitoring 1,4,12 , breath analysis 2,13 , explosive detection 3 and chemical process monitoring. Because of their sensitivity, continuous-wave (c.w.) cavity-enhanced spectroscopies have significant potential for addressing these challenging applications 6,14,15 . However, the low mechanical or thermal tuning rates of c.w. lasers have generally limited the use of these techniques to static or slowly varying analytes. Attempts to alleviate this limitation have relied on sweeping the laser frequency [8][9][10][11] , which can compromise the fidelity of the spectrum and the instrument sensitivity 16 and reduce the measurement duty cycle. We present a new approach for cavity ring-down spectroscopy 17 in which the laser frequency is rapidly stepped to successive resonances through the use of highbandwidth electro-optics. This technique, which we refer to as frequency-agile, rapid scanning (FARS) cavity ring-down spectroscopy, allows for ultrasensitive measurements in which the acquisition rate is limited only by the cavity response. Unlike earlier techniques [8][9][10][11]18,19 , FARS allows for spectra to be recorded without any dead time due to scanning of the laser frequency, offers a metrology-level frequency axis and utilizes a conventional Fabry-Pérot resonator rather than more unusual and cumbersome cavity configurations.This frequency stepping of the probe laser is enabled by the use of a microwave driver and a high-bandwidth electro-optic modulator (EOM) to generate a series of sidebands on the probe laser (Fig. 1). We then use the cavity as a spectral filter such that only a single, selected sideband is resonant. Thus, we are able to transfer the superior switching bandwidth and precision of microwave sources into the optical domain. Ring-downs are then initiated by simply switching off the microwave frequency, thus removing the need for an acousto-...