Physical processes near threshold in strong-field ionization are complex owing to the important role of many intermediate states. Two typical near-threshold processes in few-cycle laser pulses, the generations of Rydberg electrons and terahertz waves, are investigated using classical trajectory Monte Carlo simulations. Mapping final energies of electrons to the tunneling coordinates, the two processes are distinguished based on the contributions from different kinds of electron trajectories, which highlights the complex interplay between the Coulomb potential and the laser field on the electron dynamics. We clarify the dependence of momentum and energy spectra on the carrier-envelope phase. We find that the optimal phases for terahertz wave generation and creation of the Rydberg state in few-cycle laser pulses are opposite. This finding can be applied to further experiments on attosecond electron dynamics.

A theoretical study of the rotational dynamics of a linear polar molecule in a two-color nonresonant cw laser field is presented. By systematically considering the interactions of this field with the electric dipole moment, polarizability, and hyperpolarizability of the molecule, the implications that the symmetries of the Hamiltonian have on the rotational dynamics are explored in a regime where the time-average approximation does not hold. It is shown that the alignment and orientation satisfy certain symmetries as functions of the phases and field strengths, and that they can be expressed as analytic functions in terms of these main parameters of the two-color laser field.

Optical pulling forces or tractor beams can pull particles against light propagation by redirecting the incident photons forward. This is typically achieved using Bessel beams with very small half-cone angles, which considerably limits their applicability. One can circumvent such an issue by using a superposition of plane waves. In order to investigate optical pulling forces exerted by a pair of noncollinear plane waves, we develop a theoretical framework based on Mie theory, Debye potentials, and Wigner rotation matrices. We apply this framework to calculate the optical pulling force on metallodielectric composite particles, which we put forward as an alternative material platform to optimize and tailor tractor beams. Indeed, we demonstrate that by adding a few plasmonic inclusions to low-refractive-index dielectric particles of arbitrary sizes, we are able to produce polarization-dependent optical pulling forces that cannot occur in the corresponding homogeneous particles. Altogether, our findings not only provide innovative theoretical methods to compute optical pulling forces but also provide strategies to tailor and optimize them, paving the way to increase their applicability.

Multistate stimulated Raman adiabatic passage (STIRAP) is a process which allows for adiabatic population transfer between the two ends of a chainwise-connected quantum system. The process requires large temporal areas of the driving pulsed fields (pump and Stokes) in order to suppress the nonadiabatic couplings and thereby to make adiabatic evolution possible. To this end, in the present paper a variation of multistate STIRAP, which accelerates and improves the population transfer, is presented. In addition to the usual pump and Stokes fields it uses shortcut fields applied between the states, which form the dark state of the system. The shortcuts cancel the couplings between the dark state and the other adiabatic states thereby resulting (in the ideal case) in a unit transition probability between the two end states of the chain. Specific examples of five-state systems formed of the magnetic sublevels of the transitions between two degenerate levels with angular momenta $J_g=2$ (of ground or lower level) and $J_e=1$ or $J_e=2$ (of excited or upper level) are considered in detail, for which the shortcut fields are derived analytically. The proposed method is simpler than the usual “shortcuts to adiabaticity” recipe, which prescribes shortcut fields between all states of the system, while the present proposal uses shortcut fields between the sublevels forming the dark state only. The results are of potential interest in applications where high-fidelity quantum control is essential, e.g., quantum information, atom optics, formation of ultracold molecules, cavity QED, etc.

A geometry-based mechanism for inducing circulation of photons is illustrated by a metastructure consisting of three quantum dots embedded in photonic structures and forming a triangle. The coupling between the photons and the excitons in the quantum dots leads to a photon blockade and limits the number of photons participating in the transport. In the steady state described by the quantum master equation of photons, the local photonic currents exhibit distinct circulation patterns, which originate from the wave nature in a multipath geometry. The geometry-based mechanism does not require an artificial gauge field from light-matter interactions. The phase diagrams showing the regions of different patterns of circulation with and without the effective photon interactions are presented. As the number of photons allowed per site increases, the regions saturate. By using the third-quantization formalism, we show the circulation survives without any photon blockade in the noninteracting case. Moreover, we demonstrate the decoupling of the direction of the local current from the density difference and propose possible applications of the local photonic transport.

