We investigate both numerically and analytically the dynamics of out-of-time-order correlators (OTOCs) in a non-Hermitian kicked rotor model, addressing the scaling laws of the time dependence of OTOCs at the transition to the spontaneous $\mathcal{PT}$-symmetry breaking. In the unbroken phase of $\mathcal{PT}$ symmetry, the OTOCs increase monotonically and eventually saturate with time, demonstrating the freezing of information scrambling. Just beyond the phase transition points, the OTOCs increase in the power laws of time, with the exponent being larger than 2. Interestingly, the quadratic growth of OTOCs with time emerges when the system is far beyond the phase transition points. The above numerical findings are validated by our theoretical analysis, which provides a general framework with important implications for Floquet engineering and information scrambling in chaotic systems.

Quantum state sharing (QSS) is a protocol by which a (secret) quantum state may be securely split into shares, shared between multiple potentially-dishonest players, and reconstructed. Crucially, the players are each assumed to be dishonest, and so QSS requires that only a collaborating authorized subset of players can access the original secret state; any dishonest unauthorized conspiracy cannot reconstruct it. We analyze a QSS protocol involving three untrusted players and demonstrate that quantum steering is the required resource enabling the protocol to proceed securely. We analyze the level of steering required to share any single-mode Gaussian secret which enables the states to be shared with the optimal use of resources.

Quantum annealing is a contender to solve combinatorial optimization problems based on quantum dynamics. While significant efforts have been undertaken to investigate the quality of the solutions and the required run times, much less attention has been paid to understanding the dynamics of quantum annealing and the process leading to the solution during the sweep itself. In this comprehensive study, we investigate various aspects of the quantum annealing dynamics using different approaches. We perform quantum annealing, simulated quantum annealing, and classical annealing on several hundred instances of the Sherrington-Kirkpatrick model with intermediate system sizes up to 22 spins using numerical simulations. We observe qualitative differences between the quantum and classical methods, in particular at intermediate times, where a peak in the fidelity, also known as diabatic bump, appears for hard instances. Furthermore, we investigate the two-point correlation functions, which feature differences at intermediate times as well. At short times, however, the methods are similar again, which can be explained by relating the short-time expansion of quantum annealing to a high-temperature expansion, thus allowing one in principle to find the classical solution already at short times, albeit at prohibitive sampling cost.

We study the effects of the electromagnetic vacuum on the motion of a nonrelativistic electron. First we derive the equation of motion for the expectation value of the electron's position operator. We show how this equation has the same form as the classical Abraham-Lorentz equation but, at the same time, is free of the well-known runaway solution. Second, we study decoherence induced by vacuum fluctuations. We show that decoherence due to vacuum fluctuations that appears at the level of the reduced density matrix of the electron, obtained after tracing over the radiation field, does not correspond to actual irreversible loss of coherence.

We study the internal state dynamics of optically trapped polyatomic molecules subject to room-temperature blackbody radiation. Using rate equations that account for radiative decay and blackbody excitation between rovibrational levels of the electronic ground state, we model the microscopic behavior of the molecules' thermalization with their environment. As an application of the model, we describe in detail the procedure used to determine the blackbody and radiative lifetimes of low-lying vibrational states in ultracold CaOH molecules, the values of which were reported in previous work [C. Hallas et al., Phys. Rev. Lett. 130, 153202 (2023)]. Ab initio calculations are performed and are found to agree with the measured values. Vibrational-state lifetimes for several other laser-coolable molecules, including SrOH and YbOH, are also calculated.

Dissociative electron attachment is an important tool for investigating negative-ion resonances. We have studied the negative-ion resonances of ${\mathrm{H}}_2$ at 10 and 14 eV using the improved velocity slice imaging technique. We obtained modulations in the kinetic energy spectrum of ${\mathrm{H}}^{-}$ ions obtained at 12 and 14.5 eV electron energy, consistent with the earlier reported vibrational state contributions from the higher-lying bound resonances. We show that structures obtained at 12 eV are due to predissociation of the $C{}^2{\mathrm{\Sigma }}_g^{+}$ resonance consistent with the current understanding. However, based on our angular distribution measurements, we propose that the structures obtained at 14.5 eV are due to a predissociation of bound resonance of ${}^2{\mathrm{\Sigma }}_g^{+}$ symmetry as against ${\mathrm{\Delta }}_g$ that was proposed earlier. We also report that the bound ${}^2{\mathrm{\Sigma }}_g^{+}$ resonance contributes to the observed inversion symmetry breaking near 14 eV.

Optical Feshbach resonances allow one to control cold atomic scattering, produce ultracold molecules, and study atomic interactions via photoassociation spectroscopy. In the limit of ultracold $s$-wave collisions, the strength of an optical Feshbach resonance can be expressed via an energy-independent parameter called the optical length. Here we give fully analytic approximate expressions for its magnitude applicable to near-threshold bound states of an excited molecular state dominated by a single resonant-dipole or van der Waals interaction. We express these magnitudes in terms of intuitive quantities, such as the laser intensity, the excited-state binding energy, the $s$-wave scattering length, and the Condon point. Additionally, we extend the utility of the optical length to associative stimulated Raman adiabatic passage in three-dimensional optical lattices by showing that the free-bound Rabi frequency induced by a laser coupling a pair of atoms in an optical lattice site can be approximately related to the trap frequency and the optical length.

