Carbon disulfide dimers ${\left({\mathrm{CS}}_2\right)}_2$ are doubly ionized by an intense 40-fs laser pulse leading to fragmentation into a pair of ${{\mathrm{CS}}_2}^{+}$ cations. Unlike previous studies on atomic dimers, their kinetic energy, gained by mutual electrostatic repulsion, deviates strongly from the value determined by applying the Coulomb interaction approximation to the ${{\left({\mathrm{CS}}_2\right)}_2}^{2+}$ ion. An ab initio calculation shows that inclusion of non-Coulombic interactions in ${{\left({\mathrm{CS}}_2\right)}_2}^{2+}$ is crucial to account for the observed kinetic energy. These interactions result in 16 ${{\left({\mathrm{CS}}_2\right)}_2}^{2+}$ electronic microstates, which obstructs accurate determination of intermolecular bond distances in ${\left({\mathrm{CS}}_2\right)}_2$ from the fragments' kinetic energy. The situation will be similar for most other weakly bonded atomic and molecular complexes.

The threshold conditions for a rotating pancakelike asymmetric quartic-quadratic confined condensate to break in two localized fragments, as well as to produce a giant vortex at the center within the vortex-pattern distributions, are investigated within the Thomas-Fermi (TF) approximation and full-numerical solution of the corresponding Gross-Pitaevskii (GP) formalism. By comparing the TF predictions with the GP solutions, in our investigation with two different quartic-quadratic trap geometries, of particular relevance is to observe that the TF approach is not only very useful to display the averaged density distribution, but also quite realistic in establishing the critical rotational conditions for the breakup occurrence and possible giant-vortex formation. It provides almost exact results to define the contour of the condensate distribution, even for high rotating systems, after the system split into two (still confined) clouds. The applicability of the Feynman rule to the vortex distribution (full-numerical GP solutions) is also being confirmed for these nonhomogeneous asymmetric trap configurations. This study is expected to be relevant for manipulating the rotation and trap parameters in addition to Feshbach resonance techniques. It can also be helpful to define initial conditions for any further studies on the dynamical evolution of vortex pattern distributions.

In this paper, we analyze the quantum criticality of the Rabi-Stark model at finite ratios of the qubit and cavity frequencies in terms of the energy gap and the order parameter, as well as the fidelity, if the Stark coupling strength is the same as the cavity frequency. The critical exponents are derived analytically. The energy gap and the length critical exponents are different from those in the quantum Rabi model and the Dicke model. A finite-size scaling analysis for the order parameter and the fidelity susceptibility is also performed. The universal scaling behaviors are demonstrated, and several finite-size exponents then can be extracted. Furthermore, universal critical behavior also can be established in terms of the bosonic Hilbert space truncation number, and the corresponding critical scaling exponents are found. Interestingly, the critical correlation length exponents in terms of the photonic truncation number as well as the equivalently effective length scales are different in the Rabi-Stark model and the quantum Rabi model, suggesting they belong to different universality classes. The second-order quantum phase transition is convincingly corroborated in the Rabi-Stark model at finite frequency ratios; by contrast, it emerges only at the infinite frequency ratio in the original quantum Rabi model without the Stark coupling.

Entanglement generation and preservation is a key task in quantum information processing, and a variety of protocols exist to entangle remote qubits via measurement of their spontaneous emission. We here propose feedback methods, based on monitoring the fluorescence of two qubits and using only local $\pi$ pulses for control, to increase the yield and/or lifetime of entangled two-qubit states. Specifically, we describe a protocol based on photodetection of spontaneous emission (i.e., using quantum jump trajectories) which allows for entanglement preservation via measurement undoing, creating a limit cycle around Bell states. We then demonstrate that a similar modification can be made to a recent feedback scheme based on homodyne measurement (i.e., using diffusive quantum trajectories) [L. S. Martin and K. B. Whaley, arXiv:1912.00067] in order to increase the lifetime of the entanglement it creates. Our schemes are most effective for high measurement efficiencies, and the impact of less-than-ideal measurement efficiency is quantified. Our method provides a pathway towards generating and protecting entangled states, complementing others in the literature with similar aims, such that two-qubit entanglement may be used in various applications on demand.

