The propagation of photons through scattering media is one of the most salient aspects of optical science1,2,3,4. The light experiences many deleterious effects due to multiple scattering off the constituent particles in the media. The direction, path, phase, and polarization of a photon are changed when it travels in a scattering medium. The photon beam can break up into ballistic, snake, and diffusive components depending on the scattering mean free length, transport length, and anisotropic factor that are determined by particle size and structure1,2,3,4,5,6. When a photon enters a dense medium, such as brain slab, it is influenced by medium’s index of refraction dispersion, becoming itself a quantum quasi-particle. Quasi-particles involved in the coherent interactions with photons have fast dephasing time, e.g. 100 fs in water, 2 ps in solids in diamond, excitations at the 10 ps level, and non-radiative relaxation at the 1 ps level.

Medical diagnosis has employed a number of quantum physics-based technologies, such as optical spectroscopy, magnetic resonance imaging, and computerized tomography for understanding biological structure and functions and for disease treatment. These have been harnessed by our deep understanding of atomic and molecular energy levels, particle quantum properties such as position, spin, polarization, and their interaction with electromagnetic radiation.

The last few decades have seen the rise of the study of another quantum physics phenomenon: quantum entanglement. Beyond the fundamental questions that it generates, such as nonlocality and realism7,8, quantum entanglement provides the ultimate level of coherence between correlated particles, which can evolve coherently among entangled particles traveling separate paths and affect each other in the correlation when one of them is measured. The harnessing of the quantum entanglement has been at the core of a new type of technology known as quantum information9. Measurement of the exquisite level of coherence between entangled particles as they propagate through biological media could provide new medical diagnosis information not available by other methods. The method goes beyond sending single photons, let alone the coherent state of a laser, through a sample, where polarimetry and modal analysis provides information. The polarization entanglement of two photons entails a larger Hilbert space, and therefore more information. Two measures of the state of the pair are tangle (T) and linear entropy (S). They characterize distinct aspects of the state: with non-separability quantified by T and coherence by S. When one of the twin photons enters a scattering turbid medium, such as a sample with beads or a tissue, the initial entanglement of the input photon will in general be degraded by multiple scattering. The phase and polarization coherence of the pair will degrade in general to a mixed state, reducing T and increasing S of the state shared with the other twin photon. How does the change in these quantities correlate with biological sample properties? Beyond quantifying the state of the light, quantum effects provided by twin photons can reach sensitivities only reached by quantum effects, such as in imaging, where the resolution is improved below the classical wavelength limit to the one specified by the deBroglie wavelength of both particles10; by two-photon imaging via fourth-order coherence11; or by squeezing or related methods that reduce the measured uncertainty below the quantum limit12.

Optical spectroscopy has been widely employed in brain research, and is the only technique for brain imaging with the resolution at micrometer or sub-micrometer scale. In the brain there are billions of neurons, astrocytes, glia cells, vessels, and axons that form many complex tree branches or spider cobwebs. The propagating photon is virtually absorbed to upper brain states, and coherently scattered forward mainly as a ballistic photon, propagating along in thin medium4,6. Tegmark13 measured that the decoherence effects in the complex tree of the brain are extremely fast, at the order of 10 zs to 10 as. The entangled photons at 802 nm propagating in brain can interact with H 2 O and hemoglobin (Hb) in the brain vessels and undergo virtual and real transitions14. Water has little absorption at 800 nm and the effects of Hb and HbO 2 are the same so it helps the dressed ballistic photons from dephasing via scattering from the void regions in the overlapping neural trees. It is therefore hypothesized that the entanglement of a photon will be degraded when it passes through brain tissue, hence, measuring the polarization and coherence between this photon and its paired photon that not passing through any tissue/media opens the door for a new imaging technique in brain research.

The objective of this paper is to investigate the polarization change of two entangled photons as the light travels through rat brain tissue. We considered the simplest case that using twin polarization-entangled photons at 802 nm, with one photon traveling through brain tissue while the other twin photon traveling in free space to a detector. We measured the non-local correlations between the photons via quantum state tomography, and from them obtained various measures of quantum entanglement. We investigated to what degree the mixture induced by scattering depends on the depth of penetration into tissue media15. We found that despite scattering, brain tissue of various thickness shows strong photon entanglement, which is preserved through brain tissue layers up to 400 μm, and that the correlation with thickness is not as strong as with other parameters such as water content and type of tissue.