We perform laser-induced fluorescence spectroscopy on a pulsed supersonic beam of tungsten carbide (WC) molecules, which has been proposed as a candidate molecular system for a permanent electric dipole moment (EDM) search of the electron in its rovibrational ground state of the $X{}^3{\Delta }_1$ state. In particular, $\left[20.6\right]\Omega =2,v^{\prime }=4\leftarrow X{}^3{\Delta }_1,v^{\prime \prime }=0$ transition at 485 nm was used for the detection. The hyperfine structure and the $\Omega$ doublet of the transition is measured, which are essential for estimating the size of the potential systematic uncertainties for electron EDM measurement. For further suppression of the systematic uncertainty, an alternative electron EDM measurement scheme utilizing the $g$ factor crossing point of the $\Omega$-doublet levels is discussed. On the other hand, flux and internal temperature of the molecular beam are characterized, which sets the limit on the statistical uncertainty of the electron EDM experiment. With the given results, the prospect of electron EDM experiment with the $X{}^3{\Delta }_1$ state of WC molecule is discussed.

We consider spherically trapped Bose gases in three dimensions with contact interactions and investigate whether the Bose-Einstein condensate at zero temperature is stable against macroscopic fragmentation into a small number of mutually incoherent pieces. Our results are expressed in terms of a dimensionless interaction measure proportional to the Thomas-Fermi parameter. It is shown that while three-dimensional condensates are inherently much more stable against macroscopic fragmentation than their quasi-one- and quasi-two-dimensional counterparts, they fragment at a sufficiently large value of the dimensionless interaction measure, which we determine both fully numerically and semianalytically from a continuum limit of large particle numbers.

The exponential and nonexponential regimes of the buildup process in resonant tunneling structures are analyzed by considering an analytic solution of the time-dependent Schrödinger equation. It is found that the buildup exhibits a purely exponential behavior in a finite time interval followed by a clear transition to a nonexponential regime. The buildup of the probability amplitude in the nonexponential regime follows a $t^{-3/2}$ time dependence, in the same fashion as the survival amplitude in quantum decay. For incidence energies at higher isolated resonances, $E={\varepsilon }_n$ with $n>1$, it is found that the exponential regime is split into two exponential subregimes: the first one is governed by the width ${\Gamma }_n$ of the resonance chosen for the incidence energy, and the second one is dominated by the width ${\Gamma }_1$ of the lowest resonance. The transition occurs directly from $n$ to 1 without jumps to other intermediate resonant states. This dynamics is discussed in comparison with the (opposite) and well-studied process of quantum decay, with which we find that there are striking similarities in both the exponential and nonexponential regimes. We also analyze the buildup in systems with resonance doublets, where the interference effects produced by the interacting resonances play an important role. In the latter case we find that the buildup of the probability amplitude exhibits a complex oscillatory behavior that can be characterized by a mix of oscillating contributions with well-defined Rabi type frequencies of the form $|E-{\varepsilon }_n|/\hslash$, where ${\varepsilon }_n$ are the nearest resonances around the incidence energy $E$.

We demonstrate genuine three-mode nonlocality based on phase-space formalism. A Svetlichny-type Bell inequality is formulated in terms of the $s$-parametrized quasiprobability function. We test such a tool using exemplary forms of three-mode entangled states, identifying the ideal measurement settings required for each state. We thus verify the presence of genuine three-mode nonlocality that cannot be reproduced by local or nonlocal hidden variable models between any two out of three modes. In our results, GHZ- and W-type nonlocality can be fully discriminated. We also study the behavior of genuine tripartite nonlocality under the effects of detection inefficiency and dissipation induced by local thermal environments. Our formalism can be useful to test the sharing of genuine multipartite quantum correlations among the elements of some interesting physical settings, including arrays of trapped ions and intracavity ultracold atoms.

