Based on the generalized Lorenz-Mie method and Maxwell stress tensor theory, we demonstrate that a lateral optical force (OF) can be induced on a spherical particle along the translationally invariant direction of incident optical fields formed by multiple circularly polarized plane waves. It is analytically shown that the lateral OF depends on two quantities associated with the illuminating field, regardless of the particle properties such as material composition and particle size. Interestingly, for some special cases like two- or three-wave interference field, the lateral OF is rigorously proportional to the Poynting vector. As examples, the numerical results for the interference field composed of two or three circular polarized plane waves show that the spatial distribution pattern of the lateral OF is always determined by Poynting vectors of the illuminating field itself, while the particle size and material affect solely the magnitude but not the spatial profile of the lateral OF. In addition, the magnitude of the lateral OF is comparable to other components of the OF. The lateral OF is traced to the scattering force due to the interference between multipoles excited on the particle. These findings may enrich the concept of the lateral OF and also add an additional implement to the optical manipulation toolbox.

We present theoretical transmission spectra of a strongly driven, damped flux qubit coupled to a dissipative resonator in the ultrastrong-coupling regime. Such a qubit-oscillator system, described within a dissipative Rabi model, constitutes the building block of superconducting circuit QED platforms. The addition of a strong drive allows one to characterize the system properties and study novel phenomena, leading to a better understanding and control of the qubit-oscillator system. In this work, the calculated transmission of a weak probe field quantifies the response of the qubit, in frequency domain, under the influence of the quantized resonator and of the strong microwave drive. We find distinctive features of the entangled driven qubit-resonator spectrum, namely resonant features and avoided crossings, modified by the presence of the dissipative environment. The magnitude, positions, and broadening of these features are determined by the interplay among qubit-oscillator detuning, the strength of their coupling, the driving amplitude, and the interaction with the heat bath. This work establishes the theoretical basis for future experiments in the driven ultrastrong-coupling regime.

We present a closed-form analytical description for the metrological performance of a generic superposition of spin coherent states (SSCS) used as probes for quantum phase estimation. Working in this framework, we derive transparent analytical expressions for the pertinent quantum Fisher information and identify a general class of spin coherent states enabling quantum metrology with Heisenberg-limit precision. Bloch-sphere analysis of antipodal SSCS shows that the phase-estimation precision attainable with such states increases with the distance from the equator of the Bloch sphere, growing from the shot-noise limit level for the equatorial states all the way up to the Heisenberg limit for SSCS that can be represented as superpositions of the poles of the Bloch sphere.

We explore the underlying spin-orbit coupling (SOC) mechanism for the interactions between complex structured light fields in a four-level tripod electromagnetically induced transparency (EIT) system. Because one coupling field in the system is a Bessel beam, the susceptibility of the EIT medium can be modulated quasiperiodically in the radial dimension. The paraxial propagation and evolution of a rotating spinor image as the probe field in the medium can be described by a Pauli-like equation with the SOC term. We calculate the spatial distributions of the diffracted patterns of the image in the near field and clearly show the tunable SOC-induced radial splitting of the oppositely polarized pseudospin states. Our scheme may be useful for all-optical processing of spatial multimode signals in coherent media.

We discuss the role of quantum coherence in the energy fluctuations of open quantum systems. To this aim, we introduce a protocol to which we refer as the end-point measurement scheme, allowing us to define the statistics of energy changes as a function of energy measurements performed only after the evolution of the initial state. At the price of an additional uncertainty on the initial energies, this approach prevents the loss of initial quantum coherences and enables the estimation of their effects on energy fluctuations. We demonstrate our findings by running an experiment on the IBM Quantum Experience superconducting qubit platform.

Classically, electromagnetic pulses are described by real fields that couple to charged matter and propagate causally. We will show here that real fields of the form used in standard classical electromagnetic theory have a quantum mechanical interpretation in which the probability density for a photon to be at $\mathbf{x}$ is positive definite and operators representing all of the standard physical observables exist. A covariant alternative to the ${\omega }_k^{1/2}$ dependence that appears in most (but not all) presentations of quantum optics and quantum field theory is presented and real Lorentz scalar one-photon advanced and retarded potentials are derived.

