In their study of inverse Doppler shift and superluminal light [Ghafoor et al., Phys. Rev. A 91, 053807 (2015)], Ghafoor et al. consider a three-level atomic arrangement with transitions in the optical domain. In fact, the values they give to the parameters lead to a probe wavelength lying in the decimeter band. We point out that the Doppler shifts are then negligible and remark that the simulations performed by Ghafoor et al. do not evidence any superluminal effect.

We report on the electrically driven rotation of $2.4\text{−}\mu \mathrm{m}$-radius, optically levitated dielectric microspheres. Electric fields are used to apply torques to a microsphere's permanent electric dipole moment, while angular displacement is measured by detecting the change in polarization state of light transmitted through the microsphere (MS). This technique enables greater control than previously achieved with purely optical means because the direction and magnitude of the electric torque can be set arbitrarily. We measure the spin-down of a microsphere released from a rotating electric field, the harmonic motion of the dipole relative to the instantaneous direction of the field, and the phase lag between the driving electric field and the dipole moment of the MS due to drag from residual gas. We also observe the gyroscopic precession of the MS when the axis of rotation of the driving field and the angular momentum of the microsphere are orthogonal. These observations are in quantitative agreement with the equation of motion. The control offered by the electrical drive enables precise measurements of microsphere properties and torque as well as a method for addressing the direction of angular momentum for an optically levitated particle.

In this paper, we revisit the quantum Fisher information (QFI) calculation in SU(1,1) interferometer considering different phase configurations. When one of the input modes is a vacuum state, we show by using phase averaging, different phase configurations give the same QFI. In addition, by casting the phase estimation as a two-parameter estimation problem, we show that the calculation of the quantum Fisher information matrix (QFIM) is necessary in general. Particularly, within this setup, the phase-averaging method is equivalent to a two-parameter estimation problem. We also calculate the phase sensitivity for different input states using QFIM approach.

Long-range-corrected range-separated hybrid approximations of the exchange-correlation (XC) energy are proposed through the satisfaction of the local-density linear response, which requires the second-order term of the XC gradient expansion to vanish. The underlying proposition is tested with the asymptotic Perdew-Burke-Ernzerhof (APBE) [L. A. Constantin et al., Phys. Rev. Lett. 106, 186406 (2011)], PBE constructed for interfaces (PBEint) [E. Fabiano et al., Phys. Rev. B 82, 113104 (2010)], and semiclassical generalized-gradient approximation at fourth order (SG4) [L. A. Constantin et al., Phys. Rev. B 93, 045126 (2016)] functionals by constructing their corresponding exchange holes. Systematic investigation of these long-range-corrected XC hybrids shows improved performance on recovering the local-density linear response. The long-range-corrected XC hybrid based on the APBE functional and satisfying local-density linear response gives promising results compared to the popular LC-$\omega \mathrm{PBE}$ for most of the thermodynamical test sets. Most interestingly, the underlying construction and improved performance by these XC hybrids indicate that the proposed methodology can serve as a basis for developing range-separated XC hybrid functionals of greater potentialities in the future, not only long-range-corrected ones but also screened hybrid functionals.

When dealing with a fully symmetrical ground state, the symmetry dependence of the universal Hohenberg-Kohn energy functional $F\left[\gamma \right]$ of the first-order reduced density matrix (RDM) $\gamma$ can be conveniently neglected. The situation changes drastically in the case of the dissociation of a symmetrical molecule with the state crossing, in the course of which the potential energy curve of the initial non-fully symmetrical ground state is eventually crossed with that of the fully symmetrical state. In this case, as is demonstrated in the present paper, the second-order RDM ${\mathrm{\Gamma }}_{ij,kl}$ in the representation of the natural orbitals (NOs) is symmetry dependent. Since ${\mathrm{\Gamma }}_{ij,kl}$ is the goal in the design of ${\mathrm{\Gamma }}_{ij,kl}\left(n\right)$ as a functional of NO occupations $\left\{n\right\}$, which is part of a practical density matrix functional $F\left[\gamma \right],{\mathrm{\Gamma }}_{ij,kl}\left(n\right)$ must also depend on the symmetry, especially the irreducible representation of the symmetry group. The result has immediate implications for study of structural (or phase) transitions based on a single symmetry-independent functional. The demonstration is given in the minimal-base model of the dissociation of the prototype ${\mathrm{H}}_4$ molecule in the rhombic structure.

