We show that the essential properties of a Feshbach resonance in cold atomic gases can be tuned by dressing the atomic states in different scattering channels through inter-channel couplings. Such a scheme can be readily implemented in the orbital Feshbach resonance of alkaline-earth-metal-like atoms by coupling hyperfine states in the clock-state manifolds. Using ${}^{173}\mathrm{Yb}$ atoms as an example, we find that both the resonance position and the two-body bound-state energy depend sensitively on the inter-channel coupling strength, which offers control parameters in tuning the inter-atomic interactions. We also demonstrate the dramatic impact of the dressed Feshbach resonance on many-body processes such as the polaron to molecule transition and the BCS-BEC crossover.

A reinforcement algorithm solves a classical optimization problem by introducing a feedback to the system, which slowly changes the energy landscape and converges the algorithm to an optimal solution in the configuration space. Here, we use this strategy to concentrate (localize) the wave function of a quantum particle, which explores the configuration space of the problem, preferentially on an optimal configuration. We examine the method by solving numerically the equations governing the evolution of the system, which are similar to the nonlinear Schrödinger equations, for small problem sizes. In particular, we observe that reinforcement increases the minimal energy gap of the system in a quantum annealing algorithm. Our numerical simulations and the latter observation show that such kind of quantum feedback might be helpful in solving a computationally hard optimization problem by a quantum reinforcement algorithm.

Resonant Auger (RA) decay spectra of carbon $1s$ excited ${\mathrm{CH}}_3\mathrm{Cl}$ molecules are recorded with angular resolution using linearly polarized synchrotron radiation. The selected photon energies corresponding to the C $1s\to 8a_1$ core to lowest unoccupied molecular orbital and C $1s\to 4s{a}_1$, $4pe$, and $4p{a}_1$ core to Rydberg excitations of methyl chloride are used and electrons in the binding energy range of 11–37 eV are detected. The vibrationally unresolved RA electron angular distributions, recorded for participator Auger transitions populating the $X$, $A$, $B$, and $C$ states of the ${\mathrm{CH}}_3{\mathrm{Cl}}^{+}$ ion, exhibit strong variations across the selected electronic resonances. These observations are interpreted with the help of ab initio electronic structure and dynamics calculations, which account for electronic-state interference between the direct and different resonant ionization pathways. For spectator transitions, the theory predicts almost isotropic angular distributions with moderate changes of $\beta$ parameters around zero, which is in agreement with the experimental observations.

By carefully following the spatial and temporal criteria of the Debye-Hückel (DH) approximation, we present a detailed theoretical study on the redshifts of the spectroscopically isolated ${\mathrm{He}}_{\alpha }$ lines corresponding to the $1s2p{}^{1}P\to 1s^2{}^{1}S$ emission from two-electron ions embedded in external dense plasma. We first focus our study on the ratio $R=\mathrm{\Delta }{\omega }_{\alpha }/{\omega }_o$ between the redshift $\mathrm{\Delta }{\omega }_{\alpha }$ due to the external plasma environment and the energy ${\omega }_o$ of the ${\mathrm{He}}_{\alpha }$ line in the absence of the plasma. Interestingly, the result of our calculation shows that this ratio $R$ turns out to vary as a nearly universal function of a reduced Debye length ${\lambda }_D\left(Z\right)=(Z-1)D$. Since the ratio $R$ dictates the necessary energy resolution for a quantitative measurement of the redshifts and, at the same time, the Debye length $D$ is linked directly to the plasma density and temperature, the dependence of $R$ on $D$ should help to facilitate the potential experimental efforts for a quantitative measurement of the redshifts for the ${\mathrm{He}}_{\alpha }$ line of the two-electron ions. In addition, our study has led to a nearly constant redshift $\mathrm{\Delta }{\omega }_{\alpha }$ at a given $D$ for all He-like ions with $Z$ between 5 and 18 based on our recent critical assessment of the applicability of the DH approximation to atomic transitions. These two general features, if confirmed by observation, would offer a viable and easy alternative in the diagnostic efforts of the dense plasma.