Nonlocal boxes are conceptual tools that capture the essence of the phenomenon of quantum nonlocality, central to modern quantum theory and quantum technologies. We introduce network nonlocal boxes tailored for quantum networks under the natural assumption that these networks connect independent sources and do not allow signaling. Hence, these boxes satisfy the no-signaling and independence principle. For the case of boxes without inputs, connecting pairs of bipartite sources and producing binary outputs, we prove that the sources and boxes producing local random outputs and maximal two-box correlations, i.e., $E_2=\sqrt{2}-1, E_2^o=1$, are essentially unique.

Following the information-based approach to Dirac spinors under a constant magnetic field, the phase-space representation of symmetric and antisymmetric localized Dirac cat states is obtained. The intrinsic entanglement profile implied by the Dirac Hamiltonian is then investigated so as to shed a light on quantum states as carriers of qubits correlated by phase-space variables. Corresponding to the superposition of Gaussian states, cat states exhibit nontrivial elementary information dynamics which include the interplay between intrinsic entanglement and quantum superposition as reported by the corresponding Dirac archetypes. Despite the involved time evolution as nonstationary states, the Wigner function constrains the elementary information quantifiers according to a robust framework which can be consistently used for quantifying the time-dependent $\text{SU}\left(2\right)\otimes \text{SU}\left(2\right)$ (spin projection and intrinsic parity) correlation profile of phase-space localized Dirac spinor states. Our results show that the Dirac Wigner functions for cat states—described in terms of generalized Laguerre polynomials—exhibit an almost maximized timely persistent mutual information profile which is engendered by either classical- or quantumlike spin-parity correlations, depending on the magnetic field intensity.

The quantum approximate optimization algorithm (QAOA) is a near-term combinatorial optimization algorithm suitable for noisy quantum devices. However, little is known about performance guarantees for $p>2$. A recent work computing MaxCut performance guarantees for 3-regular graphs conjectures that any $d$-regular graph evaluated at particular fixed angles has an approximation ratio greater than some worst-case guarantee. In this work, we provide numerical evidence for this fixed angle conjecture for $p<12$. We compute and provide these angles via numerical optimization and tensor networks. These fixed angles serve for an optimization-free version of QAOA and have universally good performance on any 3-regular graph. Heuristic evidence is presented for the fixed angle conjecture on graph ensembles, which suggests that these fixed angles are “close” to global optimum. Under the fixed angle conjecture, QAOA has a larger performance guarantee than the Goemans Williamson algorithm on 3-regular graphs for $p\ge 11$.

We present high-accuracy relativistic investigations of the spectrum of Lr, element 103, prompted by the planned optical spectroscopy experiments on this rare and short-lived atom. Reliable predictions of the transition lines are important for the planning and success of these challenging measurements. The relativistic coupled cluster approach was used to calculate the energies of lowest excited states, while the combination of configuration-interaction method with the many-body perturbation theory was employed to address the higher-lying states and to obtain the transition strengths and the lifetimes of the levels of experimental interest. We performed similar calculations for Lu, the lighter homologue of Lr, where experimental data are available. For the lighter element, both the calculated energies and the Einstein coefficients are in excellent agreement with the previously measured values, confirming the accuracy of the performed calculations and the reliability of our predictions for Lr.

We study the ${}^{1}S_0-{}^{3}D_2$ and ${}^{1}S_0-{}^{3}D_3$ transitions in Cu ii and the ${}^{1}S_0-{}^{3}P_2^{\mathrm{o}}$ transition in Yb iii as possible candidates for the optical clock transitions. A recently developed version of the configuration interaction method, designed for a large number of electrons above the closed-shell core, is used to carry out the calculation. We calculate excitation energies, transition rates, lifetimes, and scalar static polarizabilities of the ground, clock states, and blackbody radiation shift. We demonstrate that the considered transitions have all features of the clock transition leading to prospects of highly accurate measurements. A search for new physics, such as time variation of the fine-structure constant, is also investigated.

