Supercontinuum generation has been widely applied in laser spectroscopy and few-cycle pulse generation. It is composed of complex and unmeasurable nonlinear optical effects, which influence the final broadened spectrum markedly. To describe and characterize the two key processes, the Kerr effect and ionization, we employ two nonlinear integrals, including the common B integral for the Kerr effect and a P integral for ionization or plasma effect. With these integrals, the contributions of Kerr and plasma effects in the supercontinuum generation are identified and determined quantitatively. Then we utilize machine learning to construct a multilayer perceptron neural network to obtain the solution of the propagation equation for a femtosecond pulse in a solid medium. We employ supervised and unsupervised training with both experimental and simulation data to train the neural network for a better accuracy of the calculation. The trained network can take the input and broadened spectra of the pulse to compute initial laser parameters and the B and P integrals instantly so that the contributions of Kerr and plasma effects in the supercontinuum generation may be quantified in real time, unveiling the nonlinearities behind the spectral evolution. This method provides a more accurate understanding of the physics of the entangled nonlinear optics effects and a faster and more convenient tool in the investigation of nonlinear optics.

We present a reformulation of the functional renormalization group (fRG) for many-electron systems, which relies on the recently introduced single-boson exchange (SBE) representation of the parquet equations [F. Krien, A. Valli, and M. Capone, Phys. Rev. B 100, 155149 (2019)]. The latter exploits a diagrammatic decomposition, which classifies the contributions to the full scattering amplitude in terms of their reducibility with respect to cutting one interaction line, naturally distinguishing the processes mediated by the exchange of a single boson in the different channels. We apply this idea to the fRG by splitting the one-loop fRG flow equations for the vertex function into SBE contributions and a residual four-point fermionic vertex. Similarly as in the case of parquet solvers, recasting the fRG algorithm in the SBE representation offers both computational and interpretative advantages: The SBE decomposition not only significantly reduces the numerical effort of treating the high-frequency asymptotics of the flowing vertices, but it also allows for a clear physical identification of the collective degrees of freedom at play. We illustrate the advantages of an SBE formulation of fRG-based schemes, by computing through the merger of dynamical mean-field theory and fRG the susceptibilities and the Yukawa couplings of the two-dimensional Hubbard model from weak to strong coupling, for which we also present an intuitive physical explanation of the results. The SBE formulation of the one-loop flow equations paves a promising route for future multiboson and multiloop extensions of fRG-based algorithms.

In specific open systems with collective dissipation the Liouvillian can be mapped to a non-Hermitian Hamiltonian. We here consider such a system where the Liouvillian is mapped to an XXZ Richardson-Gaudin integrable model and detail its exact Bethe ansatz solution. While no longer Hermitian, the Hamiltonian is $\text{pseudo-Hermitian}/\mathcal{P}\mathcal{T}$ symmetric, and as the strength of the coupling to the environment is increased, the spectrum in a fixed symmetry sector changes from a broken pseudo-Hermitian phase with complex conjugate eigenvalues to a pseudo-Hermitian phase with real eigenvalues, passing through a series of exceptional points and associated dissipative quantum phase transitions. The homogeneous limit supports a nontrivial steady state, and away from this limit this state gives rise to a slow logarithmic growth of the decay rate (spectral gap) with system size. Using the exact solution, it is furthermore shown how at large coupling strengths the ratio of the imaginary to the real part of the eigenvalues becomes approximately quantized in the remaining symmetry sectors.

We theoretically show that a two-band system with very different masses harbors a resonant pair scattering that leads to novel pairing properties, as highlighted by the Bardeen-Cooper-Schrieffer (BCS) to Bose-Einstein condensation (BEC) crossover. Most importantly, the interband pair-exchange coupling induces an effective intraband attraction in each band, enhancing the superfluidity/superconductivity. The effect, a kind of Suhl-Kondo mechanism, is specifically enhanced when the second band has a heavy mass and is incipient (lying close to, but just above, the chemical potential $\mu$), which we call a resonant pair scattering. By elucidating the dependence of the effective interactions and gap functions on $\mu$, we can draw an analogy between the resonant pair scattering and the Feshbach resonance.

We report exciton-mediated coupling between microwave and optical fields in cuprous oxide (${\mathrm{Cu}}_2\mathrm{O}$) at low temperatures. Rydberg excitonic states with principal quantum number up to $n=12$ were observed at 4 $\mathrm{K}$ using both one-photon (absorption) and two-photon (second harmonic generation) spectroscopy. Near resonance with an excitonic state, the addition of a microwave field significantly changed the absorption line shape, and added sidebands at the microwave frequency to the coherent second harmonic. Both effects showed a complex dependence on $n$ and angular momentum $l$. All of these features are in semiquantitative agreement with a model based on intraband electric dipole transitions between Rydberg exciton states. With a simple microwave antenna we already reach a regime where the microwave coupling (Rabi frequency) is comparable to the nonradiatively broadened linewidth of the Rydberg excitons. The results provide an additional way to manipulate excitonic states, and open up the possibility of a cryogenic microwave to optical transducer based on Rydberg excitons.

