${\mathrm{Sr}}_2{\mathrm{CuTe}}_{0.5}{\mathrm{W}}_{0.5}{\mathrm{O}}_6$ is a square-lattice magnet with superexchange between $S=\frac{1}{2}{\mathrm{Cu}}^{2+}$ spins mediated by randomly distributed Te and W ions. Here, using sub-K temperature and $20\mu \mathrm{eV}$ energy resolution neutron scattering experiments we show that this system transits from a gapless disorder-induced spin liquid to a new quantum state below ${\mathit{T}}_{\mathit{f}}=1.7(1)\mathrm{K}$, exhibiting a weak frozen moment of $⟨\mathit{S}⟩/\mathit{S}\sim 0.1$ and low energy dynamic susceptibility, ${\mathit{\chi }}^{\prime \prime }(\hslash \mathit{\omega })$, linear in energy which is surprising for such a weak freezing in this highly fluctuating quantum regime.

We report generic and tunable crossed Andreev reflection (CAR) in a superconductor sandwiched between two antiferromagnetic layers. We consider recent examples of two-dimensional magnets with hexagonal lattices, where gate voltages control the carrier type and density, and predict a robust signature of perfect CAR in the nonlocal differential conductance with one electron-doped and one hole-doped antiferromagnetic lead. The magnetic field-free and spin-degenerate CAR signal is electrically controlled and visible over a large voltage range, showing promise for solid-state quantum entanglement applications.

Self-dual structures whose dual counterparts are themselves possess unique hidden symmetry, beyond the description of classical spatial symmetry groups. Here we propose a strategy based on a nematic monolayer of attractive half-cylindrical colloids to self-assemble these exotic structures. This system can be seen as a 2D system of semidisks. By using Monte Carlo simulations, we discover two isostatic self-dual crystals, i.e., an unreported crystal with $pmg$ space-group symmetry and the twisted kagome crystal. For the $pmg$ crystal approaching the critical point, we find the double degeneracy of the full phononic spectrum at the self-dual point and the merging of two tilted Weyl nodes into one critically tilted Dirac node. The latter is “accidentally” located on the high-symmetry line. The formation of this unconventional Dirac node is due to the emergence of the critical flatbands at the self-dual point, which are linear combinations of “finite-frequency” floppy modes. These modes can be understood as mechanically coupled self-dual rhombus chains vibrating in some unique uncoupled ways. Our work paves the way for designing and fabricating self-dual materials with exotic mechanical or phononic properties.

Within simulations of molecules deposited on a surface we show that neuroevolutionary learning can design particles and time-dependent protocols to promote self-assembly, without input from physical concepts such as thermal equilibrium or mechanical stability and without prior knowledge of candidate or competing structures. The learning algorithm is capable of both directed and exploratory design: it can assemble a material with a user-defined property, or search for novelty in the space of specified order parameters. In the latter mode it explores the space of what can be made, rather than the space of structures that are low in energy but not necessarily kinetically accessible.

For two molecules to react they first have to meet. Yet, reaction times are rarely on par with the first-passage times that govern such molecular encounters. A prime reason for this discrepancy is stochastic transitions between reactive and nonreactive molecular states, which results in effective gating of product formation and altered reaction kinetics. To better understand this phenomenon we develop a unifying approach to gated reactions on networks. We first show that the mean and distribution of the gated reaction time can always be expressed in terms of ungated first-passage and return times. This relation between gated and ungated kinetics is then explored to reveal universal features of gated reactions. The latter are exemplified using a diverse set of case studies which are also used to expose the exotic kinetics that arises due to molecular gating.

The Hopf insulator is a weak topological insulator characterized by an insulating bulk with conducting edge states protected by an integer-valued linking number invariant. The state exists in three-dimensional two-band models. We demonstrate that the Hopf insulator can be naturally realized in lattices of dipolar-interacting spins, where spin exchange plays the role of particle hopping. The long-ranged, anisotropic nature of the dipole-dipole interactions allows for the precise detail required in the momentum-space structure, while different spin orientations ensure the necessary structure of the complex phases of the hoppings. Our model features robust gapless edge states at both smooth edges, as well as sharp edges obeying a certain crystalline symmetry, despite the breakdown of the two-band picture at the latter. In an accompanying paper [T. Schuster et al., Phys. Rev. A 103, AW11986 (2021)] we provide a specific experimental blueprint for implementing our proposal using ultracold polar molecules of ${}^{40}\mathrm{K}{}^{87}\mathrm{Rb}$.

Electrons on the helium surface display sharp resonant absorption lines related to the transitions between the subbands of quantized motion transverse to the surface. A magnetic field parallel to the surface strongly affects the absorption spectrum. We show that the effect results from admixing the intersubband transitions to the in-plane quantum dynamics of the strongly correlated electron liquid or a Wigner crystal. This is similar to the admixing of electron transitions in color centers to phonons. The spectrum permits a direct characterization of the many-electron dynamics and also enables testing the theory of color centers in a system with controllable coupling.