We propose an approach to the analysis of the semiclassical evolution of spinlike systems. We show that an appropriate discretization of distributions in classical phase space (in this case the two-dimensional sphere ${\mathcal{S}}_2$) allows us to describe long-time dynamics (including the Schrödinger cat times) in terms of classical trajectories, both in stable and nonstable regimes and for a variety of initial states. The discretization method is based on the Majorana stellar representation of spin states and takes into account dynamical properties of the initial distribution in the corresponding classically generated potential on ${\mathcal{S}}_2$.

Quantum state transformation under free operations plays a central role in the resource theory of coherence. In this paper, we investigate the transformation from a mixed coherent state into a pure one by using both incoherent operations and stochastic incoherent operations. We show that contrary to the strictly incoherent operations and the stochastic strictly incoherent operations, both the incoherent operations and the stochastic incoherent operations can increase the dimension of the maximal pure coherent subspace of a state. This means that the incoherent operations are generally stronger than the strictly incoherent operations when we want to transform a mixed coherent state into a pure coherent one. Our findings can also be interpreted as confirming the ability of incoherent operations to enhance the coherence of mixed states relative to certain coherence monotones under strictly incoherent operations.

Monogamy of entanglement is an indispensable feature in multipartite quantum systems. In this paper we investigate monogamy and polygamy relations with respect to any partition for generalized $W$-class states using Rényi-$\alpha$ entropy. First, we present analytical formulas of Rényi-$\alpha$ entanglement $\left(\mathrm{R}\alpha \mathrm{E}\right)$ and Rényi-$\alpha$ entanglement of assistance $\left(\mathrm{R}\alpha \mathrm{EoA}\right)$ for a reduced density matrix of an $n$-qudit pure state in a superposition of generalized $W$-class states and vacuum. Based on the analytical formulas, we show monogamy and polygamy relations in terms of $\mathrm{R}\alpha \mathrm{E}$ and $\mathrm{R}\alpha \mathrm{EoA}$. Next a reciprocal relation of $\mathrm{R}\alpha \mathrm{EoA}$ in an arbitrary three-party quantum system is found. Later, we further develop tighter monogamy relations in terms of concurrence and convex-roof extended negativity than former ones. In order to show the usefulness of our results, two partition-dependent residual entanglements are established to get a comprehensive analysis of entanglement dynamics for generalized $W$-class states. Moreover, we apply our results to an interesting quantum game and find a bound of the difference between the quantum game and the classical game.

Tensor networks are the main building blocks in various computational sciences, ranging from many-body theory and quantum computing to probability and machine learning. Here we propose a parallel algorithm for the contraction of tensor networks using probabilistic graphical models. Given a tensor network, this algorithm splits the contraction task almost optimally into subtasks, which can be contracted independently in parallel. Our approach is based on the heuristic solution of the $\mu$-treewidth deletion problem in graph theory. We apply the resulting algorithm to the simulation of random quantum circuits and discuss the extensions for general tensor network contractions. According to our estimates, the lower bound on the simulation time of 1 million amplitudes of a 49-qubit circuit of depth 40 on the Summit supercomputer is $>5$ seconds.

Field-free molecular orientation, achieved by using two-color slow turn-on and rapid turn-off laser pulses, is theoretically investigated for linear molecules. We derive analytical results for the maximum degree of field-free orientation for a given number of populated rotational states. We show that numerical results with adiabatic-turn-on and sudden-turn-off laser pulses attain the optimal orientation. Based on the adiabatic theorem, the effects of laser and molecular parameters on both the adiabatic process and the degree of molecular orientation are analyzed. The adiabaticity is well reflected through the temporal evolution of the phase difference between two adjacent rotational states.

The solid-liquid phase transition (melting) is a long-standing problem for which a complete understanding is challenging because of the strong many-body interactions. A trapped-ion Wigner crystal is a suitable platform for a controlled study of this phase transition with its unprecedented resolution of individual atomic ions. Here we study the melting dynamics of a linear chain of ${}^{174}\mathrm{Yb}^{+}$ ions, which is initially cooled to the Doppler temperature and is then periodically heated and detected by a focused laser beam. We achieve controlled melting of the ion chain and observe unusual partial melting of long chains (coexistence of solid and liquid phases). We also observe a counterintuitive nonuniform energy distribution over the chain under localized heating. These results are further confirmed by molecular dynamics simulations to rule out other heating mechanisms.