The atom-atom-anion three-body recombination (TBR) of ${}^3\mathrm{He}+{}^3\mathrm{He}+X^{-}$($X=\mathrm{H}\text{or}\mathrm{D}$) systems at ultracold temperatures ($T=0.01\sim 100$ mK) are studied by solving the Schrödinger equation in the adiabatic hyperspherical representation. It is found that for each system, ${}^3\mathrm{He}+{}^3\mathrm{He}+{\mathrm{H}}^{-}$ or ${}^3\mathrm{He}+{}^3\mathrm{He}+{\mathrm{D}}^{-}$, the $J^{\mathrm{\Pi }}=1^{-}$ symmetry dominates the TBR process, and the rates of TBR into $l=1{}^3\mathrm{He}{X}^{-}$ molecular anions are roughly three times as large as than that into $l=0{}^3\mathrm{He}{X}^{-}$ molecular anions for $T\in [0.01,10]$ mK, where $l$ denotes the two-body rotational quantum number. In addition, for a given product state, the TBR rates of the ${}^3\mathrm{He}{+}^3\mathrm{He}+{\mathrm{H}}^{-}$ system are larger than that of the ${}^3\mathrm{He}+{}^3\mathrm{He}+{\mathrm{D}}^{-}$ system by roughly two orders of magnitude which could be ascribed to the major nonadiabatic couplings between the entrance and recombination channels.

Ultrashort x-ray pulses from free-electron lasers can efficiently charge up and trigger the full fragmentation of molecules. By coincident detection of up to five ions resulting from rapid Coulomb explosion of highly charged iodomethane, we show that the full three-dimensional equilibrium geometry of this prototypical polyatomic system can be determined from the measured ion momenta with the help of a charge buildup model. Supported by simulations of how the ion momenta would reflect specific changes in molecular bond lengths and angles, we demonstrate that Coulomb-explosion imaging with ultrashort x-ray pulses is a promising technique for recording movies of multidimensional nuclear wave packets, including hydrogen motions.

We provide a topological concept to characterize energy gaps in general two-dimensional quasiperiodic systems. We show that every single gap is uniquely characterized by a set of integers, which quantize the area of the momentum space in units of multiple Brillouin zones. These integers are found to be equivalent to the second Chern numbers by considering a formal relationship between an adiabatic charge pumping under a potential sliding and the four-dimensional quantum Hall effect. The integers are independent of commensurability, and invariant under an arbitrary continuous deformation, such as a relative rotation of a twisted bilayer system.

Topological magnetic textures are of great interest in various scientific and technological fields. To allow for precise control of nanoscale magnetism, it is of great importance to understand the role of intrinsic defects in the host material. Here we use conventional and time-resolved Lorentz microscopy to study the effect of grain size in polycrystalline permalloy films on the pinning and gyration orbits of vortex cores inside magnetic nanoislands. To assess static pinning, we use in-plane magnetic fields to shift the core across the island while recording its position. This enables us to produce highly accurate two-dimensional maps of pinning sites. Based on this technique, we can generate a quantitative map of the pinning potential for the core, which we identify as being governed by grain boundaries. Furthermore, we investigate the effects of pinning on the dynamic behavior of the vortex core using stroboscopic Lorentz microscopy, harnessing a new photoemission source that accelerates image acquisition by about two orders of magnitude. We find characteristic changes to the vortex gyration in the form of increased dissipation and enhanced bistability in samples with larger grains.

We develop a minimal self-gravitating model for pulsar glitches by introducing a solid-crust potential in the three-dimensional Gross-Pitaevskii-Poisson equation, which we have used earlier to study gravitationally bound Bose-Einstein condensates, i.e., bosonic stars. In the absence of the crust potential, we show that, if we rotate such a bosonic star, it is threaded by vortices. We then show, via extensive direct numerical simulations, that the interaction of these vortices with the crust potential yields (a) stick-slip dynamics and (b) dynamical glitches. We demonstrate that, if enough momentum is transferred to the crust from the bosonic star, then the vortices are expelled from the star, and the crust's angular momentum $J_c$ exhibits features that may be interpreted as glitches. From the time series of $J_c$, we compute the cumulative probability distribution functions (CPDFs) of event sizes, event durations, and waiting times, which are consistent with the previous work. We show that these CPDFs have signatures of self-organized criticality, which are similar to those seen in observations of pulsar glitches and are consistent with previous work.

