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1. Non-local Mueller polarimetry

   https://doi.org/10.1117/12.3004822  

   You, Chan Ju; Rodriguez-Fajardo, Valeria; Francis, Leia; Galvez, Enrique J (March 2024, SPIE)

   Ramella-Roman, Jessica C; Ma, Hui; Vitkin, I Alex; Elson, Daniel S; Novikova, Tatiana (Ed.)

   We present a method to determine the Mueller matrix of a sample using polarization-entangled photon pairs. One of the photons of a pair goes through a sample and is then subject to a polarization projection measurement. The other photon, which does not go through the sample, is also subject to a polarization projection. The measured quantum correlations are equivalent to polarimetry measurements, where the initial state of the photon going through the sample is determined by the polarization projection on the entangled partner that does not go through the sample. The correspondence with the classical system is acausal because quantum measurements apply to distinct Hilbert spaces. We tested this method with standard optical elements finding excellent agreement with the expectations. Thus it can be used as an alternative to classical Mueller polarimetry for conditions that would be challenging to do otherwise. 

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2. Non-local and non-Hermitian acoustic metasurfaces

   https://doi.org/10.1088/1361-6633/acfbeb  

   Wang, Xu; Dong, Ruizhi; Li, Yong; Jing, Yun (October 2023, Reports on Progress in Physics)

   Abstract Acoustic metasurfaces are at the frontier of acoustic functional material research owing to their advanced capabilities of wave manipulation at an acoustically vanishing size. Despite significant progress in the last decade, conventional acoustic metasurfaces are still fundamentally limited by their underlying physics and design principles. First, conventional metasurfaces assume that unit cells are decoupled and therefore treat them individually during the design process. Owing to diffraction, however, the non-locality of the wave field could strongly affect the efficiency and even alter the behavior of acoustic metasurfaces. Additionally, conventional acoustic metasurfaces operate by modulating the phase and are typically treated as lossless systems. Due to the narrow regions in acoustic metasurfaces’ subwavelength unit cells, however, losses are naturally present and could compromise the performance of acoustic metasurfaces. While the conventional wisdom is to minimize these effects, a counter-intuitive way of thinking has emerged, which is to harness the non-locality as well as loss for enhanced acoustic metasurface functionality. This has led to a new generation of acoustic metasurface design paradigm that is empowered by non-locality and non-Hermicity, providing new routes for controlling sound using the acoustic version of 2D materials. This review details the progress of non-local and non-Hermitian acoustic metasurfaces, providing an overview of the recent acoustic metasurface designs and discussing the critical role of non-locality and loss in acoustic metasurfaces. We further outline the synergy between non-locality and non-Hermiticity, and delineate the potential of using non-local and non-Hermitian acoustic metasurfaces as a new platform for investigating exceptional points, the hallmark of non-Hermitian physics. Finally, the current challenges and future outlook for this burgeoning field are discussed. 

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3. Non-Local Graph Neural Networks

   https://doi.org/10.1109/TPAMI.2021.3134200  

   Liu, Meng; Wang, Zhengyang; Ji, Shuiwang (January 2022, IEEE Transactions on Pattern Analysis and Machine Intelligence)
4. Synchronicity for quantum non-local games

   https://doi.org/10.1016/j.jfa.2022.109738  

   Brannan, Michael; Harris, Samuel J.; Todorov, Ivan G.; Turowska, Lyudmila (January 2023, Journal of Functional Analysis)
5. Complexity of frustration: A new source of non-local non-stabilizerness

   https://doi.org/10.21468/SciPostPhys.15.4.131  

   Odavić, Jovan; Haug, Tobias; Torre, Gianpaolo; Hamma, Alioscia; Franchini, Fabio; Giampaolo, Salvatore Marco (January 2023, SciPost Physics)

   We advance the characterization of complexity in quantum many-body systems by examiningW $W$-states embedded in a spin chain. Such states show an amount of non-stabilizerness or “magic”, measured as the Stabilizer Rényi Entropy, that grows logarithmically with the number of qubits/spins. We focus on systems whose Hamiltonian admits a classical point with extensive degeneracy. Near these points, a Clifford circuit can convert the ground state into aW $W$-state, while in the rest of the phase to which the classical point belongs, it is dressed with local quantum correlations. Topological frustrated quantum spin-chains host phases with the desired phenomenology, and we show that their ground state’s Stabilizer Rényi Entropy is the sum of that of theW $W$-states plus an extensive local contribution. Our work reveals thatW $W$-states/frustrated ground states display a non-local degree of complexity that can be harvested as a quantum resource and has no counterpart in GHZ states/non-frustrated systems. 

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