We present measurements of the dynamical structure factor $S(q,\omega )$ of an interacting one-dimensional Fermi gas for small excitation energies. We use the two lowest hyperfine levels of the ${}^6\mathrm{Li}$ atom to form a pseudospin-$1/2$ system whose $s$-wave interactions are tunable via a Feshbach resonance. The atoms are confined to one dimension by a two-dimensional optical lattice. Bragg spectroscopy is used to measure a response of the gas to density (“charge”) mode excitations at a momentum $q$ and frequency $\omega$, as a function of the interaction strength. The spectrum is obtained by varying $\omega$, while the angle between two laser beams determines $q$, which is fixed to be less than the Fermi momentum $k_F$. The measurements agree well with Tomonaga-Luttinger theory.

Understanding multidecadal variability is an essential goal of climate dynamics. For example, the recent phenomenon referred to as the “global warming hiatus" may reflect a coupling to an intrinsic, preindustrial, multidecadal variability process. Here, using a multifractal time-series method, we demonstrate that 42 data sets of 79 proxies with global coverage exhibit pink-noise characteristics on multidecadal timescales. To quantify the persistence of this behavior, we examine high-resolution ice core and speleothem data to find pink noise in both pre- and postindustrial periods. We examine the spatial structure with an empirical orthogonal function analysis of the monthly averaged surface temperature from 1901 to 2012. The first mode clearly shows the distribution of ocean heat flux sinks located in the eastern Pacific and the Southern Ocean and has pink-noise characteristics on a multidecadal timescale. We hypothesize that this pink-noise multidecadal spatial mode may resonate with externally driven greenhouse gas forcing, driving large-scale climate processes.

Quantum mechanics allows events to happen with no definite causal order: this can be verified by measuring a causal witness, in the same way that an entanglement witness verifies entanglement. Here, we realize a photonic quantum switch, where two operations $\widehat{A}$ and $\widehat{B}$ act in a quantum superposition of their two possible orders. The operations are on the transverse spatial mode of the photons; polarization coherently controls their order. Our implementation ensures that the operations cannot be distinguished by spatial or temporal position—further it allows qudit encoding in the target. We confirm our quantum switch has no definite causal order by constructing a causal witness and measuring its value to be 18 standard deviations beyond the definite-order bound.

We establish the complete phase diagram of self-propelled hard disks in two spatial dimensions from the analysis of the equation of state and the statistics of local order parameters. The equilibrium melting scenario is maintained at small activities, with coexistence between active liquid and hexatic order, followed by a proper hexatic phase, and a further transition to an active solid. As activity increases, the emergence of hexatic and solid order is shifted towards higher densities. Above a critical activity and for a certain range of packing fractions, the system undergoes motility-induced phase separation and demixes into low and high density phases; the latter can be either disordered (liquid) or ordered (hexatic or solid) depending on the activity.

Continuously pumped passive nonlinear cavities can be harnessed for the creation of novel optical frequency combs. While most research has focused on third-order “Kerr” nonlinear interactions, recent studies have shown that frequency comb formation can also occur via second-order nonlinear effects. Here, we report on the formation of quadratic combs in optical parametric oscillator (OPO) configurations. Specifically, we demonstrate that optical frequency combs can be generated in the parametric region around half of the pump frequency in a continuously driven OPO. We also model the OPO dynamics through a single time-domain mean-field equation, identifying previously unknown dynamical regimes, induced by modulation instabilities, which lead to comb formation. Numerical simulation results are in good agreement with experimentally observed spectra. Moreover, the analysis of the coherence properties of the simulated spectra shows the existence of correlated and phase-locked combs. Our results reveal previously unnoticed dynamics of an apparently well assessed optical system, and can lead to a new class of frequency comb sources that may stimulate novel applications by enabling straightforward access to elusive spectral regions, such as the midinfrared.

We report the all-optical generation and characterization of tilted electron pulses by means of single-cycle terahertz radiation at an electron-transmitting mirror at slanted orientation. Femtosecond electron pulses with a chosen tilt angle are produced at an almost arbitrary target location. The experiments along with theory further reveal that the pulse front tilt in electron optics is directly connected to angular dispersion. Quantum mechanical considerations suggest that this relation is general for particle beams at any degree of coherence. These results indicate that ultrashort electron pulses can be shaped in space and time as versatilely as femtosecond laser pulses, but at ${10}^5$ times finer wavelength and subnanometer imaging resolution.

A flatband representing a highly degenerate and dispersionless manifold state of electrons may offer unique opportunities for the emergence of exotic quantum phases. To date, definitive experimental demonstrations of flatbands remain to be accomplished in realistic materials. Here, we present the first experimental observation of a striking flatband near the Fermi level in the layered ${\mathrm{Fe}}_3{\mathrm{Sn}}_2$ crystal consisting of two Fe kagome lattices separated by a Sn spacing layer. The band flatness is attributed to the local destructive interferences of Bloch wave functions within the kagome lattices, as confirmed through theoretical calculations and modelings. We also establish high-temperature ferromagnetic ordering in the system and interpret the observed collective phenomenon as a consequence of the synergetic effect of electron correlation and the peculiar lattice geometry. Specifically, local spin moments formed by intramolecular exchange interaction are ferromagnetically coupled through a unique network of the hexagonal units in the kagome lattice. Our findings have important implications to exploit emergent flat-band physics in special lattice geometries.

