Measurement-induced collective vibrational quantum coherence under spontaneous Raman scattering in a liquid

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.

High-Order Nanowire Resonances for High-Frequency, Large-Coupling-Strength Quantum Dot Hybrid Nanomechanics

“…In a blossoming landscape of possible implementations, ,,− building a hybrid system with a solid-state quantum light emitter − opens additional, compelling opportunities. These include microwave-to-optical conversion of photons through optomechanical wave mixing, , the realization of a long-lived mechanical memory interfaced with flying photons, and direct access to the phonon statistics thanks to powerful photon counting techniques. , Among solid-state quantum emitters, semiconductor quantum dots (QDs) offer a remarkable combination of bright, fast, and spectrally narrow emission. , Furthermore, the bandgap energy of bulk and nanostructured semiconductors is extremely sensitive to mechanical strain, which gives birth to large, built-in coupling to mechanical vibrations. ,,− Semiconductor nanowires feature appealing mechanical properties, − and strain-coupling a single QD to the sub-MHz fundamental vibration mode has enabled many early demonstrations in the field. ,,− However, achieving spectrally resolved Raman transitions in QD-based hybrid systemsthe so-called resolved-sideband regimedemands a mechanical frequency of at least a few hundred MHz. Demonstrations of such high-frequency QD hybrid systems are still scarce, , and these pioneering works report relatively modest coupling strengths.…”

“…These include microwave-to-optical conversion of photons through optomechanical wave mixing, 12,13 the realization of a long-lived mechanical memory interfaced with flying photons, and direct access to the phonon statistics thanks to powerful photon counting techniques. 14,15 Among solid-state quantum emitters, 16 semiconductor quantum dots (QDs) offer a remarkable combination of bright, fast, and spectrally narrow emission. 17,18 Furthermore, the bandgap energy of bulk 19 and nanostructured 20 semiconductors is extremely sensitive to mechanical strain, which gives birth to large, built-in coupling to mechanical vibrations.…”