In this article, we present a dynamical decoupling (DD) technique to coherently control the dynamics of a single local phonon in trapped ions. A $2\pi$ rotation at a motional sideband transition flips the sign of the relevant local-phonon state, resulting in cancellation of the phonon dynamics. In this work, we implement DD using resonant blue-sideband pulses to control the hopping of a single local phonon between two ${{}^{40}\mathrm{Ca}}^{+}$ ions in a linear Paul trap. Our proposed DD technique can be used to engineer coupling between local-phonon modes.

We investigate the harmonic generation of graphene irradiated by linearly polarized lasers with intensities in a wide range from ${10}^8\mathrm{W}/{\mathrm{cm}}^2$ to ${10}^{13}\mathrm{W}/{\mathrm{cm}}^2$. We find a striking knee structure in the laser intensity dependence of the harmonic yield, which consists of a perturbative region, followed by a plateau of saturated harmonic yield, and then a transition to nonperturbative growth. The knee structure is rather universal for the various harmonic orders and has been certified by two-band density-matrix equation calculations, as well as ab initio time-dependent density-functional theory calculations. Taking the third harmonic as an example and based on the two-band model, we reveal the underlying mechanisms: the perturbative region can be analytically depicted by perturbation theory, while the plateau of saturated harmonic yield and the transition to nonperturbative growth are caused by destructive and constructive quantum interference of harmonics generated by the electrons corresponding to the lattice momenta around Dirac points and $M$ points in the Brillouin zone, respectively. In particular, we find that tuning the Fermi energy can effectively alter the knee structure, while the profile of the knee structure is not sensitive to temperature. Our calculations of the third-order harmonic versus tuned Fermi energy are compared with recent experiments, showing good agreement. Our predicted knee structure and its associated properties are observable with the current experimental techniques.

Perturbation theory is applied to one-dimensional scattering systems consisting of a general class of inhomogeneous and isotropic slabs having size $L$ described by the relative permittivity $\varepsilon \left(z\right)=1+\alpha \chi \left(z\right)$, where $\chi \left(z\right)$ is the electric susceptibility and $\alpha$ the perturbation parameter. The transmitted and reflected amplitudes are shown to be written as Born series in powers of $\alpha$ and Padé approximants are used to obtain analytical results with a high and arbitrary degree of accuracy. The approach is very general and can handle oblique incidence. Examples are given for the transmission and reflection amplitudes of plane waves interacting with Hermitian and non-Hermitian structures with known and unknown exact solutions.

We propose a quantum electrodynamics platform where quantum emitters interact with a Hofstadter-ladder waveguide. We demonstrate several intriguing phenomena stemming from the nontrivial dispersion relation and vacuum mode properties led by the effective spin-orbit coupling. First, by assuming the emitter's frequency is resonant with the lower band, we find that the spontaneous emission is chiral, with most photonic field decaying unidirectionally. Both numerical and analytical results indicate that the Hofstadter-ladder waveguide can be engineered as a well-performed chiral quantum bus. Second, the dynamics of emitters of the giant atom form is explored by considering their frequencies below the lower band. Due to quantum interference, we find that both the emitter-waveguide interaction and the amplitudes of bound states are periodically modulated by the giant emitter's size. The periodic length depends on the positions of energy minimum points induced by the spin-orbit coupling. Last, we consider the interaction between two giant emitters mediated by bound states and find that their dipole-dipole interaction vanishes (is enhanced) when maximum destructive (constructive) interference occurs.

We have explored quantum Einstein-Podolsky-Rosen steering in symmetric two-mode Gaussian states using Gaussian and non-Gaussian measurements. For Gaussian measurements, we show that steering between the output modes of a symmetric beam splitter is possible regardless of purity when a threshold input-state quadrature variance compression is achieved. Using the non-Gaussian operators introduced in Ji et al. [Sci. Rep. 6, 29729 (2016)] we show that non-Gaussian measurements can outperform Gaussian measurements for symmetrical states. We also analyze the possibility of asymmetric measurements setups made possible by non-Gaussian measurements and provide examples where such asymmetry is optimal for revealing steering.

