We use optimal control in order to find the optimal shapes of pulses maximizing the population transfer between two bound states which are coupled via a continuum of states. We find that the optimal bounded controls acquire the bang-interior and interior-bang form, with the bang part corresponding to the maximum allowed control value and the interior part to values between zero and the maximum. Then we use numerical optimal control to obtain the switching times and the interior control values. We compare our results with those obtained using Gaussian stimulated Raman adiabatic passage pulses and find that the optimal method performs better, with the extent of improvement depending on the effective two-photon detuning and the size of incoherent losses. When we consider effective two-photon resonance, the improvement is more dramatic for larger incoherent losses, while when we take into account the effective two-photon detuning, the improvement is better for smaller incoherent losses. We also obtain how the transfer efficiency increases with increasing absolute value of the Fano factor. The present work is expected to find application in areas where the population transfer between two bound states through a continuum structure plays an important role, for example coherence effects, like population trapping and electromagnetically induced transparency, optical analogs for light waves propagating in waveguide-based photonic structures, and qubits coupled via a continuum of bosonic or waveguide modes.

We theoretically investigate the modulated high-harmonic generation (HHG) driven by an intense few-cycle infrared (IR) laser field and a weak extreme-ultraviolet (XUV) pulse at a delayed time. We establish an extended quantitative rescattering (EQRS) model to simulate the HHG streaking spectra, with the ideas of correcting the IR ionization and the transition from the ground to continuum states in the strong-field approximation. The EQRS model has an accuracy comparable to that from “exactly” solving the time-dependent Schrödinger equation (TDSE). We reveal that the fringes in the streaking spectra are caused by the interference between the attosecond XUV pulse and harmonics resulting from different recombination pathways under the intense IR laser. We then demonstrate that the XUV pulse can be accurately retrieved by treating the single-atom TDSE results or macroscopic propagation results as the “input” data. This work provides with a tool for efficiently simulating and thoroughly analyzing the XUV-assisted HHG, which could also enhance its capability for tracing the electron dynamics involved in the strong-field phenomena.

We present an energy scaling function to predict, in a specific range, the energy of bosonic trimers with large scattering lengths and finite range interactions, which is validated by quantum Monte Carlo calculations using microscopic Hamiltonians with two- and three-body potentials. The proposed scaling function depends on the scattering length, effective range, and a reference energy, which we chose as the trimer energy at unitarity. We obtained the scaling function as a limit cycle from the solution of the renormalized zero-range model with effective range corrections. We proposed a simple parametrization of the energy scaling function. Besides the intrinsic interest in theoretical and experimental investigations, this scaling function allows one to probe Efimov physics with only the trimer ground states, which may open opportunities to identify Efimov trimers whenever access to excited states is limited.

The effects of gain and loss on the band structures of a bulk topological dielectric photonic crystal (PC) with $C_{6v}$ symmetry and the PC-air-PC interface are studied based on first-principle calculation. To illustrate the importance of parity-time (PT) symmetry, three systems are considered, namely the PT-symmetric, PT-asymmetric, and lossy systems. We find that the system with gain and loss distributed in a PT symmetric manner exhibits a phase transition from a PT exact phase to a PT broken phase as the strength of the gain and loss increases, while for the PT-asymmetric and lossy systems, no such phase transition occurs. Furthermore, based on the Wilson loop calculation, the topology of the PT-symmetric system in the PT exact phase is demonstrated to keep unchanged as the Hermitian system. At last, different kinds of edge states in Hermitian systems under the influences of gain and loss are studied and we find that while the eigenfrequencies of nontrivial edge states become complex conjugate pairs, they keep real for the trivial defect states.

The Bures distance holds a special place among various distance measures due to its several distinguishing features and has applications in diverse problems in quantum information theory. It is related to fidelity, and among other things, it serves as a bona fide measure for quantifying the separability of quantum states. In this work, we calculate exact analytical results for the mean root fidelity and mean-square Bures distance between a fixed density matrix and a random density matrix and also between two random density matrices. In the course of derivation, we also obtain the spectral density for the product of the above pairs of density matrices. We corroborate our analytical results using Monte Carlo simulations. Moreover, we compare these results with the mean-square Bures distance between reduced density matrices generated using coupled kicked tops and find very good agreement.

Satellite-based quantum communication is essential for both foundational quantum physics tests and scalable quantum networks. However, the practical implementation of such application is limited due to the turbulent atmosphere, which is especially challenging for the ground-to-satellite uplink scenario. Here we consider the configuration of the continuous-variable quantum teleportation protocol to overcome both the uplink limit and background-radiation constraints with an unconditional and deterministic transfer, where the propagation of quantum light is affected by the favorable satellite-to-ground downlink (including the effects of absorption, scattering, turbulence, pointing errors, and background light). In particular, we derive the turbulent-induced fidelity that is achievable by the protocol for both nighttime and daytime operations, showing the effectiveness of this approach for various squeezing levels and orbit conditions (satellite altitudes and zenith angles). Finally, several approaches for fidelity enhancement are suggested, in terms of optimizing the transmitted Gaussian entanglement in the free-space scenario.