We investigate theoretically the photoelectron angular distribution of atoms in pulsed XUV and IR laser fields by solving the time-dependent Schrödinger equation and surprisingly find that the angular distributions of sidebands are not always straight lines (normal) in the energy-angle plot as predicted by the strong-field approximation. Comparing our results with those of the strong-field approximation, in which photoelectron nucleus Coulomb interaction is ignored, the bending of the angular distribution is attributed to the photoelectron-nucleus Coulomb interaction. The bending depends on the IR intensity, IR pulse duration, XUV photon energy, and the time delay between the two pulses.

We apply THz-field streaking to temporally resolve the ultrafast ionization of neutral cluster vs atomic targets exposed to intense ultrashort soft x-ray pulses from a free-electron laser (FEL). In the experiment pristine Xe and mixed Xe/Ar clusters as well as Xe atoms are ionized in the vicinity of the $4d\to \varepsilon f$ giant resonance of Xe. We compare the relative streaking delay between the center-of-mass oscillations of electrons that have final kinetic energies in the spectral region of the Xe($4d$) photoline. Our results show that clusters are ionized at the beginning of the 100 fs FEL pulse as supported by calculations of target frustration in the focal volume. We have identified a significantly larger 40 fs relative streaking delay between mixed Xe-Ar clusters vs Xe atoms compared to a 15 fs relative delay found for pristine Xe clusters. This is attributed to the high sensitivity of our spectroscopic measurement to the degree of condensation of the cluster target. Our results show that THz streaking is a powerful technique to temporally study electron emission from extended targets under intense FEL radiation on time scales that are significantly shorter than the FEL pulse duration.

The photodissociation of the lithium fluoride molecule gathered much attention lately. Theoretical works treating this subject usually consider the $1{}^1\mathrm{\Sigma }^{+}$ and $2{}^1\mathrm{\Sigma }^{+}$ electronic states, which are coupled nonadiabatically. As a continuation of a previous work, we intend to highlight the importance of the $1{}^1\mathrm{\Pi }$ state by investigating the kinetic energy and angular distribution of the photofragments. Besides, our results pointed out the importance of the molecular rotation.

We investigate the topological properties of spin-polarized fermionic polar molecules loaded in a multilayer structure with the electric dipole moment polarized to the normal direction. When polar molecules are paired by attractive interlayer interaction, unpaired Majorana fermions can be macroscopically generated in the top and bottom layers in dilute density regime. We show that the resulting topological state is effectively composed by a bundle of one-dimensional (1D) Kitaev ladders labeled by in-plane momenta ${\mathbf{k}}_{\parallel }$ and $-{\mathbf{k}}_{\parallel }$, and hence belongs to the BDI class characterized by the winding number $\mathbb{Z}$, protected by the time-reversal symmetry. The Majorana surface modes exhibit a flat band at zero energy, fully gapped from Bogoliubov excitations in the bulk, and hence becomes an ideal system to investigate the interaction effects on quantum degenerate Majorana fermions. We further show that additional interference fringes can be identified as a signature of such 2D Majorana surface modes in the time-of-flight experiment.

We propose a physical scheme to realize combined linear and nonlinear optical potentials with parity-time ($\mathcal{PT}$) symmetry and investigate the scattering property of optical solitons in a coherent atomic gas. We show that the combined linear and nonlinear $\mathcal{PT}$-symmetric potentials can be created through the spatial modulation of the control laser field and the inclusion of the Kerr nonlinearity of the signal laser field. We demonstrate that the imaginary part of the nonlinear $\mathcal{PT}$ potential plays a crucial role for the occurrence of the $\mathcal{PT}$ phase transition and the change of the $\mathcal{PT}$ phase diagram, which can be actively manipulated in our system. We demonstrate also that the system supports stable optical solitons, which can be managed via tuning the combined linear and nonlinear $\mathcal{PT}$ potentials; furthermore, by taking the combined linear and nonlinear $\mathcal{PT}$ potentials as a defect, the scattering of the optical solitons by the defect displays evident asymmetric behavior, controlled by the imaginary parts of the combined linear and nonlinear $\mathcal{PT}$ potentials. The results reported here may have potential applications in optical information processing and transmission.

