We study three-body collisions within ultracold mixtures with resonant interspecies $p$-wave interactions. Our results for the three-body effective interaction strength and decay rate are crucial towards understanding the stability and lifetime of these dilute quantum fluids. On resonance, we find that a class of universal scattering pathways emerges, regardless of the details of the short-range interactions. This gives rise quite generally to a remarkable regime where three-body effective interactions dominate over both inelastic decay and two-body effective interactions. Additionally, we find a series of mass-ratio-dependent trimer resonances further from resonance.

Non-Hermitian topological systems are the newest additions to the growing field of topological matter. In this work, we report the light-driven exceptional physics in a multi-Weyl semimetal. The driving is not only a key ingredient to control the position of the exceptional contours (ECs); light also has the ability to generate new ECs. Interestingly, we also demonstrate topological charge distribution and Lifshitz transition, which are controllable by the driving field in such generated ECs. Our findings present a promising platform for the manipulation and control over exceptional physics in non-Hermitian topological matter.

Reconnecting vortices in a superfluid allows for the energy transfer between different length scales and its subsequent dissipation. The present picture assumes that the dynamics of a reconnection is driven mostly by the phase of the order parameter, and this statement can be justified in the case of Bose-Einstein Condensates (BECs), where vortices have a simple internal structure. Therefore, it is natural to postulate that the reconnection dynamics in the vicinity of the reconnection moment is universal. This expectation has been confirmed in numerical simulations for BECs and experimentally for the superfluid ${}^4\mathrm{He}$. Not much has been said about this relation in the context of Fermi superfluids. In this Letter, we aim to bridge this gap, and we report our findings, which reveal that the reconnection dynamics conforms with the predicted universal behavior across the entire BCS-BEC crossover. The universal scaling also survives for spin-imbalanced systems, where unpaired fermions induce a complex structure of the colliding vortices.

We investigate the Bose-Hubbard chain in the presence of nearest-neighbor pairing. The pairing term gives rise to an unusual gapped ${\mathbb{Z}}_2$ Ising phase that has number fluctuation but no off-diagonal long range order. This phase has a strongly correlated many-body doubly degenerate ground state which is effectively a gap-protected macroscopic qubit. In the strongly interacting limit, the system can be mapped onto an anisotropic transverse spin chain, which in turn can be mapped to the better-known fermionic sister of the paired Bose-Hubbard chain: the Kitaev chain which hosts zero-energy Majorana bound states. While corresponding phases in the fermionic and bosonic systems have starkly different wave functions, they share identical energy spectra. We describe a possible cold-atom realization of the paired Bose-Hubbard model in a biased zigzag optical lattice with reservoir-induced pairing, opening a possible route towards experimental Kitaev chain spectroscopy.

In this Letter, we analyze the quantum dynamics of the perceptron model: a particle is constrained on an $N$-dimensional sphere, with $N\to \infty$, and subjected to a set of randomly placed hard-wall potentials. This model has several applications, ranging from learning protocols to the effective description of the dynamics of an ensemble of infinite-dimensional hard spheres in Euclidean space. We find that the jamming transition with quantum dynamics shows critical exponents different from the classical case. We also find that the quantum jamming transition, unlike the typical quantum critical points, is not confined to the zero-temperature axis, and the classical results are recovered only at $T=\infty$. Our findings have implications for the theory of glasses at ultralow temperatures and for the study of quantum machine-learning algorithms.

We report on the observation of the spin-exchange dynamics of ${}^{171}\mathrm{Yb}$ atoms in the ground state ${}^1{S}_0$ and in the metastable state ${}^3{P}_0$. We implement the mixed-dimensional two-orbital system using near-resonant and magic-wavelength optical lattices, where the ${}^1{S}_0$ and ${}^3{P}_0$ atoms are itinerant in a one-dimensional tube and localized in three dimensions, respectively. By exploiting an optical Stern-Gerlach method, we observe the spin depolarization of the ${}^1{S}_0$ atoms induced by the spin-exchange interaction with the ${}^3{P}_0$ atom. Our work could pave the way to the quantum simulation of the Kondo effect.

Efficient error-mitigation techniques demanding minimal resources is key to quantum information processing. We propose a generic protocol to mitigate quantum errors using detection-based quantum autoencoders. In our protocol, the quantum data are compressed into a latent subspace while leaving errors outside, the latter of which is then removed by a measurement and postselection. Compared to previously developed methods, our protocol on the one hand requires no extra qubits, and on the other hand it has a near-optimal denoising power, in which under reasonable requirements all errors detected outside of the latent subspace can be removed, while those inside the subspace cannot be removed by any means. Our detection-based quantum autoencoders are therefore particularly useful for near-term quantum devices in which controllable qubits are limited while noise reduction is important.

