Metal-cluster ionization potentials are important characteristics of these “artificial atoms,” but extracting these quantities from cluster photoabsorption spectra, especially in the presence of thermal smearing, remains a big challenge. Here we demonstrate that the classic Fowler theory of surface photoemission does an excellent job of fitting the photoabsorption profile shapes of neutral In${}_n$${}_{=3-34}$ clusters [Wucher et al., New J. Phys. 10, 103007 (2008)]. The deduced ionization potentials extrapolate precisely to the bulk work function, and the internal cluster temperatures are in close agreement with values expected for an ensemble of freely evaporating clusters. Supplementing an earlier application to potassium clusters, these results suggest that the Fowler formalism, which is straightforward and physical, may be of significant utility in metal-cluster spectroscopy. It is hoped also that the results will encourage a comprehensive theoretical analysis of the applicability of bulk-derived models to cluster photoionization behavior, and of the transition from atomic and molecular-type to surface-type photoemission.

We theoretically analyze inverse radio-frequency (rf) spectroscopy experiments in two-component Fermi gases. We consider a small number of impurity atoms interacting strongly with a bath of majority atoms. In two-dimensional geometries we find that the main features of the rf spectrum correspond to an attractive polaron and a metastable repulsive polaron. Our results suggest that the attractive polaron has been observed in a recent experiment [B. Fröhlich et al., Phys. Rev. Lett. 106, 105301 (2011)].

It was recently discovered that the quantum correlations of a pair of disentangled qubits, as measured by the quantum discord, can increase solely because of their interaction with a local dissipative bath. Here, we show that a similar phenomenon can occur in continuous-variable bipartite systems. With this aim, we consider a class of two-mode squeezed thermal states and study the behavior of Gaussian quantum discord under various local Markovian nonunitary channels. While these in general cause a monotonic drop of quantum correlations, an initial rise can take place with a thermal-noise channel.

We report an experimental demonstration of a quantum delayed-choice experiment via nuclear magnetic resonance techniques. Two spin-1/2 nuclei from each molecule of a liquid ensemble are used as target and ancilla qubits. The circuit corresponding to the recently proposed quantum delayed-choice setup has been implemented with different states of an ancilla qubit. As expected theoretically, our experiments clearly demonstrate continuous morphing of the target qubit between particle-like and wave-like behaviors. The experimental visibility of the interference patterns shows good agreement with theory.

We apply Hall and Reginatto's theory of interacting classical and quantum ensembles to harmonically coupled particles, with a view to understanding its experimental implications. This hybrid theory has no free parameters and makes distinctive predictions that should allow it to be experimentally distinguished from quantum mechanics. It also bears on the questions of quantum measurement and quantum gravity.

We consider the coherent control of a quantum bit by the use of short pulses with finite duration ${\tau }_{\mathrm{p}}$. By shaping the pulse, we perturbatively decouple the dynamics of the bath from the dynamics of the quantum bit during the pulse. Such shaped pulses provide single quantum bit gates robust against decoherence which are useful for quantum-information processing. We extend previous results in two ways: (i) we treat frequency-modulated pulses and (ii) we pass from time-independent baths to analytically time-dependent baths. First- and second-order solutions for $\pi$ and $\pi /2$ pulses are presented. They are useful in experiments where amplitude modulation is difficult to realize.

Two numerical methods are used to evaluate the relativistic spectrum of the two-center Coulomb problem (for the ${\text{H}}_2{}^{+}$ and ${\text{Th}}_2^{179+}$ diatomic molecules) in the fixed nuclei approximation by solving the single-particle time-independent Dirac equation. The first one is based on a min-max principle and uses a two-spinor formulation as a starting point. The second one is the Rayleigh-Ritz variational method combined with kinematically balanced basis functions. Both methods use a B-spline basis function expansion. We show that accurate results can be obtained with both methods and that no spurious states appear in the discretization process.