We study nonlinear dynamics of Rydberg-dressed Bose-Einstein condensates (BECs) trapped in a triple-well potential in the semiclassical limit. The Rydberg-dressed BECs experience a long-range soft-core interaction, giving rise to strong nearest- and next-nearest-neighbor interactions in the triple-well system. Using mean-field Gross-Pitaevskii (GP) equations, we show that lower branches of the eigenspectra exhibit loops and level crossings when the soft-core interaction is strong. The direct level crossings eliminate the possibility of adiabatic Landau-Zener transitions when tilting of the triple-well potential. We demonstrate that the long-range interaction allows for self-trapping in one, two, or three wells, in a far more controllable manor than BECs with short-range or dipolar interactions. Exact quantum simulations of the three-well Bose-Hubbard model indicate that self-trapping and nonadiabatic transition can be observed with less than a dozen bosons. Our study is relevant to current research into collective excitation and nonlinear dynamics of Rydberg-dressed atoms.

Vectorial solutions of Maxwell's equations describing highly focused and variously polarized vector complex source vortex beams are investigated. An analytical representation of these beams in the vector-spherical basis of electromagnetic multipoles is presented. In particular, three different families of optical vector vortex beams are studied in detail. Whereas the vortical solutions derived within spherical symmetry can be represented only by electric (magnetic) multipoles, solutions derived within cylindrical and Cartesian symmetry also exhibit magnetic (electric) multipole components. We utilize the representation of the studied beams in vector-spherical harmonics to investigate their interaction with a cluster of nanoparticles.

We study the quantum backflow problem in the noncommutative plane. In particular, we have considered a charged particle with and without an oscillator interaction with noncommuting momentum operators and examined angular momentum backflow in each case and how they differ from each other. We also propose a probability associated with the occurrence of angular momentum backflow and investigate whether or not the probability depends on a physical parameter, namely, the magnetic field.

We introduce a quantification of genuine three-party pure-state coherence for wave fields, classical and quantum, by borrowing concepts from classical optics. The tensor structure of a classical paraxial light beam composed of three principle degrees of freedom is shown to be equivalent to that of a three-qubit quantum state. The traditional basis-independent optical coherence quantity called degree of polarization is then determined to be the desired quantitative two-party coherence measure. When appropriately generalized, a set of fundamental constraint relations is derived among three two-party coherences. The constraint relations can be geometrically interpreted and visualized as tetrahedra nested within a coherence cube. A measure of three-party coherence is defined based on the constraints. We are reporting completed experimental tests and confirmations of the constraints as well as measurement of three-party coherence in the optical context. Our approach based on classical optics also opens an alternative way to analyze quantum coherence.

Solving for molecular excited states remains one of the key challenges of modern quantum chemistry. Traditional methods are constrained by existing computational capabilities, limiting the complexity of the molecules that can be studied or the accuracy of the results that can be obtained. Several quantum computing methods have been suggested to address this limitation. However, these typically have hardware requirements which may not be achieved in the near term. We propose a variational quantum machine learning based method to determine molecular excited states aiming at being as resilient as possible to the defects of early noisy intermediate scale quantum computers and demonstrate an implementation for ${\mathrm{H}}_2$ on IBM Quantum Computers. Our method uses a combination of two parametrized quantum circuits, working in tandem, combined with a variational quantum eigensolver to iteratively find the eigenstates of a molecular Hamiltonian.

We demonstrate the application of pattern recognition algorithms via hidden Markov models (HMM) for qubit readout. This scheme provides a state-path trajectory approach capable of detecting qubit-state transitions and makes for a robust classification scheme with higher starting-state assignment fidelity than when compared to a multivariate Gaussian or a support vector machine scheme. Therefore, the method also eliminates the qubit-dependent readout time optimization requirement in current schemes. Using a HMM state discriminator we estimate fidelities reaching the ideal limit. Unsupervised learning gives access to transition matrix, priors, and $IQ$ distributions, providing a toolbox for studying qubit-state dynamics during strong projective readout.

In a combined experimental and theoretical study, we probe the vibrational excitation of isocyanic acid induced by electron impact in the energy range up to 5 eV. Experimentally, we report differential elastic and vibrationally inelastic cross sections at the scattering angle of ${135}^{\circ }$. Theoretically, we characterize the involved resonant states using a regularized analytical continuation method. We also apply a nonlocal resonance model to calculate cross sections for vibrational mode that involves the N-H stretching motion. The model reproduces all the features observed in the experiment: efficient excitation at threshold, sharp cusps in the excitation functions, and the formation of an $A^{\prime }$ shape resonance. There is a second $\left(A^{\prime \prime }\right)$ resonance visible in the spectra; however, it excites only selected vibrational modes. The origins of this selectivity are discussed.