In this paper, we investigate entanglement of an experimental system of two nitrogen-vacancy-center ensembles which are initially squeezed under the one-axis twisting Hamiltonian. We take into account three scenarios in which initial squeezing and entanglement are mediated by phonons or photons: (a) the phonon-squeezed photon-entangled scenario, (b) the phonon-squeezed phonon-entangled scenario, and (c) the photon-squeezed photon-entangled scenario. For our investigation, we employ the Tavis-Cummings model, which includes dissipative decoherence of the collective spin ensemble, and analyze the system both for a relatively small number of spins and in the limit of a large number of spins using the approach of a quantum master equation. Although evidence in the literature on idealized coupled oscillator systems and coupled quantum kicked tops suggests that initial squeezing can enhance entanglement, we find that, in the realistic system studied in this paper, initial squeezing can improve entanglement overall when the field mode interacts in a particular manner with the two spin ensembles. Our analysis using the Holstein-Primakoff transformation and Wigner characteristic function in the rotating frame of reference shows that the entanglement enhancement is a subtle consequence of the way in which the dissipative decoherence rotates the state of the collective spin ensemble such that enhancement depends on the time-evolved rotated states between the presence and absence of initial squeezing.

We report on our calculations of differential hyperfine anomalies in the nuclear single-particle model for a number of atoms and ions of interest for precision atomic tests of the standard model. It is shown that a comparison with available experimental data allows one to discriminate between different nuclear magnetization models. The nuclear single-particle model leads to significantly better agreement with experiment than the routinely used uniform ball model, and we advocate its use in future studies of the hyperfine structure. These results have implications for the uncertainty analyses of atomic structure theory that forms a critical part of the interpretation of precision atomic measurements, including the atomic parity violation measurement in cesium.

We study nonsequential triple ionization of Ar in strong laser fields using a classical trajectory Monte Carlo simulation. By tracing electrons' trajectories, we reveal the mechanisms of triple ionization, including multiple recollision and sequential ionization. One electron tunnels out and rescatters with its parent ion, either kicking off two electrons together or kicking off one of them and exciting another one, which will be freed by the laser field. Besides the above-dominated channels, the cascade rescattering for triple ionization is explored, in which the first tunneling electron rescatters with the parent ion ${\mathrm{Ar}}^{+}$ and kicks off the second electron and then the second electron gains energy in the laser field and rescatters with ${\mathrm{Ar}}^{2+}$, kicking off the third electron or boosting it to excited states to be freed later. Such a cascade channel connects three electrons step by step, and ${\mathrm{Ar}}^{3+}$ may have larger ultimate momenta compared to the triple ionization mediated by only one rescattering. Our simulation shows it is possible to control the cascade channel and extract it through the ion momentum distribution.

The question of how the low-energy properties of disordered quantum systems may be connected to exotic localization phenomena at high energy is a key open question in the context of quantum glasses and many-body localization. In the preceding Letter [L. Villa, S. J. Thomson, and L. Sanchez-Palencia, preceding Letter, Phys. Rev. A 104, L021301 (2021)] we have shown that key features of the excitation spectrum of a disordered system can be efficiently probed from out-of-equilibrium dynamics following a quantum quench, providing distinctive signatures of the various phases. Here we extend this work by providing a more-in-depth study of the behavior of the quench spectral functions associated with different observables and investigating an extended parameter regime. We provide a detailed introduction to quench spectroscopy for disordered systems and show how spectral properties can be probed using both local operators and two-point correlation functions. We benchmark the technique using the one-dimensional Bose-Hubbard model in the presence of a random external potential, focusing on the low-lying excitations, and demonstrate that quench spectroscopy can distinguish the Mott insulator, superfluid, and Bose glass phases. We then explicitly reconstruct the zero-temperature phase diagram of the disordered Bose-Hubbard at fixed filling using two independent methods, experimentally accessible via both time-of-flight imaging and quantum gas microscopy, respectively, and demonstrate that quench spectroscopy can give valuable insights into the distribution of rare regions within disordered systems.

