Harnessing Quantum Entanglement to Enhance Raman Spectroscopy

A group of researchers at the City University of Hong Kong has developed a microscopic theory for ultrafast stimulated Raman spectroscopy using quantum-light fields. The study has been published in Light: Science & Application.

Two novel technologies that have gained significant traction recently are ultrafast stimulated Raman spectroscopy and quantum entangled light sources. Particles that exhibit instantaneous correlations over great distances are known as quantum entangled particles. This phenomenon is unique and is based on the concepts of quantum mechanics. In quantum sensing, quantum computing, and quantum communication, this field has attracted a lot of attention. In 2022, it was even awarded the Nobel Prize in Physics.

On the other hand, stimulated Raman spectroscopy is a contemporary analytical technique that provides important information about molecular fine structure by examining the vibrational characteristics and interactions of molecules. Its applications are found in many fields, such as materials science, environmental monitoring, chemical analysis, and biomedical research. These two methods can be combined to create an incredibly potent analytical tool for researching complex molecular materials.

The study involved a group of researchers led by Professors Zhedong Zhang and Zhe-Yu Ou of the Department of Physics at the City University of Hong Kong in Hong Kong, China. Using the quantum advantages of entangled photon sources, this novel method improves spectroscopic signals' spectral and temporal resolution. It also makes "high-speed imaging" of the incredibly quick processes that take place inside molecular systems possible. This article provides a step-by-step guide to understanding this groundbreaking concept.

What is Stimulated Raman Spectroscopy?

As a member of the Raman process family, stimulated Raman scattering is a common phenomenon in multi-photon interactions that is intimately related to quantum-light fields. It is predicated on the interaction of incident light with sample molecules, which causes the scattered light to shift in frequency. Energy is transferred from incident light to molecules in this process, and the frequency shift of the scattered light is correlated with the vibrational energy levels of the molecules.

The key breakthrough of stimulated Raman spectroscopy lies in its ultrafast processing capabilities. Traditional Raman spectroscopy necessitates significant data acquisition time. In contrast, stimulated Raman spectroscopy uses ultrashort laser pulses to quickly acquire a large number of data points, allowing for the quick retrieval of important molecular information.

Why Quantum Entangled Photon Sources?

A two-photon process called stimulated Raman scattering depends on quantum entangled photon sources. When these sources interact with matter, they release pairs of entangled photons that stimulate Raman scattering. Moreover, quantum-entangled photon sources have nonclassical characteristics like time, frequency, or polarization correlations between photon pairs. This overcomes the constraints of classical light and considerably improves spectroscopic signals' frequency and temporal resolution.

The authors highlight several benefits of using entangled photon sources, including the fact that molecules actively serve as beam mixers for Raman pump and probe fields rather than acting as passive beam splitters for light scattering alone. Using entanglement, quantum ultrafast stimulated Raman spectroscopy produces a super-resolved spectrum with time-frequency scales surpassing classical limitations. Furthermore, spectroscopic signals exhibit previously unheard-of selectivity due to multi-photon quantum interference, providing a selective transition pathway to molecular correlation functions.

“High-Speed Camera” for Molecules

Electron transfer and energy redistribution are examples of ultrafast processes that certain molecular systems exhibit. These processes happen on the femtosecond timescale (10^-15 seconds). Comprehending these swift movements is essential for the progress of imaging apparatus, energy conversion, and quantum computing. However, constraints on time and energy scales have made it difficult to study these ultrafast processes. In this work, the spectral width of the pump light and the thickness of the nonlinear crystal are two examples of nonlinear process parameters that can be tuned to produce entangled photon pairs with femtosecond-level correlation while maintaining their energy (frequency) correlations.

The produced photon pairs satisfy the necessary energy transfer requirements for stimulated Raman scattering. When this energy transfer process couples to the ultrafast processes taking place in photon-sensitive molecules, high-speed molecular imaging is made possible. The corresponding signal carrying the process information appears in the spectra.

Outlook of Quantum Spectroscopy

Quantum spectroscopy is expected to lead to significant advances in room-temperature quantum control and quantum physics in the future. These developments will fuel the development of more reliable and efficient quantum light source generation technologies, bringing new life to fields like optical communication, quantum computing, and quantum sensing.

Additionally, the highly efficient and accurate spectral measurement and analysis techniques derived from quantum spectroscopy are expected to play important roles in various fields, including biomedical research, materials science, chemical reactions, and biomedical research. This study only partially explores the potential of quantum spectroscopy. This method can significantly advance in related fields by providing deeper insights into dynamic observation and molecular structural analysis.

Journal Reference:

Zhang, Z., et al. (2024) Entangled photons enabled ultrafast stimulated Raman spectroscopy for molecular dynamics. Light: Science & Applications. doi.org/10.1038/s41377-024-01492-4