We present the first comprehensive tests of the universality of the light leptons in the angular distributions of semileptonic $B^0$-meson decays to charged spin-1 charmed mesons. We measure five angular-asymmetry observables as functions of the decay recoil that are sensitive to lepton-universality-violating contributions. We use events where one neutral $B$ is fully reconstructed in $\mathrm{\Upsilon }(4S)\to B\overline{B}$ decays in data corresponding to $189{\mathrm{fb}}^{-1}$ integrated luminosity from electron-positron collisions collected with the Belle II detector. We find no significant deviation from the standard model expectations.

We investigate the buckling dynamics of an elastic filament impacted axially by a falling liquid droplet, and identify the buckling modes through a combination of experimental and theoretical analyses. A phase diagram is constructed on a plane defined by two primary parameters: the falling height and the filament length. Two transition boundaries are observed, with one marking the occurrence of dynamic buckling and the other separating the buckling regime into two distinct modes. Notably, the hydrodynamic viscous force of the liquid dominates during the impact, with the dynamic buckling instability predicted by a single elastoviscous number. The critical load is twice the critical static load, which is, however, lower for the deformable droplet utilized in our study, as compared to a solid object. An additional time-dependent simulation on a longer filament exhibits a higher buckling mode, succeeded by a more distinct coarsening process than our experimental observations.

Linear spin wave theory (LSWT) is the standard technique to compute the spectra of magnetic excitations in quantum materials. In this Letter, we show that LSWT, even under ordinary circumstances, may fail to implement the symmetries of the underlying ordered magnetic Hamiltonian leading to spurious degeneracies. In common with pseudo-Goldstone modes in cases of quantum order by disorder these degeneracies tend to be lifted by magnon-magnon interactions. We show how, instead, the correct symmetries may be restored at the level of LSWT. In the process we give examples, supported by nonperturbative matrix product based time evolution calculations, where symmetry dictates topological features but where LSWT fails to implement them. We also comment on possible spin split magnons in ${\mathrm{MnF}}_2$ and similar rutiles by analogy to recently proposed altermagnets.

In this Letter, we use a model fluid mechanics experiment to elucidate the impact of curvature heterogeneities on two-dimensional fields deriving from harmonic potential functions. This result is directly relevant to explain the smooth stationary structures in physical systems as diverse as curved liquid crystal and magnetic films, heat and Ohmic transport in wrinkled two-dimensional materials, and flows in confined channels. Combining microfluidic experiments and theory, we explain how curvature heterogeneities shape confined viscous flows. We show that isotropic bumps induce local distortions to Darcy’s flows, whereas anisotropic curvature heterogeneities disturb them algebraically over system-spanning scales. Thanks to an electrostatic analogy, we gain insight into this singular geometric perturbation, and quantitatively explain it using both conformal mapping and numerical simulations. Altogether, our findings establish the robustness of our experimental observations and their broad relevance to all Laplacian problems beyond the specifics of our fluid mechanics experiment.

We derive a rigorous upper bound on the classical computation time of finite-ranged tensor network contractions in $d\ge 2$ dimensions. Consequently, we show that quantum circuits of single-qubit and finite-ranged two-qubit gates can be classically simulated in subexponential time in the number of gates. Moreover, we present and implement an algorithm guaranteed to meet our bound and which finds contraction orders with vastly lower computational times in practice. In practically relevant cases this beats standard simulation schemes and, for certain quantum circuits, also a state-of-the-art method. Specifically, our algorithm leads to speedups of several orders of magnitude over naive contraction schemes for two-dimensional quantum circuits on as little as an $8\times 8$ lattice. We obtain similarly efficient contraction schemes for Google’s Sycamore-type quantum circuits, instantaneous quantum polynomial-time circuits, and nonhomogeneous ($2+1$)-dimensional random quantum circuits.

Fine-tuning generic but smooth spherically symmetric initial data for general relativity to the threshold of dynamical black hole formation creates arbitrarily large curvatures, mediated by a universal self-similar solution that acts as an intermediate attractor. For vacuum gravitational waves, however, these critical phenomena have been elusive. We present, for the first time, excellent agreement among three independent numerical simulations of this collapse. Surprisingly, we find no universality, and observe approximate self-similarity for some families of initial data but not for others.