Fast and nearly lossless atomic transport, enabled by moving the confining trap, is a prerequisite for many quantum-technology applications. While theoretical studies of this problem have heretofore focused almost exclusively on simplified scenarios (one-dimensional systems, purely harmonic confining potentials, etc.), we investigate it here in the experimentally relevant setting of a moving optical lattice (optical conveyor belt). We model single-atom transport in this system by taking fully into account its three-dimensional, anharmonic confining potential. We do so using the established method of shortcuts to adiabaticity (STA), i.e., an inverse-engineering approach based on Lewis-Riesenfeld invariants, as well as its recently proposed modification known as enhanced STA (eSTA). By combining well-controlled, advanced analytical techniques and the numerical propagation of a time-dependent Schrödinger equation using the Fourier split operator method, we evaluate atom-transport fidelities within both approaches. Being obtained for realistic choices of system parameters, our results are relevant for future experiments with optical conveyor belts. Moreover, they reveal that in the system at hand the eSTA method outperforms its STA counterpart for all but the lowest optical-lattice depths.

We study the quantum dynamics of an ion confined in a radio-frequency trap in interaction with either a Bose or spin-polarized Fermi gas. To this end, we derive quantum optical master equations in the limit of weak coupling and the Lamb-Dicke approximations. For the bosonic bath, we also include the so-called “Lamb-shift” correction to the ion trap due to the coupling to the quantum gas as well as the extended Fröhlich interaction within the Bogolyubov approximation that have been not considered in previous studies. We calculate the ion kinetic energy for various atom-ion scattering lengths as well as gas temperatures by considering the intrinsic micromotion and we analyze the damping of the ion motion in the gas as a function of the gas temperature. We find that the ion's dynamics depends on the quantum statistics of the gas and that a fermionic bath enables to attain lower ionic energies.

We present the collective excitation spectrum analysis of binary Bose-Einstein condensates (BECs) with spin-orbit (SO) and Rabi couplings in a quasi-two-dimensional system. In particular, we investigate the role of SO and Rabi coupling strengths in determining the dynamical stability of the coupled BECs using Bogoliubov–de Gennes (BdG) theory. Using the eigenenergy of the BdG spectrum, we confirm the existence of phonon, roton, and maxon modes with weak repulsive intra- and interspecies contact interactions. The depth of the minimum corresponding to the roton mode strongly depends on the coupling strength. We find that the increase of the SO coupling leads to instability, while the increase in the Rabi coupling stabilizes the system. Also, the eigenvectors of the BdG spectrum indicate the presence of a densitylike mode in the stable regime and spinlike modes in the unstable regimes. A phase diagram demonstrating the stability regime in the plane of SO and Rabi coupling strengths is obtained. Finally, we complement the observation of the excitation spectrum with the direct numerical simulation results of coupled Gross-Pitaevskii equations.

Quantum spin squeezing is an important resource for quantum information processing, but its squeezing degree is not easy to preserve in an open system with decoherence. Here, we propose a scheme to implement single-photon-triggered spin squeezing with decoherence reduction in an open system. In our system, a Dicke model (DM) with an added quadratic optomechanical coupling is considered, which can be equivalent to an effective DM manipulated by the photon number. Besides, the phonon mode of the optomechanical coupling is coupled to a squeezed-vacuum reservoir with a phase matching, resulting in that its thermal noise can be suppressed completely. We show that squeezing of the phonon mode triggered by a single photon can be transferred to the spin ensemble totally, and pairwise entanglement of the spin ensemble can be realized if and only if there is spin squeezing. Importantly, when considering the impact of the environment, our system can obtain a squeezing degree better than that of the optimal squeezing that can be achieved in the traditional DM. Meanwhile, the spin squeezing generated in our system is immune to the thermal noise. This work offers an effective way to generate spin squeezing with a single photon and to reduce decoherence in an open system, which will have promising applications in quantum information processing.