In classical theory, the trajectory of a particle is entirely predetermined by the complete set of initial conditions via dynamical laws. Based on this, we formulate a no-go theorem for the dynamics of classical particles, i.e., a Bell inequality for trajectories, and discuss its possible violation in a quantum scenario. A trajectory, however, is not an outcome of a quantum measurement, in the sense that there is no observable associated with it, and thus there is no “direct” experimental test of the Bell inequality for trajectories. Nevertheless, we show how to overcome this problem by considering a special case of our generic inequality that can be experimentally tested point-by-point in time. Such inequality is indeed violated by quantum mechanics, and the violation persists during an entire interval of time and not just at a particular singular instant. We interpret the violation to imply that trajectories (or at least pieces thereof) cannot exist predetermined, within a local-realistic theory.

The usual Su-Schrieffer-Heeger model with an even number of lattice sites possesses two degenerate zero-energy modes. The degeneracy of the zero-energy modes leads to the mixing between the topological left- and right-edge states, which makes it difficult to implement the state transfer via topological edge channel. Here, enlightened by the Rice-Male topological pumping, we find that the staggered periodic next-nearest-neighbor hoppings can also separate the initial mixed edge states, which ensures the state transfer between topological left- and right-edge states. Significantly, we construct a unique topological state transfer channel by introducing the staggered periodic onsite potentials and the periodic next-nearest-neighbor hoppings added only on the odd sites simultaneously, and we find that the state initially prepared at the last site can be transferred to the first two sites with the same probability distribution. This special topological state transfer channel is expected to realize a topological beam splitter, whose function is to make the initial photon at one position appear at two different positions with the same probability. Further, we demonstrate the feasibility of implementing the topological beam splitter based on the circuit quantum electrodynamic lattice. Our scheme opens up a new way for the realization of topological quantum information processing and provides a path toward the engineering of new type of quantum optical device.

We consider the task of secure multiparty distributed quantum computation on a quantum network. We propose a protocol based on quantum error correction which reduces the number of necessary qubits. That is, each of the $n$ nodes in our protocol requires an operational workspace of $n^2+4n$ qubits, as opposed to the previously shown $\mathrm{\Omega }\left((n^3+n^2{s}^2)logn\right)$ qubits, where $s$ is a security parameter. Additionally, we reduce the communication complexity by a factor of $O(n^{3}log\left(n\right))$ qubits per node compared to existing protocols. To achieve universal computation, we develop a distributed procedure for verifying magic states, which allows us to apply distributed gate teleportation and which may be of independent interest. We showcase our protocol in a small example for a seven-node network.

We demonstrate lasing in arrays of microring resonators with underlying para-Fermi symmetry. The properties of the algebra allow the analytic prediction of propagation constants and normal modes of an array of passive resonators that show the optical analog of a zero-energy mode that is its own chiral pair. The rest of the normal modes in the linear model form chiral pairs that are evenly distributed around the pseudo-zero-energy mode. We use this information to construct a class-B laser model where even and odd sites are driven with different strength in a pattern following para-Fermi algebra chirality to demonstrate lasing with enhancement or suppression of the zero-energy mode. The former leads to lasing in the zero-energy mode for a large set of driving ratios and is mostly independent of initial field configurations. In contrast, the latter can lead to vanishing fields or lasing with either steady or strongly fluctuating fields depending on the driving ratios and initial field configurations.

A $\mathcal{PT}$-symmetry-breaking-enhanced cavity optomechanical magnetometer is proposed, which is achieved by monitoring the change in intensity of a nonlinear four-wave mixing (FWM) process in a gain-cavity-assisted cavity optomechanical system (COMS). Compared with the traditional single loss COMS, the FWM intensity can be enhanced by two orders of magnitude when the gain-cavity-assisted COMS operates on the $\mathcal{PT}$-symmetry-breaking phase. Meanwhile, the sensitivity of magnetic-field sensing can be increased from ${10}^{-9}\mathrm{T}$ to ${10}^{-11}\mathrm{T}$. This originally comes from the fact that the effective detuning and decay of the loss cavity can be effectively modified in the $\mathcal{PT}$-symmetry-breaking phase. Our work shows that an ultrahigh-sensitivity magnetometer can be achieved in the $\mathcal{PT}$-symmetry-breaking COMS, which will have wide applications in the field of quantum sensing.