We present a calculation of the hyperfine splitting of the $2^{3}S$ state in the ${}^3\mathrm{He}$ atom with inclusion of all QED effects up to ${\alpha }^3{E}_F$, where $E_F$ is the Fermi splitting. Using the experimental value of the $1S$ hyperfine splitting in ${}^3{\mathrm{He}}^{+}$, we eliminate uncertainties from the nuclear structure and obtain the theoretical prediction for ${}^3\mathrm{He}$ of ${\nu }_{\mathrm{hfs}}=-6739701181(41)\mathrm{Hz}$, which is in perfect agreement with the experimental value $-6739701177(16)\mathrm{Hz}$ [S. D. Rosner and F. M. Pipkin, Phys. Rev. A 1, 571 (1970)]. This result constitutes a 40-fold improvement in precision as compared to the previous value and is the most accurate theoretical prediction ever obtained for a nonhydrogenic system.

Careful control of quantum states is a gateway to research in many areas of science such as quantum information, quantum-controlled chemistry, and astrophysical processes. Precise optical control of molecular ions remains a challenge due to the scarcity of suitable level schemes, and direct laser cooling has not yet been achieved for either positive or negative molecular ions. Using a cryogenic wire trap, we show how the internal quantum states of ${\mathrm{C}}_2^{-}$ anions can be manipulated using optical pumping and inelastic quenching collisions with ${\mathrm{H}}_2$ gas. We obtained optical pumping efficiencies of about 96% into the first vibrational level of ${\mathrm{C}}_2^{-}$ and determined the absolute inelastic rate coefficient from $v=1$ to 0 to be $k_{\mathrm{q}}=(3.2\pm {0.2}_{\mathrm{stat}}\pm {1.3}_{\mathrm{sys}})\times {10}^{-13}{\mathrm{cm}}^3/\mathrm{s}$ at 20(3) K, over 3 orders of magnitude smaller than the capture limit. Reduced-dimensional quantum scattering calculations yield a small rate coefficient as well, but significantly larger than the experimental value. Using optical pumping and inelastic collisions, we also realized fluorescence imaging of negative molecular ions. Our work demonstrates high control of a cold ensemble of ${\mathrm{C}}_2^{-}$, providing a solid foundation for future work on laser cooling of molecular ions.

We build a model to elucidate the high harmonic generation in combined EUV and midinfrared laser fields by embodying the spin-resolved three-electron dynamics. The EUV pulse ionizes an inner-shell electron, and the midinfrared laser drives the photoelectron and steers the electron-ion rescattering. Depending on the spin of the photoelectron, the residual ion including two bound electrons can be either in a single spin configuration or in a coherent superposition of different spin configurations. In the latter case, the two electrons in the ion swap their orbits, leading to a deep valley in the harmonic spectrum. The model results agree with the time-dependent Schrödinger equation simulations including three active electrons. The intriguing picture explored in this work is fundamentally distinguished from all reported scenarios relied on spin-orbit coupling, but originates from the exchanges asymmetry of two-electron wave functions.

We demonstrate long rotational coherence of individual polar molecules in the motional ground state of an optical trap. In the present, previously unexplored regime, the rotational eigenstates of molecules are dominantly quantized by trapping light rather than static fields, and the main source of decoherence is differential light shift. In an optical tweezer array of NaCs molecules, we achieve a three-orders-of-magnitude reduction in differential light shift by changing the trap’s polarization from linear to a specific “magic” ellipticity. With spin-echo pulses, we measure a rotational coherence time of 62(3) ms (one pulse) and 250(40) ms (up to 72 pulses), surpassing the projected duration of resonant dipole-dipole entangling gates by orders of magnitude.