We present the results of a search for dark matter weakly interacting massive particles (WIMPs) in the mass range below $20\mathrm{GeV}/c^2$ using a target of low-radioactivity argon with a 6786.0 kg d exposure. The data were obtained using the DarkSide-50 apparatus at Laboratori Nazionali del Gran Sasso. The analysis is based on the ionization signal, for which the DarkSide-50 time projection chamber is fully efficient at $0.1\mathrm{keV}\text{ee}$. The observed rate in the detector at $0.5\mathrm{keV}\text{ee}$ is about $1.5\mathrm{event}/\mathrm{keV}\text{ee}/\mathrm{kg}/\mathrm{d}$ and is almost entirely accounted for by known background sources. We obtain a 90% C.L. exclusion limit above $1.8\mathrm{GeV}/c^2$ for the spin-independent cross section of dark matter WIMPs on nucleons, extending the exclusion region for dark matter below previous limits in the range $1.8–6\mathrm{GeV}/c^2$.

We report on nondestructive imaging of optically trapped calcium monofluoride molecules using in situ $\mathrm{\Lambda }$-enhanced gray molasses cooling. 200 times more fluorescence is obtained compared to destructive on-resonance imaging, and the trapped molecules remain at a temperature of $20\mu \mathrm{K}$. The achieved number of scattered photons makes possible nondestructive single-shot detection of single molecules with high fidelity.

We study a minimal Hubbard model for electronically driven superconductivity in a correlated flat miniband resulting from the superlattice modulation of a twisted graphene multilayer. The valley degree of freedom drastically modifies the nature of the preferred pairing states, favoring spin triplet $d+id$ order with a valley singlet structure. We identify two candidates in this class, which are both topological superconductors. These states support half-vortices carrying half the usual superconducting flux quantum $hc/(4e)$, and have topologically protected gapless edge states.

Transition-metal dichalcogenides showing type-II Dirac fermions are emerging as innovative materials for nanoelectronics. However, their excitation spectrum is mostly unexplored yet. By means of high-resolution electron energy loss spectroscopy and density functional theory, here, we identify the collective excitations of type-II Dirac fermions (3D Dirac plasmons) in ${\mathrm{PtTe}}_2$ single crystals. The observed plasmon energy in the long-wavelength limit is $\sim 0.5\mathrm{eV}$, which makes ${\mathrm{PtTe}}_2$ suitable for near-infrared optoelectronic applications. We also demonstrate that interband transitions between the two Dirac bands in ${\mathrm{PtTe}}_2$ give rise to additional excitations at $\sim 1$ and $\sim 1.4\mathrm{eV}$. Our results are crucial to bringing to fruition type-II Dirac semimetals in optoelectronics.

The bulk-boundary correspondence is among the central issues of non-Hermitian topological states. We show that a previously overlooked “non-Hermitian skin effect” necessitates redefinition of topological invariants in a generalized Brillouin zone. The resultant phase diagrams dramatically differ from the usual Bloch theory. Specifically, we obtain the phase diagram of the non-Hermitian Su-Schrieffer-Heeger model, whose topological zero modes are determined by the non-Bloch winding number instead of the Bloch-Hamiltonian-based topological number. Our work settles the issue of the breakdown of conventional bulk-boundary correspondence and introduces the non-Bloch bulk-boundary correspondence.

Classical charge transport, such as longitudinal and Hall currents in weak magnetic fields, is usually not affected by quantum phenomena. Yet relativistic quantum mechanics is at the heart of the spin-orbit interaction, which has been at the forefront of efforts to realize spin-based electronics, new phases of matter, and topological quantum computing. In this work we demonstrate that quantum spin dynamics induced by the spin-orbit interaction is directly observable in classical charge transport. We determine the Hall coefficient $R_H$ of two-dimensional hole systems at low magnetic fields and show that it has a sizable spin-orbit contribution, which depends on the density $p$, is independent of temperature, is a strong function of the top gate electric field, and can reach $\sim 20\%$ of the total. We provide a general method for extracting the spin-orbit parameter from magnetotransport data, applicable even at higher temperatures where Shubnikov–de Haas oscillations and weak antilocalization are difficult to observe. Our work will enable experimentalists to measure spin-orbit parameters without requiring large magnetic fields, ultralow temperatures, or optical setups.

Experimental data on thin films of cylinder-forming block copolymers (BC)—free-standing BC membranes as well as supported BC films—strongly suggest that the local orientation of the BC patterns is coupled to the geometry in which the patterns are embedded. We analyze this phenomenon using general symmetry considerations and numerical self-consistent field studies of curved BC films in cylindrical geometry. The stability of the films against curvature-induced dewetting is also analyzed. In good agreement with experiments, we find that the BC cylinders tend to align along the direction of curvature at high curvatures. At low curvatures, we identify a transition from perpendicular to parallel alignment in supported films, which is absent in free-standing membranes. Hence both experiments and theory show that curvature can be used to manipulate and align BC patterns.