We present the absolute triple differential cross section (TDCS) for single ionization of $\mathrm{Ne}\left(2p\right)$ at an impact energy of 599.6 eV and $\mathrm{Ar}\left(3p\right)$ at 195 eV. The role of the postcollision interaction (PCI) is studied using a high-order distorted-wave Born approximation model with a continuum distorted-waves expansion. Both the second- and third-order effects are considered in the present calculations, and the third-order distorted wave Born approximation model is reported in the $(e,2e)$ reaction. The calculated results show satisfactory agreement with experimental data. The magnitude of the absolute TDCS is enhanced by a factor 2–3 when the strength factor $\gamma$ of the PCI amplitude is summarized just from 0 to 2. This proves that the PCI plays an important role in the absolute TDCS of the $(e,2e)$ reaction in the intermediate-energy region.

We present a method for generating precise, dynamically tunable magnetic potentials that can be described by a polynomial series along the axis of a cold-atom waveguide near the surface of an atom chip. With a single chip design consisting of several wire pairs, various axial potentials can be created by changing the ratio of the currents in the wires, including double wells, triple wells, and pure harmonic traps with suppression of higher-order terms. We use this method to design and fabricate a chip with modest experimental requirements. Finally, we use the chip to demonstrate a double-well potential.

We study computationally the effects of dissipations on correlated two-photon transport in waveguide–quantum-electrodynamic systems. We show that in the dissipative regime, the two-photon correlation signatures remain prominent even when the two-photon transport can be described by a single-photon picture. Moreover, by varying the dissipation, the photonic correlation can have an induced transition from bunching to antibunching, and vice versa, depending on the input quantum state.

We study a pattern-forming instability in a laser-driven optically thick cloud of cold two-level atoms with a planar feedback mirror. We develop a theoretical model, enabling a full analysis of transverse patterns in a medium with saturable nonlinearity, taking into account diffraction within the medium, and both the transmission and reflection gratings. The focus of the analysis is on the combined treatment of nonlinear propagation in a diffractively and optically thick medium and the boundary condition given by feedback. We demonstrate explicitly how diffraction within the medium breaks the degeneracy of Talbot modes inherent in thin-slice models. We predict the existence of envelope curves bounding all possible pattern-formation thresholds and illustrate their interaction with threshold curves by experimental observation of a sudden transition between length scales as mirror displacement is varied.

We employ phase-sensitive amplification to perform homodyne detection of the resonance fluorescence from a driven superconducting artificial atom. Entanglement between the emitter and its fluorescence allows us to track the individual quantum state trajectories of the emitter conditioned on the outcomes of the field measurements. We analyze the ensemble properties of these trajectories by considering trajectories that connect specific initial and final states. By applying the stochastic path-integral formalism, we calculate equations of motion for the most-likely path between two quantum states and compare these predicted paths to experimental data. Drawing on the mathematical similarity between the action formalism of the most-likely quantum paths and ray optics, we study the emergence of caustics in quantum trajectories: places where multiple extrema in the stochastic action occur. We observe such multiple most-likely paths in experimental data and find these paths to be in reasonable quantitative agreement with theoretical calculations.

We investigate theoretically a driven dissipative model of strongly interacting photons in a nonlinear optical cavity in the presence of a synthetic magnetic field. We show the possibility of using a frequency-dependent incoherent pump to create a strongly correlated $\nu =1/2$ bosonic Laughlin state of light: Due to the incompressibility of the Laughlin state, fluctuations in the total particle number and excitation of edge modes can be tamed by imposing a suitable external potential profile for photons. We further propose angular-momentum-selective spectroscopy of the emitted light as a tool to obtain unambiguous signatures of the microscopic physics of the quantum Hall liquid of light.

An analytical approximation for the eigenvalues of $\mathcal{PT}$-symmetric Hamiltonian $\mathsf{H}=-d^2/d{x}^2-{\left(ix\right)}^{\epsilon +2}$, $\epsilon >-1$ is developed via simple basis sets of harmonic-oscillator wave functions with variable frequencies and equilibrium positions. We demonstrate that our approximation provides high accuracy for any given energy level for all values of $\epsilon >-1$.

One-dimensional spin-$\frac{1}{2}$ systems are well-known candidates to study the quantum correlations between particles. In condensed matter physics, studies often are restricted to first-neighbor particles. In this work, we consider the one-dimensional $XXZ$ model in a transverse magnetic field (TF) which is not integrable except at specific points. Analytical expressions for quantum correlations (entanglement and quantum discord) between spin pairs at any distance are obtained for both zero and finite temperature by using the analytical approach proposed by Caux et al. [Phys. Rev. B 68, 134431 (2003)]. We compare the efficiency of the quantum discord (QD) with respect to the entanglement in the detection of critical points as the neighboring spin pairs go farther than the next-nearest neighbors. In the absence of the TF and at zero temperature, we show that the QD for spin pairs farther than the second neighbors is able to capture the critical points while the pairwise entanglement is absent. In contrast with the pairwise entanglement, two-site QD is effectively long range in the critical regimes where it decays algebraically with the distance between pairs. We also show that the thermal QD between neighbor spins possesses strong distinctive behavior at the critical point that can be seen at finite temperature and, therefore, spotlights the critical point while the entanglement fails in this task.