The dynamics of a three-state quantum walk with amplitude-dependent phase shifts is investigated. We consider two representative inputs whose linear evolution is known to display either full dispersion of the wave packet or intrinsic localization on the initial position. The nonlinear counterpart presents much more involved dynamics featuring self-trapping, solitonic pulses, radiation, and chaoticlike behavior. We show that nonlinearity leads to a metastable self-trapped wave-packet component that radiates in the long-time regime with a survival probability $\propto t^{-1/2}$. A sudden dynamical transition from such a metastable state to the point when the radiation process is triggered is found for a set of parameters.

Spin measurement is studied as a unitary time evolution of the spin coupled to an environment representing the meter and the apparatus. Modeling the environment as a heat bath comprising only a finite number of boson modes and represented in a basis of coherent states, following the Davydov ansatz, it can be fully included in the quantum time evolution of the total system. We perform numerical simulations of projective measurements of the polarization, with the spins prepared initially in a neutral pure state. The likewise pure initial state of the environment is constructed as a product of coherent states of the boson modes with a random distribution of their centroids around the origin of phase space. Switching the self-energy of the spin and the coupling to the heat bath on and off by a time-dependent modulation, we observe the outcome of the measurement in terms of the long-time behavior of the spin. Interacting with the heat bath, the spins get entangled with it and lose coherence, thus reproducing the collapse of the wave function. The expected quantum randomness in the final state is manifest in our simulations as a tendency of the spin to approach either one of the two eigenstates of the measured spin operator, recovering an almost pure state. The unitary time evolution allows us to reproducibly relate these random final states to the respective initial states of the environment and to monitor the exchange of information between the two subsystems in terms of their purity and mutual entropy.

In designing quantum control, it is generally required to simulate the controlled system evolution with a classical computer. However, computing the time evolution operator can be quite resource consuming since the total Hamiltonian is often hard to diagonalize. In this paper, we mitigate this issue by substituting the time evolution segments with their Trotter decompositions, which reduces the propagator into a combination of single-qubit operations and fixed-time system evolutions. The resulting procedure can provide substantial speed gain with acceptable costs in the propagator error. As a demonstration, we apply the proposed strategy to improve the efficiency of the gradient ascent pulse engineering algorithm for searching optimal control fields. Furthermore, we show that the higher-order Trotter decompositions can provide efficient Ansätze for the variational quantum algorithm, leading to improved performance in solving the ground-state problem. The strategy presented here is also applicable for many other quantum optimization and simulation tasks.

Einstein-Podolsky-Rosen (EPR) steering is a kind of quantum nonlocal correlation characterizing the ability of one party to remotely affect another party's state by local measurements. It is regarded as an essential resource for one-sided device-independent quantum information processing. Although identification of EPR steerable states has been widely studied, it remains a hard issue under arbitrary local measurements. In this paper, we propose a steerability criterion for arbitrary dimensional bipartite systems via correlation matrices of local observables. In particular, we apply the steering criterion to three classes of local measurements which are of special significance: local orthogonal observables, mutually unbiased measurements, and general symmetric informationally complete measurements. We obtain some useful criteria that are operational and experimentally testable. We further illustrate the criteria through several examples, compare them with some steering criteria in the literature, and exhibit their power and advantages in certain cases.

The distinguishability of photons in nonoperational degrees of freedom compromises the unconditional security of quantum key distribution since an eavesdropper can improve attack strategies by exploiting this distinguishability. However, the optimal eavesdropping strategies in the presence of light-source side channels are not known. Here we provide several explicit attack strategies on the Bennett-Brassard 1984 (BB84) protocol with partially distinguishable photons. In particular, we consider the phase-covariant cloning attack, which is optimal in the absence of side channels, and show that there are even better strategies in the presence of side channels. The improved strategies exploit a measurement of the side-channel state followed by an attack on the signal photon. Our results explicitly demonstrate a reduction of the critical error rate and set an upper bound on the practical secret key rate.

