Quantum Light-Enhanced Two-Photon Imaging of Breast Cancer Cells

Exploiting Quantum Light-Matter Interaction for Probing and Controlling Molecules

Quantum mechanical properties of light, such as time-energy entanglement, quadrature squeezing, and non-Poisson statistics, can be exploited to develop novel spectroscopic signals that enhance the signal strength and spectrotemporal resolution. Moreover, quantum light also provides nonclassical control knobs for controlling the outcome of a chemical reaction. Here, we provide a perspective on how quantum light-matter interaction can be exploited to probe and control molecular events.

Imaging Brain Tissue with Quantum Light at Low Power

Light-induced tissue damage is a crucial limitation for traditional microscopy of the living brain, underscoring the need for new techniques that minimize exposure of samples to light. Here, we tested the hypothesis that quantum light, i.e., entangled photons, could detect brain structures at a lower excitation energy. In a proof of principle, we show microscopic images of fixed brain tissue in the hippocampus area created by fluorescence selective excitation in the process of entangled two-photon absorption in a scanning microscope. Quantum-enhanced entangled two-photon microscopy (TPM) had brain imaging capabilities at an unprecedented low excitation intensity of ∼3.6 × 107 photons/s, orders of magnitude lower than the excitation level for the classical two-photon fluorescence image obtained in the same microscope. The extremely low light probe intensity demonstrated in entangled TPM is of critical importance in the investigation of neural activity to minimize heating and photobleaching during repetitive imaging. It may have important functional implications in optogenetic technology, removing unintended heating and accumulated photodamage effects. This technology also opens avenues in spatially resolved brain tissue investigations with quantum light, providing new capabilities in local spectroscopy.

Harnessing quantum light for microscopic biomechanical imaging of cells and tissues

The biomechanical properties of cells and tissues play an important role in our fundamental understanding of the structures and functions of biological systems at both the cellular and subcellular levels. Recently, Brillouin microscopy, which offers a label-free spectroscopic means of assessing viscoelastic properties in vivo, has emerged as a powerful way to interrogate those properties on a microscopic level in living tissues. However, susceptibility to photodamage and photobleaching, particularly when high-intensity laser beams are used to induce Brillouin scattering, poses a significant challenge. This article introduces a transformative approach designed to mitigate photodamage in biological and biomedical studies, enabling nondestructive, label-free assessments of mechanical properties in live biological samples. By leveraging quantum-light-enhanced stimulated Brillouin scattering (SBS) imaging contrast, the signal-to-noise ratio is significantly elevated, thereby increasing sample viability and extending interrogation times without compromising the integrity of living samples. The tangible impact of this methodology is evidenced by a notable three-fold increase in sample viability observed after subjecting the samples to three hours of continuous squeezed-light illumination, surpassing the traditional coherent light-based approaches. The quantum-enhanced SBS imaging holds promise across diverse fields, such as cancer biology and neuroscience where preserving sample vitality is of paramount significance. By mitigating concerns regarding photodamage and photobleaching associated with high-intensity lasers, this technological breakthrough expands our horizons for exploring the mechanical properties of live biological systems, paving the way for an era of research and clinical applications.

Impact of Classical and Quantum Light on Donor-Acceptor-Donor Molecules

Investigations of entangled and classical two-photon absorption have been carried out for six donor (D)-acceptor (A)-donor (D) compounds containing the dithieno pyrrole (DTP) unit as donor and acceptors with systematically varied electronic properties. Comparing ETPA (quantum) and TPA (classical) results reveals that the ETPA cross section decreases with increasing TPA cross section for molecules with highly off-resonant excited states for single-photon excitation. Theory (TDDFT) results are in semiquantitative agreement with this anticorrelated behavior due to the dependence of the ETPA cross section but not TPA on the two-photon excited state lifetime. The largest cross section is found for a DTP derivative that has a single photon excitation energy closest to resonance with half the two-photon excitation energy. These results are important for the possible use of quantum light for low-intensity energy-conversion applications.

Investigations of Astrocyte Calcium Signaling and Imaging with Classical and Nonclassical Light

The application of light in studying and influencing cellular behavior with improved temporal and spatial resolution remains a key objective in fields such as chemistry, physics, medicine, and engineering. In the brain, nonexcitable cells called astrocytes play essential roles in regulating homeostasis and cognitive function through complex calcium signaling pathways. Understanding these pathways is vital for deciphering brain physiology and neurological disorders like Parkinson's and Alzheimer's. Despite challenges in selectively targeting astrocyte signaling pathways due to shared molecular equipment with neurons, recent advancements in laser technology offer promising avenues. However, the effort to use laser light properties to study astroglial cell function is still limited. This work aims to exploit an in-depth pharmacological analysis of astrocyte calcium channels to determine the physiological mechanism induced by exposure to classical nanosecond-pulsed light. We herein report molecular clues supporting the use of visible-nanosecond laser pulses as a promising approach to excite primary rat neocortical astrocytes and unprecedentedly report on the implementation of entangled two-photon microscopy to image them.

Fiber-integrated quantum microscopy system for cells

Quantum entanglement serves as an essential resource across various fields, including quantum communication, quantum computing, and quantum precision measurement. Quantum microscope, as one of the significant applications in quantum precision measurement, could bring revolutionary advancements in both signal-to-noise ratio (SNR) and spatial resolution of imaging. Here, we present a quantum microscopy system that relies on a fully fiber-integrated high-performance energy-time entangled light source operating within the near-infrared II (NIR-II) window. Complemented by tailored real-time data acquisition and processing software, we successfully demonstrate the quantum imaging of a standard target, achieving a SNR of 131.51 ± 6.74 and a spatial resolution of 4.75 ± 0.27 µm. Furthermore, we showcase quantum imaging of cancer cells, unveiling the potential of quantum entanglement in biomedical applications. Our fiber-integrated quantum microscope, characterized by high imaging SNR, instantaneous image capture, and analysis capabilities, marks an important step toward the practical application in life sciences.

