Remote Infrared Absorption Spectroscopy with Undetected Photons

We demonstrate a novel method for remote open-path Fourier-transform infrared spectroscopy with undetected photons. Similar to previous quantum spectroscopy works, a mid-infrared spectrum is reconstructed by detecting a near-infrared radiation only, thus bypassing important limitations of infrared detectors. Our study however relies on the co-propagation of the photon-pair and the pump-laser over the same optical path, which allows the probing of the open atmosphere over long distances. By sending the photons over unprecedented distances of up-to 43.4m in the outdoor atmosphere, we were able to detect butane released in the open-path, as well as natural atmospheric methane, thus demonstrating the first use of infrared spectroscopy with undetected photons for atmospheric measurements.