Theoretical explorations have revealed that quantum batteries can exploit quantum correlations to achieve faster charging, thus promising exciting applications in future technologies. Using NMR architecture, here we experimentally investigate various aspects of quantum batteries with the help of nuclear spin systems in a star-topology configuration. We first carry out numerical analysis to study how charging a quantum battery depends on the relative purity factors of charger and battery spins. By experimentally characterizing the state of the battery spin undergoing charging, we estimate the battery energy as well as the ergotropy, the maximum amount of work that is unitarily available for extraction. The experimental results thus obtained establish the quantum advantage in charging the quantum battery. We propose using the quantum advantage, gained via quantum correlations among chargers and the battery, as a measure for estimating the size of the correlated cluster. We develop a simple iterative method to realize asymptotic charging that avoids the oscillatory behavior of charging and discharging. Finally, we introduce a load spin and realize a charger-battery-load circuit and experimentally demonstrate battery energy consumption after varying the duration of battery storage, for up to 2 min.

We develop the theory of nuclear ring currents induced by external magnetic fields in torsional molecules. The theory is applied to toluene, whose torsion axis is oriented along the magnetic field. We obtain magnetically induced diatropic and paratropic contributions to the nuclear ring current flowing in the classical and nonclassical directions, respectively. In the electronic and torsional and rotational ground state of toluene, the strengths of the diatropic and paratropic nuclear ring-current susceptibilities are $-19.9$ pA/T and 0.4 fA/T, respectively, yielding a net current strength of $-19.9$ pA/T. The paratropic contribution is very small because the torsional barrier of toluene is very low. The study suggests criteria for observing significant magnetically induced nuclear ring currents in torsional molecules whose axis is oriented along the magnetic field.

The theory of nuclear ring currents of torsional molecules induced by an external magnetic field along the torsion axis was developed in the preceding paper [D. Jia et al., preceding paper, Phys. Rev. A 106, 042801 (2022)]. Here we study the magnetically induced electronic current density (MIC) for toluene in the presence of an external magnetic field that is aligned with the torsion axis of the methyl group. Properties of the MIC are studied in detail at the density-functional theory (DFT) level using our gauge-including magnetically induced current method, the derivation of which is briefly outlined. The strength of the MIC is determined by numerical integration and compared to the estimated strength of the magnetically induced nuclear ring current reported in the preceding paper. Spatial contributions to the diatropic and paratropic MICs are discussed in detail, where the diatropic MIC flows in the classical direction and the paratropic MIC flows in the opposite direction. The MIC in the vicinity of the methyl group is mainly diatropic, whereas the phenyl group ring is dominated by a paratropic MIC of $-14.90\mathrm{nA}{\mathrm{T}}^{-1}$ localized to the carbon atoms. The strength of the MIC near the methyl group is $10.41\mathrm{nA}{\mathrm{T}}^{-1}$, which is of about the same size as the strength of the ring current of benzene when the magnetic field is perpendicular to the molecular ring. The strength of the magnetically induced nuclear ring current of the whole toluene molecule is $19.9\mathrm{pA}{\mathrm{T}}^{-1}$, which is two orders of magnitude smaller than the electronic one of $-1.93\mathrm{nA}{\mathrm{T}}^{-1}$ calculated for the eclipsed structure at the employed DFT level. This value is in perfect agreement with the current strength of $-1.93\mathrm{nA}{\mathrm{T}}^{-1}$ calculated at the coupled-cluster singles and doubles level with a perturbative treatment of the triple excitations. The current strength calculated at the second-order Møller-Plesset perturbation level is $1.20\mathrm{nA}{\mathrm{T}}^{-1}$, which is of the same size with opposite sign.

We identify laser-accessible transitions in group-16-like highly charged ions as candidates for high-accuracy optical clocks, including S-, Se-, and Te-like systems. For this class of ions, the ground ${}^{3}P_J$ fine-structure manifold exhibits irregular (nonmonotonic in $J$) energy ordering for large enough ionization degree. We consider the $|{}^{3}P_2\rangle ⟷|{}^{3}P_0\rangle$ (ground to first-excited state) electric quadrupole transition, performing relativistic many-body calculations of several atomic properties important for optical clock development. All ions discussed are suitable for production in small-scale ion sources and lend themselves to sympathetic cooling and quantum-logic readout with singly charged ions.

