High-harmonic generation (HHG) and quantum optics are historically two distant fields of science. HHG as a central technique to generate attosecond pulses via atoms, molecules, or solids has been mostly treated as a classical source of light. Entanglement and squeezing of HHG light would open fascinating perspectives in quantum technology, with the emergence of a field of basic and applied research: attosecond quantum optics. Here, we measure entanglement in a tripartite solid-state HHG system through the violation of a Cauchy-Schwarz inequality. Displaced squeezing is experimentally observed and supported by numerical simulations with excellent agreement to the data. The Schmidt number is estimated, which indicates a low-dimensional multimode structure for each harmonic, an important property for most quantum technologies. At the applied level, the semiconductor HHG source is scalable, integrable, operates at room temperature with compact lasers, and thus represents a useful and exploitable resource in quantum photonics.

Leveraging quantum information geometry, we derive generalized quantum speed limits on the rate of change of the expectation values of observables. These bounds subsume and, for Hilbert space dimension $\ge 3$, tighten existing bounds—in some cases by an arbitrarily large multiplicative constant. Our theoretical results are supported by illustrative examples and an experimental demonstration using a superconducting qutrit. We also derive two upper bounds on the generalized quantum Fisher information in terms of the condition number of the density matrix. One of these bounds applies only to coherent dynamics and depends also on the variance of the Hamiltonian. The other bound depends also on the so-called Wigner-Yanase skew information. These bounds generalize well-known bounds on the symmetric logarithmic derivative quantum Fisher information and are tighter than the existing bounds for sufficiently mixed states (e.g., for sufficiently high temperature thermal states).

We propose a method for converting single microwave photons to single optical sideband photons based on spinful impurities in magnetic materials. This hybrid system is advantageous over previous proposals because (i) the implementation allows much higher transduction rates (${10}^3$ times faster at the same optical pump Rabi frequency) than state-of-the-art devices, (ii) high-efficiency transduction is found to happen in a significantly larger space of device parameters (in particular, over $1\text{GHz}$ microwave detuning), and (iii) it does not require mode volume matching between optical and microwave resonators. We identify the needed magnetic interactions as well as potential materials systems to enable this speed up using erbium dopants for telecom compatibility. This is an important step towards realizing high-fidelity entangling operations between remote qubits and will provide additional control of the transduction through perturbation of the magnet.

We investigate the different degrees of freedom underlying far-from-equilibrium scaling behavior in a relativistic, single-component $\mathrm{O}\left(1\right)$ scalar field theory in two and three spatial dimensions. In such a strongly correlated many-body system, identifying the respective roles of nonlinear wave excitations and defect dynamics is a prerequisite for understanding the universal character of time evolution far from equilibrium and thus the different possible universality classes of nonthermal fixed points. Using unequal-time two-point correlation functions, we extract information about the dominant infrared excitations and study their connections to the turbulent dynamics of topological defects created in the system. In three dimensions, the primary excitations are identified as kelvon quasiparticles, which are quantized Kelvin waves propagating along vortex lines, while in two dimensions, vortices are point defects, and the infrared dynamics is dominated by bound-state-like excitations similar to Kelvin waves. In both cases, the kelvon excitations are found to be characterized by distinct time-evolving dispersion relations, subject to the coarsening dynamics close to the respective nonthermal fixed point and, thus, to the decay of superfluid turbulence in the system. Our results underline the role of topological defects and their influence on the universal dynamics of strongly correlated systems near nonthermal fixed points, complementing the analysis of large-$N$ models in $\mathrm{O}\left(N\right)$ systems.

When two point sources are separated by less than the Abbe-Rayleigh limit, conventional resolution methods fail to distinguish whether there is one source or two. Consequently, it becomes nearly impossible to resolve important relative properties of the two sources, such as mutual coherence, relative phase, and brightness contrast. In this regime, the incoming composite quantum signal must be treated as a single entity described by an overall quantum wave function. In this study, we explore the superresolution of such two-point sources by examining a special wave property, i.e., entanglement between spatial degree of freedom (DoF) and remaining DoFs (both internal and external). Remarkably, we find that increasing this inter-DoF entanglement enhances the precision of superresolution, as quantified by the quantum Fisher information (QFI) in single-parameter estimation. Moreover, we show that this entanglement reliably ensures a nonvanishing QFI in two-parameter estimation, highlighting its role as a valuable resource for quantum superresolution.

