This project aims to investigate the possibility to use entangled photon sources to probe brain activity. Current photonic approaches for measuring brain activity, ranging from techniques such as diffuse correlation spectroscopy to functional near infrared spectroscopy, utilise relatively intense laser beams, at the edge of or just beyond eye-safety limits. These are injected into the head, non-invasively at one point using eg a fibre and then collected by another fibre placed 2-3 cm away from the input. The collected light has followed trajectories that have probed the gray matter and are sensitive to changes in absorption due to changes in blood oxygenation resulting from neuronal activity. However, there is mounting evidence that photobiomodulation, i.e. the modulation of brain activity as a result of illumination and absorption of light in the brain. In other words, the light used to measure brain activity is also simultaneously modifying that same activity. This is a very similar situation encountered in spectroscopy where e.g. one tries to use intense laser pulses to probe photosensitive molecule dynamics, e.g. photosynthesis: the probing technique is also modifying the system under study. We recently demonstrated that it is possible to use entangled photons to measure energy transfer dynamics on picosecond timescales in photosynthetic molecules with high SNR, in sub-second measurement times and using only a CW laser (to generate the entangled photon pairs) [Mendiza et al. Nat. Commun. (2025)]. The goal is to now extend this technique to the human brain. This will allow: to probe the brain at the lowest possible illumination levels; obtain high precision time-domain fNIRS measurements using only a CW laser rather that the usual pulsed lasers. Comparing the results from a classical a system with the quantum system will also allow to directly probe the effects of photobiomodulation with the goal of demonstrating that the quantum approach is therefore effectively more precise. In the later stages of the project we will aim to extend this technique to new wavelength regimes (for brain sensing), e.g. in infrared and to more complex devices where entangled photon measurements are combined with electrical or magnetic recordings.