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1. Time-dependent Hamiltonian simulation with L1 -norm scaling

   https://doi.org/10.22331/q-2020-04-20-254  

   Berry, Dominic W. ; Childs, Andrew M. ; Su, Yuan ; Wang, Xin ; Wiebe, Nathan ( April 2020 , Quantum)

   The difficulty of simulating quantum dynamics depends on the norm of the Hamiltonian. When the Hamiltonian varies with time, the simulation complexity should only depend on this quantity instantaneously. We develop quantum simulation algorithms that exploit this intuition. For sparse Hamiltonian simulation, the gate complexity scales with the L 1 norm ∫ 0 t d τ ‖ H ( τ ) ‖ max , whereas the best previous results scale with t max τ ∈ [ 0 , t ] ‖ H ( τ ) ‖ max . We also show analogous results for Hamiltonians that are linear combinations of unitaries. Our approaches thus provide an improvement over previous simulation algorithms that can be substantial when the Hamiltonian varies significantly. We introduce two new techniques: a classical sampler of time-dependent Hamiltonians and a rescaling principle for the Schrödinger equation. The rescaled Dyson-series algorithm is nearly optimal with respect to all parameters of interest, whereas the sampling-based approach is easier to realize for near-term simulation. These algorithms could potentially be applied to semi-classical simulations of scattering processes in quantum chemistry. 

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2. An optical atomic clock using 4DJ states of rubidium

   https://doi.org/10.1088/2058-9565/ad77ef  

   Duspayev, A. ; Owens, C. ; Dash, B. ; Raithel, G. ( September 2024 , Quantum Science and Technology)

   We analyze an optical atomic clock using two-photon$5S_{1/2}\to 4D_J$transitions in rubidium. Four one- and two-color excitation schemes to probe the$4D_{3/2}$and$4D_{5/2}$fine-structure states are considered in detail. We compare key characteristics of Rb$4D_J$and$5D_{5/2}$two-photon clocks. The$4D_J$clock features a high signal-to-noise ratio due to two-photon decay at favorable wavelengths, low dc electric and magnetic susceptibilities, and minimal black-body shifts. Ac Stark shifts from the clock interrogation lasers are compensated by two-color Rabi-frequency matching. We identify a ‘magic’ wavelength near 1060 nm, which allows for in-trap, Doppler-free clock-transition interrogation with lattice-trapped cold atoms. From our analysis of clock statistics and systematics, we project a quantum-noise-limited relative clock stability at the${10}^{-13}/\sqrt{\tau (s)}$-level, with integration timeτin seconds, and a relative accuracy of$\sim {10}^{-13}$. We describe a potential architecture for implementing the proposed clock using a single telecom clock laser at 1550 nm, which is conducive to optical communication and long-distance clock comparisons. Our work could be of interest in efforts to realize small and portable Rb clocks and in high-precision measurements of atomic properties of Rb$4D_J$-states.

    

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3. Electronic structure of Co 3d states in the Kitaev material candidate honeycomb cobaltate Na3Co2SbO6 probed with x-ray dichroism

   https://doi.org/10.1103/PhysRevB.107.214443  

   van Veenendaal, M. ; Poldi, E. H. ; Veiga, L. S. ; Bencok, P. ; Fabbris, G. ; Tartaglia, R. ; McChesney, J. L. ; Freeland, J. W. ; Hemley, R. J. ; Zheng, H. ; et al ( June 2023 , Physical Review B)

   none (Ed.)

   The recent prediction that honeycomb lattices of Co2+ (3d7) ions could host dominant Kitaev interactions provides an exciting direction for exploration of new routes to stabilizing Kitaev’s quantum spin liquid in real materials. Na3Co2SbO6 has been singled out as a potential material candidate provided that spin and orbital moments couple into a Jeff = 1/2 ground state, and that the relative strength of trigonal crystal field and spin-orbit coupling acting on Co ions can be tailored. Using x-ray linear dichroism (XLD) and x-ray magnetic circular dichroism (XMCD) experiments, alongside configuration interaction calculations, we confirm the counterintuitive positive sign of the trigonal crystal field acting on Co2+ ions and test the validity of the Jeff = 1/2 description of the electronic ground state. The results lend experimental support to recent theoretical predictions that a compression (elongation) of CoO6 octahedra along (perpendicular to) the trigonal axis would drive this cobaltate toward the Kitaev limit, assuming the Jeff = 1/2 character of the electronic ground state is preserved. 

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4. Spin liquid nature in the Heisenberg J1−J2 triangular antiferromagnet

   https://doi.org/10.1103/PhysRevB.93.144411  

   Iqbal, Yasir ; Hu, Wen-Jun ; Thomale, Ronny ; Poilblanc, Didier ; Becca, Federico ( April 2016 , Physical Review B)
5. Mott insulating low thermal expansion perovskite TiF3

   https://doi.org/10.1103/PhysRevB.108.235140  

   Sheets, Donal ; Lyszak, Kaitlin ; Jain, Menka ; Fernando, Gayanath W. ; Sochnikov, Ilya ; Franklin, Jacob ; Hancock, Jason N. ; Geilhufe, R. Matthias ( December 2023 , Physical Review B)

   We characterize perovskite TiF3, a material which is reported to display significant negative thermal expansion at elevated temperatures above its cubic-to-rhombohedral structural phase transition at 330 K. We find the optical response favors an insulating state in both structural phases, which we show can be produced in density functional theory calculations only through the introduction of an on-site Coulomb repulsion. Analysis of the magnetic susceptibility data gives a S = 21 local moment per Ti+3 ion and an antiferromagnetic exchange coupling. Together, these results show that TiF3 is a strongly correlated electron system, a fact which constrains possible mechanisms of strong negative thermal expansion in the Sc1−x Tix F3 system. We consider the relative strength of the Jahn-Teller and electric dipole interactions in driving the structural transition. 

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