"Two-photon fluorescence microscopy has evolved to one of the most powerful tools in biological imaging, especially in the field of neuroscience. It provides improved depth penetration and reduced photo-bleaching of fluorophores and has thus enabled three dimensional structural and functional imaging of biological samples. However, in typical media and with uncorrelated (classical) photons, the two-photon absorption (TPA) process has small interaction strengths and therefore requires large optical intensities, which can lead to photo-damage in the media or prohibit its use with highly light sensitive biological samples.In this fellowship, we will address this shortcoming by using the process of entangled two-photon absorption (ETPA). Due to the energy-time correlation of entangled photon pairs, the effective cross-section, and therefore the efficiency of the absorption, can be increased by many orders of magnitude. This represents a fundamentally new approach to microscopy and if successful, will have a significant impact on fundamental and applied biology, as it allows for imaging and probing biological processes in a highly non-invasive fashion.For this project, we aim to design and build an entangled photon source engineered for optimal ETPA and identify ideal molecules that can function as labels and fluorescent proteins in biological samples. This highly interdisciplinary project will combine and exploit advantages of quantum optics and two-photon fluorescence microscopy and pave the way for novel, entanglement-based biological imaging and spectroscopy tools. This highly interdisciplinary project will combine the candidate’s expertise in quantum optics and quantum information with the host’s extensive background in bio-medical imaging. This fellowship will catalyze a significant development in the candidate’s career and enable him to attain a leading independent position in this new field at the interface of quantum physics and biology."