Physicists have demonstrated a method for detecting atmospheric gases from a distance using infrared light, without ever detecting the infrared photons themselves. By exploiting quantum correlations, the technique reconstructs a mid-infrared absorption spectrum using only near-infrared detectors, opening a new path for remote sensing of pollutants and greenhouse gases.
The study, published as a preprint on arXiv by researchers at the University of Geneva, demonstrates remote open-path Fourier-transform infrared spectroscopy using “undetected photons,” a technique that relies on quantum entanglement to infer the properties of light that is never directly measured.
How it works
The method begins with a photon-pair source, typically a nonlinear crystal that splits a single high-energy photon into two lower-energy photons through spontaneous parametric down-conversion. The two photons are quantum-correlated: one in the mid-infrared range, the range that carries distinctive absorption signatures for most molecules, and one in the near-infrared range, where detectors are cheap, sensitive, and operate at room temperature.
The mid-infrared photon travels along an open path through the atmosphere, where it may be absorbed by target molecules. Its near-infrared twin, the “undetected” photon in the technique’s name, is sent to a standard detector. Because the two are entangled, the absorption information carried by the mid-infrared photon is imprinted on the detectable near-infrared partner.
A Fourier-transform interferometer scans the path difference, and the mid-infrared absorption spectrum is reconstructed from the interference pattern of the detected near-infrared photons. The mid-infrared photons themselves are never directly sensed.
“The mid-infrared spectrum is reconstructed by detecting near-infrared radiation only, thus bypassing important limitations of infrared detectors,” the authors explain.
The breakthrough: outdoor operation
Previous demonstrations of this technique were confined to benchtop laboratory setups. The Geneva team achieved something fundamentally different: they sent the photons over 43.4 meters in the outdoor atmosphere, an unprecedented distance for this method.
They were able to detect butane released intentionally into the open path, as well as natural atmospheric methane present in ambient air.
“This demonstrates the first use of infrared spectroscopy with undetected photons for atmospheric measurements,” the researchers write.
The key engineering innovation is the co-propagation of the photon pair and the pump laser over the same optical path. This design simplifies alignment, makes the system robust for field deployment, and allows the probe beam to travel long distances while maintaining the quantum correlation needed for the technique to work.
Why it matters
Infrared spectroscopy is the gold standard for identifying and quantifying gases, every molecule has a unique infrared fingerprint, but mid-infrared detectors are expensive, often require cryogenic cooling, and suffer from lower sensitivity than their near-infrared counterparts.
By shifting the detection to the near-infrared, where off-the-shelf silicon detectors perform near the quantum limit, the technique sidesteps these limitations entirely. The result is a system that is potentially cheaper, more robust, and more sensitive than conventional approaches.
The implications extend across environmental monitoring (continuous methane leak detection at industrial sites), industrial safety (remote sensing of fugitive emissions), and atmospheric science (mapping greenhouse gas concentrations at high spatial resolution).
What comes next
The 43-meter demonstration is a proof of concept, not a finished instrument. Practical deployment would require extending the path length to hundreds or thousands of meters, improving the signal-to-noise ratio for trace concentrations, and miniaturizing the optical system for field use.
But the core advance, demonstrating that quantum-correlated sensing works in real outdoor conditions, not just in a climate-controlled lab, marks the transition of “undetected photon” spectroscopy from a quantum curiosity to a candidate technology for real-world applications.
Reference: Neves, S., Armbruster, F., & Wolf, J.-P. (2026). “Remote Infrared Absorption Spectroscopy with Undetected Photons.” arXiv:2607.16419 [quant-ph].