We propose a generalized Mathieu equation (GME) which describes well the dynamics for two different models in spin-1 Bose-Einstein condensates. The stability chart of this GME differs significantly from that of Mathieu's equation and the unstable dynamics under this GME is called generalized parametric resonance. A typical region of $\epsilon \gtrsim 1$ and $\delta \approx 0.25$ can be used to distinguish these two equations. The GME we propose not only explains the experimental results of Hoang et al. [Nat. Commun. 7, 11233 (2016)] in nematic space with a small driving strength, but predicts the behavior in the regime of large driving strength. In addition, the model in spin space we propose, whose dynamics also obeys this GME, can be well tuned such that it is easily implemented in experiments.

Planar thermal equilibration is studied using direct numerical simulations of ultracold two-dimensional ion crystals in a Penning trap with a rotating wall. The large magnetic field of the trap splits the modes that describe in-plane motion of the ions into two branches: high-frequency cyclotron modes dominated by kinetic energy and low-frequency $\mathbf{E}\times \mathbf{B}$ modes dominated by potential energy associated with thermal position displacements. Using an eigenmode analysis we extract the equilibration rate between these two branches as a function of the ratio of the frequencies that characterize the two branches and observe this equilibration rate to be exponentially suppressed as the ratio increases. Under experimental conditions relevant for current work at NIST, the predicted equilibration time is orders of magnitude longer than any relevant experimental timescales. We also study the coupling rate dependence on the initial energy of the planar modes and the number of ions. In addition, we show how increasing the rotating wall strength improves crystal stability. These details of in-plane mode dynamics help set the stage for developing strategies to efficiently cool the in-plane modes and improve the performance of single plane ion crystals for quantum information processing.

Chaos and synchronization are important and ubiquitous phenomena in nature. Chaos synchronization spontaneously emerges in the collective dynamics of coupled nonlinear oscillators; however, its control remains a challenge at the nanoscale. Here, we present an experimental and theoretical investigation of synchronization on the route to chaos and in the chaotic regime in mechanically coupled electro-optomechanical nanocavities. In this system an integrated electroactuation enables one to drive the mechanical normal modes in the nonlinear regime while the mechanical displacement is transferred to an optical probe which provides a direct readout access to both their amplitude and phase responses. Using a single low-frequency modulation of the force applied to each normal mode, we induce a chaotic dynamics in the coupled nanoresonators. By applying a simultaneous force modulation on each of the normal modes, we generate chaos at two distinct carrier frequencies referred to as bichromatic chaos. The mode amplitudes are synchronized all the way on the route to chaos as well as in the chaotic regime. Meanwhile, their phases experience complex dynamics with imperfect synchronization in the chaotic regime. These results set the ground for applications of mechanical nanoresonators in areas such as metrology, multispectral chaotic encryption, and analog computing.

Recent studies show that hybrid quantum systems based on magnonics provide a new and promising platform for generating macroscopic quantum states involving a large number of spins. Here, we show how to entangle two magnon modes in two massive yttrium-iron-garnet (YIG) spheres using cavity optomagnonics, where magnons couple to high-quality optical whispering gallery modes supported by the YIG sphere. The spheres can be as large as 1 mm in diameter and each sphere contains more than ${10}^{18}$ spins. The proposal is based on the asymmetry of the Stokes and anti-Stokes sidebands generated by the magnon-induced Brillouin light scattering in cavity optomagnonics. This allows one to utilize the Stokes and anti-Stokes scattering process, respectively, for generating and verifying the entanglement. Our work indicates that cavity optomagnonics could be a promising system for preparing macroscopic quantum states.

We study single-photon scattering via a giant $\mathrm{\Lambda }$-type atom, where both atomic transitions are coupled with the modes of a single waveguide at two separated points. The giant-atom structure introduces phase-dependent interference effects to both elastic (frequency-preserving) and inelastic (frequency-converting) scattering processes, which modify the corresponding decay rates (as well as the transition frequencies) such that the giant atom is capable of accessing the various limits of a small one. The condition of the optimal frequency conversion is also identified and shown to be phase dependent. Moreover, we consider the combination of the giant-atom interference and the Sagnac quantum interference by further inserting a Sagnac interferometer at each of the coupling points. It is shown that the two kinds of interference effects are compatible and play independent roles such that efficient frequency conversion with unit efficiency can be achieved in addition to the phase-dependent phenomena induced by the giant-atom structure.

