Helical conductors with spin-momentum locking are promising platforms for Majorana fermions. Here we report observation of two topologically distinct phases supporting helical edge states in charge neutral Bernal-stacked tetralayer graphene in Hall bar and Corbino geometries. As the magnetic field $B_{\perp }$ and out-of-plane displacement field $D$ are varied, we observe a phase diagram consisting of an insulating phase and two metallic phases, with 0, 1, and 2 helical edge states, respectively. These phases are accounted for by a theoretical model that relates their conductance to spin-polarization plateaus. Transitions between them arise from a competition among interlayer hopping, electrostatic and exchange interaction energies. Our work highlights the complex competing symmetries and the rich quantum phases in few-layer graphene.

Quantum interference is typically detected through the dependence of the interference signal on certain parameters (path length, Aharonov-Bohm flux, etc.), which can be varied in a controlled manner. The destruction of interference by a which-path measurement is a paradigmatic manifestation of quantum effects. Here we report on a novel measurement protocol that realizes two objectives: (i) certifying that a measured signal is the result of interference avoiding the need to vary parameters of the underlying interferometer, and (ii) certifying that the interference signal at hand is of quantum nature. In particular, it yields a null outcome in the case of classical interference. Our protocol comprises measurements of cross-correlations between the readings of which-path weakly coupled detectors positioned at the respective interferometer’s arms and the current in one of the interferometer’s drains. We discuss its implementation with an experimentally available platform: an electronic Mach-Zehnder interferometer (MZI) coupled electrostatically to “detectors” (quantum point contacts).

Optical spectroscopy constitutes the historical path to accumulate basic knowledge on the atom and its structure. Former work based on fluorescence and resonance ionization spectroscopy enabled identifying optical spectral lines up to element 102, nobelium. The new challenges faced in this research field are the refractory nature of the heavier elements and the decreasing production yields. A new concept of ion-mobility-assisted laser spectroscopy is proposed to overcome the sensitivity limits of atomic structure investigations persisting in the region of the superheavy elements. The concept offers capabilities of both broadband-level searches and high-resolution hyperfine spectroscopy of synthetic elements beyond nobelium.

On January 14, 2019, the Major Atmospheric Gamma Imaging Cherenkov telescopes detected GRB 190114C above 0.2 TeV, recording the most energetic photons ever observed from a gamma-ray burst. We use this unique observation to probe an energy dependence of the speed of light in vacuo for photons as predicted by several quantum gravity models. Based on a set of assumptions on the possible intrinsic spectral and temporal evolution, we obtain competitive lower limits on the quadratic leading order of speed of light modification.

Direct characterizations of the two component surfaces of a solid-solid interface are essential for understanding its various interfacial mechanical, physical, and electrical behaviors. Particularly, the fascinating phenomenon termed structural superlubricity, a state of nearly zero friction and wear, is sensitively dependent on the interface structure. Here we report a controllable pick-and-flip technique to separate a microscale contact pair for the characterization of its two component surfaces for van der Waals layered materials. With this technique, the interface of a graphite superlubric contact is characterized with resolution from microscale down to the atomic level. Imaging of the graphite lattice provides direct proof that this superlubric interface consists of two monocrystalline surfaces incommensurate with each other. More importantly, the structure-property relationship for this contact is investigated. Friction measurements combined with fully atomistic molecular dynamics reveal that internal structures [internals steps, pits, and bulges buried underneath the topmost graphene sheet(s)] have negligible contribution to the total friction; in contrast, external defects lead to a high friction. These results help us to better understand the structure of highly oriented pyrolytic graphite and the fundamental mechanisms of structural superlubricity, as well as to guide the design of superlubricity-based devices.

Bacterial ribosomes are composed of one-third protein and two-thirds RNA by mass. The predominance of RNA is often attributed to a primordial RNA world, but why exactly two-thirds remains a long-standing mystery. Here we present a quantitative analysis, based on the kinetics of ribosome self-replication, demonstrating that the $1:2$ protein-to-RNA mass ratio uniquely maximizes cellular growth rates in E. coli. A previously unrecognized growth law, and an invariant of bacterial growth, also follow from our analysis. The growth law reveals that the ratio between the number of ribosomes and the number of polymerases making ribosomal RNA is proportional to the cellular doubling time. The invariant is conserved across growth conditions and specifies how key microscopic parameters in the cell, such as transcription and translation rates, are coupled to cellular physiology. Quantitative predictions from the growth law and invariant are shown to be in excellent agreement with E. coli data despite having no fitting parameters. Our analysis can be readily extended to other bacteria once data become available.

