Stimulated by the recent progress in engineering topological band structures in cold atomic gases, we study the dynamic topological phenomena for atoms loaded in a periodically driven optical lattice. When the frequency of the periodic modulation is low, the time-dependent Hamiltonian can be mapped to a two-dimensional topological insulator, with the discretized frequency components playing the role of an additional, synthetic dimension. In the high-frequency limit, we derive the effective Floquet Hamiltonian of the system, and reveal the occurrence of Floquet dynamic quantum phase transitions—an emergent topological phenomenon in the micromotion of the Floquet dynamics. Addressing the relation between the topology of the effective Floquet Hamiltonian and the presence of dynamic topological phenomena, we demonstrate that the topologically nontrivial nature of the Floquet Hamiltonian is a sufficient but not necessary condition for the onset of the Floquet dynamic quantum phase transition. We further discuss the relation of the topology of the Floquet Hamiltonian with the existence of dynamic skyrmion structures in the emergent momentum-time manifold of the micromotion, as well as the fate of these dynamic topological phenomena when the modulation frequency decreases away from the high-frequency limit. Finally, making use of the rich level structures of ${}^{171}\mathrm{Yb}$ atoms, we show that the system under study can be implemented in a one-dimensional Raman lattice where states in the ${}^1{S}_0$ ground-state manifold are coupled by Raman beams with periodically modulated amplitudes.

The relativistic plane-wave Born approximation is applied to the study of inelastic collisions of charged particles with atoms, by considering atomic wave functions calculated from the independent-electron approximation with the self-consistent Dirac-Hartree-Fock-Slater potential. A database of longitudinal and transverse generalized oscillator strengths (GOSs) has been computed by using accurate numerical methods for all the subshells of the ground-state configurations of the elements with atomic numbers from 1 (hydrogen) to 99 (einsteinium). The calculated GOS do not satisfy the Bethe sum rule; departures from the sum rule are in accordance with previous theoretical estimates. Asymptotic high-energy formulas for the total cross section, the stopping cross section, and the energy-straggling cross section are derived with proper account of the relativistic departure from the Bethe sum rule. The shell correction is calculated as the energy-dependent term that, when added to the asymptotic formula, reproduces the value of the atomic cross section calculated by integrating the energy-loss differential cross section. Shell corrections to the stopping cross section obtained from the present approach are presented and compared with previous estimates.

The coherent excitation and manipulation of a two-level system with ultrashort intense extreme-ultraviolet laser fields is investigated theoretically, based on numerically solving the time-dependent Schrödinger equation. We are particularly interested in the dynamical phase excursion of the energy states over the course of the interaction and the resulting spectral modifications. Fitting the absorption line shapes with the Fano profile quantifies the asymmetry parameter and the corresponding dipole phase offset, capturing the phase difference of the state coefficients after the interaction. Nonperturbative analytical calculations using rectangular driving pulses are employed, yielding physical insights into the dependence of the dipole phase shift on the external field. The validity of the formulas is validated by comparing their predictions with numerical results, which proves to be robust against the variation of laser parameters. The present investigation of strong-field dressing effects complements recent attosecond transient absorption studies assuming weak excitation, and marks a ubiquitous phenomenon that should be generally considered for the interaction of matter with intense laser fields.

We proposed and analyzed a scheme to generate large-size Schrödinger cat states based on linear operations of Fock states, squeezed vacuum states, and conditional measurements. By conducting conditional measurements via photon number detectors, two unbalanced small-amplitude Schrödinger kitten states combined by a beam splitter can be amplified to a large-size cat state with the same parity. According to simulation results, two Schrödinger odd kitten states with amplitudes of $\left|\beta \right|=1.06$ and $\left|\beta \right|=1.11$ generated from one-photon-subtracted 3 dB squeezed vacuum states, are amplified to an odd cat state of $\left|\beta \right|=1.73$ with a fidelity of $F=99\%$. A large-size Schrödinger odd cat state with $\left|\beta \right|=2.51$ and $F=97.30\%$ is predicted when 5.91 dB squeezed vacuum states are employed. According to the analysis on the impacts of experimental imperfections in practice, Schrödinger odd cat states of $\left|\beta \right|>2$ are available. A feasible configuration based on a quantum frequency comb is developed to realize the large-size cat state generation scheme we proposed.