The approach of variational measurement is applied to the cavityless dissipative coupling (when test mass displacement changes transmittance of a single mirror). The result obtained is compared to the cavityless dispersive coupling (when the mass of the displaced single mirror changes the phase in the reflected wave). These types of coupling are compared for ponderomotive squeezing. In addition, the cavityless variant of combined dissipative and dispersive coupling is explored. It is established that combined coupling results in a stable optical rigidity even in the case of a single pump. Finally, it is confirmed that variational measurement can be applied to combined coupling.

In view of recent proposals for the realization of anisotropic light-matter interaction in such platforms as (i) nonstationary or inductively and capacitively coupled superconducting qubits, (ii) atoms in crossed fields, and (iii) semiconductor heterostructures with spin-orbital interaction, the concept of a generalized Dicke model, where coupling strengths of rotating wave and counter-rotating wave terms are unequal, has attracted great interest. For this model we study photon fluctuations in the critical region of normal-to-superradiant phase transition when both the temperatures and numbers of two-level systems are finite. In this case, the superradiant quantum phase transition is changed to a fluctuational region in the phase diagram that reveals two types of critical behaviors. These are regimes of Dicke model (with discrete ${\mathbb{Z}}_2$ symmetry), and that of anti-Tavis-Cummings and Tavis-Cummings $U\left(1\right)$ models. We show that squeezing parameters of photon condensate in these regimes show distinct temperature scalings. Besides, relative fluctuations of a photon number take universal values. We also find a temperature scale below which one approaches a zero-temperature quantum phase transition where quantum fluctuations dominate. Our effective theory is provided by a non-Goldstone functional for condensate mode and by Majorana representation of Pauli operators. We also discuss the Bethe ansatz solution for integrable $U\left(1\right)$ limits.

Correlation self-testing of a theory addresses the question of whether we can identify the set of correlations realizable in a theory from its performance in a particular information processing task. Applied to quantum theory it aims to identify an information processing task whose optimal performance is achieved only by theories realizing the same correlations as quantum theory in any causal structure. In M. Weilenmann and R. Colbeck [Phys. Rev. Lett. 125, 060406 (2020)], we introduce a candidate task for this—the adaptive Clauser-Horne-Shimony-Holt (CHSH) game. Here, we analyze the maximum probability of winning this game in different generalized probabilistic theories. We show that theories with a joint state space given by the minimal or the maximal tensor product are inferior to quantum theory, before considering other tensor products in theories whose elementary systems have various two-dimensional state spaces. For these, we find no theories that outperform quantum theory in the adaptive CHSH game and prove that it is impossible to recover the quantum performance in various cases. This is the first step towards a general solution that, if successful, will have wide-ranging consequences, in particular, enabling an experiment that could rule out all theories in which the set of realizable correlations does not coincide with the quantum set.

The interaction of monochromatic electromagnetic radiation with a photoionized plasma formed during multiphoton ionization of inert gas atoms is studied. It is shown how electron collisions with neutral atoms affect the surface impedance and absorption coefficient. The possibility of a significant absorption coefficient growth in the regimes of high-frequency and normal skin effects due to the manifestation of the Ramsauer-Townsend effect was revealed. The conditions under which the interaction of test low-frequency radiation with a photoionized inert gas plasma is similar to the interaction with a dielectric with a permittivity having a small imaginary part are established.

Electrons have much shorter timescales of movement than nuclei, and thus electron dynamics is generally averaged out in the study of molecular rotation. However, our numerical study shows that the electron dynamical localization on different nuclei during the molecular dissociation may determine the molecular rotation directions. Taking $\mathrm{D}{{}_2}^{+}$ as the prototype, an isolated linearly polarized attosecond pulse initiates the molecular dissociation, and then a time-delayed linearly polarized middle-infrared pulse, with the polarization cross angle $\pi /4$ to the attosecond pulse, exerts opposite torques on the molecule when the electron localizes on different nuclei, resulting in the clockwise or counterclockwise rotation of the dissociating $\mathrm{D}{{}_2}^{+}$. The time-dependent analysis explores the complex behavior of molecular rotation determined by the ultrafast electron dynamics, and sheds light on quantum control of molecular rotation.