The process of reconstruction of attosecond beating by interference of two-photon transitions (RABBITT) can become resonant with a discrete atomic level either in the intermediate or the final continuous states. Experimental observations of the former [Phys. Rev. Lett. 104, 103003 (2010)] and latter [Nature Commun. 7, 10566 (2016)] resonant processes revealed modification of only those parts of the photoelectron spectrum that overlapped directly with the resonance. In the lithium atom and other members of the alkali-metal family, the valence shell $ns\to np$ transition to the intermediate RABBITT state affects the whole photoelectron spectrum in the final state. The strong additional resonant channel modifies entirely the ionization dynamics and opens direct access to the resonant phase of the two-photon transitions, which is common for various single and multiple electron ionization processes. Elucidation of this phase has wider implications for strongly resonant laser-matter interaction.

Above-threshold ionization (ATI) of atoms in intense laser pulses produces photoelectron momentum distributions that exhibit discrete concentric circles related to the absorption of more photons than required to exceed the ionization threshold. Using numerical solutions of the one-electron time-dependent Schrödinger equation beyond the electric dipole approximation for circularly polarized laser pulses, we show that the geometric centers of ATI rings are counterintuitively shifted against the propagation direction of the light, i.e., in the direction opposite to the on-average transferred photon momentum. From a comparison with a simple model based on energy considerations, we identify a nondipole modification of the ac Stark effect for electron continuum states as the origin of our observation.

The characterization of excitations in disordered quantum systems is a central issue in connection with glass physics and many-body localization. Here we show that quench spectroscopy of a disordered model, as realized from its out-of-equilibrium dynamics following a global quench, allows us to fully characterize the spectral properties of the disordered phases. In the Bose-Hubbard model, a clear signature of gapless excitations in momentum-resolved spectroscopy enables us to accurately locate the Mott insulator to Bose glass transition, while the presence or absence of a well-defined soundlike mode distinguishes the superfluid from the Bose glass phase. Moreover, spatially resolved spectroscopy provides local spectral properties and allows us to extract the typical spacing of gapless regions, giving a second independent way to uniquely identify all three phases. Our findings have far-ranging implications for a variety of experimental platforms and offer a powerful and versatile probe of the low-energy phases of disordered systems.

Parity-time $\left(\mathcal{PT}\right)$ symmetric systems are classical, gain-loss systems whose dynamics are governed by non-Hermitian Hamiltonians with exceptional-point (EP) degeneracies. The eigenvalues of a $\mathcal{PT}$-symmetric Hamiltonian change from real to complex conjugates at a critical value of gain-loss strength that is called the $\mathcal{PT}$ breaking threshold. Here, we obtain the $\mathcal{PT}$ threshold for a one-dimensional, finite Kitaev chain—a prototype for a $p$-wave superconductor—in the presence of a single pair of gain and loss potentials as a function of the superconducting order parameter, on-site potential, and the distance between the gain and loss sites. In addition to a robust, nonlocal threshold, we find a rich phase diagram for the threshold that can be qualitatively understood in terms of the band structure of the Hermitian Kitaev model. In particular, for an even chain with zero on-site potential, we find a re-entrant $\mathcal{PT}$-symmetric phase bounded by second-order EP contours. Our numerical results are supplemented by analytical calculations for small system sizes.

Assigning the variations of internal energy into heat or work contributions is a challenging task due to the fact that these properties are trajectory dependent. A number of proposals have been put forward for open quantum systems following an arbitrary dynamics. We here focus on nonequilibrium thermodynamics of a two-level system and explore in addition to the conventional approach, two definitions motivated by either classical work or heat in which the driving Hamiltonian or the trajectory itself, respectively, are used to set up a reference basis. We first give the thermodynamic properties for an arbitrary dynamics and illustrate the results on the Bloch sphere. Then, we solve the particular example of a periodically driven qubit interacting with a dissipative and decoherence bath. Our results illustrate the trajectory-dependent character of heat and work and how contributions originally assigned to dissipation in the Lindblad equation can become a coherent part assigned to work.

Entanglement is an essential ingredient for building a quantum network that can have many applications. Understanding how entanglement is distributed in a network is a crucial step to move forward. Here we study the conservation and distribution of Gaussian entanglement in a linear network using a new quantifier for bipartite entanglement. We show that the entanglement can be distributed through a beam splitter in the same way as the transmittance and reflectance. The requirements on the entangled states and the type of networks to satisfy this relation are presented explicitly. Our results provide a useful quantification for quantum entanglement and further insights into the structure of entanglement in a network.