Controlling the photon statistics of light is paramount for quantum science and technologies. Recently, we demonstrated that transmitting resonant laser light past an ensemble of two-level emitters can result in a stream of single photons or excess photon pairs. This transformation is due to quantum interference between the transmitted laser light and the incoherently scattered photon pairs [Prasad et al., Nat. Photonics 14, 719 (2020)]. Here, using the dispersion of the atomic medium, we actively control the relative quantum phase between these two components. We thereby realize a tunable two-photon interferometer and observe interference fringes in the normalized photon coincidence rate. When tuning the relative phase, the coincidence rate varies periodically, giving rise to a continuous modification of the photon statistics from antibunching to bunching. Beyond the fundamental insight that there exists a tunable quantum phase between incoherent and coherent light that dictates the photon statistics, our results lend themselves to the development of novel quantum light sources.

We demonstrate time-of-flight measurements for an ultracold levitated nanoparticle and reveal its velocity for the translational motion brought to the quantum ground state. We discover that the velocity distributions obtained with repeated release-and-recapture measurements are significantly broadened via librational motions of the nanoparticle. Under feedback cooling on all the librational motions, we recover the velocity distributions in reasonable agreement with an expectation from the occupation number, with approximately twice the width of the quantum limit. The strong impact of librational motions on the translational motions is understood as a result of the deviation between the libration center and the center of mass, induced by the asymmetry of the nanoparticle. Our results elucidate the importance of the control over librational motions and establish the basis for exploring quantum mechanical properties of levitated nanoparticles in terms of their velocity.

Antibubbles are ephemeral objects composed of a liquid drop encapsulated by a thin gas shell immersed in a liquid medium. When the drop is made of a volatile liquid and the medium is superheated, the gas shell inflates at a rate governed by the evaporation flux from the drop. This thermal process represents an alternate strategy for delaying the antibubble collapse. We model the dynamics of such “thermal” antibubbles by incorporating to the film drainage equation the heat-transfer-limited evaporation of the drop, which nourishes the gas shell with vapor, as for Leidenfrost drops. We demonstrate that the inflation of the gas shell is drastically inhibited by the thermalization of the initially colder drop. Because of this thermalization effect, smaller drops evaporate much faster than larger ones.

High-performance fusion plasmas, requiring high pressure $\beta$, are not well understood in stellarator-type experiments. Here, the effect of $\beta$ on ion-temperature-gradient-driven (ITG) turbulence is studied in Wendelstein 7-X (W7-X), showing that subdominant kinetic ballooning modes (KBMs) are unstable well below the ideal MHD threshold and get strongly excited in the turbulence. By zonal-flow erosion, these subthreshold KBMs (stKBMs) affect ITG saturation and enable higher heat fluxes. Controlling stKBMs will be essential to allow W7-X and future stellarators to achieve maximum performance.

We study the motion of a heavy impurity in a one-dimensional Bose gas. The impurity experiences the friction force due to scattering off thermally excited quasiparticles. We present detailed analysis of an arbitrarily strong impurity-boson coupling in a wide range of temperatures within a microscopic theory. Focusing mostly on weakly interacting bosons, we derive an analytical result for the friction force and uncover new regimes of the impurity dynamics. Particularly interesting is the low-temperature $T^2$ dependence of the friction force obtained for a strongly coupled impurity, which should be contrasted with the expected $T^4$ scaling. This new regime applies to systems of bosons with an arbitrary repulsion strength. We finally study the evolution of the impurity with a given initial momentum. We evaluate analytically its nonstationary momentum distribution function. The impurity relaxation towards the equilibrium is a realization of the Ornstein-Uhlenbeck process in momentum space.

Under certain experimental conditions, the deposition of ${\mathrm{C}}_{60}$ molecules onto an atomically flat copper surface gives rise to the formation of corrugated islands. This corrugation, which reflects a molecular displacement perpendicular to the surface plane, presents an astonishing pattern: It is well described by a frustrated Ising spin Hamiltonian whose thermodynamics is compatible with a spin liquid about to transit toward an ordered zigzag state. Here we study the statistical properties of such a molecular corrugation using a structure factor analysis, a tool generally employed in frustrated magnetism. More specifically, the real and reciprocal space analysis of pairwise molecule correlations allows us to demonstrate that the ${\mathrm{C}}_{60}/\mathrm{Cu}$ system, in which magnetism is totally absent, has all the characteristics of a triangular Ising antiferromagnet. Our results indicate that the organization of two-dimensional matter, at the molecular length scale, sometimes turns out to be particularly close to that encountered in highly frustrated magnets.