In this paper we introduce a way to quantify the noise level associated to a given quantum transformation. The key mechanism lying at the heart of the proposal is noise addition: in other words we compute the amount of extra noise we need to add to the system through convex combination with a reference noisy map or by reiterative applications of the original map, before the resulting transformation becomes entanglement breaking. We also introduce the notion of entanglement-breaking channels of order $n$ (i.e., maps which become entanglement breaking after $n$ iterations), and the associated notion of amendable channels (i.e., maps which can be prevented from becoming entanglement breaking after iterations by interposing proper quantum transformations). Explicit examples are analyzed in the context of qubit and one-mode Gaussian channels.

The electric fields near the heterogeneous metal-dielectric surface of an atom chip were measured using cold atoms. The atomic sensitivity to electric fields was enhanced by exciting the atoms to Rydberg states that are 10${}^8$ times more polarizable than the ground state. We attribute the measured fields to charging of the insulators between the atom chip wires. Surprisingly, it is found that although the chip wire currents were turned off before Rydberg excitation, the measured fields were strongly influenced by how the wire currents had been applied. These fields may be dramatically lowered with appropriate voltage biasing, suggesting configurations for the future development of hybrid quantum systems.

The eigenvalue condition for computing the Fabry-Perot eigenmodes of $n$-identical equally spaced metallic parallel plates is obtained. The frequency and decay rate of the plasmonic resonances for gold and silver are computed for quarter-wave metallic plates. I show that the system's plasmonic frequency can be tuned in silver to any desired value in the optical regime through a specific choice of the ratio of the dielectric to metal plate thickness. I also show that the eigenmodes with wave vectors having $\mathrm{Re}\left(v\right)\cong 0$ form narrow resonances.

We provide analytical lower and upper bounds for entanglement of formation for bipartite systems, which give a direct relation between the bounds of entanglement of formation and concurrence, and improve the previous results. Detailed examples are presented.

In a previous work [Shchukin, Phys. Rev. A 84, 052325 (2011)] a lower bound was found for the multipartite uncertainty product of the position and momentum observables over all separable states. In this work we are trying to minimize this uncertainty product over a broader class of states to find the fundamental limits imposed by nature on the observable quantities. We show that it is necessary to consider pure states only and find the infimum of the uncertainty product over a special class of pure states (states with spherically symmetric wave functions). It is shown that this infimum is not attained. We also explicitly construct a parametrized family of states that approaches the infimum by varying the parameter. Since the constructed states beat the lower bound for separable states, they are entangled. We thus show that there is a gap that separates the values of a simple measurable quantity for separable states from entangled ones and we also try to find the size of this gap.

We propose a test of quantum contextuality for a single three-level system that uses nine projective measurements. It has the form of an inequality that has to be satisfied by any noncontextual theory. We show that for any qutrit state, except for the maximally mixed state, one can always find a set of nine observables for which the corresponding inequality is violated. This inequality is a test of contextuality that may offer insight into the relation between state-dependent and state-independent contextuality tests present in the literature.

Topological subsystem codes were proposed by Bombin based on 3-face-colorable cubic graphs. Suchara, Bravyi, and Terhal generalized this construction and proposed a method to construct topological subsystem codes using 3-valent hypergraphs that satisfy certain constraints. Finding such hypergraphs and computing their parameters, however, is a nontrivial task. We propose families of topological subsystem codes that were previously not known. In particular, our constructions give codes which cannot be derived from Bombin's construction. We also study the error recovery schemes for the proposed subsystem codes and give detailed schedules for the syndrome measurement that take advantage of the 2-locality of the gauge group. The study also leads to a new and general construction for color codes.

We propose a scheme for the production of hyperentangled photon pairs as well as for the complete differentiation of 16 hyperentangled Bell states in both polarization and spatial-mode degrees of freedom using the quantum-dot cavity systems. This hyperentangled-Bell-state photon generation and the complete hyperentangled-Bell-state-analysis device can serve as crucial components of the high-capacity, long-distance quantum communication. We use the hyperentanglement quantum repeater as an example to show the application of this device. When the hyperentanglement quantum repeater operation is complete, two parties that are far from each other in quantum communication can share two Bell-state spin pairs simultaneously. By using numerical calculations we prove that the present scheme can both work in the weak- and strong-coupling regimes with current technology.

This paper presents a set of highly efficient quantum circuits for discrete-time quantum walks on non-degree-regular graphs. In particular, we describe a general procedure for constructing highly efficient quantum circuits for quantum walks on star graphs of any degree and Cayley trees with an arbitrary number of layers, which are nonsparse in general. We also show how to modify these circuits to implement a full quantum-walk search algorithm on these graphs, without reference to a “black-box” oracle. This provides a practically implementable method to explore quantum-walk-based algorithms with the aim of eventual real-world applications.

Most of known multipartite Bell inequalities involve correlation functions for all subsystems. They are useless for entangled states without such correlations. We give a method of derivation of families of Bell inequalities for $N$ parties, which involve, e.g., only $(N-1)$-partite correlations, but still are able to detect proper $N$-partite entanglement. We present an inequality which reveals five-partite entanglement despite using only four-partite correlations. Classes of inequalities introduced here can be put into a handy form of a single nonlinear inequality. An example is given of an $N$-qubit state, which strongly violates such an inequality, despite having no $N$-qubit correlations. This surprising property might be of potential value for quantum information tasks.