Quantum walks function as essential means to implement quantum simulators, allowing one to study complex and often directly inaccessible quantum processes in controllable systems. In this contribution, the notion of a driven Gaussian quantum walk is introduced. In contrast to typically considered quantum walks in optical settings, we describe the operation of the walk in terms of a nonlinear map rather than a unitary operation, e.g., by replacing a beam-splitter-type coin with a two-mode squeezer, being a process that is controlled and driven by a pump field. This opens previously unattainable possibilities for quantum walks that include nonlinear elements as core components of their operation, vastly extending their range of applications. A full framework for driven Gaussian quantum walks is developed, including methods to dynamically characterize nonlinear, quantum, and quantum-nonlinear effects. Moreover, driven Gaussian quantum walks are compared with their classically interfering and linear counterparts, which are based on classical coherence of light rather than quantum superpositions. In particular, the generation and boost of highly multimode entanglement, squeezing, and other quantum effects are studied over the duration of the nonlinear walk. Importantly, we prove the quantumness of the evolution itself, regardless of the input state. A scheme for an experimental realization is proposed. Furthermore, nonlinear properties of driven Gaussian quantum walks are explored, such as amplification that leads to an ever increasing number of correlated quantum particles, constituting a source of new walkers during the walk. Therefore, a concept for quantum walks is proposed that leads to—and even produces—directly accessible quantum phenomena, and that renders the quantum simulation of nonlinear processes possible.

A detailed comparison of experimental and theoretical elastic cross sections for low-energy electron scattering by ethyne (${\mathrm{C}}_2{\mathrm{H}}_2$), taken earlier in our group by Gauf et al. [Phys. Rev. A 87, 012710 (2013)], and some of its methylated derivatives, propyne (${\mathrm{C}}_3{\mathrm{H}}_4$), and the isomers 1-butyne and 2-butyne ($n-{\mathrm{C}}_4{\mathrm{H}}_6, n=1,2$), taken here, are presented. The present differential cross sections were measured at incident electron energies ranging from 1 eV to 30 eV and for scattering angles from $5^{\circ }$ to ${130}^{\circ }$ using the relative flow method with an aperture gas source. Our earlier work was taken over a larger energy range of up to $100\mathrm{eV}$. The theoretical calculations were carried out for impact energies up to $30\mathrm{eV}$, employing the Schwinger multichannel method with pseudopotentials in the static-exchange plus polarization approximation. In addition to the differential cross sections, we present the integral and momentum transfer cross sections with which we discuss the shape resonances present in these systems and other physical phenomena, such as the presence of Ramsauer-Townsend minimum. We also compare our theoretical and experimental results with previous data that are available in the literature.

There are major discrepancies between recent [J. D. Savee et al., J. Phys. Chem. A 125, 3874 (2021)] and the two early [B. L. FitzPatrick et al., J. Chem. Phys. 133, 094306 (2010); V. A. Shubert and S. T. Pratt, J. Phys. Chem. A 114, 11238 (2010)] measurements regarding an absolute cross section for the photoionization of formyl radicals. To estimate the accuracy of the various results, we carried out calculations with the $R$-matrix method employing the configuration-interaction method to describe electronic correlation. The quality of the description of the continuum is assessed by using the Gaussian, $B$-spline, and their mixed bases. The convergence of the results is checked by changing the partial waves and active spaces. Accurate cross sections and asymmetry parameters for the valence orbitals are reported for photon energies from the ionization thresholds to 90 eV. Extensive resonance structures are observed near the ionization threshold.