Maximally entangled states play a central role in quantum information processing. Despite much progress throughout the years, robust protocols for manipulations of such states in many-level systems are still scarce. Here we present a control scheme that allows efficient manipulation of complete population inversion between two maximally entangled states. Exploiting the self-duality of $\mathrm{SU}\left(2\right)$, we present in this work a family of $2^{2N}$-level systems with couplings related to Pythagorean triples that make a complete population inversion from one state to another (orthogonal) state, using very few couplings and generators. We relate our method to the recently developed retrograde-canon scheme and derive a more general complete transfer recipe. We also discuss the cases of ${\left(2n\right)}^2$-level systems, ${(2n+1)}^2$-level systems, and other unitary groups, and give a geometrical description of the inversion via the Majorana sphere.

We perform accurate calculations of energy levels as well as fine and hyperfine splittings of the lowest ${}^{1,3}P_J, {}^{3}S_1, {}^{3}P_J^e$, and ${}^{1,3}D_J$ excited states of the ${}^9\mathrm{Be}$ atom using explicitly correlated Gaussian functions, and we report on the breakdown of the standard hyperfine structure theory. Because of the strong hyperfine mixing, which prevents the use of common hyperfine constants, we formulate a description of the fine and hyperfine structure that is valid for an arbitrary coupling strength and may have wide applications in many other atomic systems.

The dynamics of molecules exposed to intense x-ray radiation involve a large number of multiply ionized and highly excited electronic configurations. To model these dynamics a reliable and efficient electronic structure model is imperative. Employing the Hartree-Fock-Slater electronic structure model in combination with the maximum overlap method, we quantify the associated convergence failures when calculating electronic states of carbon monoxide with multiple vacancies in the core and valence levels. We characterize these cases and describe strategies to overcome the convergence problems. The described techniques not only eliminate all convergence issues for CO but also result in a significant reduction of convergence failures for simulations of the x-ray-induced multiple ionization dynamics of the phenol molecule.

Abruptly autofocusing beams have attracted more and more attention because of their potential applications in optical manipulation. Here, we not only experimentally and theoretically investigate dynamical propagation properties of two-dimensional (2D) Airy–beam superpositions in free space and disordered media, but also study their stability, dynamical power flow, and trapping force properties from a different point. 2D Airy-beam superpositions have superior stability and can easily control their autofocusing propagations and trapping forces through a changing number of superimposing beams, truncated factor, scale factor, initial position, and the angle between two wings of 2D Airy beams. Based on the analysis of the evolution of power flow and trapping force, it is demonstrated that 2D Airy-beam superpositions repeat many autofocusing propagations, leading to a series of focuses and stable trapping positions to appear in propagation. Compared to traditional autofocusing beams, i.e., circular Airy beams and one-dimensional Airy-beam superpositions, 2D Airy-beam superpositions can show a better optical trapping performance in theory. Moreover, we used 2D Airy-beam superpositions as an optical tweezer to trap particles in the experiment and quantitatively analyzed trapping force properties such as trap stiffness via the power spectra method. The experimental results further demonstrate that 2D Airy-beam superposition can show better optical trapping ability than the circular Airy beam. Our work provides a systematic understanding of the propagation and trapping properties of 2D Airy-beam superpositions in theory and experiment. It is helpful for the further applications of autofocusing beams on the optical tweezer, especially in biomedical research.

To understand the dynamical origin of the measurement in quantum mechanics, several models have been put forward which have a quantum system coupled to an apparatus. The system and the apparatus evolve in time, and the Born rule for the system to be in various eigenstates of the observable is naturally obtained. In this paper, we show that the effect of the drive-induced dissipation in an open quantum system can lead to the Born rule, even if there is no separate apparatus. The applied drive needs to be much stronger than the system-environment coupling. In this condition, we show that the dynamics of the driven-dissipative system could be reduced to a Milburn-like form, using a recently proposed fluctuation-regulated quantum master equation [A. Chakrabarti and R. Bhattacharyya, Phys. Rev. A 97, 063837 (2018).]. The irreversible part of the dynamics is caused by the drive-induced dissipation. The resulting mixed state is identical to that obtained by using the Born rule.

