Entangled-photon SRS Achieves Signal Intensity Comparable to Entangled Two-Photon Absorption

Scientists are increasingly harnessing entanglement to boost the sensitivity of spectroscopic techniques. Mingran Zhang, Jiahao Joel Fan, both from the Department of Physics at City University of Hong Kong, and Frank Schlawin, spanning the Max Planck Institute for the Structure and Dynamics of Matter, the University of Hamburg, and with contributions from Zhedong Zhang at the Shenzhen Research Institute, City University of Hong Kong, have now developed a microscopic theory explaining stimulated Raman scattering using entangled photons. Their research demonstrates how the unique time-energy correlation within these photon pairs can significantly optimise signals when probing complex polyatomic molecules. Crucially, the team reveals that the intensity of entangled-photon stimulated Raman scattering is comparable to that of entangled two-photon absorption, identifying key parameters for achieving this , and highlighting the vital role of vibrational coherence in maximising the signal. This work establishes a strong foundation for advancing molecular spectroscopy using light, building upon recent experimental observations of entangled two-photon absorptio.

The team achieved a detailed theoretical comparison between entangled-photon SRS (ESRS) and entangled two-photon absorption (ETPA), identifying a crucial parameter window where ESRS intensity matches that of ETPA, a finding with profound implications for experimental design. Crucially, the study unveils the significant role of vibrational coherence in boosting ESRS signals relative to ETPA, offering a strategy to overcome limitations previously encountered in quantum spectroscopy.

Researchers developed a comprehensive theoretical framework to model the interaction between entangled photons and polyatomic molecules undergoing SRS. This involved constructing a microscopic theory that accounts for the quantum correlations present in entangled photon pairs generated via parametric down conversion. The work meticulously considers the spectral properties of these photons, specifically focusing on how their time-energy correlations influence the efficiency of the SRS process. Experiments utilising squeezed light have previously demonstrated ESRS, but this study extends the understanding to entangled photon pairs, offering greater experimental control and facilitating more accurate theoretical modelling.
The theoretical approach employed a three-energy level system to represent molecular transitions, allowing for a detailed analysis of the energy exchange during both ETPA and ESRS. The results show that the spectral-line intensity of ESRS is comparable to that of ETPA, identifying a specific range of parameters where both processes exhibit similar signal strengths. This parameter window is critical for designing experiments aimed at observing and exploiting ESRS with entangled photons. Furthermore, the study highlights that manipulating the vibrational coherence within the molecule can substantially enhance the ESRS signal, effectively suppressing the competing ETPA process.

This enhancement is achieved by carefully controlling the frequency correlations of the entangled photons, favouring positively correlated pairs for ESRS and anti-correlated pairs for ETPA. The team numerically compared the signal strengths of ETPA and ESRS to evaluate the feasibility of observing SRS with entangled photon pairs under realistic conditions. This work paves the way for extending molecular spectroscopy with quantum light, building upon the established observation of ETPA in experimental settings. By demonstrating the potential for strong ESRS signals using entangled photons, the research opens exciting possibilities for applications in molecular vibration sensing, live-cell imaging, chemical mapping, and drug delivery monitoring. The ability to control and optimise the spectral properties of entangled photons, coupled with the enhancement provided by vibrational coherence, promises to overcome the limitations of classical light sources and unlock new levels of sensitivity and precision in spectroscopic measurements. The findings establish a firm theoretical foundation for future experimental investigations aimed at harnessing the power of quantum entanglement for advanced molecular spectroscopy and imaging.

Entangled Photon SRS and Two-Photon Absorption Comparison reveals

Scientists engineered a theoretical framework to explore stimulated Raman scattering (SRS) using entangled photons, demonstrating how time-energy correlations within photon pairs can optimise signal detection in polyatomic molecules. The study pioneered a numerical comparison between entangled two-photon absorption (ETPA) and entangled SRS (ESRS) signal intensities, revealing that ESRS possesses comparable spectral-line intensity to ETPA, thereby identifying a viable parameter window for its observation. Researchers established a three-energy level system, defining transition frequencies ωeg and ωfe between the ground state |g⟩, the first excited state |e⟩, and the final state |f⟩ for the two-photon absorption process. Similarly, two lower states |g1⟩ and |g2⟩, alongside a virtual upper state |e⟩ with transition frequencies ωeg1 and ωeg2, were defined to model the nonlinear interactions.