Chiral quantum systems have received intensive attention in fundamental physics and applications in quantum information processing including optical isolation and photon unidirectional emission. Here, we design an on-chip emitter-resonator system with strong chiral light-matter interaction for a chiral single-photon interface. The system includes a microring resonator with a strong evanescent field and a near-unity optical chirality along both of the whole outside and inside walls, allowing a strong and chiral coupling of the whispering-gallery mode to a quantum emitter. By initializing a quantum dot in a specific spin ground state or shifting the transition energy with a polarization-selective optical Stark effect, we show a broadband optical isolation at the single-photon level over several gigahertz. Furthermore, a quantum emitter chirally coupling to the microring resonator can emit single photons unidirectionally. Our protocol paves the way to realize a multifunctional chiral single-photon interface for on-chip quantum information processing and quantum networks.

The locally constant field approximation (LCFA) has to date underpinned the numerical simulation of quantum processes in laser-plasma physics and astrophysics, but its validity has recently been questioned in the parameter regime of current laser experiments. While improvements are needed, literature corrections to the LCFA show inherent problems. Using nonlinear Compton scattering in laser fields to illustrate, we show here how to overcome the problems in LCFA corrections. We derive an LCFA+, which, compared with the full QED result, shows an improvement over the LCFA across the whole photon emission spectrum. We also demonstrate an implementation of our results in the type of numerical code used to design and analyze intense laser experiments.

The three-spin-$1/2$ decoherence-free subsystem defines a logical qubit protected from collective noise and supports exchange-only universal gates. Such logical qubits are well suited for implementation with electrically defined quantum dots. Exact exchange-only entangling logical gates exist but are challenging to construct and understand. We use a decoupling strategy to obtain straightforward approximate entangling gates. A benefit of the strategy is that, if the physical spins are aligned, then it can implement evolution under entangling Hamiltonians. Hamiltonians expressible as linear combinations of logical Pauli products not involving ${\sigma }_y$ can be implemented directly. Self-inverse gates that are constructible from these Hamiltonians, such as the controlled-not (cnot) gate, can be implemented without the assumption on the physical spins. We compare the control complexity of implementing cnot to previous methods and find that the complexity for fault-tolerant fidelities is competitive.

We investigate the stability of two-electron ions interacting with Yukawa potentials with varying $Z$ using correlated exponential wave functions based on the Ritz variational method. The critical screening parameter ${\mu }_C$ as a function of the nuclear charge $Z$, the critical nuclear charge $Z_C$ as a function of the screening parameter $\mu$, and the ionization energies in terms of the screening parameter $\mu$ and $Z$ are reported. The critical charge for the bare Coulomb system ($Z$, $e$, $e$) obtained using 700-term correlated exponential wave functions is in reasonable agreement with the reported results in the literature. The critical screening parameter, critical nuclear charge, and ionization energy for the Yukawa system ($Z$, $e$, $e$) exhibit interesting behaviors. The possible existence of Borromean states and quasibound resonant states is also discussed.

We propose how to prepare a molecular ensemble with macroscopic threefold symmetry. By utilizing the special laser electric field trajectory with threefold symmetry, which can be formed by superposing a counterrotating circularly polarized fundamental pulse and its second harmonic pulse, sample molecules with threefold symmetry such as $\mathrm{B}{X}_3$ ($X=$ F, Cl, Br, I) can be aligned with their three arms along (or in between) the laser electric fields with threefold symmetry depending on the sign of the hyperpolarizability of the sample molecule. We show that this method is feasible with practical experimental conditions as for the rotational temperature of the sample molecules and the intensities of the two wavelengths. This method will open up physics of symmetry concerning a molecular ensemble with macroscopic threefold symmetry.

We study the stochastic dynamics of a particle in a periodically driven potential. For atomic ions trapped in radio-frequency Paul traps, noise heating and laser cooling typically act slowly in comparison with the unperturbed motion. These stochastic processes can be accounted for in terms of a probability distribution defined over the action variables, which would otherwise be conserved within the regular regions of the Hamiltonian phase space. We present a semiclassical theory of low-saturation laser cooling applicable from the limit of low-amplitude motion to large-amplitude motion, accounting fully for the time-dependent and anharmonic trap. We employ our approach to a detailed study of the stochastic dynamics of a single ion, drawing general conclusions regarding the nonequilibrium dynamics of laser-cooled trapped ions. We predict a regime of anharmonic motion in which laser cooling becomes diffusive (i.e., it is equally likely to cool the ion as it is to heat it), and can also turn into effective heating. This implies that a high-energy ion could be easily lost from the trap despite being laser cooled; however, we find that this loss can be counteracted using a laser detuning much larger than Doppler detuning.