The two-body Coulomb scattering problem is solved using the standard complex-scaling method. The explicit enforcement of the scattering boundary condition is avoided. Splitting of the scattering wave function based on the Coulomb modified plane wave is considered. This decomposition leads to a three-dimensional Schrödinger equation with a source term. Partial-wave expansion is carried out and the asymptotic form of the solution is determined. This splitting does not lead to simplification of the scattering boundary condition if complex scaling is invoked. An alternative splitting carried out only on the partial-wave level is introduced and this method is proven to be very useful. The scattered part of the wave function tends to zero at large interparticle distances. This property permits easy numerical solution: the scattered part of the wave function can be expanded on a bound-state-type basis. The method can be applied not only for a pure Coulomb potential but also in the presence of a short-range interaction.

Simulation of conditional master equations is important to describe systems under continuous measurement and for the design of control strategies in quantum systems. For large bosonic systems, such as Bose-Einstein condensates and atom lasers, full quantum-field simulations must rely on scalable stochastic methods. Currently, these methods have a convergence time that is restricted by the use of representations based on coherent states. Here, we show that typical measurements on atom-optical systems have a common form that allows for an efficient simulation using the number-phase Wigner (NPW) phase-space representation. We demonstrate that a stochastic method based on the NPW can converge orders of magnitude longer and more precisely than its coherent equivalent. We then examine how these methods can be used in multimode simulations, demonstrated by a simulation of a two-mode Bose-Hubbard model. Finally, we combine these techniques to demonstrate a full-field simulation of a realistic multimode quantum system controlled by active feedback.

In this paper we propose a model of polariton condensation with Kerr-type nonlinear photons. We introduce a generalized Dicke Hamiltonian to describe our system. By constructing the partition function as a path integral, the analytical and numerical solutions are presented. On the mean-field level, it is shown that the polariton condensation can occur and the Kerr nonlinearity affects the character of the polariton condensate. As the nonlinear coefficient increases, the condensate evolves from more photon-like to more exciton-like. Although the photon nonlinearity gives rise to a chemical potential greater than the photon energy, the quasiparticle excitation spectrum is still fully gapped. For the condensate collective excitations, the nonlinearity destroys the Goldstone modes and mixes the phase modes with the amplitude modes, resulting four non-zero-frequency collective modes. In addition, the influence of the photon-exciton detuning on the polariton condensate is also discussed.

We theoretically study an optical system in which the back-reaction of quantum vacuum manifests itself as a sizable suppression of the absorption by a three-level emitter embedded in an optical cavity. A theoretical model including the anti-rotating-wave terms of the light-matter interaction Hamiltonian is developed to describe the conversion of zero-point fluctuations into observable radiation, as well as the back-reaction of the quantum vacuum onto the emitter.

Differential scattering of left- and right-handed circularly polarized waves is a general property of scattering media lacking space-inversion symmetry. We show in this paper that this scattering difference has remarkable properties in the case of an almost transparent chiral random medium. By tuning the refractive indices of the materials composing the medium, this scattering difference can be varied and even inverted. Within the limit of a dense scattering material, the difference of scattering cross sections between polarizations of opposite handedness can be largely adjusted. These properties are illustrated by measurement of the difference between the transmission of right- and left-handed polarized waves through a chiral Christiansen filter.

We present a study of the relation between wave-particle duality and quantum uncertainty in a two-path interferometer and derive equalities and inequalities involving the visibility (representing wave-like behavior), the predictability (representing particle-like behavior), and their variances. We experimentally demonstrate that, for a single photon in a Mach-Zehnder interferometer, these quantities are related via an equation that connects both duality and uncertainty. This relation holds for the single-photon source prepared either in a pure state or a mixed state.

Atom number states are a valuable resource for ultracold chemistry, atom interferometry, and quantum information processing. Recent experiments have achieved their deterministic preparation in trapped few-fermion systems. We analyze the tunneling decay of these states, in terms of both the survival probability and the nonescape probability, which can be extracted from measurements of the full counting statistics. At short times, both probabilities exhibit deviations from the exponential law. The decay is governed by the multiparticle Zeno time, which exhibits a signature of quantum statistics and contact interactions. The subsequent exponential regime governs most of the dynamics, and we provide accurate analytical expressions for the associated decay rates. Both dynamical regimes are illustrated in a realistic model. Finally, a global picture of multiparticle quantum decay is presented.