Turbulent flow restricted to two dimensions can spontaneously develop order on large scales, defying entropy expectations and in sharp contrast with turbulence in three dimensions where nonlinear turbulent processes act to destroy large-scale order. In this work we report the observation of unusual turbulent behavior in steady-state flow of superfluid ${}^4\mathrm{He}$—a liquid with vanishing viscosity and discrete vorticity—in a nearly two-dimensional channel. Surprisingly, for a range of experimental parameters, turbulence is observed to exist in two bistable states. This bistability can be well explained by the appearance of large-scale regions of flow of opposite vorticity.

We measure the mode-resolved direction of the precessional motion of the magnetic order, i.e., magnon polarization, via the chiral term of inelastic polarized neutron scattering spectra. The magnon polarization is a unique and unambiguous signature of magnets and is important in spintronics, affecting thermodynamic properties such as the magnitude and sign of the spin Seebeck effect. However, it has never been directly measured in any material until this work. The observation of both signs of magnon polarization in ${\mathrm{Y}}_3{\mathrm{Fe}}_5{\mathrm{O}}_{12}$ also gives direct proof of its ferrimagnetic nature. The experiments agree very well with atomistic simulations of the scattering cross section.

One of the characteristic features of many marine dinoflagellates is their bioluminescence, which lights up nighttime breaking waves or seawater sliced by a ship’s prow. While the internal biochemistry of light production by these microorganisms is well established, the manner by which fluid shear or mechanical forces trigger bioluminescence is still poorly understood. We report controlled measurements of the relation between mechanical stress and light production at the single cell level, using high-speed imaging of micropipette-held cells of the marine dinoflagellate Pyrocystis lunula subjected to localized fluid flows or direct indentation. We find a viscoelastic response in which light intensity depends on both the amplitude and rate of deformation, consistent with the action of stretch-activated ion channels. A phenomenological model captures the experimental observations.

An angular analysis of the $B^0\to K^{*0}(\to K^{+}{\pi }^{-}){\mu }^{+}{\mu }^{-}$ decay is presented using a dataset corresponding to an integrated luminosity of $4.7{\mathrm{fb}}^{-1}$ of $pp$ collision data collected with the LHCb experiment. The full set of $CP$-averaged observables are determined in bins of the invariant mass squared of the dimuon system. Contamination from decays with the $K^{+}{\pi }^{-}$ system in an $S$-wave configuration is taken into account. The tension seen between the previous LHCb results and the standard model predictions persists with the new data. The precise value of the significance of this tension depends on the choice of theory nuisance parameters.

With growing interest in quantum technologies, possibilities of synchronizing quantum systems have garnered significant recent attention. In experiments with dilute ensemble of laser cooled spin-1 ${}^{87}\mathrm{Rb}$ atoms, we observe phase difference of spin coherences to synchronize with phases of external classical fields. An initial limit-cycle state of a spin-1 atom localizes in phase space due to dark-state polaritons generated by classical two-photon tone fields. In particular, when the two couplings fields are out of phase, the limit-cycle state synchronizes only with two artificially engineered, anisotropic decay rates. Furthermore, we observe a blockade of synchronization due to quantum interference and emergence of Arnold-tongue-like features. Such anisotropic decay induced synchronization of spin-1 systems with no classical analog can provide insights in open quantum systems and find applications in synchronized quantum networks.

Spin-momentum locking is a direct consequence of bulk topological order and provides a basic concept to control a carrier’s spin and charge flow for new exotic phenomena in condensed matter physics. However, up to date the research on spin-momentum locking solely focuses on its in-plane transport properties. Here, we report an emerging out-of-plane radiation feature of spin-momentum locking in a non-Hermitian topological photonic system and demonstrate a high performance topological vortex laser based on it. We find that the gain saturation effect lifts the degeneracy of the paired counterpropagating spin-momentum-locked edge modes enabling lasing from a single topological edge mode. The near-field spin and orbital angular momentum of the topological edge mode lasing has a one-to-one far-field radiation correspondence. The methodology of probing the near-field topology feature by far-field lasing emission can be used to study other exotic phenomena. The device can lead to applications in superresolution imaging, optical tweezers, free-space optical sensing, and communication.