Linear and Nonlinear Optical Properties of All-cis and All-trans Poly(p-phenylenevinylene)

Poly(p-phenylenevinylene) (PPV) is a staple of the family of conjugated polymers with desirable optoelectronic properties for applications including light-emitting diodes (LEDs) and photovoltaic devices. Although the significant impact of olefin geometry on the steady-state optical properties of PPVs has been extensively studied, PPVs with precise stereochemistry have yet to be investigated using nonlinear optical spectroscopy for quantum sensing, as well as light harvesting for biological applications. Herein, we report our investigation of the influence of olefin stereochemistry on both linear and nonlinear optical properties through the synthesis of all-cis and all-trans PPV copolymers. We performed two-photon absorption (TPA) using a classical and entangled light source and compared both classical TPA and entangled two-photon absorption (ETPA) cross sections of these stereodefined PPVs. Whereas the TPA cross section of the all-trans PPV was expectedly higher than that of all-cis PPV, presumably because of the larger transition dipole moment, the opposite trend was measured via ETPA, with the all-cis PPV exhibiting the highest ETPA cross section. DFT calculations suggest that this difference might stem from the interaction of entangled photons with lower-lying electronic states in the all-cis PPV variant. Additionally, we explored the photoinduced processes for both cis and trans PPVs through time-resolved fluorescence upconversion and femtosecond transient absorption techniques. This study revealed that the sensitivity of PPVs in two-photon absorption varies with classical versus quantum light and can be modulated through the control of the geometry of the repeating alkenes, which is a key stepping stone toward their use in quantum sensing, bioimaging, and the design of polymer-based light-harvesting systems.

Broadband Two-Photon Absorption Spectroscopy with Stimulated Raman Scattering as an Internal Standard

Two-photon absorption (2PA) spectroscopy provides valuable information about the nonlinear properties of molecules. In contrast with single-wavelength methods, broadband 2PA spectroscopy using a pump-probe approach gives a continuous 2PA spectrum across a wide range of transition energies without tuning the excitation laser. This contribution shows how stimulated Raman scattering from the solvent can be used as a convenient and robust internal standard for obtaining accurate absolute 2PA cross sections using the broadband approach. Stimulated Raman scattering has the same pump-probe overlap dependence as 2PA, thus eliminating the need to measure the intensity-dependent overlap of the pump and probe directly. Eliminating the overlap represents an important improvement because intensity profiles are typically the largest source of uncertainty in the measurement of absolute 2PA cross sections using any method. Raman scattering cross sections are a fundamental property of the solvent and therefore provide a universal standard that can be applied any time the 2PA and Raman signals are present within the same probe wavelength range. We demonstrate this approach using sample solutions of coumarin 153 in methanol, DMSO, and toluene, as well as fluorescein in water.

Noninvasive Three-dimensional Assessment of Single Molecular Crystals Using Multiphoton Microscopic Observation and Their Deformation-induced Emission Characteristics

Distinguishing the luminescence contribution from the surface and bulk of a crystal is a long-standing challenge in crystal materials. Herein, three-dimensional, multiphoton, luminescence microscope imaging of the elastic molecular single crystal 1,4-bis(4-methylthien-2-yl)-2,3,5,6-tetrafluorobenzene, was conducted. Further, the luminescence contribution from the surface and bulk of the crystal was experimentally distinguished. Strong luminescence was observed only from the surface of the crystal, while the bulk did not emit strongly. Furthermore, the surface and bulk luminescence behavior responded well to the mechanical shape change of the crystal; i.e., strong luminescence was observed for the elongated side of the crystal.

Quantum-enhanced stimulated Raman scattering microscopy in a high-power regime

Quantum-enhanced stimulated Raman scattering (QESRS) microscopy is expected to realize molecular vibrational imaging with sub-shot-noise sensitivity, so that weak signals buried in the laser shot noise can be uncovered. Nevertheless, the sensitivity of previous QESRS did not exceed that of state-of-the-art stimulated Raman scattering (SOA-SRS) microscopes mainly because of the low optical power (3 mW) of amplitude squeezed light [Nature594, 201 (2021)10.1038/s41586-021-03528-w]. Here, we present QESRS based on quantum-enhanced balanced detection (QE-BD). This method allows us to operate QESRS in a high-power regime (>30 mW) that is comparable to SOA-SRS microscopes, at the expense of 3 dB sensitivity drawback due to balanced detection. We demonstrate QESRS imaging with 2.89 dB noise reduction compared with classical balanced detection scheme. The present demonstration confirms that QESRS with QE-BD can work in the high-power regime, and paves the way for breaking the sensitivity of SOA-SRS microscopes.

Manipulating Two-Photon Absorption of Molecules through Efficient Optimization of Entangled Light

We report how the unique temporal and spectral features of pulsed entangled photons from a parametric downconversion source can be utilized for manipulating electronic excitations through the optimization of their spectral phase. A new comprehensive optimization protocol based on Bayesian optimization has been developed in this work to selectively excite electronic states accessible by two-photon absorption. Using our optimization method, the entangled two-photon absorption probability for a thiophene dendrimer can be enhanced by up to a factor of 20, while classical light turns out to be nonoptimizable. Moreover, the optimization involving photon entanglement enables selective excitation that would not be possible otherwise. In addition to optimization, we have explored entangled two-photon absorption in the small entanglement time limit showing that entangled light can excite molecular electronic states that are vanishingly small for classical light. We demonstrate these opportunities with an application to a thiophene dendrimer.