State space structure of tripartite quantum systems is analyzed. In particular, it has been shown that the set of states separable across all the three bipartitions [say, ${\mathcal{B}}^{int}\left(ABC\right)$] is a strict subset of the set of states having positive partial transposition across the three bipartite cuts [say, ${\mathcal{P}}^{int}\left(ABC\right)$] for all the tripartite Hilbert spaces ${\mathbb{C}}_A^{d_1}\otimes {\mathbb{C}}_B^{d_2}\otimes {\mathbb{C}}_C^{d_3}$ with $min\{d_1,d_2,d_3\}\ge 2$. The claim is proved by constructing a state belonging to the set ${\mathcal{P}}^{int}\left(ABC\right)$ but not to ${\mathcal{B}}^{int}\left(ABC\right)$. For ${\left({\mathbb{C}}^d\right)}^{\otimes 3}$ with $d\ge 3$, the construction follows from a specific type of multipartite unextendible product bases. However, such a construction is not possible for ${\left({\mathbb{C}}^2\right)}^{\otimes 3}$ since bipartite systems ${\mathbb{C}}^2\otimes {\mathbb{C}}^n$ do not allow any unextendible product bases for arbitrary $n$ [Phys. Rev. Lett. 82, 5385 (1999)]. For the three-qubit system, we therefore come up with a different construction.

Quantum information scrambling describes the delocalization of local information to global information in the form of entanglement throughout all possible degrees of freedom. A natural measure of scrambling is the tripartite mutual information, which quantifies the amount of delocalized information for a given quantum channel with its state representation, i.e., the Choi state. In this work, we show that quantum information scrambling can also be witnessed by temporal quantum steering for qubit systems. We can do so because there is a fundamental equivalence between the Choi state and the pseudodensity matrix formalism used in temporal quantum correlations. In particular, we propose a quantity as a scrambling witness, based on a measure of temporal steering called temporal steerable weight. We justify the scrambling witness for unitary qubit channels by proving that the quantity vanishes whenever the channel is nonscrambling.

We propose a temporal multislit interference model of high-order harmonic generation (HHG). By using the interference picture, we investigate the polarization features of harmonics generated by atoms and the two-dimensional synthesized driving laser fields. The result shows that the selection rules and ellipticity of harmonics can be understood by intracycle interference. Based on quantum path analysis, we show the harmonic polarization can be controlled by modifying the structure of the temporal interferometer. We perform a quantitative analysis of the harmonic polarization spectrum in terms of electron dynamics and provide an intuitive picture and a general tool for HHG investigation.

We report on the experimental characterization of a spatially extended Josephson junction realized with a coherently coupled two-spin-component Bose-Einstein condensate. The cloud is trapped in an elongated potential such that transverse spin excitations are frozen. We extract the nonlinear parameter with three different manipulation protocols. The outcomes are all consistent with a simple local density approximation of the spin hydrodynamics, i.e., of the so-called Bose-Josephson junction equations. We also identify a method to produce states with a well-defined uniform magnetization.

Driven-dissipative many-body systems are difficult to analyze analytically due to their nonequilibrium dynamics, dissipation, and many-body interactions. In this paper, we consider a driven-dissipative infinite-range Ising model with local spontaneous emission, which naturally emerges from the open Dicke model in the large-detuning limit. Utilizing an adaptation of the Suzuki-Trotter quantum-to-classical mapping, we develop an exact field-theoretical analysis and a diagrammatic representation of the spin model that can be understood from a simple scattering picture. With this representation, we are able to analyze critical behavior, finite-size scaling, and the effective temperature near the respective phase transition. Our formalism further allows a detailed study of the ordered phase where we find a “heating” region within which the effective temperature becomes negative, thereby exhibiting a truly nonequilibrium behavior. At the phase transition, we find two distinct critical behaviors with overdamped and underdamped critical dynamics at generic and weakly dissipative critical points, respectively. We further show that the underdamped critical behavior is robust against short-range perturbations and is not an artifact of the mean-field nature of the model. To treat such perturbations, we extend our diagrammatic representation to include the coupling to spin waves due to the short-range interactions. The field-theoretical approach and the diagrammatics developed in this work should prove useful in applications to generic short-range driven-dissipative spin systems.

Quantum Fisher information and signal-to-noise ratio bounds are derived for the estimation of moments of general partially coherent objects. Under an asymptotic analysis in the sub-Rayleigh regime, these bounds are shown to be less restrictive than those of the incoherent case. This leaves open the possibility of more accurately estimating object distribution moments when the object is partially coherent.

It is well known that energy-time entanglement can enhance two-photon absorption (TPA) by simultaneously optimizing the two-photon resonance and the coincidence rate of photons at the absorber. However, the precise relation between entanglement and the TPA rate depends on the coherences of intermediate states involved in the transition, making it a rather challenging task to identify universal features of TPA processes. In the present paper, we show that the theory can be simplified greatly by separating the two-photon resonance from the temporal dynamics of the intermediate levels. The result is a description of the role of entanglement in the TPA process by a one-dimensional coherence in the Hilbert space defined by the arrival-time difference of the two photons. Transformation into the frequency-difference basis results in Kramers-Kronig relations for the TPA process, separating off-resonant contributions of virtual levels from resonant contributions. In particular, it can be shown that off-resonant contributions are insensitive to the frequencies of the associated virtual states, indicating that virtual-state spectroscopy of levels above the final two-photon excited state is not possible.