In this study, we reveal noncontact frictional forces between surfaces in the presence of peristaltic permittivity modulation. Our setup comprises a conducting medium, an air gap, and a dielectric substrate on which we have a space-time-modulated grating that emits electromagnetic radiation. The radiation receives energy and momentum from the grating, which is eventually absorbed by the conducting medium or propagates away from the grating on the dielectric side, resulting in electromagnetic power loss and lateral forces at the surfaces.

The coupling constant between two nearby waveguides is usually predicted using formulas derived from the perturbative theory applied to the electromagnetic Maxwell equations. These formulas, however, fail to provide any reliable estimate when the index contrast between the core of the waveguide and its cladding becomes large. We demonstrate in this paper that a good accuracy can be retrieved if the local field effect is taken into account. Moreover, we show that in case of structured and inhomogeneous cladding, an effective background index must be taken into account so that the local field effect correction remains accurate. This theoretical study is the occasion for physics-oriented discussions regarding the impact of the substrate on the interwaveguide coupling constant.

We study the task of coherence filtration under strictly incoherent operations. The aim of this task is to transform a given state $\rho$ into another one ${\rho }^{\prime }$ whose fidelity with the maximally coherent state is maximal by using stochastic strictly incoherent operations. We find that the maximal fidelity between ${\rho }^{\prime }$ and the maximally coherent state is given by a multiple of the $\mathrm{\Delta }$ robustness of coherence, $R(\rho \parallel \mathrm{\Delta }\rho ):=min\left\{\lambda \right|\rho \le \lambda \mathrm{\Delta }\rho \}$, which provides $R(\rho \parallel \mathrm{\Delta }\rho )$ an operational interpretation. Finally, we provide a coherence measure based on the task of coherence filtration.

The effect of quantum steering arises from the judicious combination of an entangled state with a set of incompatible measurements. Recently, it was shown that this form of quantum correlations can be quantified in terms of a dimension, leading to the notion of genuine high-dimensional steering. While this naturally connects to the dimensionality of entanglement (Schmidt number), we show that this effect also directly connects to a notion of dimension for measurement incompatibility. More generally, we present a general connection between the concepts of steering and measurement incompatibility, when quantified in terms of dimension. From this connection, we propose an alternative twist on the problem of simulating quantum correlations. Specifically, we show how the correlations of certain high-dimensional entangled states can be exactly recovered using only shared randomness and lower-dimensional entanglement. Finally, we derive criteria for testing the dimension of measurement incompatibility and discuss the extension of these ideas to quantum channels.

The electronic energy loss of proton in platinum (Pt) is studied through nonadiabatic electron-ion coupling dynamic simulations within time-dependent density functional formalism. We have clarified the counterintuitive deviation from velocity proportionality due to the existence of kink velocity of the $d$ electron excitation for a slow proton moving in metal Pt, whose valence $d$ band indeed extends up above the Fermi energy. We have also quantitatively investigated the involvement of the host core electron excitation in the ion-target interaction, through monitoring the evolution of the bound electron number on a specific orbital of the host atom during the close encounter with the flying incident ion. It is found that the low-lying $5p$, $5s$, and $4f$ configuration excitations play significant roles in determining the profile of the stopping curve around and above the stopping maximum.

The ionization of a hydrogenlike heavy ion by the impact of a charged projectile under simultaneous irradiation by a short laser pulse is investigated within the nonperturbative approach, based on numerical solutions of the time-dependent Dirac equation. Emphasis is placed on the question of whether the laser- and impact-ionization channels interfere with each other and how this interference affects the ionization probability. To answer this question we perform detailed calculations for the laser-assisted collisions between hydrogenlike ${\mathrm{Pb}}^{81+}$ and $\alpha$ particles. The results of the calculations clearly indicate that for the experimentally relevant set of (collision and laser) parameters, the interference contribution can reach 10% and can be easily controlled by varying the laser frequency.

We investigate dissociative ionization of methanol, $\mathrm{C}{\mathrm{H}}_3\mathrm{OH}\to {\mathrm{CH}}_3{}^{+}+\mathrm{OH}+e^{-}$, in a linearly polarized intense ultraviolet laser field (400 nm, 67 fs, $3.1\times {10}^{13}\mathrm{W}/\mathrm{c}{\mathrm{m}}^2$) by photoelectron-photoion coincidence three-dimensional momentum imaging. The photoelectron angular distributions (PADs) are recorded as a function of the orientation angle between the molecular C-O bond axis and the laser polarization direction. Photoelectrons are emitted dominantly along the laser polarization direction when ${\mathrm{CH}}_3{\mathrm{OH}}^{+}$ is produced in the $\tilde{X}$ state via the four-photon absorption. By contrast, the photoelectron emission becomes prominent along the direction perpendicular to the laser polarization direction when ${\mathrm{CH}}_3{\mathrm{OH}}^{+}$ is produced in the $\tilde{B}$ state via the five-photon absorption. The orientation-angle-resolved PADs are expanded in terms of spherical harmonics in the laboratory frame, and then the anisotropy of the PADs varying as a function of the orientation angle of the C-O bond axis with respect to the laser polarization direction is examined quantitatively. We discuss how the shape of the molecular orbital from which an electron is emitted is reflected in the recorded PADs.

