We study the process of nonlinear stimulated Raman adiabatic passage within a classical mean-field framework. Depending on the sign of interaction, the breakdown of adiabaticity in the interacting nonintegrable system is not related to bifurcations in the energy landscape, but rather to the emergence of quasistochastic motion that drains the followed quasistationary state. Consequently, faster sweep rate, rather than quasistatic variation of parameters, is better for adiabaticity.

Entangled-photon sources with simultaneously near-unity heralding efficiency and indistinguishability are the fundamental elements for scalable photonic quantum technologies. We design and realize a degenerate telecommunication wavelength entangled-photon source from an ultrafast pulsed laser pumped spontaneous parametric down-conversion (SPDC), which shows simultaneously 97% heralding efficiency and 96% indistinguishability between independent single photons without narrow-band filtering. Such a beamlike and frequency-uncorrelated SPDC source allows generation of the first 12-photon genuine entanglement with a state fidelity of $0.572\pm 0.024$. We further demonstrate a blueprint of scalable scattershot boson sampling using 12 SPDC sources and a $12\times 12$ mode interferometer for three-, four-, and five-boson sampling, which yields count rates more than 4 orders of magnitude higher than all previous SPDC experiments.

Device-independent certification refers to the characterization of an apparatus without reference to the internal description of other devices. It is a trustworthy certification method, free of assumption on the underlying Hilbert space dimension and on calibration methods. We show how it can be used to quantify the quality of a Bell-state measurement, whether deterministic, partial, or probabilistic. Our certification is noise resistant and opens the way towards the device-independent self-testing of Bell-state measurements in existing experiments.

Self-testing refers to the possibility of characterizing an unknown quantum device based only on the observed statistics. Here we develop methods for self-testing entangled quantum measurements, a key element for quantum networks. Our approach is based on the natural assumption that separated physical sources in a network should be considered independent. This provides a natural formulation of the problem of certifying entangled measurements. Considering the setup of entanglement swapping, we derive a robust self-test for the Bell-state measurement, tolerating noise levels up to $\sim 5\%$. We also discuss generalizations to other entangled measurements.

Cosmic ray electrons and positrons are tracers of particle propagation in the interstellar medium (ISM). A recent measurement performed using the High Energy Stereoscopic System extends the all-electron ($\mathrm{electron}+\mathrm{positron}$) spectrum up to 20 TeV, probing very local sources and transport due to the $\sim 10\mathrm{kyr}$ cooling time of these particles. An additional key local measurement was the recent estimation of the ISM diffusion coefficient around Geminga performed using the High-Altitude Water Cherenkov Observatory. The inferred diffusion coefficient is much lower than typically assumed values. It has been argued that if this diffusion coefficient is representative of the local ISM, pulsars would not be able to account for the all-electron spectrum measured at Earth. Here we show that a low diffusion coefficient in the local ISM is compatible with a pulsar wind nebula origin of the highest energy electrons, if a so-far-undiscovered pulsar with spin-down power $\sim 10^{33–34}\mathrm{erg}/\mathrm{s}$ exists within 30–80 pc of Earth. The existence of such a pulsar is broadly consistent with the known population and may be detected in near future survey observations.

We use Holevo information in the two-dimensional conformal field theory (CFT) with a large central charge $c$ to distinguish microstates from the underlying thermal state. Holographically, the CFT microstates of a thermal state are dual to black hole microstate geometries in anti–de Sitter space. It was found recently that holographic Holevo information shows plateau behaviors at both short and long interval regions. This indicates that the black hole microstates are indistinguishable from the thermal state by measuring over a small region, and perfectly distinguishable over a region with its size comparable to the whole system. In this Letter, we demonstrate that the plateaus are lifted by including the $1/c$ corrections from both the vacuum and nonvacuum conformal families of CFT in either the canonical ensemble or microcanonical ensemble thermal state. Our results imply that the aforementioned indistinguishability and distinguishability of black hole microstate geometries from the underlying black hole are spoiled by higher order Newton constant $G_N$ corrections of quantum gravity.

In this Letter, we study the equivalence between planar Ising networks and cells in the positive orthogonal Grassmannian. We present a microscopic construction based on amalgamation, which establishes the correspondence for any planar Ising network. The equivalence allows us to introduce two recursive methods for computing correlators of Ising networks. The first is based on duality moves, which generate networks belonging to the same cell in the Grassmannian. This leads to fractal lattices where the recursion formulas become the exact renormalization group equations of the effective couplings. The second, we use an amalgamation in which each iteration doubles the size of the seed lattice. This leads to an efficient way of computing the correlator where the complexity scales logarithmically with respect to the number of spin sites.