We theoretically study the dynamics of an optomechanical system, consisting of a passive optical mode and an active mechanical mode, in the $\mathcal{PT}$- and broken-$\mathcal{PT}$-symmetric regimes. By fully analytical treatments for the dynamics of the average displacement and particle numbers, we reveal the phase diagram under different conditions and the various regimes of both $\mathcal{PT}$ symmetry and stability of the system. We find that by appropriately tuning either mechanical gain or optomechanical coupling, both phase transitions of the $\mathcal{PT}$ symmetry and stability of the system can be flexibly controlled. As a result, the dynamical behaviors of the average displacement, photons, and phonons are radically changed in different regimes. The presented physical mechanism is general and this method can be extended to a general model of dissipative and amplified coupled systems. Our study shows that $\mathcal{PT}$-symmetric optomechanical devices can serve as a powerful tool for the manipulation of mechanical motion, photons, and phonons.

We report experimental results from a study of nonlinear Thomson scattering of elliptically polarized light. Polarization-resolved radiation patterns of the scattered light are measured as a function of the elliptical polarization state of the incident laser light. The relativistic electron trajectory in intense elliptically polarized fields leads to the formation of unique radiated polarization states, which are observed by our measurements and predicted by a theoretical model. The polarization of Thomson scattered light depends strongly on the intensity of the incident light due to nonlinearity. The results are relevant to high-field electrodynamics and to research and development of light sources with novel capabilities.

We present an alternative route to track the motion of a single atom traversing a high-finesse optical cavity by means of the coherence statistics of photon antibunching. Here the single atom strongly couples to the high-order transverse Laguerre-Gaussian (LG) mode (e.g., ${\text{LG}}_{01}$) of optical cavity, instead of the high-order transverse Hermite-Gaussian (HG) mode, which is beneficial to the single-atom trajectory measurement utilizing such a photon antibunching effect. With this aim, we characterize the position-dependent quantum correlations of the transmitted light in such a cavity quantum electrodynamics (QED) system subject to various kinds of the LG modes, finding that the photon antibunching effect is closely related to the position of the atom in the transverse plane and therefore the degree of the antibunching carries the information about the position of the atom within the cavity. The numerical results of the second-order correlation function agree well with the analytical calculations. Thanks to the tilted transverse ${\text{LG}}_{01}$ mode of the cavity, which is inclined to the vertical direction by an angle of $\sim {15}^{\circ }$, the LG-mode cavity QED architecture helps us to eliminate the degenerate trajectory of the single atom falling through the cavity and to obtain a unique atomic trajectory. In a $10\text{−}\mu \mathrm{s}$-long time interval as an example, our in-depth analysis displays that the atomic position with the spatial resolution of $\sim 4.0\mu \mathrm{m}$ in the vertical direction (axis $x$) can be achieved. The present study is useful for well understanding and researching both the robust generation of single-photon source and the precision measurement of single-atom trajectory.

We demonstrate the efficiency of a recent exact-gradient optimal control methodology by applying it to a challenging many-body problem, crossing the superfluid to Mott-insulator phase transition in the Bose-Hubbard model. The system size necessitates a matrix product state representation and this seamlessly integrates with the requirements of the algorithm. We observe fidelities in the range 0.99–0.9999 with associated minimal process duration estimates displaying an exponential fidelity-duration trade-off across several orders of magnitude. The corresponding optimal solutions are characterized in terms of a predominantly linear sweep across the critical point followed by bang-bang-like structure. This is quite different from the smooth and monotonic solutions identified by earlier gradient-free optimizations which are hampered in locating the higher complexity protocols in the regime of high fidelities at low process durations. Overall, the comparison suggests significant methodological improvements also for many-body systems in the ideal open-loop setting. Acknowledging that idealized open-loop control may deteriorate in actual experiments, we discuss the merits of using such an approach in combination with closed-loop control—in particular, high-fidelity physical insights extracted with the former can be used to formulate practical, low-dimensional search spaces for the latter.