Quantum harmonic oscillators are central to many modern quantum technologies. We introduce a method to determine the frequency noise spectrum of oscillator modes through coupling them to a qubit with continuously driven qubit-state-dependent displacements. We reconstruct the noise spectrum using a series of different drive phase and amplitude modulation patterns in conjunction with a data-fusion routine based on convex optimization. We apply the technique to the identification of intrinsic noise in the motional frequency of a single trapped ion with sensitivity to fluctuations at the sub-Hz level in a spectral range from quasi-dc up to 50 kHz.

Characterization and suppression of noise are essential for the control of harmonic oscillators in the quantum regime. We measure the noise spectrum of a quantum harmonic oscillator from low frequency to near the oscillator resonance by sensing its response to amplitude modulated periodic drives with a qubit. Using the motion of a trapped ion, we experimentally demonstrate two different implementations with combined sensitivity to noise from 500 Hz to 600 kHz. We apply our method to measure the intrinsic noise spectrum of an ion trap potential in a previously unaccessed frequency range.

A fiducial cross section for $W\gamma$ production in proton-proton collisions is measured at a center-of-mass energy of 13 TeV in $137{\mathrm{fb}}^{-1}$ of data collected using the CMS detector at the LHC. The $W\to e\nu$ and $\mu \nu$ decay modes are used in a maximum-likelihood fit to the lepton-photon invariant mass distribution to extract the combined cross section. The measured cross section is compared with theoretical expectations at next-to-leading order in quantum chromodynamics. In addition, 95% confidence level intervals are reported for anomalous triple-gauge couplings within the framework of effective field theory.

The ${\mathrm{\Xi }}_b^{-}{\pi }^{+}{\pi }^{-}$ invariant mass spectrum is investigated with an event sample of proton-proton collisions at $\sqrt{s}=13\mathrm{TeV}$, collected by the CMS experiment at the LHC in 2016–2018 and corresponding to an integrated luminosity of $140{\mathrm{fb}}^{-1}$. The ground state ${\mathrm{\Xi }}_b^{-}$ is reconstructed via its decays to ${J/\psi \mathrm{\Xi }}^{-}$ and $J/\psi \mathrm{\Lambda }{K}^{-}$. A narrow resonance, labeled ${\mathrm{\Xi }}_b(6100{)}^{-}$, is observed at a ${\mathrm{\Xi }}_b^{-}{\pi }^{+}{\pi }^{-}$ invariant mass of $6100.3\pm 0.2(\mathrm{stat})\pm 0.1(\mathrm{syst})\pm 0.6({\mathrm{\Xi }}_b^{-})\mathrm{MeV}$, where the last uncertainty reflects the precision of the ${\mathrm{\Xi }}_b^{-}$ baryon mass. The upper limit on the ${\mathrm{\Xi }}_b(6100{)}^{-}$ natural width is determined to be $1.9\mathrm{MeV}$ at 95% confidence level. The low ${\mathrm{\Xi }}_b(6100{)}^{-}$ signal yield observed in data does not allow a measurement of the quantum numbers of the new state. However, following analogies with the established excited ${\mathrm{\Xi }}_c$ baryon states, the new ${\mathrm{\Xi }}_b(6100{)}^{-}$ resonance and its decay sequence are consistent with the orbitally excited ${\mathrm{\Xi }}_b^{-}$ baryon, with spin and parity quantum numbers $J^P=3/2^{-}$.

We experimentally demonstrate the steady-state generation of propagating Wigner-negative states from a continuously driven superconducting qubit. We reconstruct the Wigner function of the radiation emitted into propagating modes defined by their temporal envelopes, using digital filtering. For an optimized temporal filter, we observe a large Wigner logarithmic negativity, in excess of 0.08, in agreement with theory. The fidelity between the theoretical predictions and the states generated experimentally is up to 99%, reaching state-of-the-art realizations in the microwave frequency domain. Our results provide a new way to generate and control nonclassical states, and may enable promising applications such as quantum networks and quantum computation based on waveguide quantum electrodynamics.

Achieving Bloch oscillations of free carriers under a direct current, a long-sought-after collective many-body behavior, has been challenging due to stringent constraints on the band properties. We argue that the flat bands in moiré graphene fulfill the basic requirements for observing Bloch oscillations, offering an appealing alternative to the stacked quantum wells used in previous work aiming to access this regime. Bloch-oscillating moiré superlattices emit a comblike spectrum of incommensurate frequencies, a property of interest for converting direct currents into high-frequency currents and developing broadband amplifiers in terahertz domain. The oscillations can be synchronized through coupling to an oscillator mode in a photonic or plasmonic resonator. Phase-coherent collective oscillations in the resonant regime provide a realization of current-pumped terahertz lasing.