Relativistic distorted-wave studies by Fontes et al. [Phys. Rev. A 47, 1009 (1993)] demonstrated that the generalized Breit interaction (equivalently, the M$ø$ller interaction) can affect electron-impact excitation cross sections of hydrogenlike ${\mathrm{U}}^{91+}$ by more that 50$\%$ in comparison to calculations that employ the Coulomb interaction alone. We present calculations that investigate the effects of both the M$ø$ller interaction and close coupling in the calculation of electron-impact excitation cross sections. Electron scattering from ${\mathrm{U}}^{91+}$ is used as a test case. The relativistic convergent close-coupling (RCCC) method is nonperturbative and we emphasize the restrictions and subsequent limitations associated with employing the M$ø$ller interaction in the RCCC method.

We present theoretical studies of a two-step resonant Auger process at high x-ray intensity. Tuning a short x-ray pulse to the initially closed resonant channel of the $1s-2p$ transition in singly ionized neon, the initially neutral neon target is valence ionized. Subsequently, the strong resonant x-ray field transfers an inner-shell electron to the created outer valence vacancy, thereby creating a core-excited state. The strong resonant coupling, giving rise to Rabi oscillations involving a core transition, results in a modification of the resonant Auger-electron spectral line profile. If the valence photoelectron remains unobserved, the system of the residual ion undergoing the resonant Auger decay can be treated by an open quantum system approach. The resonant Auger-electron spectral line shape is shown to be determined by an analog of the reduced density matrix that depends on two time arguments. The equations of motion of this reduced density matrix are derived and numerical results are presented, in support of the recent experimental verification [E. Kanter et al., Phys. Rev. Lett. 107, 233001 (2011)] of this nonlinear x-ray optical effect.

We investigate numerically the structural dynamics of ion Coulomb crystals confined in a three-dimensional harmonic trap when influenced by an additional one-dimensional optically induced periodical potential. We demonstrate that transitions between thermally excited crystal structures, such as body-centered cubic and face-centered cubic, can be suppressed by a proper choice of the potential depth and periodicity. Furthermore, by varying the harmonic trap parameters and/or the optical potential in time, controlled transitions between crystal structures can be obtained with close to unit efficiency.

Quantum control strategies that provide shortcuts to adiabaticity are increasingly considered in various contexts including atomic cooling. Recent studies have emphasized practical issues in order to reduce the gap between idealized models and actual ongoing implementations. We rephrase the cooling features in terms of a peculiar squeezing effect and use it to parametrize the robustness of frictionless cooling techniques with respect to noise-induced deviations from the ideal time-dependent trajectory for the trapping frequency. We finally discuss qualitative issues for the experimental implementation of this scheme using bichromatic optical traps and lattices, which seem especially suitable for cooling Fermi-Bose mixtures and for investigating equilibration of negative temperature states, respectively.

We present a theoretical study of the impact of an electrostatic field combined with nonresonant linearly polarized laser pulses on the rotational dynamics of linear molecules. Within the rigid rotor approximation, we solve the time-dependent Schrödinger equation for several field configurations. Using the carbonyl sulfide molecule as the prototype, the field-dressed dynamics is analyzed in detail for experimentally accessible static-field strengths and laser pulses. Results for directional cosines are presented and compared to the predictions of the adiabatic theory. We demonstrate that for the prototypical field configuration used in current mixed-field orientation experiments, the molecular field dynamics is, in general, nonadiabatic, and a time-dependent description of these systems is mandatory. We investigate several field regimes identifying the sources of nonadiabatic effects and provide the field parameters under which the adiabatic dynamics would be achieved.

We present a scheme to embed molecular anions in a gas of ultracold rubidium atoms as a route towards the preparation of cold molecular ions by collisional cooling with ultracold atoms. Associative detachment as an important loss process in collisions between OH${}^{-}$ molecules and rubidium atoms is studied. The density distribution of trapped negative ions in the multipole radio frequency trap is measured by photodetachment tomography, which allows us to derive absolute rate coefficients for the process. We define a regime where translational and internal cooling of molecular ions embedded into the ultracold atomic cloud can be achieved.

A cold rubidium atom fountain interferometry gravimeter with an active vibration isolator is demonstrated. The natural resonance frequency of the active vibration isolator is 0.016 Hz, and the vertical vibration noise is greatly reduced by a factor of 100 from 0.1 to 1 Hz. After substantial suppression of the vibration noise, the gravimeter reaches a sensitivity of ${5.5\times 10}^{-8}$ g/Hz${}^{1/2}$. We measured the local gravitational acceleration $g$ by this sensitive gravimeter with a resolution of ${6.5\times 10}^{-9}$ g after 60 s and ${1.5\times 10}^{-9}$ g after 2000 s integration time, which is comparable to the resolution of state-of-the-art atom gravimeters.

We investigate the dynamical properties of anyons confined in one-dimensional optical lattices combined with a weak harmonic trap using the exact numerical method based on a generalized Jordan-Wigner transformation. The evolving density profiles, momentum distributions, occupation distributions, and occupations of the lowest natural orbital after quench of the harmonic trap are obtained for different statistical parameters. The density profiles of anyons display the same behaviors irrespective of statistical parameter in the full evolving period, while the behaviors dependent on statistical property are shown in the momentum distributions and occupations of natural orbitals.