Strong-field atomic processes, driven by long-wavelength laser beams, are known to be affected by magnetic forces. In such beams, the Lorentz force pushes the photoelectrons along the beam direction and prevents their rescattering or recombination with the parent ions. In high-order harmonic generation (HHG), therefore, the yield of energetic photons is markedly suppressed, rendering x-ray radiation sources from high harmonics so far impractical. To compensate these magnetic forces and to reenable HHG at long wavelengths, a setup of two not quite collinear beams has been suggested recently but not much analyzed beyond classical arguments and with respect to accessible laser parameters. Using the nondipole strong-field approximation, we here investigate when the longitudinal momentum of the photoelectrons vanishes and how this noncollinear setup explicitly depends on the wavelength and intensity of the driving beams. We also demonstrate that an optimal crossing angle ${\delta }_0$ between these beams always exists for which the fraction of the returning electrons is maximized. This rather simple steering of the longitudinal momentum will allow an efficient HHG with driving beams deep in the midinfrared.

We consider a minimal model to investigate the quantum phases of hardcore, polarized dipolar atoms confined in multilayer optical lattices. The model is a variant of the extended Bose-Hubbard model, which incorporates intralayer repulsion and interlayer attraction between the atoms in nearest-neighbor sites. We study the phases of this model emerging from the competition between the attractive interlayer interaction and the interlayer hopping. Our results from the analytical and cluster-Gutzwiller mean-field theories reveal that multimer formation occurs in the regime of weak intra- and interlayer hopping due to the attractive interaction. In addition, intralayer isotropic repulsive interaction results in the checkerboard ordering of the multimers. This leads to an incompressible checkerboard multimer phase at half-filling. At higher interlayer hopping, the multimers are destabilized to form resonating valence-bond-like states. Furthermore, we discuss the effects of thermal fluctuations on the quantum phases of the system.

We study the two-body bound states in a spin-orbital-angular-momentum (SOAM) coupled quantum gas of fermions within a tightly confined ring geometry in two dimensions. Two different configurations are considered: an attractive $s$-wave interaction exists between two spin species that are SOAM coupled and an atom with SOAM coupled internal spins interacts state selectively with another atom. For both cases, we identify the condition for the emergence of molecular states with finite orbital angular momenta. These molecular states with quantized orbital angular momenta correspond to the SOAM-coupling-induced vortices in the corresponding Fermi superfluid. We propose to detect the molecules through Raman spectroscopy with Laguerre-Gaussian lasers. As the molecular states can form above the superfluid temperature, they offer an experimentally more accessible route toward the study of the underlying pairing mechanism under SOAM coupling.

We propose the effective generation of entangled and squeezed states in an optoelectromechanical system comprising a macroscopic $LC$ electrical circuit and an optomechanical system. We notice an enhancement in the optoelectrical entanglement. This boost is obtained by application of modulations in the laser drive amplitude, the voltage drive amplitude, and the spring constant of the movable end mirror. The maximum amount of entanglement is observed to be primarily dependent on the voltage modulation. Alongside the generated entanglement, we also study the variation of squeezing in the mechanical mode for different parameter regimes. The observation of maximum squeezing and maximum entanglement windows for the system, when all three types of modulations are applied, is reported.

We report a detailed study on soliton steering dynamics in a parity-time-symmetric directional coupler in the femtosecond domain, which requires incorporation of higher-order perturbative effects such as third-order and fourth-order dispersions, self-steepening, and intrapulse Raman scattering. With a high gain/loss, the combination of all these effects is found to stabilize the soliton pulse evolution in the coupler from the chaotic behavior of unperturbed evolution. This work demonstrates that efficient soliton steering can be achieved at very low critical power and a relatively higher gain/loss even in the femtosecond regime.