We numerically demonstrate the control of motional degrees of freedom of an ensemble of neutral atoms in an optical lattice with a shallow trapping potential. Taking into account the range of quasimomenta across different Brillouin zones results in an ensemble whose members effectively have inhomogeneous control fields as well as spectrally distinct control Hamiltonians. We present an ensemble-averaged optimal control technique that yields high-fidelity control pulses, irrespective of quasimomentum, with average fidelities above 98$\%$. The resulting controls show a broadband spectrum with gate times in the order of several free oscillations to optimize gates with up to 13.2$\%$ dispersion in the energies from the band structure. This can be seen as a model system for the prospects of robust quantum control. This result explores the limits of discretizing a continuous ensemble for control theory.

We consider two independent quantum walks on separate lines augmented by partial or full swapping of coins after each step. For classical random walks, swapping or not swapping coins makes little difference to the random-walk characteristics, but we show that quantum walks with partial swapping of coins have complicated yet elegant interwalker correlations. Specifically we study the joint position distribution of the reduced two-walker state after tracing out the coins and analyze total, classical, and quantum correlations in terms of mutual information, quantum mutual information, and measurement-induced disturbance. Our analysis shows intriguing quantum features without classical analogs.

We propose a fast and nondestructive spectroscopic method for single molecular ions that implements quantum logic schemes between an atomic ion and the molecular ion of interest. Our proposal relies on a hybrid coherent manipulation of the two-ion system, using optical or magnetic forces depending on the types of molecular levels to be addressed (e.g., Zeeman, rotational, vibrational or electronic degrees of freedom). The method is especially suited for the nondestructive precision spectroscopy of single molecular ions and sets a starting point for new hybrid quantum computation schemes that combine molecular and atomic ions, covering the measurement and entangling steps.

Deciding whether a mixed quantum state is separable or entangled is a difficult problem in general. Separable states are positive under partial transposition [they are positive-partial-transpose (PPT) states], but this simple test does not exclude all entangled states. In order to understand the entangled PPT states, having so-called bound entanglement, we want to study the extremal PPT states. An extremal PPT state is either a pure product state, then it is separable, or it is entangled, in which case the state and its partial transpose must still be rather low-rank density matrices, although it is known that the rank must be at least 4. In a previous paper we presented a complete classification of the rank-4 entangled PPT states in dimension $3\times 3$, generalizing the construction by Bennett et al. from unextendible sets of orthogonal product vectors. In the present paper we continue our investigations of the low-rank entangled PPT states, mostly in dimension $3\times 3$, using a combination of analytical and numerical methods. We use perturbation theory in order to construct rank-5 entangled PPT states close to the known rank-4 states, and in order to compute dimensions and study the geometry of surfaces of low-rank PPT states. We exploit the close connection between low-rank PPT states and product vectors. In particular, we show how to reconstruct a low-rank PPT state from a sufficient number of product vectors in its kernel. It may seem surprising that the number of product vectors needed may be smaller than the dimension of the kernel.

A systematic study of Sc iii atomic properties is carried out using a high-precision relativistic all-order method where all single, double, and partial-triple excitations of the Dirac-Fock wave functions are included to all orders of perturbation theory. Reduced matrix elements, oscillator strengths, transition rates, and lifetimes are determined for the $ns$, $n{p}_j$, $n{d}_j$, $n{f}_j$, and $n{g}_j$ levels with $n⩽7$. Recommended values and estimates of their uncertainties are provided for a large number of electric-dipole transitions. Electric-quadrupole and magnetic-dipole matrix elements are evaluated to determine lifetimes of the $3d_{5/2}$ and $4s$ metastable levels. These calculations provide recommended values critically evaluated for their accuracy for a number of Sc iii atomic properties for use in theoretical modeling as well as planning and analysis of various experiments. We hope that the present study will stimulate further exploration of Sc iii for various applications owing to its interesting structure of different low-lying metastable levels.