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Quantum Tools for Imaging

In our Quantum Tools for Imaging project, we exploit advantages of quantum optics and two-photon fluorescence microscopy to pave the way for novel, entanglement-based bio-imaging and spectroscopy tools.

Two-photon fluorescence microscopy has evolved to one of the most powerful tools in biological imaging. It provides improved depth penetration and reduced photo-bleaching of chromophores and has thus enabled three dimensional structural and functional imaging of biological samples. These improvements are due to the non-linear, intensity squared dependence of the two-photon absorption process on the molecules. 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. Hence a tool for imaging and probing biological processes in a highly non-invasive fashion would be of importance and have a significant impact on fundamental and applied biology.

The process of entangled two-photon absorption (ETPA) provides a fundamentally new approach to achieve this goal. The energy-time correlation of entangled photon pairs produced in spontaneous parametric down-conversion (SPDC) results in orders of magnitude larger cross sections than for conventional TPA. Although pioneering work on the theory of TPA with entangled photons and some proof of principle experiments demonstrating the effect in atomic vapors and molecules have been achieved, so far the unique advantages of ETPA has not been exploited for any technological breakthroughs.

Fig. 1: a) Two-photon absorption and fluorescence. Two photons at wavelength λex excite a two-photon transition in the absorbing medium if they arrive within a narrow time interval τ. b) Photon statistic of laser vs. SPDC photons. The top shows a conventional Ti:Sa laser which is routinely used for two-photon imaging. Due to the random arrival time of the photons, only a small portion of the photons falls within τ. In stark contrast, photons produced in SPDC are highly correlated in time.

We have design and build an entangled photon source engineered to exploit the high temporal correlations of the down-converted photons. Our source features high entangled photon yield, up to several uW, at wavelenghts relevant for biological markers and fluorophores. Its spectrally broad-band emission leads to short SPDC “pulse length” which allows to maintain high energy-time correlations even for uW of SPDC power.

SPDC spectrum
Fig. 2: SPDC spectrum measured with a sensitive spectrometer and EMCCD camera. The FWHM bandwidth of the down-conversion is ~90nm.

Parallel to this efforts, alternative applications of quantum states of light in the biomedical research fields have been identified. In this respect, a number of light-matter interactions with light absorbing proteins such as opsins, cryptochrome or photosynthesis complexes at the low light levels provide an exciting avenue for future research. In this regime the unique characteristic features of entangled photons could be employed to get a better understanding of the light absorption process.