When a quantum emitter (QE) is placed in close proximity to a plasmonic metal nanoparticle within an external optical field, a dipole-dipole coupling arises, resulting in a highly tunable hybrid nanosystem that surpasses the optical manipulation capabilities of the individual components. These hybrid systems enable the exploration and manipulation of optical fields at the intersection between classical and quantum phenomena. Theoretical models of this interaction have typically been limited to the extreme orientations, where the QE and plasmonic dipoles are polarized either along or perpendicular to the interparticle axis, for analytical tractability. In this work, we generalize the semiclassical optical dipole-dipole interaction model for a two-level quantum emitter in a plasmonic field for arbitrary polarization angles. We show that the total field experienced by the quantum emitter at intermediate angles does not necessarily align with the external input field and discuss the implications of varying the polarization angle of the external input field.

Structural superlubricity is an intriguing physical phenomenon, whereby sliding at a structurally incommensurate, atomically flat interface yields vanishingly small friction forces. Despite its recent experimental validation under ambient conditions, critical questions remain regarding the physical limitations of the concept. In particular, it is not known whether the ultralow friction state would persist at high sliding speeds relevant for practical, small-scale mechanical systems. Here, we perform sliding experiments via atomic force microscopy on gold nanoislands on graphite at increasing speeds, extracting interfacial friction forces under ambient conditions. A heterodyne detection methodology enables the extraction of extremely weak friction signals buried deep in the noise, revealing that the structurally superlubric regime extends over 100 µm/s with minimal changes in friction force, spanning three orders of magnitude in sliding speed. It is proposed that molecular contaminants at the gold-graphite interface result in the observed insensitivity of friction forces on sliding speed, i.e., Coulombic behavior. Our results contribute significantly to the pursuit of functional, superlubric mechanical devices.

We investigate a dilute Bose gas with both a short-range contact and an effective long-range interaction between the atoms. The latter is induced by the strong coupling to a cavity light mode and is spatially characterized by a periodic signature and a tunable envelope rooted in the pumping of the cavity. We formulate a Bogoliubov theory based on a homogeneous mean-field description and quantum fluctuations around it. The competition between the repulsive contact interaction and the long-range interaction allows the formation of self-bound quantum droplets. This generic approach is applied to two cavity setups, one without and one with a momentum-conserving effective long-range interaction between the atoms in the form of a driven dispersive cavity mode and a multimode cavity, respectively. For both cases we show analytically how the size and the central density of the cavity-induced quantum droplets depend on the contact interaction strength and on the shape of the spatial envelope of the long-range interaction.

Understanding the high-dimensional chaotic dynamics occurring in complex biological systems such as recurrent neural networks or ecosystems remains a conceptual challenge. For low-dimensional dynamics, fixed points provide the geometric scaffold of the dynamics. However, in high-dimensional systems, even the location of fixed points is unknown. Here, we analytically determine the number and distribution of fixed points for a canonical model of a recurrent neural network that exhibits high-dimensional chaos. This distribution reveals that fixed points and dynamics are confined to separate shells in state space. Furthermore, the distribution enables us to determine the eigenvalue spectra of the Jacobian at the fixed points, showing that each fixed point has a low-dimensional unstable manifold. Despite the radial separation of fixed points and dynamics, we find that the principal components of fixed points and dynamics align and that nearby fixed points act as partially attracting landmarks for the dynamics. Our results provide a detailed characterization of the fixed point geometry and its interplay with the dynamics, thereby paving the way towards a geometric understanding of high-dimensional chaos through their skeleton of unstable fixed points.

Complex multiscale flows associated with instabilities and turbulence are commonly induced under high-energy density (HED) conditions, but accurate measurement of their transport properties has been challenging. X-ray photon correlation spectroscopy (XPCS) with coherent x-ray sources can, in principle, probe material dynamics to infer transport properties using time autocorrelation of density fluctuations. Here we develop a theoretical framework for utilizing XPCS to study material diffusivity in multiscale flows. We extend single-scale shear flow theories to broadband flows using a multiscale analysis that captures shear and diffusion dynamics. Our theory is validated with simulated XPCS for Brownian particles advected in multiscale flows. We demonstrate the versatility of the method over several orders of magnitude in timescale using sequential-pulse XPCS, single-pulse x-ray speckle visibility spectroscopy (XSVS), and double-pulse XSVS.