The first evidence of spin alignment of vector mesons ($K^{*0}$ and $\varphi$) in heavy-ion collisions at the Large Hadron Collider (LHC) is reported. The spin density matrix element ${\rho }_{00}$ is measured at midrapidity ($|y|<0.5$) in Pb-Pb collisions at a center-of-mass energy ($\sqrt{s_{\mathrm{NN}}}$) of 2.76 TeV with the ALICE detector. ${\rho }_{00}$ values are found to be less than $1/3$ ($1/3$ implies no spin alignment) at low transverse momentum ($p_T<2\mathrm{GeV}/c$) for $K^{*0}$ and $\varphi$ at a level of $3\sigma$ and $2\sigma$, respectively. No significant spin alignment is observed for the $K_S^0$ meson ($\text{spin}=0$) in Pb-Pb collisions and for the vector mesons in $pp$ collisions. The measured spin alignment is unexpectedly large but qualitatively consistent with the expectation from models which attribute it to a polarization of quarks in the presence of angular momentum in heavy-ion collisions and a subsequent hadronization by the process of recombination.

A pentagonal covalent network consisting of ${\mathrm{sp}}^2$ and ${\mathrm{sp}}^3$ C atoms has been investigated based on the density functional theory. Our theoretical investigations clarified that the pentagonal covalent network is a metastable three-dimensional carbon allotrope with the $Fm\overline{3}m$ space group possessing remarkable mechanical properties: relatively high bulk modulus of 381 GPa together with a negative Poisson’s ratio of $-0.241$. Accordingly, the pentagonal covalent network possesses extremely high Young’s and shear moduli of 1691 and 1113 GPa, respectively, surpassing those of the diamond. The electronic structure of the pentagonal network is a semiconductor with an indirect band gap of 2.52 eV between $L$ and $X$ points for valence and conduction band edges, respectively, with the relatively small carrier masses.

Subgap states in semiconducting-superconducting nanowire hybrid devices are controversially discussed as potential topologically nontrivial quantum states. One source of ambiguity is the lack of an energetically and spatially well defined tunnel spectrometer. Here, we use quantum dots directly integrated into the nanowire during the growth process to perform tunnel spectroscopy of discrete subgap states in a long nanowire segment. In addition to subgap states with a standard magnetic field dependence, we find topologically trivial subgap states that are independent of the external magnetic field, i.e., that are pinned to a constant energy as a function of field. We explain this effect qualitatively and quantitatively by taking into account the strong spin-orbit interaction in the nanowire, which can lead to a decoupling of Andreev bound states from the field due to a spatial spin texture of the confined eigenstates. This result constitutes an important step forward in the research on superconducting subgap states in nanowires, such as Majorana bound states.

Nanofluidic systems show great promise for applications in energy conversion, where their performance can be enhanced by nanoscale liquid-solid slip. However, efficiency is also controlled by surface charge, which is known to reduce slip. Combining molecular dynamics simulations and analytical developments, we show the dramatic impact of surface charge distribution on the slip-charge coupling. Homogeneously charged graphene exhibits a very favorable slip-charge relation (rationalized with a new theoretical model correcting some weaknesses of the existing ones), leading to giant electrokinetic energy conversion. In contrast, slip is strongly affected on heterogeneously charged surfaces, due to the viscous drag induced by counterions trapped on the surface. In that case slip should depend on the detailed physical chemistry of the interface controlling the fraction of bound ions. Our numerical results and theoretical models provide new fundamental insight into the molecular mechanisms of liquid-solid slip, and practical guidelines for searching new functional interfaces with optimal energy conversion properties, e.g., for blue energy or waste heat harvesting.

