Scientists have developed a novel mid-infrared (IR) spectroscopy technique that leverages quantum photons to analyze biomolecules without directly detecting the IR light. This method, termed IR spectroscopy with undetected photons (IR-UDP), overcomes the limitations of traditional IR detectors, which are often noisy, slow, or require cryogenic cooling. The key lies in generating entangled photon pairs, one in the IR range and the other in the visible, allowing the absorption of the IR photon by the sample to be inferred through changes in the easily detectable visible photon.

The process begins with a nonlinear crystal that, when pumped by a laser, generates entangled photon pairs via spontaneous parametric down-conversion (SPDC). One photon from each pair is in the visible range (the "idler") and the other in the mid-infrared (the "signal"). The IR photon interacts with the biomolecular sample, while the visible photon passes directly to a high-efficiency detector. The absorption of the IR photon by the sample manifests as a reduction in the intensity of the entangled visible photon, enabling the reconstruction of the sample's IR absorption spectrum.

This technique offers several significant advantages. By circumventing direct IR photon detection, it achieves superior signal-to-noise ratios and faster acquisition times compared to conventional methods. Furthermore, IR-UDP spectroscopy enables the use of low-power light sources and room-temperature detectors, simplifying instrumentation and reducing costs. This advancement has the potential to revolutionize biomolecular analysis, opening new avenues for characterizing proteins, nucleic acids, and other biological molecules with unprecedented sensitivity and resolution in the mid-infrared range.