We report on the discovery and rationale to devise bright single optical eigenmodes that feature quantum-optical mode volumes of about $1{\mathrm{nm}}^3$. Our findings rely on the development and application of a quasinormal mode theory that self-consistently treats fields and electron nonlocality, spill-out, and Landau damping around atomistic protrusions on a metallic nanoantenna. By outpacing Landau damping with radiation via properly designed antenna modes, the extremely localized modes become bright with radiation efficiencies reaching 30% and could provide up to $4\times {10}^7$ times intensity enhancement.

Localized electronic and nuclear spin qubits in the solid state constitute a promising platform for storage and manipulation of quantum information, even at room temperature. However, the development of scalable systems requires the ability to entangle distant spins, which remains a challenge today. We propose and analyze an efficient, heralded scheme that employs a parity measurement in a decoherence free subspace to enable fast and robust entanglement generation between distant spin qubits mediated by a hot mechanical oscillator. We find that high-fidelity entanglement at cryogenic and even ambient temperatures is feasible with realistic parameters and show that the entangled pair can be subsequently leveraged for deterministic controlled-NOT operations between nuclear spins. Our results open the door for novel quantum processing architectures for a wide variety of solid-state spin qubits.

We perform the three-dimensional lattice simulation of the magnetic field and gravitational wave productions from bubble collisions during the first-order electroweak phase transition. Except for the gravitational wave, the power-law spectrum of the magnetic field strength is numerically calculated for the first time, which is of a broken power-law spectrum: $B_{\xi }\propto f^{0.91}$ for the low-frequency region of $f<f_{\star }$ and $B_{\xi }\propto f^{-1.65}$ for the high-frequency region of $f>f_{\star }$ in the thin-wall limit, with the peak frequency being $f_{\star }\sim 5\mathrm{Hz}$ at the phase transition temperature 100 GeV. When the hydrodynamics is taken into account, the generated magnetic field strength can reach $B_{\xi }\sim {10}^{-7}\mathrm{G}$ at a correlation length $\xi \sim {10}^{-7}\mathrm{pc}$, which may seed the large scale magnetic fields. Our study shows that the measurements of cosmic magnetic field strength and gravitational waves are complementary to probe new physics admitting electroweak phase transition.

Chiral optical effects are generally quantified along some specific incident directions of exciting waves (especially for extrinsic chiralities of achiral structures) or defined as direction-independent properties by averaging the responses among all structure orientations. Though of great significance for various applications, chirality extremization (maximized or minimized) with respect to incident directions or structure orientations has not been explored, especially in a systematic manner. In this study we examine the chiral responses of open photonic structures from perspectives of quasinormal modes and polarization singularities of their far-field radiations. The nontrivial topology of the momentum sphere secures the existence of generic singularity directions along which mode radiations are either circularly or linearly polarized. When plane waves are incident along those directions, the reciprocity ensures ideal maximization and minimization of optical chiralities, for corresponding mode radiations of circular and linear polarizations, respectively. For directions of general elliptical polarizations, we have unveiled the subtle equality of a Stokes parameter and the circular dichroism, showing that an intrinsically (geometrically) chiral structure can unexpectedly exhibit no optical chirality at all or even optical chiralities of opposite handedness for different incident directions. The framework we establish can be applied to not only finite scattering bodies but also infinite structures, encompassing both intrinsic and extrinsic optical chiralities. We have effectively merged two vibrant disciplines of chiral and singular optics, which can potentially trigger more optical chirality-singularity related interdisciplinary studies.

Tantalum was once thought to be the canonical bcc metal, but is now predicted to transition to the Pnma phase at the high pressures and temperatures expected along the principal Hugoniot. Furthermore, there remains a significant discrepancy between a number of static diamond anvil cell experiments and gas gun experiments in the measured melt temperatures at high pressures. Our in situ x-ray diffraction experiments on shock compressed tantalum show that it does not transition to the Pnma phase or other candidate phases at high pressure. We observe incipient melting at approximately $254\pm 15\mathrm{GPa}$ and complete melting by $317\pm 10\mathrm{GPa}$. These transition pressures from the nanosecond experiments presented here are consistent with what can be inferred from microsecond gas gun sound velocity measurements. Furthermore, the observation of a coexistence region on the Hugoniot implies the lack of significant kinetically controlled deviation from equilibrium behavior. Consequently, we find that kinetics of phase transitions cannot be used to explain the discrepancy between static and dynamic measurements of the tantalum melt curve. Using available high pressure thermodynamic data for tantalum and our measurements of the incipient and complete melting transition pressures, we are able to infer a melting temperature ${8070}_{-750}^{+1250}\mathrm{K}$ at $254\pm 15\mathrm{GPa}$, which is consistent with ambient and a recent static high pressure melt curve measurement.