The magnetic effect on the bound state induced by the laser pulse is investigated in strong-field ionization. We find that the interaction of magnetic field of the laser pulse with atoms causes a time-dependent energy shift of the bound state, and the energy shift depends on the magnetic quantum numbers of the electrons, which is the so-called Zeeman effect. This time-dependent energy shift adds an extra phase to the tunneling electron wave packets. By solving the three-dimensional time-dependent Schrödinger equation, we demonstrate that this extra phase results in a shift of the interference structure originating from the interference of short and long trajectories of the backward-rescattering photoelectrons. The shift of the interference structure is also confirmed by simulations with the three-step model beyond the dipole approximation.

Experiments with ultracold atoms involve interatomic interactions, which are essential for cooling the atoms. The $s$-wave interaction between atoms can be tuned via the Feshbach resonance, potentially enabling the creation of a noninteracting system when the interaction reaches its vanishing limit. Although feasible at zero temperature, eliminating the interaction at a finite temperature in an isolated system prevents the system from reaching equilibrium. In this study, we used a Bose-Fermi mixture to create equilibrated noninteracting Bose gas at a finite temperature. First, we used the Bose-Fermi superfluid mixture of a dilute lithium gas in an optical dipole trap to determine the zero crossing of the boson-boson interaction at near-zero temperature. Thereafter, we showed that the noninteracting Bose gas created at a finite temperature represents an ideal Bose gas under the canonical description. The results of this study provides an avenue for experimental investigations of the fundamental properties of an ideal Bose gas in a harmonic trap.

In recent years, the idea of reachable-set characterization based on the quantum speed limit (QSL) was proposed by Arenz et al. [New J. Phys. 19, 103015 (2017)]; that is, the reachable set of the target unitary gate in a closed qubit system can be characterized by considering the QSL as the necessary condition that the control setup must satisfy in order to achieve the goal. Inspired by this idea, in this paper we characterize a general Markovian open quantum system based on the QSL derived by Kobayashi and Yamamoto [Phys. Rev. A 102, 042606 (2020)]. Note that this bound is not only explicitly computable with respect to system parameters but also tighter than the other bounds. Some examples for demonstrating this analysis will be given.

Simulating quantum phenomena in extreme spacetimes in the laboratory represents a powerful approach to explore fundamental physics in the interplay of quantum mechanics, quantum field theory, and general relativity. Here we propose to simulate the movement of a Dirac particle propagating with a superluminal velocity caused by the emergent Alcubierre warp drive spacetime using trapped ions. We demonstrate that the platform allows for observing the tilted light cone that manifests as a superluminal velocity, which is in agreement with the prediction of general relativity. Furthermore, the Zitterbewegung effect arising from relativistic quantum mechanics persists with the superluminal propagation and is experimentally measurable. The present scheme can be extended to simulate the Dirac equation in other exotic curved spacetimes, thus providing a versatile tool to gain insights into the fundamental limit of these extreme spacetimes.

One-way quantum computing is a promising candidate for fault-tolerant quantum computing. Here, we propose protocols to realize a deterministic one-way controlled-not (cnot) gate and one-way $X$ rotations on current quantum-computing platforms. By applying a delayed-choice scheme, we overcome a limit of most currently available quantum computers, which are unable to implement further operations on measured qubits or operations conditioned on measurement results from other qubits. Moreover, we decrease the error rate of the one-way logic gates, compared to the original protocol using local operations and classical communication. In addition, we apply our deterministic one-way cnot gate in the Deutsch-Jozsa algorithm to show the feasibility of our proposal. We demonstrate all these one-way gates and algorithms by running experiments on the cloud quantum-computing platform IBM Quantum Experience.

While the recently realized dissipative time crystal in a laser-pumped atom-cavity system in the experiment of Keßler et al. [Phys. Rev. Lett. 127, 043602 (2021)] is qualitatively consistent with a theoretical description in an idealized limit, here, we investigate the stability of this dissipative time crystal in the presence of an inhomogeneous potential provided by a harmonic trap, and competing short- and infinite-range interactions. We note that these features are ubiquitous in any realization of atom-cavity systems. By mapping out the dynamical phase diagram and studying how it is modified by the harmonic trap and short-range interactions, we demonstrate the persistence of long-lived dissipative time crystals beyond the idealized limit. We show the emergence of metastable dissipative time crystals with and without prethermalization plateaus for tight harmonic confinement and strong contact interaction, respectively.