We use rubidium atoms with specified mass and internal energy to carry out a joint mass-energy test of the equivalence principle (EP). We improve the four-wave double-diffraction Raman transition method (4WDR) we proposed before to select atoms with a certain mass and angular momentum state, and form a dual-species atom interferometer. By using the extended 4WDR to ${}^{85}\mathrm{Rb}$ and ${}^{87}\mathrm{Rb}$ atoms with different angular momenta, we measure their differential gravitational acceleration, and we determine the value of the Eötvös parameter, $\eta$, which measures the strength of the violation of EP. The Eötvös parameters of the four paired combinations ${}^{85}\mathrm{Rb}|F=2\rangle \text{−}{}^{87}\mathrm{Rb}|F=1\rangle$, ${}^{85}\mathrm{Rb}|F=2\rangle \text{−}{}^{87}\mathrm{Rb}|F=2\rangle$, ${}^{85}\mathrm{Rb}|F=3\rangle \text{−}{}^{87}\mathrm{Rb}|F=1\rangle$, and ${}^{85}\mathrm{Rb}|F=3\rangle \text{−}{}^{87}\mathrm{Rb}|F=2\rangle$ were measured to be ${\eta }_1=(1.5\pm 3.2)\times {10}^{-10}$, ${\eta }_2=(-0.6\pm 3.7)\times {10}^{-10}$, ${\eta }_3=(-2.5\pm 4.1)\times {10}^{-10}$, and ${\eta }_4=(-2.7\pm 3.6)\times {10}^{-10}$, respectively. The violation parameter of mass is constrained to ${\eta }_0=(-0.8\pm 1.4)\times {10}^{-10}$, and that of internal energy to ${\eta }_E=(0.0\pm 0.4)\times {10}^{-10}$ per reduced energy ratio $a$ ($a=h{\nu }_0/m_i^{85}{c}^2$, and ${\nu }_0=1$ GHz). This work opens a door for joint tests of two attributes beyond the traditional pure mass or energy tests of EP with quantum systems.

The ionization probabilities and photoelectron distributions of the hydrogen atom and hydrogenlike ions ${\mathrm{He}}^{+}$ and ${\mathrm{Ne}}^{9+}$ in superstrong electromagnetic fields are obtained by solving the time-dependent Dirac equation with the help of the generalized pseudospectral method in spherical coordinates. A simple transformation of the original radial Dirac Hamiltonian is suggested that removes the spurious states. The ionization probabilities are calculated both within and beyond the dipole approximation for two pulse durations and various peak field strengths scaled with respect to the nuclear charge for each target. Performance of two analytic parameters previously suggested to estimate the nondipole effects is assessed against the numerical data obtained in this study. The photoelectron energy and angular distributions exhibit the above-threshold ionization peaks as well as the low-energy structure. In the superstrong field regime, the low-energy structure builds up with increasing field strength while the above-threshold ionization peaks decrease. The nondipole effects in the photoelectron distributions are also revealed and analyzed.

Three-dimensional (3D) analysis on the growth process of high-order harmonic generation (HHG) is achieved by integrating in situ measurement of a 3D phase-matching profile with interaction length control. Incorporating the phase-matching profile, calculation of high-order harmonic wave generation, and propagation reproduces the measured harmonics growth curve as well as distinctive features of the far-field beam profiles. The method resolves contributions of HHG from different positions of the driving beam affected by different sources of phase mismatch, and thus provides illuminating insight into the HHG process.

The status of the quantum wave function is one of the most debated issues in quantum foundations—whether it corresponds directly to the reality or just represents knowledge or information about some aspect of reality. We propose a $\psi$-ontology theorem that excludes a class of ontological explanations where the quantum wave function is treated as mere information. Our result, unlike the acclaimed Pusey-Barrett-Rudolph's theorem, does not presume the absence of holistic ontological properties for product quantum preparations. At the core of our derivation, we utilize the seminal no-go result by Bell that rules out any local realistic world view for quantum theory. We show that the observed phenomenon of quantum nonlocality cannot be incorporated in a class of $\psi$-epistemic models. Using the well-known Clauser-Horne-Shimony-Holt inequality, we obtain a threshold bound on the degree of epistemicity above which the ontological models are not compatible with quantum statistics.