Spontaneous Raman scattering is classically understood as an incoherent process. Here, we evidence that a macroscopic quantum coherence among billions of vibrating molecules is generated when detecting a single Stokes photon collected into a single-mode fiber. Our results show that at the quantum level spontaneous Raman scattering is an intrinsically collective effect, even in the absence of optical cavity or stimulating laser beam.
Full circles represent the measured data points while the solid red line indicates the model prediction. The observed quantum beats are perfectly consistent with a macroscopic quantum superposition of four sub-ensembles of CS2 molecules (inset) sharing a single quantum of vibration. The dotted red line represents the multi-exponential decay that would result from a statistical mixture of single vibrating molecules.
Reference: “Measurement-induced collective vibrational quantum coherence under spontaneous Raman scattering in a liquid”, Valeria Vento, Santiago Tarrago Velez, Anna Pogrebna， Christophe Galland，Nature Communications volume 14, Article number: 2818 (2023); https://www.nature.com/articles/s41467-023-38483-9