We report the first plausible optical electromagnetic counterpart to a (candidate) binary black hole merger. Detected by the Zwicky Transient Facility, the electromagnetic flare is consistent with expectations for a kicked binary black hole merger in the accretion disk of an active galactic nucleus [B. McKernan, K. E. S. Ford, I. Bartos et al., Astrophys. J. Lett. 884, L50 (2019)] and is unlikely [$<O(0.01\%)$)] due to intrinsic variability of this source. The lack of color evolution implies that it is not a supernova and instead is strongly suggestive of a constant temperature shock. Other false-positive events, such as microlensing or a tidal disruption event, are ruled out or constrained to be $<O(0.1\%)$. If the flare is associated with S190521g, we find plausible values of total mass $M_{\mathrm{BBH}}\sim 100M_{\odot }$, kick velocity $v_k\sim 200\mathrm{km}{\mathrm{s}}^{-1}$ at $\theta \sim 60°$ in a disk with aspect ratio $H/a\sim 0.01$ (i.e., disk height $H$ at radius $a$) and gas density $\rho \sim {10}^{-10}\mathrm{g}{\mathrm{cm}}^{-3}$. The merger could have occurred at a disk migration trap ($a\sim 700r_g$; $r_g\equiv G{M}_{\mathrm{SMBH}}/c^2$, where $M_{\mathrm{SMBH}}$ is the mass of the active galactic nucleus supermassive black hole). The combination of parameters implies a significant spin for at least one of the black holes in S190521g. The timing of our spectroscopy prevents useful constraints on broad-line asymmetry due to an off-center flare. We predict a repeat flare in this source due to a reencountering with the disk in $\sim 1.6\mathrm{yr}(M_{\mathrm{SMBH}}/{10}^8{M}_{\odot })(a/{10}^3{r}_g{)}^{3/2}$.

We demonstrate the formation of a single NaCs molecule in an optical tweezer by magnetoassociation through an $s$-wave Feshbach resonance at 864.11(5) G. Starting from single atoms cooled to their motional ground states, we achieve conversion efficiencies of 47(1)%, and measure a molecular lifetime of 4.7(7) ms. By construction, the single molecules are predominantly [77(5)%] in the center-of-mass motional ground state of the tweezer. Furthermore, we produce a single $p$-wave molecule near 807 G by first preparing one of the atoms with one quantum of motional excitation. Our creation of a single weakly bound molecule in a designated internal state in the motional ground state of an optical tweezer is a crucial step towards coherent control of single molecules in optical tweezer arrays.

Structure in the Universe is widely believed to have originated from quantum fluctuations during an early epoch of accelerated expansion. Yet, the patterns we observe today do not distinguish between quantum or classical primordial fluctuations; current cosmological data are consistent with either possibility. We argue here that a detection of primordial non-Gaussianity can resolve the present situation, and provide a litmus test for the quantum origin of cosmic structure. Unlike in quantum mechanics, vacuum fluctuations cannot arise in classical theories and therefore long-range classical correlations must result from (real) particles in the initial state. Similarly to flat-space scattering processes, we show how basic principles require these particles to manifest themselves as poles in the $n$-point functions, in the so-called folded configurations. Following this observation, and assuming fluctuations are (i) correlated over large scales and (ii) generated by local evolution during an inflationary phase, we demonstrate that the absence of a pole in the folded limit of non-Gaussian correlators uniquely identifies the quantum vacuum as the initial state. In the same spirit as Bell’s inequalities, we discuss how this can be circumvented if locality is abandoned.

In an experiment performed at Lawrence Berkeley National Laboratory’s 88-inch cyclotron, the isotope ${}^{244}\mathrm{Md}$ was produced in the ${}^{209}\mathrm{Bi}({}^{40}\mathrm{Ar},5n)$ reaction. Decay properties of ${}^{244}\mathrm{Md}$ were measured at the focal plane of the Berkeley Gas-filled Separator, and the mass number assignment of $A=244$ was confirmed with the apparatus for the identification of nuclide $A$. The isotope ${}^{244}\mathrm{Md}$ is reported to have one, possibly two, $\alpha$-decaying states with $\alpha$ energies of 8.66(2) and 8.31(2) MeV and half-lives of ${0.4}_{-0.1}^{+0.4}$ and $\sim 6\mathrm{s}$, respectively. Additionally, first evidence of the $\alpha$ decay of ${}^{236}\mathrm{Bk}$ was observed and is reported.