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1. Liouvillian exceptional points of an open driven two-level system

   https://doi.org/10.1063/5.0177714  

   Seshadri, Nikhil; Li, Anqi; Galperin, Michael (January 2024, The Journal of Chemical Physics)

   We study the applicability of the Liouvillian exceptional points (LEPs) approach to nanoscale open quantum systems. A generic model of the driven two-level system in a thermal environment is analyzed within the nonequilibrium Green’s function (NEGF) and Bloch quantum master equation formulations. We derive the latter starting from the exact NEGF Dyson equations and highlight the qualitative limitations of the LEP treatment by examining the approximations employed in its derivation. We find that the non-Markov character of evolution in open quantum systems does not allow for the introduction of the concept of exceptional points for a description of their dynamics. Theoretical analysis is illustrated with numerical simulations. 

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2. Speeding up squeezing with a periodically driven Dicke model

   https://doi.org/10.1103/PhysRevResearch.6.033090  

   Reilly, Jarrod T; Jäger, Simon B; Wilson, John Drew; Cooper, John; Eggert, Sebastian; Holland, Murray J (July 2024, Physical Review Research)

   We present a simple and effective method to create highly entangled spin states on a faster timescale than that of the commonly employed one-axis twisting (OAT) model. We demonstrate that by periodically driving the Dicke Hamiltonian at a resonance frequency, the system effectively becomes a two-axis countertwisting Hamiltonian, which is known to quickly create Heisenberg limit scaled entangled states. For these states we show that simple quadrature measurements can saturate the ultimate precision limit for parameter estimation determined by the quantum Cramér-Rao bound. An example experimental realization of the periodically driven scheme is discussed with the potential to quickly generate momentum entanglement in a recently described experimental vertical cavity system. We analyze effects of collective dissipation in this vertical cavity system and find that our squeezing protocol can be more robust than the previous realization of OAT. Published by the American Physical Society2024 

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3. Room temperature polaritonic soft-spin XY Hamiltonian in organic–inorganic halide perovskites

   https://doi.org/10.1515/nanoph-2023-0818  

   Peng, Kai; Li, Wei; Berloff, Natalia G; Zhang, Xiang; Bao, Wei (February 2024, Nanophotonics)

   Abstract Exciton–polariton condensates, due to their nonlinear and coherent characteristics, have been employed to construct spin Hamiltonian lattices for potentially studying spin glass, critical dephasing, and even solving optimization problems. Here, we report the room-temperature polariton condensation and polaritonic soft-spin XY Hamiltonian lattices in an organic–inorganic halide perovskite microcavity. This is achieved through the direct integration of high-quality single-crystal samples within the cavity. The ferromagnetic and antiferromagnetic couplings in both one- and two-dimensional condensate lattices have been observed clearly. Our work shows a nonlinear organic–inorganic hybrid perovskite platform for future investigations as polariton simulators. 

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4. Light–matter interaction Hamiltonians in cavity quantum electrodynamics

   https://doi.org/10.1063/5.0225932  

   Taylor, Michael_A D; Mandal, Arkajit; Huo, Pengfei (March 2025, Chemical Physics Reviews)

   When matter is strongly coupled to an optical cavity, new hybrid light–matter states are formed, the so-called polariton states. These polaritons can qualitatively change the physical properties of the matter coupled to the cavity by completely altering its energy eigenspectrum. Fueled by experimental innovations in recent years, much progress has been made in simulating the intrinsic quantum behavior of these hybrid states. At the heart of each simulation is the choice of Hamiltonian to represent the total light–matter hybrid system. Even at this fundamental level, there has been significant progress in developing new gauges and representations for this Hamiltonian, whether exact or under approximations. As such, this review aims to discuss several different forms of Hamiltonians for the researcher trying to enter this field by clearly and concisely deriving each different representation from the fundamental Minimal Coupling Hamiltonian. In addition, this review provides commentary on the optimal usage and extent of approximations for each individual representation to assist the reader in choosing the appropriate Hamiltonian for their work. 

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5. Role of microstructure on flux expulsion of superconducting radio frequency cavities

   https://doi.org/10.1088/1361-6668/ad9ad7  

   Khanal, B D; Balachandran, S; Chetri, S; Barron, M; Mullinix, R; Williams, A; Xu, P; Ingrole, A; Lee, P J; Ciovati, G; et al (December 2024, Superconductor Science and Technology)

   The trapped residual magnetic flux during the cool-down due to the incomplete Meissner state is a significant source of radio frequency losses in superconducting radio frequency cavities. Here, we clearly correlate the niobium microstructure in elliptical cavity geometry and flux expulsion behavior. In particular, a traditionally fabricated Nb cavity half-cell from an annealed poly-crystalline Nb sheet after an 800 C heat treatment leads to a bi-modal microstructure that ties in with flux trapping and inefficient flux expulsion. This non-uniform microstructure is related to varying strain profiles along the cavity shape. A novel approach to prevent this non-uniform microstructure is presented by fabricating a 1.3 GHz single cell Nb cavity with a cold-worked sheet and subsequent heat treatment leading to better flux expulsion after 800 ^∘C/3 h. Microstructural evolution by electron backscattered diffraction-orientation imaging microscopy on cavity cutouts, and flux pinning behavior by dc-magnetization on coupon samples confirms a reduction in flux pinning centers with increased heat treatment temperature. The heat treatment temperature-dependent mechanical properties and thermal conductivity are reported. The significant impact of cold work in this study demonstrates clear evidence for the importance of the microstructure required for high-performance superconducting cavities with reduced losses caused by magnetic flux trapping. 

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