The team harnessed the parametric down conversion (PDC) process to generate twin-entangled photons, mathematically described as |ψent⟩= ZZ +∞ −∞ dωsdωiFPDC(ωs, ωi)a†(ωs)a†(ωi) |0⟩, where ‘s’ and ‘i’ denote signal and idler photons, and ωj represents the frequency of photon j. The function FPDC, representing the two-photon wave function, was defined as 1 p πΩmΩp f(Ωs + Ωi 2Ωp )f(Ωs −Ωi 2Ωm ), with f(x) = e−x2, to characterise the generated entangled photon pairs. To facilitate a meaningful comparison, scientists employed both frequency-correlated and frequency-anti-correlated entangled photon pairs, manipulating the parameter Ωp to control spectral bandwidth. A smaller Ωp narrowed the sum-frequency bandwidth ω+ = ω(0) s + ω(0) i, creating anti-correlated pairs suitable for ETPA, as depicted in the study’s figures.

Conversely, for ESRS, a higher Ωp was set, prioritising the difference frequency ω−= ω(0) s −ω(0) i, generating positively correlated photon pairs. Frequency-correlated photons were achieved through techniques including time lenses, pump pulse modification, or counter-propagating phase-matching, enabling spectral modulation via nonlinear interference and phase control devices. The full three-level system was described by the atomic Hamiltonian Hatom = hωg |g⟩⟨g| + hωe |e⟩⟨e| + hωf |f⟩⟨f|, coupled with the interaction Hamiltonian HI under the rotating wave approximation, defined as −V †(t) · E(t) + h. c, where V is the transition dipole moment operator μegeiωegt |e⟩⟨g| + μfeeiωfet |f⟩⟨e|. This rigorous methodology provides a firm foundation for extending molecular spectroscopy with light, building upon existing ETPA experiments and paving the way for advanced quantum spectroscopy techniques.

Entangled Photons Enhance Raman Scattering Signals

Scientists have developed a microscopic theory for stimulated Raman scattering (SRS) utilizing entangled photons, demonstrating how the time-energy correlation of these photon pairs can optimize signals for polyatomic molecules. Experiments reveal that the spectral-line intensity of entangled-photon SRS (ESRS) is comparable to that of entangled two-photon absorption (ETPA), identifying a crucial parameter window for achieving this equivalence. Moreover, the research confirms that vibrational coherence significantly enhances ESRS intensity relative to ETPA, paving the way for advancements in molecular spectroscopy with quantum light. The.

Scientists generated twin-entangled. The spectral-line intensity of entangled-photon SRS (ESRS) is comparable to that of entangled two-photon absorption (ETPA), identifying a parameter window where this equivalence holds true. Vibrational coherence is also shown to significantly enhance ESRS relative to ETPA intensity, offering a pathway to extend molecular spectroscopy techniques currently based on ETPA observations. These results establish that, under specific conditions, ETPA and ESRS signals can achieve comparable intensities with carefully selected input photon frequencies. The study highlights that while optimal central frequencies differ between the two processes, a fixed photon pair can still yield comparable signal power, particularly within the green region of the spectrum, without substantial loss.

Analysis of molecular systems incorporating both high and low frequency vibrations reveals a unique advantage for ESRS, even when molecules are less suited for ETPA experiments, specifically, those with larger Huang-Rhys factors and small low frequency decay rates. Furthermore, the research indicates a broad tolerance for intermediate-state detuning, with signal intensities remaining comparable within a range of ±0.5 for both ETPA and ESRS. The authors acknowledge that signal intensities are not perfectly symmetrical with respect to detuning, potentially due to contributions from additional vibrational states. They also note that the ESRS signal is inversely proportional to temperature, while ETPA remains relatively unaffected by temperature changes, meaning the ratio of ETPA to ESRS signals varies linearly with temperature. Future research could explore the implications of these temperature dependencies and further investigate the role of vibrational states in optimising ESRS signals for diverse molecular systems. This work provides a firm foundation for advancing molecular spectroscopy using entangled light, potentially leading to more sensitive and efficient analytical techniques.