The interface separating a liquid from its vapor phase is diffuse; the composition varies continuously from one phase to the other over a finite length. Recent experiments on dynamic jamming fronts in two dimensions [Waitukaitis et al., Europhys. Lett. 102, 44001 (2013)] identified a diffuse interface between jammed and unjammed disks. In both cases, the thickness of the interface diverges as a critical transition is approached. We investigate the generality of this behavior using a third system: A model of cyclically sheared non-Brownian suspensions. As we sediment the particles toward a boundary, we observe a diffuse traveling front that marks the interface between irreversible and reversible phases. We argue that the front width is linked to a diverging correlation length scale in the bulk, which we probe by studying avalanches near criticality. Our results show how diffuse interfaces may arise generally when an incompressible phase is brought to a critical point.

Combining gravity with quantum mechanics remains one of the biggest challenges of physics. In the past years, experiments with optomechanical systems have been proposed that may give indirect clues about the quantum nature of gravity. In a recent variation of such tests [Carney et al., PRX Quantum 2, 030330 (2021)], the authors propose to gravitationally entangle an atom interferometer with a mesoscopic oscillator. The interaction results in periodic drops and revivals of the interferometeric visibility, which under specific assumptions indicate the gravitational generation of entanglement. Here, we study semiclassical models of the atom interferometer that can reproduce the same effect. We show that the core signature—periodic collapses and revivals of the visibility—can appear if the atom is subject to a random unitary channel, including the case where the oscillator is fully classical and situations even without explicit modeling of the oscillator. We also show that the nonclassicality of the oscillator vanishes unless the system is very close to its ground state, and even when the system is in the ground state, the nonclassicality is limited by the coupling strength. Our results thus indicate that deducing entanglement from the proposed experiment is very challenging, since fulfilling and verifying the nonclassicality assumptions constitute a significant challenge in their own right.

Finding a feasible scheme for testing the quantum mechanical nature of the gravitational interaction has been attracting an increasing level of attention. Gravity mediated entanglement generation so far appears to be the key ingredient for a potential experiment. In a recent proposal [D. Carney et al., PRX Quantum 2, 030330 (2021)] combining an atom interferometer with a low-frequency mechanical oscillator, a coherence revival test is proposed for verifying this entanglement generation. With measurements performed only on the atoms, this protocol bypasses the need for correlation measurements. Here, we explore formulations of such a protocol, and specifically find that in the envisioned regime of operation with high thermal excitation, semiclassical models, where there is no concept of entanglement, also give the same experimental signatures. We elucidate in a fully quantum mechanical calculation that entanglement is not the source of the revivals in the relevant parameter regime. We argue that, in its current form, the suggested test is only relevant if the oscillator is nearly in a pure quantum state, and in this regime the effects are too small to be measurable. We further discuss potential open ends. The results highlight the importance and subtleties of explicitly considering how the quantum case differs from the classical expectations when testing for the quantum mechanical nature of a physical system.

Split-pulse x-ray photon correlation spectroscopy using x-ray free-electron laser is a promising tool to probe atomic dynamics in liquid and soft-matter in picosecond time scale, which has been accessible only by spectroscopy. However, sample heating by x-ray beam is a major obstacle for this technique. Using molecular dynamics and the two-temperature model we examine the atomic level response of water to x-ray laser pulse at picosecond time scale and compare with observations from recent x-ray free-electron laser experiments. We investigate the effects of the heating due to x-ray laser pulses of different energies and the effects of the heat dissipation on the structure and dynamics of water through the atomic density correlation and the dynamic structure factor. Our results indicate, in agreement with experiment that in addition to the beam energy the time delay between the two pulses is a critical factor for obtaining reliable information on the atomic level dynamics of water.

Deep learning techniques have opened a new venue for electronic structure theory in recent years. In contrast to traditional methods, deep neural networks provide much more expressive and flexible wave function Ansätze, resulting in better accuracy and timescale behavior. In order to study larger systems while retaining sufficient accuracy, we integrate a powerful neural-network-based model (FermiNet) with the effective core potential method, which helps to reduce the complexity of the problem by replacing inner core electrons with additional semilocal potential terms in the Hamiltonian. In this work, we calculate the ground-state energy of $3d$ transition metal atoms and their monoxides, which is quite challenging for the original FermiNet work, and the results are consistent with both experimental data and other state-of-the-art computational methods. Our work is an important step for a broader application of deep learning in the electronic structure calculation of molecules and materials.

We present an optimization framework based on Lagrange duality and the scattering $\mathbb{T}$-operator of electromagnetism to construct limits on the possible features that may be imparted to a collection of output fields from a collection of given input fields, i.e., constraints on achievable optical transformations and the characteristics of structured materials as communication channels. Implications of these bounds on the performance of representative optical devices having multiwavelength or multiport functionalities are examined in the context of electromagnetic shielding, focusing, near-field resolution, and linear math kernels.