Entangled photons, as the dominant quantum light source, have been widely used in quantum communications, quantum imaging, quantum computing, optical interferometry, and other fields.
Natural down-shifting results in entangled photon pairs with momentum and energy conserved so that the quantum correlation is encoded in space-time. This feature greatly supports the quantum benefit of bypassing the classical pulse diffraction limit in imaging and detection.
One of the long-standing bottlenecks in molecular spectroscopy is the sensing of ultra-fast electronic processes on the femtosecond (10-15 second) scale. The dynamics of electron coherence is generally vital. However, limited by the incoherent channels of the excited states and temporal frequency resolution, the current Raman technique cannot be used for this end.
In a recent article published in the magazine Light science and applicationProfessor Zhedong Zhang of the Department of Physics at the City University of Hong Kong and colleagues created femtosecond coherent and time-resolved Raman spectroscopy with entangled photons. This paves the way for quantitative Raman spectroscopy (QFRS).
The ultra-resolution nature of the Raman signal generated by exploiting photon entanglement is demonstrated in their work—both spectral and temporal resolutions can be achieved concurrently. QFRS is only sensitive to electronic coherence. This makes it exclusively suitable for sensing electronically excited state dynamics within a short ~50 femtosecond time scale.
This kind of advantage cannot be achieved in the previously explored Raman techniques, which have been hampered by either rapid decay or time-frequency precision. The study provides a new perspective for examining ultrafast processes in multifaceted materials such as 2D materials, excitons, molecules and polaritons, which can be extracted using preferred relaxation and radiation processes.
Quantum Raman spectroscopy replaces the classical probe pulse with a photon signal beam from the entangled photon source. The idling photon beam acts as the beam indicated for the coincidence measurement. Thus the spectral and temporal resolution can be controlled individually.
This results in the ultra-fine nature of more than the uniformity of the time-frequency relationship. Covariance detection can be developed to track the phase of electrons.
“We design a quantum version of femtosecond Raman spectroscopy for three purposes: (1) to perform high-resolution anti-stroke Raman spectroscopy in the real-time domain; (2) to be able to image electron dynamics over an extremely short period of time; and (3) to be sensitive to the phase Molecular excitation such that it allows detection sensitivity to overcome the quantum limit.”
“Our work greatly expands the horizon of entangled light and complements the spectral advances made by entangled light in the context of optimal two-photon absorption processes in complex molecules. This work will aid future experimental and theoretical efforts.” Scientists predict.
Chang, Z.; , and others. (2022) Entangled photons have enabled time-resolved coherent Raman spectroscopy and its applications to femtosecond-scale electronic coherence. Light science and its applications. doi.org/10.1038/s41377-022-00953-y.