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1. Quantum Computation Toolbox for Decoherence‐Free Qubits Using Multi‐Band Alkali Atoms

   https://doi.org/10.1002/qute.201900132  

   Mamaev, Mikhail; Thywissen, Joseph_H; Rey, Ana_Maria (April 2020, Advanced Quantum Technologies)

   Abstract Protocols for designing and manipulating qubits with ultracold alkali atoms in 3D optical lattices are introduced. These qubits are formed from two‐atom spin superposition states that create a decoherence‐free subspace immune to stray magnetic fields, dramatically improving coherence times while still enjoying the single‐site addressability and Feshbach resonance control of state‐of‐the‐art alkali atom systems. The protocol requires no continuous driving or spin‐dependent potentials, and instead relies upon the population of a higher motional band to realize naturally tunable in‐site exchange and cross‐site superexchange interactions. As a proof‐of‐principle example of their utility for entanglement generation for quantum computation, it is shown that the cross‐site superexchange interactions can be used to engineer 1D cluster states. Explicit protocols for experimental preparation and manipulation of the qubits are also discussed, as well as methods for measuring more complex quantities such as out‐of‐time‐ordered correlation functions (OTOCs). 

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2. Integrability and quench dynamics in the spin-1 central spin XX model

   https://doi.org/10.21468/SciPostPhys.15.1.030  

   Tang, Long Hin; Long, David; Polkovnikov, Anatoli; Chandran, Anushya; Claeys, Pieter W (January 2023, SciPost Physics)

   Central spin models provide an idealized description of interactions between a central degree of freedom and a mesoscopic environment of surrounding spins. We show that the family of models with a spin-1 at the center and XX interactions of arbitrary strength with surrounding spins is integrable. Specifically, we derive an extensive set of conserved quantities and obtain the exact eigenstates using the Bethe ansatz. As in the homogenous limit, the states divide into two exponentially large classes: bright states, in which the spin-1 is entangled with its surroundings, and dark states, in which it is not. On resonance, the bright states further break up into two classes depending on their weight on states with central spin polarization zero. These classes are probed in quench dynamics wherein they prevent the central spin from reaching thermal equilibrium. In the single spin-flip sector we explicitly construct the bright states and show that the central spin exhibits oscillatory dynamics as a consequence of the semilocalization of these eigenstates. We relate the integrability to the closely related class of integrable Richardson-Gaudin models, and conjecture that the spin-s central spin XX model is integrable for any s. 

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3. Sensing spin wave excitations by spin defects in few-layer-thick hexagonal boron nitride

   https://doi.org/10.1126/sciadv.adk8495  

   Zhou, Jingcheng; Lu, Hanyi; Chen, Di; Huang, Mengqi; Yan, Gerald Q; Al-matouq, Faris; Chang, Jiu; Djugba, Dziga; Jiang, Zhigang; Wang, Hailong; et al (May 2024, Science Advances)

   Optically active spin defects in wide bandgap semiconductors serve as a local sensor of multiple degrees of freedom in a variety of “hard” and “soft” condensed matter systems. Taking advantage of the recent progress on quantum sensing using van der Waals (vdW) quantum materials, here we report direct measurements of spin waves excited in magnetic insulator Y₃Fe₅O₁₂(YIG) by boron vacancy $V_{\mathrm{B}}^{-}$spin defects contained in few-layer-thick hexagonal boron nitride nanoflakes. We show that the ferromagnetic resonance and parametric spin excitations can be effectively detected by $V_{\mathrm{B}}^{-}$spin defects under various experimental conditions through optically detected magnetic resonance measurements. The off-resonant dipole interaction between YIG magnons and $V_{\mathrm{B}}^{-}$spin defects is mediated by multi-magnon scattering processes, which may find relevant applications in a range of emerging quantum sensing, computing, and metrology technologies. Our results also highlight the opportunities offered by quantum spin defects in layered two-dimensional vdW materials for investigating local spin dynamic behaviors in magnetic solid-state matters. 

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4. Tailoring magnon modes by extending square, kagome, and trigonal spin ice lattices vertically via interlayer coupling of trilayer nanomagnets

   https://doi.org/10.1088/1361-648X/ad5d3f  

   de_Rojas, Julius; Atkinson, Del; Adeyeye, Adekunle O (July 2024, Journal of Physics: Condensed Matter)

   Abstract In this work high-frequency magnetization dynamics and statics of artificial spin-ice lattices with different geometric nanostructure array configurations are studied where the individual nanostructures are composed of ferromagnetic/non-magnetic/ferromagnetic trilayers with different non-magnetic thicknesses. These thickness variations enable additional control over the magnetic interactions within the spin-ice lattice that directly impacts the resulting magnetization dynamics and the associated magnonic modes. Specifically the geometric arrangements studied are square, kagome and trigonal spin ice configurations, where the individual lithographically patterned nanomagnets (NMs) are trilayers, made up of two magnetic layers of ${\text{Ni}}_{81}{\text{Fe}}_{19}$of 30 nm and 70 nm thickness respectively, separated by a non-magnetic copper layer of either 2 nm or 40 nm. We show that coupling via the magnetostatic interactions between the ferromagnetic layers of the NMs within square, kagome and trigonal spin-ice lattices offers fine-control over magnetization states and magnetic resonant modes. In particular, the kagome and trigonal lattices allow tuning of an additional mode and the spacing between multiple resonance modes, increasing functionality beyond square lattices. These results demonstrate the ability to move beyond quasi-2D single magnetic layer nanomagnetics via control of the vertical interlayer interactions in spin ice arrays. This additional control enables multi-mode magnonic programmability of the resonance spectra, which has potential for magnetic metamaterials for microwave or information processing applications. 

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5. Probing dynamics of a two-dimensional dipolar spin ensemble using single qubit sensor

   https://doi.org/10.48550/arXiv.2207.10688  

   Rezai, K.; Choi, S.; Lukin, M. D.; Sushkov, A. O. (July 2022, ArXivorg)

   Understanding the thermalization dynamics of quantum many-body systems at the microscopic level is among the central challenges of modern statistical physics. Here we experimentally investigate individual spin dynamics in a two-dimensional ensemble of electron spins on the surface of a diamond crystal. We use a near-surface NV center as a nanoscale magnetic sensor to probe correlation dynamics of individual spins in a dipolar interacting surface spin ensemble. We observe that the relaxation rate for each spin is significantly slower than the naive expectation based on independently estimated dipolar interaction strengths with nearest neighbors and is strongly correlated with the timescale of the local magnetic field fluctuation. We show that this anomalously slow relaxation rate is due to the presence of strong dynamical disorder and present a quantitative explanation based on dynamic resonance counting. Finally, we use resonant spin-lock driving to control the effective strength of the local magnetic fields and reveal the role of the dynamical disorder in different regimes. Our work paves the way towards microscopic study and control of quantum thermalization in strongly interacting disordered spin ensembles. 

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