Laser-induced electron tunneling ionization from atoms and molecules plays as the trigger for a broad class of interesting strong-field phenomena in attosecond community. Understanding the time of electron tunneling ionization is vital to achieving the ultimate accuracy in attosecond metrology. We propose a novel attosecond photoelectron interferometer, which is based on the interference of the direct and near-forward rescattering electron wave packets, to determine the time information characterizing the tunneling process. Adding a weak perturbation in orthogonal to the strong fundamental field, the phases of the direct and the near-forward rescattering electron wave packets are modified, leading to the shift of the interferogram in the photoelectron momentum distributions. By analyzing the response of the interferogram to the perturbation, the real part of the ionization time, which denotes the instant when the electron exits the potential barrier, and the associated rescattering time are precisely retrieved. Moreover, the imaginary part of the ionization time, which has been interpreted as a quantity related to electron motion under the potential barrier, is also unambiguously determined.

We realize a mechanical analogue of the Dicke model, achieved by coupling the spin of individual neutral atoms to their quantized motion in an optical trapping potential. The atomic spin states play the role of the electronic states of the atomic ensemble considered in the Dicke model, and the in-trap motional states of the atoms correspond to the states of the electromagnetic field mode. The coupling between spin and motion is induced by an inherent polarization gradient of the trapping light fields, which leads to a spatially varying vector light shift. We experimentally show that our system reaches the ultrastrong coupling regime; i.e., we obtain a coupling strength that is a significant fraction of the trap frequency. Moreover, with the help of an additional light field, we demonstrate the in situ tuning of the coupling strength. Beyond its fundamental interest, the demonstrated one-to-one mapping between the physics of optically trapped cold atoms and the Dicke model paves the way for implementing protocols and applications that exploit extreme coupling strengths.

The breakup of low-viscosity droplets like water is a ubiquitous and rich phenomenon. Theory predicts that in the inviscid limit one observes a finite-time singularity, giving rise to a universal power law, with a prefactor that is universal for a given density and surface tension. This universality has been proposed as a powerful tool to determine the dynamic surface tension at short time scales. We combine high-resolution experiments and simulations to show that this universality is unobservable in practice: in contrast to previous studies, we show that fluid and system parameters do play a role; notably a small amount of viscosity is sufficient to alter the breakup dynamics significantly.

The qualitative behavior of charged particles in a vacuum is given by Earnshaw’s theorem, which states that there is no steady configuration of charged particles in a vacuum that is asymptotically stable to perturbations. In a viscous fluid, examples of stationary configurations of sedimenting uncharged particles are known, but they are unstable or neutrally stable—they are not attractors. In this Letter, it is shown by example that two charged particles settling in a fluid may have a configuration that is asymptotically stable to perturbations for a wide range of charges, radii, and densities. The existence of such “bound states” is essential from a fundamental point of view and it can be significant for dilute charged particulate systems in various biological, medical, and industrial contexts.

Magnesium thin films covered with a layer of Pd absorb hydrogen at much higher pressures than bulk Mg. Such an effect was originally explained as a consequence of elastic clamping on Mg by the capping Pd layer. An alternative interpretation later suggested that the pressure increase could originate from simple alloying between Mg and Pd. Here we resolve this controversy by measuring the hydrogenation and dehydrogenation isotherms of Mg-Pd thin film alloys over a wide range of compositions. Our results disentangle the effects of elastic clamping and alloying and highlight the role of plastic deformations.

When adsorbed on solids, water molecules are usually arranged into a honeycomb hydrogen-bond network. Here we report the discovery of a novel monolayer ice built exclusively from water hexamers but without shared edges, distinct from all conventional ice phases. Water grown on graphite crystalizes into a robust monolayer ice after annealing, attaining an exceedingly high density of $0.134{\AA }^{-2}$. Unlike chemisorbed ice on metal surfaces, the ice monolayer can translate and rotate on graphite terraces and grow across steps, confirming its two-dimensional nature. First-principles calculations identify the monolayer ice structure as a robust self-assembly of closely packed water hexamers without edge sharing, whose stability is maintained by maximizing the number of intralayer hydrogen bonds on inert surfaces.

Structure-based coarse graining of molecular systems offers a systematic route to reproduce the many-body potential of mean force. Unfortunately, common strategies are inherently limited by the molecular mechanics force field employed. Here, we extend the concept of multisurface dynamics, initially developed to describe electronic transitions in chemical reactions, to accurately sample the conformational ensemble of a classical system in equilibrium. In analogy to describing different electronic configurations, a surface-hopping scheme couples distinct conformational basins beyond the additivity of the Hamiltonian. The incorporation of more surfaces leads systematically toward improved cross-correlations. The resulting models naturally achieve consistent long-time dynamics for systems governed by barrier-crossing events.

We investigate the electronic and transport properties of topological and nontopological ${\mathrm{InAs}}_{0.85}{\mathrm{Bi}}_{0.15}$ quantum dots (QDs) described by a $\sim 30\mathrm{meV}$ gapped Bernevig-Hughes-Zhang (BHZ) model with cylindrical confinement, i.e., “BHZ dots.” Via modified Bessel functions, we analytically show that nontopological dots quite unexpectedly have discrete helical edge states, i.e., Kramers pairs with spin-angular-momentum locking similar to topological dots. These unusual nontopological edge states are geometrically protected due to confinement for a wide range of parameters and remarkably contrast with the bulk-edge correspondence in topological insulators, as no bulk topological invariant guarantees their existence. Moreover, for a conduction window with four edge states, we find that the two-terminal conductance $\mathcal{G}$ versus the QD radius $R$ and the gate $V_g$ controlling its levels shows a double peak at $2e^2/h$ for both topological and trivial BHZ QDs. This is in stark contrast to conductance measurements in 2D quantum spin Hall and trivial insulators. All of these results were also found in HgTe QDs. Bi-based BHZ dots should also prove important as hosts to room temperature edge spin qubits.

