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1. Decoherence of photon entanglement by transmission through brain tissue with Alzheimer’s disease

   https://doi.org/10.1364/BOE.474469  

   Galvez, E_J; Sharma, B.; Williams, F_K; You, C-J; Khajavi, B.; Castrillon, J.; Shi, L.; Mamani, S.; Sordillo, L_A; Zhang, L.; et al (November 2022, Biomedical Optics Express)

   The generation, manipulation and quantification of non-classical light, such as quantum-entangled photon pairs, differs significantly from methods with classical light. Thus, quantum measures could be harnessed to give new information about the interaction of light with matter. In this study we investigate if quantum entanglement can be used to diagnose disease. In particular, we test whether brain tissue from subjects suffering from Alzheimer’s disease can be distinguished from healthy tissue. We find that this is indeed the case. Polarization-entangled photons traveling through brain tissue lose their entanglement via a decohering scattering interaction that gradually renders the light in a maximally mixed state. We found that in thin tissue samples (between 120 and 600 micrometers) photons decohere to a distinguishable lesser degree in samples with Alzheimer’s disease than in healthy-control ones. Thus, it seems feasible that quantum measures of entangled photons could be used as a means to identify brain samples with the neurodegenerative disease. 

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2. Polarization Sensitive Imaging with Qubits

   https://doi.org/10.3390/app12042027  

   Sukharenko, Vitaly; Dorsinville, Roger (February 2022, Applied Sciences)

   We compare reconstructed quantum state images of a birefringent sample using direct quantum state tomography and inverse numerical optimization technique. Qubits are used to characterize birefringence in a flat transparent plastic sample by means of polarization sensitive measurement using density matrices of two-level quantum entangled photons. Pairs of entangled photons are generated in a type-II nonlinear crystal. About half of the generated photons interact with a birefringent sample, and coincidence counts are recorded. Coincidence rates of entangled photons are measured for a set of sixteen polarization states. Tomographic and inverse numerical techniques are used to reconstruct the density matrix, the degree of entanglement, and concurrence for each pixel of the investigated sample. An inverse numerical optimization technique is used to obtain a density matrix from measured coincidence counts with the maximum probability. Presented results highlight the experimental noise reduction, greater density matrix estimation, and overall image enhancement. The outcome of the entanglement distillation through projective measurements is a superposition of Bell states with different amplitudes. These changes are used to characterize the birefringence of a 3M tape. Well-defined concurrence and entanglement images of the birefringence are presented. Our results show that inverse numerical techniques improve overall image quality and detail resolution. The technique described in this work has many potential applications. 

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3. Direct measurement of the density matrix of a two-photon polarization qubit

   https://doi.org/10.1117/12.2666182  

   Galvez, Enrique J.; Goldstein, Andrew D.; You, Chan Ju; Rodriguez-Fajardo, Valeria; Shi, Lingyan; Alfano, Robert R. (March 2023, Proceedings of SPIE)

   Andrews, D.; Galvez, EJ; Rubinsztein-Dunlop, H. (Ed.)

   There is interest in using photon entanglement in biomedical applications. In one application, polarization-entangled photons pass through brain tissue. The effect of the brain tissue on the photon entanglement is measured via the decoherence that is imparted on the entangled state. Our current method to obtain a measure of the decoherence involves quantum state tomography, where a minimum of 16 measurements are used in conjunction with tomographic optimization to obtain the density matrix representing the state of the photons. In this work we report on a method to avoid tomographic optimization on behalf of a direct measurement of the elements of the density matrix. We make preliminary comparisons between the two methods. 

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4. Pathway Selectivity in 2D Electronic‐Vibrational Spectroscopy with Quantum Light

   https://doi.org/10.1002/lpor.202401576  

   Jadoun, Deependra; Yadalam, Hari_K; Harbola, Upendra; Chernyak, Vladimir_Y; Kizmann, Matthias; Mukamel, Shaul (January 2025, Laser & Photonics Reviews)

   Abstract Pathway selectivity in quantum spectroscopy with entangled photons is a powerful spectroscopic tool. Phase‐matched signals involving classical light contain contributions from multiple material pathways, whereas quantum spectroscopy may allow the selection of individual pathways. 2D electronic‐vibrational spectroscopy (2DEVS) is a four‐wave mixing technique which employs visible and infrared entangled photons. It is showed how the three contributing pathways—ground state bleach, excited state absorption, and excited state emission—can be separated by photon‐number‐resolved coincidence measurements. Entangled photons thus reveal spectral features not visible in the classical signal, with an enhanced spectral resolution. 

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5. Colors of entangled two-photon absorption

   https://doi.org/10.1073/pnas.2307719120  

   Varnavski, Oleg; Giri, Sajal Kumar; Chiang, Tse-Min; Zeman, Charles J.; Schatz, George C.; Goodson, Theodore (August 2023, Proceedings of the National Academy of Sciences)

   Multiphoton absorption of entangled photons offers ways for obtaining unique information about chemical and biological processes. Measurements with entangled photons may enable sensing biological signatures with high selectivity and at very low light levels to protect against photodamage. In this paper, we present a theoretical and experimental study of the excitation wavelength dependence of the entangled two-photon absorption (ETPA) process in a molecular system, which provides insights into how entanglement affects molecular spectra. We demonstrate that the ETPA excitation spectrum can be different from that of classical TPA as well as that for one-photon resonant absorption (OPA) with photons of doubled frequency. These results are modeled by assuming the ETPA cross-section is governed by a two-photon excited state radiative linewidth rather than by electron-phonon interactions, and this leads to excitation spectra that match the observed results. Further, we find that the two-photon-allowed states with highest TPA and ETPA intensities have high electronic entanglements, with ETPA especially favoring states with the longest radiative lifetimes. These results provide concepts for the development of quantum light–based spectroscopy and microscopy that will lead to much higher efficiency of ETPA sensors and low-intensity detection schemes. 

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