Promising searches for new physics beyond the current standard model (SM) of particle physics are feasible through isotope-shift spectroscopy, which is sensitive to a hypothetical fifth force between the neutrons of the nucleus and the electrons of the shell. Such an interaction would be mediated by a new particle which could in principle be associated with dark matter. In so-called King plots, the mass-scaled frequency shifts of two optical transitions are plotted against each other for a series of isotopes. Subtle deviations from the expected linearity could reveal such a fifth force. Here, we study experimentally and theoretically six transitions in highly charged ions of Ca, an element with five stable isotopes of zero nuclear spin. Some of the transitions are suitable for upcoming high-precision coherent laser spectroscopy and optical clocks. Our results provide a sufficient number of clock transitions for—in combination with those of singly charged ${\mathrm{Ca}}^{+}$—application of the generalized King plot method. This will allow future high-precision measurements to remove higher-order SM-related nonlinearities and open a door to yet more sensitive searches for unknown forces and particles.

Nonlinear topological photonics, which explores topics common to the fields of topological phases and nonlinear optics, is expected to open up a new paradigm in topological photonics. Here, we demonstrate second-harmonic generation (SHG) via nonlinear interaction of double topological valley-Hall kink modes in all-dielectric photonic crystals (PhCs). We first show that two topological frequency band gaps can be created around a pair of frequencies, ${\omega }_0$ and $2{\omega }_0$, by gapping out the corresponding Dirac points in two-dimensional honeycomb PhCs. Valley-Hall kink modes along a kink-type domain wall interface between two PhCs placed together in a mirror-symmetric manner are generated within the two frequency band gaps. Importantly, through full-wave simulations and mode dispersion analysis, we demonstrate that tunable, bidirectional phase-matched SHG via nonlinear interaction of the valley-Hall kink modes inside the two band gaps can be achieved. In particular, by using Stokes parameters associated with the magnetic part of the valley-Hall kink modes, we introduce the concept of SHG directional dichroism, which is employed to characterize optical probes for sensing chiral molecules. Our work opens up avenues toward topologically protected nonlinear frequency mixing and active photonic devices implemented in all-dielectric material platforms.

A canonical quantization scheme for localized surface plasmons (LSPs) in a metal nanosphere is presented based on a microscopic model composed of electromagnetic fields, oscillators that describe plasmons, and a reservoir that describes excitations other than plasmons. The eigenmodes of this fully quantum electrodynamic theory show a spectrum that includes radiative depolarization and broadening, including redshifting from the quasistatic LSP modes with increasing particle size. These spectral profiles correctly match those obtained with exact classical electrodynamics (Mie theory). The present scheme provides the electric fields per plasmon in both near- and far-field regions whereby its utility in the fields of quantum plasmonics and nano-optics is demonstrated.

We characterize entanglement subject to its definition over real and complex, composite quantum systems. In particular, a method is established to assess quantum correlations with respect to a selected number system, illuminating the deeply rooted, yet rarely discussed question of why quantum states are described via complex numbers. With our experiment, we then realize two-photon polarization states that are entangled with respect to the notion of two rebits, comprising two two-level systems over real numbers. At the same time, the generated states are separable with respect to two complex qubits. Among other results, we reconstruct the best approximation of the generated states in terms of a real-valued, local expansion and show that this yields an incomplete description of our data. Conversely, the generated states are shown to be fully decomposable in terms of tensor-product states with complex wave functions. Thereby, we probe paradigms of quantum physics with modern theoretical tools and experimental platforms that are relevant for applications in quantum information science and technology and connected to the fundamentals of the quantum description of nature.

Time-resolved buildup of a Fano resonance in a macroscopic gaseous medium is theoretically studied by attosecond transient absorption spectroscopy. An intense near-infrared field is employed as an adjustable gate to confine the temporal dynamics, enabling one to resolve the line-shape formation in real time. Beyond the region where the single-atom approximation holds, we observe the emergence of spectral spikes with narrow linewidths for certain time delays and propagation distances, which represents that the medium can be delicately structured to be an efficient resonant absorber. Spectral substructures appear on the shoulder of the absorption lines after additional pulse propagation. The links between these features imprinted in spectral profiles and the formation of temporal characteristics in the excitation field are identified. The collective response of the medium versus time delay and propagation distance, in this laser-gated case, provides clues to the time-domain information on autoionization state and the microscopic dynamics occurring at each layer of the medium.

We propose a scheme comprising an array of anisotropic optical waveguides, embedded in a gas of cold atoms, which can be tuned from a Hermitian to an odd-$\mathcal{P}\mathcal{T}$-symmetric configuration through the manipulation of control and assistant laser fields. We show that the system can be controlled by tuning intra- and intercell coupling coefficients, enabling the creation of topologically distinct phases and linear topological edge states. The waveguide array, characterized by a quadrimer primitive cell, allows for implementing transitions between Hermitian and odd-$\mathcal{P}\mathcal{T}$-symmetric configurations, broken and unbroken $\mathcal{P}\mathcal{T}$-symmetric phases, topologically trivial and nontrivial phases, as well as transitions between linear and nonlinear regimes. The introduced scheme generalizes the Rice-Mele Hamiltonian for a nonlinear non-Hermitian quadrimer array featuring odd-$\mathcal{P}\mathcal{T}$ symmetry and makes accessible unique phenomena and functionalities that emerge from the interplay of non-Hermiticity, topology, and nonlinearity. We also show that in the presence of nonlinearity the system sustains nonlinear topological edge states bifurcating from the linear topological edge states and the modes without a linear limit. Each nonlinear mode represents a doublet of odd-$\mathcal{P}\mathcal{T}$-conjugate states. In the broken $\mathcal{P}\mathcal{T}$ phase, the nonlinear edge states may be effectively stabilized when an additional absorption is introduced into the system.

