One molecule, one photon: Entanglement makes an imperceptible recoil measurable

For decades, light has been used to understand the molecular structures of matter. A sample is irradiated with light, and measurements determine the wavelengths at which it is absorbed. Since each molecule absorbs light at very specific wavelengths that depend on its structure, the resulting absorption spectrum acts like a molecular fingerprint. For individual molecules, however, this signal is vanishingly small and mostly indistinguishable from noise.