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1. Polarization-Sensitive Quantum Optical Coherence Tomography: Birefringence Profiles of Biological Samples

   https://doi.org/10.3390/app14031168  

   Sukharenko, Vitaly; Dorsinville, Roger (February 2024, Applied Sciences)

   Polarization-sensitive quantum optical coherence tomography (PS-QOCT) is used to image and characterize birefringence effects in biological samples. Entangled photons are generated via spontaneous parametric down-conversion and split into a reference arm and a sample arm of a Mach Zehnder interferometer. Interferometric patterns between two entangled photons reveal information about tissue birefringence. Biological tissue samples are imaged and characterized, and their quantum interference patterns and birefringence profiles are presented. 

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2. 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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3. Experimental storage of photonic polarization entanglement in a broadband loop-based quantum memory

   https://doi.org/10.1103/PhysRevA.108.L050601  

   Evans, C J; Nunn, C M; Cheng, S_W L; Franson, J D; Pittman, T B (November 2023, Physical Review A)

   We experimentally study the ability of a broadband “loop-and-switch” type quantum memory device to store entanglement. We find that one active loop-based memory and one passive fiber delay line can be used to faithfully store two polarization-entangled photons and demonstrate a rudimentary entanglement distribution protocol. The entangled photons are produced by a conventional spontaneous parametric down-conversion source with center wavelengths at 780 nm and bandwidths of ∼10 THz, while the memory has an even wider operational bandwidth that is enabled by the weakly dispersive nature of the Pockels effect used for polarization-insensitive active switching. These results help demonstrate the utility of loop-based quantum memories for quantum networking applications. 

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4. Generation of entangled-photons by a quantum dot cascade source in polarized cavities: Using cavity resonances to boost signals and preserve the entanglements

   https://doi.org/10.1063/5.0144364  

   Nasiri Avanaki, K.; Schatz, George C. (April 2023, The Journal of Chemical Physics)

   Motivated by recent advances in the development of single photon emitters for quantum information sciences, here we design and formulate a quantum cascade model that describes cascade emission by a quantum dot (QD) in a cavity structure while preserving entanglement that stores information needed for single photon emission. The theoretical approach is based on a photonic structure that consists of two orthogonal cavities in which resonance with either the first or second of the two emitted photons is possible, leading to amplification and rerouting of the entangled light. The cavity–QD scheme uses a four-level cascade emitter that involves three levels for each polarization, leading to two spatially entangled photons for each polarization. By solving the Schrodinger equation, we identify the characteristic properties of the system, which can be used in conjunction with optimization techniques to achieve the “best” design relative to a set of prioritized criteria or constraints in our optical system. The theoretical investigations include an analysis of emission spectra in addition to the joint spectral density profile, and the results demonstrate the ability of the cavities to act as frequency filters for the photons that make up the entanglements and to modify entanglement properties. The results provide new opportunities for the experimental design and engineering of on-demand single photon sources. 

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5. Quantum phase-sensitive diffraction and imaging using entangled photons

   https://doi.org/10.1073/pnas.1904839116  

   Asban, Shahaf; Dorfman, Konstantin E.; Mukamel, Shaul (June 2019, Proceedings of the National Academy of Sciences)

   We propose a quantum diffraction imaging technique whereby one photon of an entangled pair is diffracted off a sample and detected in coincidence with its twin. The image is obtained by scanning the photon that did not interact with matter. We show that when a dynamical quantum system interacts with an external field, the phase information is imprinted in the state of the field in a detectable way. The contribution to the signal from photons that interact with the sample scales as ∝ I p 1 / 2 , where I p is the source intensity, compared with ∝ I p of classical diffraction. This makes imaging with weak fields possible, providing high signal-to-noise ratio, avoiding damage to delicate samples. A Schmidt decomposition of the state of the field can be used for image enhancement by reweighting the Schmidt modes contributions. 

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