In this Letter, we present a cosmic Bell experiment with polarization-entangled photons, in which measurement settings were determined based on real-time measurements of the wavelength of photons from high-redshift quasars, whose light was emitted billions of years ago; the experiment simultaneously ensures locality. Assuming fair sampling for all detected photons and that the wavelength of the quasar photons had not been selectively altered or previewed between emission and detection, we observe statistically significant violation of Bell’s inequality by 9.3 standard deviations, corresponding to an estimated $p$ value of $\lesssim 7.4\times {10}^{-21}$. This experiment pushes back to at least $\sim 7.8\mathrm{Gyr}$ ago the most recent time by which any local-realist influences could have exploited the “freedom-of-choice” loophole to engineer the observed Bell violation, excluding any such mechanism from 96% of the space-time volume of the past light cone of our experiment, extending from the big bang to today.

Inspired by the recent remarkable progress in the experimental test of local realism, we report here such a test that achieves an efficiency greater than $(78\%{)}^2$ for entangled photon pairs separated by 183 m. Further utilizing the randomness in cosmic photons from pairs of stars on the opposite sides of the sky for the measurement setting choices, we not only close the locality and detection loopholes simultaneously, but also test the null hypothesis against local hidden variable mechanisms for events that took place 11 years ago (13 orders of magnitude longer than previous experiments). After considering the bias in measurement setting choices, we obtain an upper bound on the $p$ value of $7.87\times {10}^{-4}$, which clearly indicates the rejection with high confidence of potential local hidden variable models. One may further push the time constraint on local hidden variable mechanisms deep into the cosmic history by taking advantage of the randomness in photon emissions from quasars with large aperture telescopes.

X-ray two-photon absorption (TPA) spectrum of metallic copper is measured using a free-electron laser (XFEL). The spectrum differs from that measured by the conventional one-photon absorption (OPA), and characterized by a peak below the Fermi level, which is assigned to the transition to the $3d$ state. The impact of the XFEL pulse on the OPA spectrum is discussed by analyzing the pulse-energy dependence, which indicates that the intrinsic TPA spectrum is measured.

Using an intense negative muon (${\mu }^{-}$) source, we have studied the internal magnetic fields in a powder sample of magnesium hydride (${\mathrm{MgH}}_2$). By extracting the signal from the ${\mu }^{-}$ captured on Mg nuclei, we found that the negative muon spin rotation and relaxation (${\mu }^{-}\mathrm{SR}$) spectra clearly showed a Kubo-Toyabe-type relaxation, which indicates a random magnetic field at the Mg site. The field distribution width obtained is very consistent with the predicted value at the Mg site estimated by dipole field calculations, supporting our claim to have observed the nuclear magnetic fields of hydrogens in ${\mathrm{MgH}}_2$. As is the case with ${\mu }^{+}\mathrm{SR}$, ${\mu }^{-}\mathrm{SR}$ promises to soon be an indispensable tool for materials analyses.

Nonthermal pickup ions (PUIs) are created in the solar wind (SW) by charge-exchange between SW ions (SWIs) and slow interstellar neutral atoms. It has long been theorized, but not directly observed that PUIs should be preferentially heated at quasiperpendicular shocks compared to thermal SWIs. We present in situ observations of interstellar hydrogen (${\mathrm{H}}^{+}$) PUIs at an interplanetary shock by the New Horizons’ Solar Wind Around Pluto (SWAP) instrument at $\sim 34\mathrm{au}$ from the Sun. At this shock, ${\mathrm{H}}^{+}$ PUIs are only a few percent of the total proton density but contain most of the internal particle pressure. A gradual reduction in SW flow speed and simultaneous heating of ${\mathrm{H}}^{+}$ SWIs is observed ahead of the shock, suggesting an upstream energetic particle pressure gradient. ${\mathrm{H}}^{+}$ SWIs lose $\sim 85\%$ of their energy flux across the shock and ${\mathrm{H}}^{+}$ PUIs are preferentially heated. Moreover, a PUI tail is observed downstream of the shock, such that the energy flux of all ${\mathrm{H}}^{+}$ PUIs is approximately six times that of ${\mathrm{H}}^{+}$ SWIs. We find that ${\mathrm{H}}^{+}$ PUIs, including their suprathermal tail, contain almost half of the total downstream energy flux in the shock frame.

We present a discrete-time, one-dimensional quantum walk based on the entanglement between the momentum of ultracold rubidium atoms (the walk space) and two internal atomic states (the “coin” degree of freedom). Our scheme is highly flexible and can provide a platform for a wide range of applications such as quantum search algorithms, the observation of topological phases, and the realization of walks with higher dimensionality. Along with the investigation of the quantum-to-classical transition, we demonstrate the distinctive features of a quantum walk and contrast them to those of its classical counterpart. Also, by manipulating either the walk or coin operator, we show how the walk dynamics can be steered or even reversed.