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Quantum Optics Tools for Biomedical Imaging

Final Report Summary - QUANTUM4BIO (Quantum Optics Tools for Biomedical Imaging)

Optical imaging techniques have undergone a renaissance over the recent years and are powerful and omnipresent tools in current biological research. They provide three dimensional structural and functional information of biological samples and have led to new and important insights that help to understand the structure and dynamic of cellular processes at different levels. One widely used technique is two-photon fluorescence microscopy (TPFM), which has the advantage of improved depth penetration due to the longer wavelengths used and reduced photo-bleaching of chromophores. Unfortunately, it requires expensive laser light sources and high light intensities. In this project, we investigated the use of alternative, non classical light sources for their use in biological research.

In typical biological samples, the two-photon absorption coefficient is very small and high light intensities are required to facilitate two-photon interactions. However, there exist light sources that might be better suited to this type of imaging. Non-classical sources of light, also known as entangled light sources, feature intrinsic energy-time correlation which significantly increases the absorption efficiency of TPFM. Such correlated light sources further have a non-Poissonian emission statistics, which allows controlling the exact amount of light as measured by individual light quanta, or photons, to be directed to the biological sample.

During the reporting period of this project, the researchers were successful in designing and setting-up a broadband source of correlated photons based on spontaneous parametric down-conversion (SPDC). It provides up to 0.2uW (or 10^13 photons per second) of optical power, and due to it’s high bandwidth of ~100nm, features ultra-fast coherence times of ~18fs.

Using these novel light sources we have set up an international collaboration with researchers at the University of Waterloo, Canada (IQC), which are aimed at investigating the two-photon excitation of different fluorescent molecules under different conditions. Our results so far have indicated that compensation of dispersion properties of these photons are extremely important and highly delicate.

Parallel to this efforts,alternative applications of quantum states of light in the biomedical research fields had been identified, for which we have designed and built a SPDC source that allows to investigate the interaction of individual photons from the SPDC source with bio-molecules.

In this context the efficient generation and detection of time-energy entangled SPDC light offers the advantage of well-defined photon number states, so-called Fock states. Detection of photons in the idler beam heralds a Fock-state in the signal arm. Therefore one can infer with low uncertainty the number of photons present in a given “pulse”. This is in contrast to classical laser light, which – even when appropriately attenuated – retains a Poissonian and therefore quasi-random distribution of photon numbers. Using quantum states of light with well-defined photon numbers could shed light onto the exact timing and interaction mechanism of light sensitive biological molecules. Experiments in this direction are currently underway in our lab.