Relativistic spin-polarized electron beams are important for fundamental research and the industry, but their generation currently requires conventional accelerators or ultrastrong laser facilities, limiting their accessibility and broad applications. Here, we put forward a method for constructing a compact efficient “polarizer” that achieves direct ultrafast conversion of relativistic dense electron beams into polarized ones, based on a beam “self-polarization” mechanism via beam-target interactions. In this scheme, as the electron beam grazes through the polarizer (a double-layer solid target), it ionizes the target and excites an asymmetric plasma field due to the plasma backflows. This field then reacts on the beam itself, triggering spontaneous radiative polarization and reflecting approximately 10% of the beam, and thereby yielding a dense polarized electron beam. Moreover, the double-layer target setup enhances the plasma field and induces a plasma bubble that focuses the polarized beam and reshapes its polarization distribution. Our method is robust with respect to the beam and target parameters, and opens an avenue for relativistic beam polarization with compact accessible devices, which would facilitate their broad applications and the development of related polarized beam experiments.

Nonclassicality in composite quantum systems depicts several puzzling manifestations, with Einstein-Podolsky-Rosen entanglement, Schrödinger steering, and Bell nonlocality being the most celebrated ones. In addition to those, an unentangled quantum state can also exhibit nonclassicality, as evidenced from notions such as quantum discord and work deficit. In this paper, we present a general framework for exploring quantumness of correlations in multipartite quantum states. By exploiting the different signatures reflected on observable quantities depending on whether subsystems of a composite system are probed jointly or independently, we introduce an operational quantifier of nonclassicality, termed ergodiscord. As we show, this proposed quantifier faithfully captures nonclassicality in any bipartite quantum state while being fundamentally distinct from the original quantum discord. Moreover, ergodiscord uncovers an intriguing phenomenon called nonlocal energy locking, where a useful form of energy (i.e., work) gets locked in correlations of nonclassical states. We also show that a mixed nonclassical state can lock more work than the maximally entangled state of the corresponding system, establishing an interesting superadditivity phenomenon of nonlocal energy locking. This paper may inspire designs of quantum energy storage devices by utilizing nonclassical correlations in composite quantum systems.

Long-distance communication is one of the main bottlenecks in the development of quantum communication networks. Free-space communication is a way to circumvent exponential fiber loss and to allow longer communication distances. Satellite nodes are the main devices currently studied for free-space communication, but they come with downsides such as high cost and low availability. In this work, we study an alternative to satellites, namely, aerial platforms such as high-altitude balloons. We provide a loss model to simulate the channel efficiency of balloon-to-ground, ground-to-balloon, and balloon-to-balloon communication channels, considering a large set of hardware parameters. We perform a parameter exploration to exhibit important trade-offs in these channels, as well as simulations of different quantum key distribution network architectures including balloon nodes. We demonstrate that balloons are a realistic alternative to satellites for free-space communications in national network architectures.

Realizing Kitaev interactions on triangular lattices offers a compelling platform for exploring quantum spin liquid physics beyond the conventional honeycomb-lattice framework. Here, we investigate the triangular-lattice antiferromagnet ${\mathrm{KCeSe}}_2$, where multiple probes reveal strong magnetic anisotropy, suggesting significant Kitaev physics. Through detailed and combined analysis of magnetization, neutron scattering, and thermodynamic experiments, we identify dominant ferromagnetic Kitaev $(K=-1.82\mathrm{K})$ and antiferromagnetic Heisenberg $(J=1.34\mathrm{K})$ interactions that stabilize a stripe-$yz$ ordered ground state via an order-by-disorder mechanism. Magnetic fields applied along the Kitaev bond direction induce two phase transitions at 1.67 and 3.8 T, consistent with density matrix renormalization group calculation predictions of a progression from stripe-$yz$ to stripe-canted and spin-polarized phases. Near the 1.67 T quantum critical point, enhanced quantum fluctuations suggest conditions favorable for exotic excitations. These results establish ${\mathrm{KCeSe}}_2$ as a platform for exploring Kitaev physics on triangular lattices.