To trace the origin of time-reversal symmetry breaking (TRSB) in Re-based superconductors, we performed comparative muon-spin rotation and relaxation ($\mu \mathrm{SR}$) studies of superconducting noncentrosymmetric ${\mathrm{Re}}_{0.82}{\mathrm{Nb}}_{0.18}$ ($T_c=8.8\mathrm{K}$) and centrosymmetric Re ($T_c=2.7\mathrm{K}$). In ${\mathrm{Re}}_{0.82}{\mathrm{Nb}}_{0.18}$, the low–temperature superfluid density and the electronic specific heat evidence a fully gapped superconducting state, whose enhanced gap magnitude and specific-heat discontinuity suggest a moderately strong electron-phonon coupling. In both ${\mathrm{Re}}_{0.82}{\mathrm{Nb}}_{0.18}$ and pure Re, the spontaneous magnetic fields revealed by zero-field $\mu \mathrm{SR}$ below $T_c$ indicate time-reversal symmetry breaking and thus unconventional superconductivity. The concomitant occurrence of TRSB in centrosymmetric Re and noncentrosymmetric $\mathrm{Re}T$ ($T=\text{transition}$ metal), yet its preservation in the isostructural noncentrosymmetric superconductors ${\mathrm{Mg}}_{10}{\mathrm{Ir}}_{19}{\mathrm{B}}_{16}$ and ${\mathrm{Nb}}_{0.5}{\mathrm{Os}}_{0.5}$, strongly suggests that the local electronic structure of Re is crucial for understanding the TRSB superconducting state in Re and $\mathrm{Re}T$. We discuss the superconducting order parameter symmetries that are compatible with the experimental observations.

Breathers are localized waves in nonlinear systems that undergo a periodic variation in time or space. The concept of breathers is useful for describing many nonlinear physical systems including granular lattices, Bose-Einstein condensates, hydrodynamics, plasmas, and optics. In optics, breathers can exist in either the anomalous or the normal dispersion regimes, but they have only been characterized in the former, to our knowledge. Here, externally pumped optical microresonators are used to characterize the breathing dynamics of localized waves in the normal dispersion regime. High-$Q$ optical microresonators featuring normal dispersion can yield mode-locked Kerr combs whose time-domain waveform corresponds to circulating dark pulses in the cavity. We show that with relatively high pump power these Kerr combs can enter a breathing regime, in which the time-domain waveform remains a dark pulse but experiences a periodic modulation on a time scale much slower than the microresonator round trip time. The breathing is observed in the optical frequency domain as a significant difference in the phase and amplitude of the modulation experienced by different spectral lines. In the highly pumped regime, a transition to a chaotic breathing state where the waveform remains dark-pulse-like is also observed, for the first time to our knowledge; such a transition is reversible by reducing the pump power.

Two-dimensional electronic materials such as graphene and transition metal dichalgenides feature unique electrical and optical properties due to the conspirative effect of band structure, orbital coupling, and crystal symmetry. Synthetic matter, as accomplished by artificial lattice arrangements of cold atoms, molecules, electron patterning, and optical cavities, has emerged to provide manifold intriguing frameworks to likewise realize such scenarios. Exciton polaritons have recently been added to the list of promising candidates for the emulation of system Hamiltonians on a semiconductor platform, offering versatile tools to engineer the potential landscape and to access the nonlinear electro-optical regime. In this work, we introduce an electronically driven square and honeycomb lattice of exciton polaritons, paving the way towards real world devices based on polariton lattices for on-chip applications. Our platform exhibits laserlike emission from high-symmetry points under direct current injection, hinting at the prospect of electrically driven polariton lasers with possibly topologically nontrivial properties.

Electrically tunable graphene plasmons are anticipated to enable strong light-matter interactions with resonant quantum emitters. However, plasmon resonances in graphene are typically limited to infrared frequencies, below those of optical excitations in robust quantum light sources and many biologically interesting molecules. Here we propose to utilize near fields generated by the plasmon-assisted nonlinear optical response of nanostructured graphene to resonantly couple with proximal quantum emitters operating in the near infrared. We show that the nonlinear near field produced by a graphene nanodisk can strongly excite and coherently control quantum states in two- and three-level atomic systems when the third harmonic of its plasmon resonance is tuned to a particular electronic transition. In the present scheme, emitter and plasmon resonances are nondegenerate, circumventing strong enhancement of spontaneous emission. We envision potential applications for the proposed nonlinear plasmonic coupling scheme in sensing and temporal quantum control.