The dynamics of entanglement and quantum discord between two identical qubits strongly interacting with a common single mode leaky cavity field have been investigated beyond the rotating wave approximation (RWA) by using a recently derived Lindblad type quantum optical master equation [F. Beaudoin, J. M. Gambetta, and A. Blais, Phys. Rev. A 84, 043832 (2011)] that can describe the losses of the cavity field in a strong atom-field coupling regime. Contrary to previous investigations of the same model in the dissipative regime by using the standard Lindblad quantum optical master equation in a strong-coupling regime, the atom-field steady states are found to be cavity decay rate independent and have a very simple structure determined solely by the overlap of initial atomic state with the subradiant state which is valid for all coupling regimes. Non-RWA dynamics are found to have remarkable effects on the steady state quantum discord and entanglement that cannot be achieved under RWA conditions, for instance, they can induce steady state entanglement even for the initial states that have no overlap with the subradiant state. Moreover, the non-RWA dynamics are found to reverse the initial state dependence of steady state entanglement and quantum discord contrary to the RWA case.

A systematic numerical approach to approximate high-dimensional Lindblad equations is described. It is based on a deterministic rank $m$ approximation of the density operator, the rank $m$ being the only parameter to adjust. From a known initial density operator, this rank $m$ approximation gives at each time step an estimate of its largest $m$ eigenvalues with their associated eigenvectors. A numerical integration scheme is also proposed. Its numerical efficiency in the case of a rank $m=12$ approximation is demonstrated for oscillation revivals of 50 atoms interacting resonantly with a slightly damped coherent quantized field of 200 photons.

It has been recently conjectured that the state-independency of quantum contextuality may be lost when the indistinguishability of identical particles is taken into account. Here, we show that quantum state-independent contextuality exists for any system of more than one identical bosonic qudit, and for most systems of fermionic qudits. The only exception is the case of $d$ fermionic qudits, since there the dimension of the antisymmetric subspace is 1, which is insufficient for contextuality. For all the other cases, either the symmetry precludes the existence of physical states or we provide an explicit method to produce quantum state-independent contextual correlations.

We have developed methods for performing qudit quantum computation in the Jaynes-Cummings model with the qudits residing in a finite subspace of individual harmonic oscillator modes, resonantly coupled to a spin-1/2 system. The first method determines analytical control sequences for the one- and two-qudit gates necessary for universal quantum computation by breaking down the desired unitary transformations into a series of state preparations implemented with the Law-Eberly scheme [Law and Eberly, Phys. Rev. Lett. 76, 1055 (1996)]. The second method replaces some of the analytical pulse sequences with more rapid numerically optimized sequences. In our third approach, we directly optimize the evolution of the system, without making use of any analytic techniques. While limited to smaller dimensional qudits, the third approach finds pulse sequences which carry out the desired gates in a time which is much shorter than either of the other two approaches.

In entanglement-based quantum key distribution (QKD), the generation and detection of multiphoton modes leads to a trade-off between entanglement visibility and twofold coincidence events when maximizing the secure key rate. We produce a predictive model for the optimal twofold coincidence probability per coincidence window given the channel efficiency and detector dark count rate of a given system. This model is experimentally validated and used in simulations for QKD with satellites as well as optical fibers.

The common approach to topological quantum computation is to implement quantum gates by adiabatically moving non-Abelian anyons around each other. Here we present an alternative perspective based on the possibility of realizing the exchange (braiding) operators of anyons by adiabatically varying pairwise interactions between them rather than their positions. We analyze a system composed by four anyons whose couplings define a T junction, and we show that the braiding operator of two of them can be obtained through a particular adiabatic cycle in the space of the coupling parameters. We also discuss how to couple this scheme with anyonic chains in order to recover the topological protection.

Locating the boundaries of consecutive blocks of quantum information is a fundamental building block for advanced quantum computation and quantum communication systems. We develop a coding theoretic method for properly locating boundaries of quantum information without relying on external synchronization when block synchronization is lost. The method also protects qubits from decoherence in a manner similar to conventional quantum error-correcting codes, seamlessly achieving synchronization recovery and error correction. A family of quantum codes that are simultaneously synchronizable and error-correcting is given through this approach.