Photon subtraction from squeezed states is a powerful scheme to create good approximation of so-called Schrödinger cat states. However, conventional continuous-wave-based methods actually involve some impurity in squeezing of localized wave packets, even in the ideal case of no optical losses. Here, we theoretically discuss this impurity by introducing mode match of squeezing. Furthermore, here we propose a method to remove this impurity by filtering the photon-subtraction field. Our method in principle enables creation of pure photon-subtracted squeezed states, which was not possible with conventional methods.

In this paper we present a protocol for the implementation of a positive-operator-valued measure (POVM) on massive fermionic qubits. We present methods for implementing nondispersive qubit transport, spin rotations, and spin polarizing beam-splitter operations. Our scheme attains linear opticslike control of the spatial extent of the qubits by considering ground-state electrons trapped in the minima of surface acoustic waves in semiconductor heterostructures. Furthermore, we numerically simulate a high-fidelity POVM that carries out Procrustean entanglement distillation in the framework of our scheme, using experimentally realistic potentials. Our protocol can be applied not only to pure ensembles with particle pairs of known identical entanglement, but also to realistic ensembles of particle pairs with a distribution of entanglement entropies. This paper provides an experimentally realizable design for future quantum technologies.

This paper proposes an efficient method for the simultaneous estimation of the state of a quantum system and the classical parameters that govern its evolution. This hybrid approach benefits from efficient numerical methods for the integration of stochastic master equations for the quantum system, and efficient parameter estimation methods from classical signal processing. The classical techniques use sequential Monte Carlo (SMC) methods, which aim to optimize the selection of points within the parameter space, conditioned by the measurement data obtained. We illustrate these methods using a specific example, an SMC sampler applied to a nonlinear system, the Duffing oscillator, where the evolution of the quantum state of the oscillator and three Hamiltonian parameters are estimated simultaneously.

We investigate a quasi-one-dimensional periodic array of coupled waveguides, with one extended and one bound dimension, incorporating both first- and second-order coupling. We study the evolution of optical fields in this system, and measure quantum correlations when path-entangled photon pairs are launched into them. We observe a surprisingly large and nontrivial effect of second-order coupling on these correlations—while quantum correlations are symmetric when only first-order coupling is present, the introduction of next-nearest-neighbor coupling often breaks the symmetry to reflections.

Architectures based on waveguide quantum electrodynamics have emerged as promising candidates for quantum networks. In this paper, we analyze the propagation of single photons in disordered many-atom waveguides. We pay special attention to the influence of chirality (directionality of photon transport) on the formation of localized photonic states, considering separately the cases of disorder in the atomic positions and in the atomic transition frequencies.

Compressed sensing (CS) has been verified as an effective technique in the reconstruction of quantum state; however, it is still unknown if CS can reconstruct quantum states given the incomplete data measured by nuclear magnetic resonance (NMR). In this paper, we propose an effective NMR quantum state reconstruction method based on CS. Different from the conventional CS-based quantum state reconstruction, our method uses the actual observation data from NMR experiments rather than the data measured by the Pauli operators. We implement measurements on quantum states in practical NMR computing experiments and reconstruct states of two, three, and four qubits using fewer number of measurement settings, respectively. The proposed method is easy to implement and performs more efficiently with the increase of the system dimension size. The performance reveals both efficiency and accuracy, which provides an alternative for the quantum state reconstruction in practical NMR.

We consider the effect of relativistic boosts on single-particle Gaussian wave packets. The coherence of the wave function as measured by the boosted observer is studied as a function of the momentum and the boost parameter. Using various formulations of coherence, it is shown that in general the coherence decays with the increase of the momentum of the state, as well as the boost applied to it. Employing a basis-independent formulation, we show, however, that coherence may be preserved even for large boosts applied on narrow uncertainty wave packets. Our result is exemplified quantitatively for practically realizable neutron wave functions.