Imaging Brain Tissue\nwith Quantum Light at Low Power

Light-induced tissue damage is a crucial limitation for\ntraditional\nmicroscopy of the living brain, underscoring the need for new techniques\nthat minimize exposure of samples to light. Here, we tested the hypothesis\nthat quantum light, i.e., entangled photons, could detect brain structures\nat a lower excitation energy. In a proof of principle, we show microscopic\nimages of fixed brain tissue in the hippocampus area created by fluorescence\nselective excitation in the process of entangled two-photon absorption\nin a scanning microscope. Quantum-enhanced entangled two-photon microscopy\n(TPM) had brain imaging capabilities at an unprecedented low excitation\nintensity of ∼3.6 × 10<sup>7</sup> photons/s, orders of\nmagnitude lower than the excitation level for the classical two-photon\nfluorescence image obtained in the same microscope. The extremely\nlow light probe intensity demonstrated in entangled TPM is of critical\nimportance in the investigation of neural activity to minimize heating\nand photobleaching during repetitive imaging. It may have important\nfunctional implications in optogenetic technology, removing unintended\nheating and accumulated photodamage effects. This technology also\nopens avenues in spatially resolved brain tissue investigations with\nquantum light, providing new capabilities in local spectroscopy.