A physical mechanism of stimulated light scattering on nanoscale objects in a water suspension is proposed. The proposed mechanism is based on the dipole interaction between the light wave and the inevitable uncompensated electrical charge on a nanoscale object (e.g., a virus or nanoparticle) in a water environment. Experimental data on the tobacco mosaic virus are presented to support the proposed physical mechanism. It is demonstrated that stimulated amplification spectral line frequencies observed experimentally are well explained by the proposed mechanism. In particular, the absence of lower-frequency lines and the shifting of generation lines when the pH changes are due to ion friction in the ionic solution environment. The selection rules observed experimentally also confirm the dipole interaction type. It is shown that microwave radiation on the nanoscale object acoustic vibrations frequency should appear under such scattering conditions. We demonstrate that such conditions also allow for local selective heating of nanoscale objects from dozens to hundreds of degrees Celsius. This effect is controlled by the optical irradiation parameters, and it can be used to selectively affect a specific type of virus.

Single photons and single-photon pairs, inherently nonclassical in their nature, are fundamental elements of quantum sciences and technologies. Here, we propose to realize the nonclassical correlated deterministic photon pairs at the single-photon level for a single atom trapped in bimodal cavities. It is shown that the photon emissions for bimodal cavities exhibit a high single-photon purity and relatively large intracavity photon numbers, which are ascribed to the combination of optical Stark-shift enhanced energy-spectrum anharmonicity and quantum interference suppressed two-photon excitation. Our scheme generates photon-photon pairs with strong photon blockade for cavity modes via constructing a highly tunable cavity-enhanced deterministic parametric down-conversion process. Furthermore, we show that the cross-correlation function between the two cavity modes vastly violates the Cauchy-Schwarz inequality of the classical boundary, which unambiguously demonstrates the nonclassicality of the single-photon pairs. Our result reveals a prominent strategy to generate the high-quality two-mode single-photon sources with strong nonclassical correlation, which could provide versatile applications in distribution of quantum networks, long-distance quantum communication, and fundamental tests of quantum physics.

We experimentally demonstrate an anti-parity-time (anti-$PT$)-symmetric photonic lattice in thermal cesium atoms driven by two standing-wave laser fields. Such a lattice possesses anti-$PT$-symmetric susceptibility, of which the real (imaginary) part is an odd (even) function of position. By manipulating the intensities and phases of the two standing-wave fields, we experimentally observe unidirectional reflectionless propagation at the exceptional points. Our work provides a platform to study the scattering property of anti-$PT$-symmetric systems.

Given the evolution of an arbitrary open quantum system, we formulate a general and unambiguous method to separate the internal energy change of the system into an entropy-related contribution and a part causing no entropy change, identified as heat and work, respectively. We also demonstrate that heat and work admit geometric and dynamical descriptions by developing a universal dynamical equation for the given trajectory of the system. The dissipative and coherent parts of this equation contribute exclusively to heat and work, where the specific role of a work contribution from a counterdiabatic drive is underlined. Next we define an expression for the irreversible entropy production of the system which does not have explicit dependence on the properties of the ambient environment; rather, it depends on a set of the system's observables excluding its Hamiltonian and is independent of internal energy change. We illustrate our results with three examples.

We explore a general feature of the interaction mediated by the gravitational fields of spatially superposed masses. For this purpose, based on quantum information theory, we characterize the evolution of two particles each in a superposition state of paths. The evolution is assumed to be given by a completely positive trace-preserving (CPTP) map. We further assume that the probability of a particle being on each path is unchanged during the evolution. This property is called population preserving. We examine when a population-preserving CPTP map can create entanglement in terms of separable operations, which form a large class of local operations and classical communication (LOCC). In general, entanglement is not always generated by inseparable or non-LOCC operations, and one can consider a model of gravity described by an inseparable operation which does not create entanglement. However, we find that a population-preserving CPTP map is inseparable if and only if it can create entanglement. This means that the above model of gravity is incompatible with the possible evolution of spatially superposed masses.