Many-body physics in dimensional crossover systems has attracted much attention, but is yet to explore intensively. Cold atoms provide an excellent experimental platform to address the questions in this area, but technical difficulties still exist. When an optical lattice is applied to tune the dimensionality of trapped cold atoms, it is usually difficult to precisely tune the occupation ratio of the atom in the different lattice bands. In this paper we report the method to tune the occupation ratio of the energy band when transferring a three-dimensional Fermi gas into a one-dimensional optical lattice, where we could tune the occupation ratio in the lowest band from unity to $50\%$ quantitatively by jointly varying the trapping potentials of the optical dipole trap and the one-dimensional lattice. This method provides a route to study the dependence of the many-body interaction on the dimensionality by continuously changing the dimensionality of a Fermi gas in the crossover from two dimensions to three dimensions.

Ultracold fermionic Feshbach molecules are promising candidates for exploring quantum matter with strong $p$-wave interactions; however, their lifetimes were measured to be short. Here we characterize the $p$-wave collisions of ultracold fermionic ${}^{23}\mathrm{Na}{}^{40}\mathrm{K}$ Feshbach molecules for different scattering lengths and temperatures. By increasing the binding energy of the molecules, the two-body loss coefficient reduces by three orders of magnitude, leading to a second-long lifetime 20 times longer than that of ground-state NaK molecules. We exploit the scaling of elastic and inelastic collisions with the scattering length and temperature to identify a regime where the elastic collisions dominate over the inelastic ones, allowing the molecular sample to thermalize. Our results provide a benchmark for four-body calculations of molecular collisions and pave the way for investigating quantum many-body phenomena with fermionic Feshbach molecules.

In a multiband Hubbard model the self-consistency relations for the two-body bound-state bands are in the form of a nonlinear eigenvalue problem. Assuming that the resultant eigenvectors form an orthonormal set, e.g., in the strong-binding regime, here we reformulate their Berry curvatures and the associated Chern numbers. As an illustration we solve the two-body problem in a Haldane-Hubbard model with attractive on-site interactions and analyze its topological phase diagrams from weak to strong couplings, i.e., by keeping track of the gap closings in between the low-lying two-body bands. The resultant Chern numbers are consistent with the lobe structure of the phase diagrams in the strong-coupling regime.

Hong-Ou-Mandel (HOM) interference with two independent photons of different wavelengths is a primary tool for controlled transfer of quantum information across a hybrid quantum network such as the quantum internet, in which the diverse material nodes operate at different resonant frequencies. The key challenge of realizing two-photon interference in the frequency domain is the need for linear frequency conversion between spectrally distinct photons, which, in contrast to frequency mixing in nonlinear optical processes, is free from multiphoton noise arising in strong laser fields. Here, we present a linear HOM scheme based on a lossless three-wave mixing between two photons of different frequencies in the laser-controlled tripod-type atoms, where the parametric interaction between photons is triggered by the induced coherence between two metastable states of the atoms [S. Petrosyan et al., Phys. Rev. A 105, 052606 (2022)]. We give the general derivation of the HOM effect and examine it for two single-pulse photons and in the important case of time-bin encoded single photons, which are well protected from decoherence in optical fibers. We show that two-photon coincidence measurements display the HOM-type “dip and peak” fringes according to the number of time bins in the incident photon state, allowing full recovery of the original quantum information. An essential feature of our scheme is the neutrality of the HOM effect implementation to the polarization indistinguishability of photons, which makes our model flexible for use in hybrid quantum networks.

Multichannel synchronous amplifications are an inevitable key problem in quantum communication process, which can broaden the bandwidth of transmitted signals and establish correlation among different optical channels. Here we study a nonreciprocal system with four concurrent amplification channels using hot cesium atoms, both theoretically and experimentally. For the forward probe field, the double–electromagnetically induced transparency structure is formed and the phase-matching condition of the multiwave mixing process is satisfied, which are both destroyed when the probe field is reversed. In addition, the four-channel nonreciprocal amplifications are formed in the Zeeman sublevels of the system with special selection of light field polarization, which will also dramatically enhance the signal-to-noise ratio by suppressing the spontaneous emission noise of the system. In our experiment, the quadruple nonreciprocal amplifications are achieved with the maximum forward gain reaching 30 dB and the reverse suppression reaching–23 dB. The gain adjustability allows the construction of a gain-loss balanced system, providing a scheme for an atomic system to engineer a parity-time-symmetric (or -antisymmetric) structure.