We investigate the dynamical properties of one-dimensional dissipative Fermi-Hubbard models, which are described by the Lindblad master equations with site-dependent jump operators. The corresponding non-Hermitian effective Hamiltonians with pure loss terms possess parity-time ($\mathcal{PT}$) symmetry if we compensate the system additionally with an overall gain term. By solving exactly the two-site Lindblad equation with fixed dissipation, we find that the dynamics of the rescaled density matrix shows an instability as the interaction increases over a threshold, which can be equivalently described in the scheme of non-Hermitian effective Hamiltonians. This instability is also observed in multisite systems and closely related to the $\mathcal{PT}$-symmetry breaking accompanied by appearance of complex eigenvalues of the effective Hamiltonian. Moreover, we unveil that the dynamical instability of the antiferromagnetic Mott phase comes from the $\mathcal{PT}$-symmetry breaking in highly excited bands, although the low-energy effective model of the non-Hermitian Hubbard model in the strongly interacting regime is always Hermitian. We also provide a quantitative estimation of the time for the observation of dynamical $\mathcal{PT}$-symmetry breaking which could be probed in experiments.

We present extensive ab initio path-integral Monte Carlo (PIMC) simulations of two-dimensional quantum dipole few-body systems ($2\le N\le 7$) in a harmonic confinement, taking into account both Bose- and Fermi-statistics. This allows us to study the nonclassical rotational inertia, which can lead to a negative superfluid fraction in the case of fermions [Phys. Rev. Lett. 112, 235301 (2014)]. Moreover, we study in detail the structural characteristics of such systems and are able to clearly resolve the impact of quantum statistics on density profiles and the respective shell structure. Further, we present results for a more advanced center-two-particle correlation function [Phys. Rev. E 91, 043104 (2015)], which allows detection of differences between Fermi and Bose systems even when they are almost absent in other observables like the density. Overall, we find that bosonic systems sensitively react to even small values of the dipole-dipole coupling strength, whereas such a weak interaction is effectively masked for fermions by the Pauli exclusion principle. In addition, the abnormal superfluid fraction for fermions is not reflected by the structural properties of the system, which are equal to the bosonic case even though the moments of inertia diverge from each other. Lastly, we have explored the possibility of fermionic PIMC simulations of quantum dipole systems despite the notorious fermion sign problem, which can be further extended in future investigations in this field.

We explore the topological properties of non-Hermitian nodal-link semimetals with dissipative cold atoms in a three-dimensional optical lattice. We construct a two-band continuum model in three dimensions with a spin-dependent gain and loss, where the exceptional points in the energy spectrum can comprise a double Hopf link. The topology of the bulk band is characterized by a winding number defined for a one-dimensional loop in the momentum space and a topological transition of the nodal structures emerges as the change of the non-Hermiticity strength. A non-Bloch theory is built to describe the corresponding lattice model which has anomalous bulk-boundary correspondence. Furthermore, we propose that the model can be realized by using ultracold fermionic atoms in an optical lattice, and the exceptional nodal links as well as the topological properties can be detected by measuring the atomic spin textures.

We characterize the high-temperature thermodynamics of rotating bosons and fermions in two-dimensional (2D) and three-dimensional (3D) isotropic harmonic trapping potentials. We begin by calculating analytically the conventional virial coefficients $b_n$ for all $n$ in the noninteracting case, as functions of the trapping and rotational frequencies. We also report on the virial coefficients for the angular momentum and associated moment of inertia. Using the $b_n$ coefficients, we analyze the deconfined limit (in which the angular frequency matches the trapping frequency) and derive explicitly the limiting form of the partition function, showing from the thermodynamic standpoint how both the 2D and 3D cases become effectively homogeneous 2D systems. To tackle the virial coefficients in the presence of weak interactions, we implement a coarse temporal lattice approximation and obtain virial coefficients up to third order.