We explicitly demonstrate the universality of critical dynamics through unprecedented large-scale Graphics Processing Units (GPU)-based simulations of two out-of-equilibrium processes, comparing the behavior of spin-$1/2$ Ising and spin-1 Blume-Capel models on a square lattice. In the first protocol, a completely disordered system is instantaneously brought into contact with a thermal bath at the critical temperature, allowing it to evolve until the coherence length exceeds ${10}^3$ lattice spacings. Finite-size effects are negligible due to the mesoscopic scale of the lattice sizes studied, with linear dimensions up to $L=2^{22}$ and $2^{19}$ for the Ising and Blume-Capel models, respectively. Our numerical data, and the subsequent analysis, demonstrate a strong dynamic universality between the two models and provide the most precise estimate to date of the dynamic critical exponent for this universality class, $z=2.1676\left(1\right)$. In the second protocol, we corroborate the role of the universal ratio of dynamic and static length scales in achieving an exponential acceleration in the approach to equilibrium just above the critical temperature, through a time-dependent variation of the thermal bath temperature. The results presented in this work leverage our Compute Unified Device Architecture (CUDA)-based numerical code, breaking the world record for the simulation speed of the Ising model.

We present a Bayesian algorithm to identify explainable processes generating open quantum system dynamics, described by a Lindblad master equation, that are compatible with measured experimental data. The algorithm, based on a Markov chain Monte Carlo approach, assumes the energy levels of the system are known and outputs a ranked list of interpretable master-equation models that produce predicted measurement traces that closely match experimental data. We benchmark our algorithm on quantum optics experiments performed on single and pairs of quantum emitters. The latter case opens the possibility of cooperative emission effects and additional complexity due to the possible interplay between photon and phonon influences on the dynamics. Our algorithm retrieves various minimal models that are consistent with the experimental data, and which can provide a closer fit to measured data than previously suggested and physically expected approximate models. Our results represent an important step towards automated systems characterization with an approach that is capable of working with diverse and tomographically incomplete input data. This may help with the development of theoretical models for unknown quantum systems as well as providing scientists with alternative interpretations of the data that they might not have originally envisioned and enabling them to challenge their original hypotheses.

Quantum droplets are formed in quantum many-body systems when the competition of quantum corrections with the mean-field interaction yields a stable self-bound quantum liquid. We predict the emergence of a quantum droplet when a Bose-Einstein condensate is placed in a transverse-pumped optical cavity. The strong coupling between the atoms and a cavity mode induces long-range interactions in the atoms, and a roton mode for negative cavity detuning emerges. Using Bogoliubov theory, we show that the roton mode competes with the repulsive atomic $s$-wave scattering. Due to the favorable scaling of the quantum fluctuations with respect to the volume, a self-bound stable quantum liquid emerges.

During gravitational collapse of dust in spherical symmetry, matter particles may collide forming shell crossing singularities (SCSs) at which the Einstein equations become indeterminate. We show that in the case of marginally bound dust collapse, there is a unique evolution beyond SCSs such that a propagating shock wave forms, the metric remains continuous, and the stress-energy tensor dynamically becomes that of a thin shell. We give numerical simulations that exhibit this result.

Non-Hermitian skin effect (NHSE), which is a localization phenomenon in non-Hermitian systems, has recently been of great interest. Most previous works in various systems studied periodic systems. However, electromagnetic waves do not feel the periodicity of a system if the wavelength is sufficiently long, and it has not been clear that electromagnetic waves exhibit the localization caused by the non-Hermiticity itself even in uniform systems without the periodicity. Here, we establish the theory of the optical NHSE in non-Hermitian uniform media and show that the optical NHSE occurs even in uniform media. We apply our theory to subwavelength metamaterials and demonstrate the existence of a non-Hermitian skin mode in a multilayer metamaterial. Finally, we propose a concept of a stationarily excited skin mode to describe a lossy system under stationary excitation with real excitation frequency.