A method involving intensity correlation measurements is described, which allows for the complete removal of Doppler broadening in the emission of electromagnetic radiation from faraway sources that are inaccessible to conventional Doppler-free measurements. The technique, relying on a correction to $g^{\left(2\right)}$ of order $N^{-1}$, probes the separation between neighboring spectral lines and is also applicable to the elimination of broadening due to collisions ($N$ is the number of emitting particles and $g^{\left(2\right)}$ is the second-order field correlation function). Possible applications include a determination of cosmological parameters from redshifts of gravitationally lensed quasars.

In the noisy intermediate-scale quantum era, optimal digitized pulses are requisite for efficient quantum control. This goal is translated into dynamic programming, in which a deep reinforcement learning (DRL) agent is gifted. As a reference, shortcuts to adiabaticity (STA) provide analytical approaches to adiabatic speedup by pulse control. Here, we select the single-component control of qubits, resembling the ubiquitous two-level Landau-Zener problem for gate operation. We aim at obtaining fast and robust digital pulses by combining the STA and DRL algorithm. In particular, we find that DRL leads to robust digital quantum control with the operation time bounded by quantum speed limits dictated by STA. In addition, we demonstrate that robustness against systematic errors can be achieved by DRL without any input from STA. Our results introduce a general framework of digital quantum control, leading to a promising enhancement in quantum information processing.

Classical $(1+1)$-dimensional (D) cellular automata, as for instance Domany-Kinzel cellular automata, are paradigmatic systems for the study of nonequilibrium phenomena. Such systems evolve in discrete time steps, and are thus free of time-discretization errors. Moreover, they display nonequilibrium phase transitions which can be studied by simulating the evolution of an initial seed. At any finite time, this has support only on a finite light cone. Thus, essentially numerically exact simulations free of finite-size errors or boundary effects are possible, leading to high-accuracy estimates of critical exponents. Here, we show how similar advantages can be gained in the quantum regime: The many-body critical dynamics occurring in $(1+1)\mathrm{D}$ quantum cellular automata with an absorbing state can be studied directly on an infinite lattice when starting from seed initial conditions. This can be achieved efficiently by simulating the dynamics of an associated one-dimensional, nonunitary quantum cellular automaton using tensor networks. We apply our method to a model introduced recently and find accurate values for universal exponents, suggesting that this approach can be a powerful tool for precisely studying nonequilibrium universal physics in quantum systems.

We study the repulsive Fermi polaron in a two-component, two-dimensional system of fermionic atoms inspired by the results of a recent experiment with ${}^{173}\mathrm{Yb}$ atoms [N. Darkwah Oppong et al., Phys. Rev. Lett. 122, 193604 (2019)]. We use the diffusion Monte Carlo method to report properties such as the polaron energy and the quasiparticle residue that have been measured in that experiment. To provide insight into the quasiparticle character of the problem, we also report results for the effective mass. We show that the effective range, together with the scattering length, is needed in order to reproduce the experimental results. Using different model potentials for the interaction between the Fermi sea and the impurity, we show that it is possible to establish a regime of universality, in terms of these two parameters, that includes the whole experimental regime. This illustrates the relevance of quantum fluctuations and beyond mean-field effects to correctly describe the Fermi polaron problem.

Numerical simulations are frequently required for quantum scattering problems and often face difficulties with finite grids and unwanted, unphysical reflections. For decades, improved complex absorbing potentials (CAPs) have been sought. Today, the rise of ultracold physics makes a solution essential as CAP errors increase at lower energies. We present a method that provides a physical, semiclassical picture of how to improve CAPs based on the behavior of classical trajectories. The method does not rely on the mathematical formalism often required by existing methods and reduces the error associated with CAP-based calculation of the low-energy scattering wavefunctions by up to several orders of magnitude relative to the standard Woods-Saxon approach, as demonstrated via the distorted-wave Born approximation. This indicates the method may be applied to the numerical simulation of collisions in the ultracold regime.

Large clouds of cold atoms prepared in a magneto-optical trap are known to present spatiotemporal instabilities when the frequency of the trapping lasers is brought close to the atomic resonance. This system bears similarities with trapped plasmas where the role of the Coulomb interaction is played by the exchange of scattered photons, and astrophysical objects, such as stars whose size is dependent on radiative forces. We present in this paper a study of the phase space of such instabilities and reveal different dynamical regimes. Three-dimensional simulations of the highly nonlinear atomic dynamics permit a detailed analysis of the experimental observations.