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1. An optical atomic clock using $4D_J$ 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)

   Abstract 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 ${10}^{-13}/\sqrt{\tau \left(s\right)}$-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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2. Evolution of the Interplanetary Turbulence and the Associated Turbulence Anisotropy in the Outer Heliosphere: VOYAGER 2 Observations

   https://doi.org/10.3847/1538-4357/ad2eae  

   Zhu, Xingyu; He, Jiansen; Zank, Gary P; Verscharen, Daniel; Zhao, Ling-Ling; Duan, Die; Lin, Rong (April 2024, The Astrophysical Journal)

   Abstract We study the radial evolution of the inertial-range solar wind plasma turbulence and its anisotropy in the outer heliosphere. We use magnetic field (B) measurements from the Voyager 2 spacecraft for heliocentric distancesRfrom 1 to 33 au. We find that the perpendicular and trace power spectral densities (PSDs) of the magnetic field ( $E_{{\mathrm{B}}_{\perp }}$and $E_{{\mathrm{B}}_{\mathrm{Tr}}}$) still follow a Kolmogorov-like spectrum until 33 au. The parallel magnetic field PSD, $E_{B_{\parallel }}$, transits from a power-law index of −2 to −5/3 as the distance crossesR∼ 10 au. The PSD at frequencies 0.01 Hz <f< 0.2 Hz flattens atR> 20 au, gradually approaching anf⁻¹spectrum, probably due to instrument noise. At 0.002 Hz <f< 0.1 Hz, quasi-parallel propagation dominates at 1 au <R< 7 au, with quasi-perpendicular propagation gradually emerging atR> 5 au. ForR> 7 au, oblique propagation becomes the primary mode of propagation. At smaller frequencies off< 0.01 Hz, $E_{{\mathrm{B}}_{\perp }}$increases with propagation angle at 1 au <R< 5 au, and in contrast decreases with propagation angle atR> 5 au due to the enhanced power level at propagation angles smaller than 20°. Such enhancement may derive from the injection of wave energy from the pickup ion source into the background turbulent cascade, and the injected wave energy is transferred across scales without leaving local enhancements in $E_{{\mathrm{B}}_{\perp }}$or $E_{{\mathrm{B}}_{\mathrm{Tr}}}$. 

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3. Planck Dust Polarization Power Spectra Are Consistent with Strongly Supersonic Turbulence

   https://doi.org/10.3847/1538-4357/ad571b  

   Stalpes, Kye_A; Collins, David_C; Huffenberger, Kevin_M (August 2024, The Astrophysical Journal)

   Abstract The polarization of the cosmic microwave background is rich in information but obscured by foreground emission from the Milky Way’s interstellar medium (ISM). To uncover relationships between the underlying turbulent ISM and the foreground power spectra, we simulated a suite of driven, magnetized, turbulent models of the ISM, varying the fluid properties via the sonic Mach number, ${\mathcal{M}}_{\mathcal{S}}$, and magnetic (Alfvén) Mach number, ${\mathcal{M}}_{\mathrm{A}}$. We measure the power spectra of density (ρ), velocity (v), magnetic field (H), total projected intensity (T), parity-even polarization (E), and parity-odd polarization (B). We find that the slopes of all six quantities increase with ${\mathcal{M}}_{\mathcal{S}}$. Most increase with ${\mathcal{M}}_{\mathrm{A}}$, while the magnetic field spectrum steepens with ${\mathcal{M}}_{\mathrm{A}}$. By comparing spectral slopes ofEandBto those measured by Planck, we infer typical values of ${\mathcal{M}}_{\mathcal{S}}$and ${\mathcal{M}}_{\mathrm{A}}$for the ISM. As the fluid velocity increases, ${\mathcal{M}}_{\mathcal{S}}>4$, the ratio of BB power to EE power increases to approach a constant value near the Planck-observed value of ∼0.5, regardless of the magnetic field strength. We also examine correlation coefficients between projected quantities, and find thatr^TE≈ 0.3, in agreement with Planck, for appropriate combinations of ${\mathcal{M}}_{\mathcal{S}}$and ${\mathcal{M}}_{\mathrm{A}}$. Finally, we consider parity-violating correlationsr^TBandr^EB. 

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4. Bounds on eigenfunctions of quantum cat maps

   https://doi.org/10.1088/1402-4896/ad0331  

   Kim, Elena; Koirala, Robert (October 2023, Physica Scripta)

   Abstract We studyℓ^∞norms ofℓ²-normalized eigenfunctions of quantum cat maps. For maps with short quantum periods (constructed by Bonechi and de Biévre in F Bonechi and S De Bièvre (2000,Communications in Mathematical Physics,211, 659–686)) we show that there exists a sequence of eigenfunctionsuwith $\parallel u{\parallel }_{\infty }\gtrsim {\left(\log N\right)}^{-1/2}$. For general eigenfunctions we show the upper bound $\parallel u{\parallel }_{\infty }\lesssim {\left(\log N\right)}^{-1/2}$. Here the semiclassical parameter is $h={\left(2\pi N\right)}^{-1}$. Our upper bound is analogous to the one proved by Bérard in P Bérard (1977,Mathematische Zeitschrift,155, 249-276) for compact Riemannian manifolds without conjugate points. 

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5. In vivo estimation of anisotropic mechanical properties of the gastrocnemius during functional loading with MR elastography

   https://doi.org/10.1088/1361-6560/acb482  

   Smith, Daniel R; Caban-Rivera, Diego A; Williams, L Tyler; Van_Houten, Elijah_E W; Bayly, Phil V; Paulsen, Keith D; McGarry, Matthew_D J; Johnson, Curtis L (February 2023, Physics in Medicine & Biology)

   Abstract Objective.In vivoimaging assessments of skeletal muscle structure and function allow for longitudinal quantification of tissue health. Magnetic resonance elastography (MRE) non-invasively quantifies tissue mechanical properties, allowing for evaluation of skeletal muscle biomechanics in response to loading, creating a better understanding of muscle functional health.Approach. In this study, we analyze the anisotropic mechanical response of calf muscles using MRE with a transversely isotropic, nonlinear inversion algorithm (TI-NLI) to investigate the role of muscle fiber stiffening under load. We estimate anisotropic material parameters including fiber shear stiffness ( ${\mu }_1$), substrate shear stiffness ( ${\mu }_2$), shear anisotropy ( $\varphi$), and tensile anisotropy ( $\zeta$) of the gastrocnemius muscle in response to both passive and active tension.Main results. In passive tension, we found a significant increase in ${\mu }_1,$ $\varphi ,$and $\zeta$with increasing muscle length. While in active tension, we observed increasing ${\mu }_2$and decreasing $\varphi$and $\zeta$during active dorsiflexion and plantarflexion—indicating less anisotropy—with greater effects when the muscles act as agonist.Significance. The study demonstrates the ability of this anisotropic MRE method to capture the multifaceted mechanical response of skeletal muscle to tissue loading from muscle lengthening and contraction. 

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