We demonstrate that a quantum field theory (QFT) in general two-dimensional curved spacetimes can be realized by a system of quantum spins or qubits. We consider a spin-1/2 model on a one-dimensional ring with spatially and temporally varying exchange couplings and magnetic fields. This model reduces to a QFT of Majorana fermions in the continuum limit. From this correspondence, we establish a dictionary for translating between the spacetime-dependent parameters of the spin model and the general metric on which the QFT is defined. After addressing the general case, we consider the Friedmann-Lemaître-Robertson-Walker (FLRW) metric as a simple example. According to the dictionary, the QFT of Majorana fermions on the FLRW metric corresponds to the Ising model with a time-dependent transverse magnetic field. We demonstrate that the production of Majorana particles in the expanding universe can be simulated with the transverse-field Ising model by increasing the strength of the magnetic field. Furthermore, we examine the Unruh effect through the spin system by using our prescription and show the direct relation between the entanglement (or modular) Hamiltonian in the spin system and the Rindler Hamiltonian. This approach provides an experimentally viable system for probing various phenomena in QFT within curved spacetime, while also opening the door to uncovering nontrivial phenomena in spin systems inspired by curved spacetime physics. It offers fresh perspectives on both QFT in curved spacetimes and quantum many-body spin systems, revealing profound connections between these fields.

We study a cylindrical plasmonic waveguide consisting of a magnetic Weyl semimetal embedded in a dielectric medium. We determine the dispersion relation of the surface plasmon polaritons and show how it depends on the plasma frequency, the radius of the semimetal core, and the separation between the nodes. We show that the band structure, which modifies the electrodynamics in the medium, manifests itself through a pronounced asymmetry in the dispersion curves and a giant splitting in the group velocity, with the orbital angular momentum as a control parameter for the direction of propagation.

Using tensor network methods, we simulate the real-time evolution of the lattice Thirring model quenched out of equilibrium in both the critical and massive phases and study the appearance of dynamical quantum phase transitions, as nonanalyticities in the Loschmidt rate. Although the presence of a dynamical quantum phase transition in the model does not correspond to quenches across the critical line of the equilibrium phase diagram at zero temperature, we identify a threshold in the energy density of the initial state, necessary for a dynamical quantum phase transition to be present. Moreover, in the case of the gapped quench Hamiltonian, we unveil a connection of this threshold to a transition between different regions in the finite-temperature phase diagram.

We describe our implementation of fermionic tensor network contraction on arbitrary lattices within both a globally ordered and a locally ordered formalism. We provide a pedagogical description of these two conventions as implemented for the quimb library. Using hyperoptimized approximate contraction strategies, we present benchmark fermionic projected entangled pair state simulations of finite Hubbard models defined on the three-dimensional diamond lattice and random regular graphs.

Quantum technologies present new opportunities for fundamental tests of nature. One potential application is to probe the interplay between quantum physics and general relativity—a field of physics with no empirical evidence yet. Here we show that quantum networks open a new window to test this interface. We demonstrate how photon mediated entanglement between atomic or atomlike systems can be used to probe time-dilation-induced entanglement and interference modulation. Key are nonlocal measurements between clocks in a gravitational field, which can be achieved either through direct photon interference or by using auxiliary entanglement. The resulting observable depends on the interference between different proper times, and can only be explained if both quantum theory and general relativity are taken into account. The proposed protocol enables clock interferometry on kilometer-scale separations and beyond. Our work thus shows a realistic experimental route for a first test of quantum theory on curved spacetime, opening up new scientific opportunities for quantum networks.

We analyze the feasibility of cooling of classical motion of a micro- to nano-sized magnetic particle, levitated inside a hollow-core photonic crystal fiber. The cooling action is implemented by means of controlling the relative phase between counterpropagating fiber guided waves. Direct imaging of the particle's position allows dynamic phase adjustments that produce a Stokes-type cooling force. We provide estimates of cooling efficiency and final achievable temperature, taking into account thermal and detection noise sources. Our results bring forward an important step towards using trapped micromagnets in sensing, testing the fundamental physics and preparing the quantum states of magnetization.

Laser plasma acceleration techniques hold promise for generating compact, high-flux relativistic proton bunches. However, the inherent instability of laser-plasma interactions and the requirement for reliable proton source stability often hinder their practical applications. In this paper, we explore the potential of a water-leaf target irradiated by 27 fs laser pulses with energies ranging from 1.1 to 9.9 J and peak intensities spanning from ${10}^{20}$ to ${10}^{21}{\text{W/cm}}^2$. We finally conducted a series of 400 shots with a peak power of 300 TW, producing proton energies up to 30 MeV and peak flux beyond ${10}^9\text{protons}{\text{MeV}}^{-1}{\text{msr}}^{-1}$. These results demonstrate the possibility of long-term, stable, and efficient proton acceleration at high repetition rates, addressing a key challenge in laser-plasma acceleration.