We put forward an approach for the development of a nonlocal density functional by a direct modeling of the shape of exchange-correlation (xc) hole in inhomogeneous systems. The functional is aimed at giving an accurate xc energy and an accurate corresponding xc potential even in difficult near-degeneracy situations such as molecular bond breaking. In particular we demand that: (1) the xc hole properly contains $-1$ electron, (2) the xc potential has the asymptotic $-1/r$ behavior outside finite systems, and (3) the xc potential has the correct step structure related to the derivative discontinuities of the xc energy functional. None of the currently existing functionals satisfies all these requirements. These demands are achieved by screening the exchange hole in such a way that the pair-correlation function is symmetric and satisfies the sum rule. These two features immediately imply (1) and (2) while the explicit dependence of the exchange hole on the Kohn-Sham orbitals implies (3). Preliminary calculations show an improved physical description of the dissociating hydrogen molecule. Though the total energy is still far from perfect, the binding curve from our nonlocal density functional provides a significant improvement over the local density approximation.

Dipole matrix elements have been calculated with different methods in the intermediate-coupling scheme to study their impact on the Stark effect of highly charged ions. Special emphasis has been devoted to the 1$s$3$l$ fine structure of He-like ions that is widely employed for Stark broadening analysis in dense plasmas. Apart from a wavelength shift in the total He${}_{\beta }$ group in the x-ray energy range, important differences in the Stark width, induced line dips, and relative intensities of maxima are demonstrated for different methods of calculation. We found that these differences were related to the precision of the fine-structure dipole matrix elements and related wavelengths and explored the particularities of atomic structure precision for line broadening purposes that was distinct from the current database approaches. Based on advanced multi-configuration-Dirac-Fock simulations, we propose a complete set of high-precision matrix elements and wavelengths. Detailed numerical results are presented for He-like aluminum. We also discuss the influence of relativistic approximations in atomic structure on the line broadening.

A theoretical treatment of dissociative recombination (DR), vibrational excitation, and vibrational deexcitation of the BeH${}^{+}$ ion in its four lowest vibrational states ($X{}^1{\Sigma }^{+},v_i^{+}=0,1,2,3$) is reported. The multichannel quantum defect theory is used to determine cross sections and rate coefficients. Three electronic symmetries of BeH (${}^2\Pi$, ${}^2{\Sigma }^{+}$, and ${}^2\Delta$) have been included in the calculations. At low energies the DR is dominated by capture into states of ${}^2\Pi$ symmetry. Satisfactory agreement with results obtained using the wave packet approach is reached at intermediate energies despite significant differences at low energies. Cross sections and rate coefficients suitable for the modeling of the kinetics of BeH${}^{+}$ in fusion plasmas and in the stellar atmospheres are presented and discussed.

We report molecular-frame ($e$,2$e$) cross sections for N${}_2$ at large momentum transfer, obtained using the electron-electron-fragment ion triple-coincidence technique. The measured angular distribution of the ($e$,2$e$) cross section for the inner-valence 2${\sigma }_{\mathrm{g}}$ orbital appears to show that the spatial character of the orbital has been directly observed in momentum space. On the other hand, experimental results for ionization to states above the double-ionization threshold suggested a larger intensity in the direction perpendicular to the molecular axis rather than parallel, an observation that our plane-wave impulse-approximation calculations fail to reproduce.

Radio-frequency quantum engineering of spins is based on the dressing by a nonresonant electromagnetic field. Radio-frequency dressing occurs also for the motion of particles, electrons, or ultracold atoms, within a periodic spatial potential. The dressing, producing a renormalization and also a freeze of the system energy, is described by different approaches: dressed atom, magnetic resonance semiclassical treatment, and continued-fraction solution of the Schrödinger equation. A comparison between these solutions points out that the semiclassical treatment, to be denoted as the $S$ solution, represents the most convenient tool to evaluate the tunneling renormalization of ultracold atoms.

We study hydrodynamic fluctuations in a nonrelativistic fluid. We show that in three dimensions, fluctuations lead to a minimum in the shear viscosity to entropy density ratio $\eta /s$ as a function of the temperature. The minimum provides a bound on $\eta /s$ which is independent of the conjectured bound in string theory, $\eta /s\ge \hslash /\left(4\pi k_B\right)$, where $s$ is the entropy density. For the dilute Fermi gas at unitarity, we find $\eta /s\gtrsim 0.2\hslash$. This bound is not universal—it depends on the thermodynamic properties of the unitary Fermi gas and on empirical information about the range of validity of hydrodynamics. We also find that the viscous relaxation time of a hydrodynamic mode with frequency $\omega$ diverges as $1/\sqrt{\omega }$, and that the shear viscosity in two dimensions diverges as $\ln (1/\omega )$.