Measurement-Induced Collective Vibrational Quantum Coherence under Spontaneous Raman Scattering in a Liquid

TL;DR

This study demonstrates that spontaneous Raman scattering in a liquid can induce a collective vibrational quantum coherence, revealing that the vibrational state’s coherence depends on optical setup rather than intrinsic material properties.

Contribution

The paper provides experimental evidence that spontaneous Raman scattering can generate collective vibrational coherence in liquids, challenging the view of it as an incoherent process.

Findings

Detection of Stokes--anti-Stokes correlations indicates collective vibrational coherence.

Coherence depends on excitation and detection geometry, not just material properties.

Results suggest a new way to control vibrational quantum states in liquids.

Abstract

Spontaneous vibrational Raman scattering is a ubiquitous form of light-matter interaction whose description necessitates quantization of the electromagnetic field. It is usually considered as an incoherent process because the scattered field lacks any predictable phase relationship with the incoming field. When probing an ensemble of molecules, the question therefore arises: What quantum state should be used to describe the molecular ensemble following spontaneous Stokes scattering? We experimentally address this question by measuring time-resolved Stokes--anti-Stokes two-photon coincidences on a molecular liquid consisting of several sub-ensembles with slightly different vibrational frequencies. When spontaneously scattered Stokes photons and subsequent anti-Stokes photons are detected into a single spatiotemporal mode, the observed dynamics is inconsistent with a statistical mixture of individually excited molecules. Instead, we show that the data are reproduced if Stokes--anti-Stokes correlations are mediated by a collective vibrational quantum, i.e. a coherent superposition of all molecules interacting with light. Our results demonstrate that the degree of coherence in the vibrational state of the liquid is not an intrinsic property of the material system, but rather depends on the optical excitation and detection geometry.