We propose an experimental protocol for using cold atoms to create and probe quantum dimer models, thereby exploring the Pauling-Anderson vision of a macroscopic collection of resonating bonds. This process can allow the study of exotic crystalline phases, fractionalization, topological spin liquids, and the relationship between resonating dimers and superconductivity subjects which have been challenging to address in solid-state experiments. Our key technical development is considering the action of an off-resonant photoassociation laser on large spin atoms localized at the sites of a deep optical lattice. The resulting superexchange interaction favors nearest-neighbor singlets. We derive an effective Hamiltonian in terms of these dimer degrees of freedom, finding that it is similar to well-known quantum dimer models, which boast a rich variety of valence bond crystal and spin liquid phases. We numerically study the ground state, explain how to tune the parameters, and develop a protocol to directly measure the dimers and their resonating patterns.

Emitters strongly coupled to a photonic crystal provide a powerful platform for realizing novel quantum light-matter interactions. Here we study the optical properties of a three-level artificial atomic chain coupled to a one-dimensional superconducting microwave photonic crystal. A sharp minimum-energy dip appears in the transmission spectrum of a weak input field, which reveals rich behavior of the long-range interactions arising from localized bound states. We find that the dip frequency scales linearly with both the number of the artificial atoms and the characteristic strength of the long-range interactions when the localization length of the bound state is sufficiently large. Motivated by this observation, we present a simple model to calculate the dip frequency with system parameters, which agrees well with the results from exact numerics for large localization lengths. We observe oscillation between bunching and antibunching in photon-photon correlation function of the output field. Furthermore, we find that the model remains valid even though the coupling strengths between the photonic crystal and artificial atoms are not exactly equal and the phases of external driving fields for the artificial atoms are different. Thus we may infer valuable system parameters from the dip location in the transmission spectrum, which provides an important measuring tool for the superconducting microwave photonic crystal systems in experiment. With remarkable advances to couple artificial atoms with microwave photonic crystals, our proposal may be experimentally realized in currently available superconducting circuits.

In this study, the design of a directional cloaking based on the Luneburg lens system is proposed and its operating principle is experimentally verified. The cloaking concept is analytically investigated via geometrical optics and numerically realized with the help of the finite-difference time-domain method. In order to benefit from its unique focusing and/or collimating characteristics of light, the Luneburg lens is used. We show that by the proper combination of Luneburg lenses in an array form, incident light bypasses the region between junctions of the lenses, i.e., the “dark zone.” Hence, direct interaction of an object with propagating light is prevented if one places the object to be cloaked inside that dark zone. This effect is used for hiding an object which is made of a perfectly electric conductor material. In order to design an implementable cloaking device, the Luneburg lens is discretized into a photonic crystal structure having gradually varying air cylindrical holes in a dielectric material by using Maxwell Garnett effective medium approximations. Experimental verifications of the designed cloaking structure are performed at microwave frequencies of around 8 GHz. The proposed structure is fabricated by three-dimensional printing of dielectric polylactide material and a brass metallic alloy is utilized in place of the perfectly electric conductor material in microwave experiments. Good agreement between numerical and experimental results is found.

The notion of a quantum speed limit (QSL) refers to the fundamental fact that two quantum states become completely distinguishable upon dynamical evolution only after a finite amount of time, called the QSL time. A different but related concept is that of minimum control time (MCT), which is the minimum evolution time needed for a state to be driven (by suitable, generally time-dependent, control fields) to a given target state. While the QSL can give information about the MCT, it usually imposes little restrictions on it and is thus impractical for control purposes. In this work, we revisit this issue by first presenting a theory of geometrical QSL for unitary transformations, rather than for states, and discuss its implications and limitations. Then, we propose a framework for bounding the MCT for realizing unitary transformations that goes beyond the QSL results and gives much more meaningful information to understand the controlled dynamics of the system at short times.