Generative diffusion models are described by time-forward and -backward stochastic differential equations to connect the data and prior distributions. While conventional diffusion models (e.g., score-based models) only learn the backward process, more flexible frameworks have been proposed to also learn the forward process by employing the Schrödinger bridge (SB). However, due to the complexity of the mathematical structure behind SB-type models, we cannot easily give an intuitive understanding of their objective function. In this work, we propose a unified framework to construct diffusion models by reinterpreting the SB-type models as an extension of variational autoencoders. In this context, the data processing inequality plays a crucial role. As a result, we find that the objective function consists of the prior loss and drift matching parts, which enable us to reduce the numerical cost of training the forward process. Furthermore, we discuss the overfitting problem in the SB-type models in this framework.

We propose to combine neutral atom and trapped ion qubits in one scalable modular architecture that uses shuttling of individual neutral atoms in optical tweezers to realize atomic interconnects between trapped ion quantum registers. These interconnects are deterministic, and thus may be performed on-demand. The proposed protocol is as follows: A tweezer-trapped neutral atom qubit is brought close to a trapped ion in an ion chain serving as a module of a larger quantum computer, and an entangling gate is performed between the two qubits. Then, the neutral atom is quickly moved to another, nearby trapped ion chain in the same modular ion trap and entangled with an ion in that chain, thus entangling the two separate ion chains. The optical dipole potential of the tweezer beam for the neutral atom does not measurably affect the trapped ions, while the rf ion trap does not affect the neutral atom. With realistic tweezer trap parameters, the neutral atom can be moved over millimeter-scale distance in a few tens of microseconds, thus enabling a remote entanglement generation rate of over ${10}^3\mathrm{s}{}^{-1}$ even with very modest assumptions for the atom-ion quantum gate speed, and possibly up to ${10}^4\mathrm{s}{}^{-1}$, which is two orders of magnitude higher than the current state-of-the-art with photonic interconnects.

Decades of scientific inquiry have sought to understand how evolution fosters cooperation, a concept seemingly at odds with the belief that evolution should produce rational, self-interested individuals. Most previous work has focused on the evolution of cooperation among boundedly rational individuals whose decisions are governed by behavioral rules that do not need to be rational. Here, using an evolutionary model, we study how altruism can evolve in a community of rational agents and promote cooperation. We show that in both well-mixed and structured populations, a population of objectively rational agents is readily invaded by mutant individuals who make rational decisions but evolve a distorted (i.e., subjective) perception of their payoffs. This promotes behavioral diversity and gives rise to the evolution of rational, other-regarding agents who naturally solve all the known strategic problems of two-person two-strategy games by perceiving their games as pure coordination games.

Using a shared microwave resonator, we propose a transduction scheme between superconducting qubits and qubit states encoded in the low-lying internal levels of trapped atomic systems. The approach employs atomic Rydberg levels together with laser pulses that connect them to the low-lying qubit states. We explore two coupling protocols: one based on resonant interactions between the subsystems and another operating in the dispersive regime, where the resonator is far detuned from the transition frequencies of the matter qubits. These protocols enable the transfer of a general qubit state from the superconducting qubit to the atomic qubit. Transfer fidelity is evaluated across different coupling regimes and under various sources of dissipation and decoherence, including resonator decay, Rydberg state decay, and both relaxation and pure dephasing of the superconducting qubit, providing a detailed assessment of the performance of the proposed scheme.

The switching mechanisms in artificial spin ice systems are investigated with focus on shakti and vertex-modified shakti lattices. Minimum energy paths are calculated using the geodesic nudged elastic band method implemented with a micromagnetic description of the system, including the internal magnetic structure of the islands and edge deviations. Two switching mechanisms, uniform magnetization rotation and domain wall formation, are found to have comparable activation energies. The preference for one over the other depends strongly on the saturation magnetization and the magnetic ordering of neighboring islands. Surprisingly, these mechanisms can coexist, leading to an enhanced probability of magnetization reversal. These results provide valuable insight that can help control internal magnetization switching processes in spin ice systems and help predict their thermodynamic properties.

We show that magnons in two-dimensional altermagnets can spontaneously decay at zero temperature. The decay rate is determined by quantum fluctuations and scattering processes involving the decay of a single magnon into three. These processes are kinematically allowed due to the convexity of the altermagnetic magnon dispersion. For small wave vectors $k$, the decay rate is proportional to $k^5$ with a direction-dependent prefactor, which is maximal along the diagonals of the Brillouin zone. Moreover, for a given momentum only magnons with one specific chirality can spontaneously decay.