We consider the phenomenon of mutual synchronization in a fundamental quantum system of two detuned quantum harmonic oscillators dissipating into the environment. We identify the conditions leading to this spontaneous phenomenon, showing that the ability of the system to synchronize is related to the existence of disparate decay rates and is accompanied by robust quantum discord and mutual information between the oscillators, preventing the leak of information from the system.

In this paper, we present the concept of the one-way unlocalizable quantum discord and investigate its properties. We provide a polygamy inequality for it in a tripartite pure quantum system of arbitrary dimension. Several tradeoff relations between the one-way unlocalizable quantum discord and other correlations are given. If the von Neumann measurement is made on a part of the system, we give two expressions of the one-way unlocalizable quantum discord in terms of partial distillable entanglement and quantum disturbance. Finally, we also provide a lower bound for bipartite shareability of quantum correlation beyond entanglement in a tripartite system.

The four-state continuous-variable quantum key distribution protocol has a long practical secure distance [A. Leverrier and P. Grangier, Phys. Rev. Lett. 102, 180504 (2009)], while it poses the difficulty of parameter estimation. We propose an improved four-state protocol where the covariance matrix can be estimated from experimental data without using the linear channel assumption, thus ensuring its unconditional security in the asymptotical limit. Our scheme maintains the advantage of high reconciliation efficiency of the four-state protocol, which ensures long practical secure distance its. Our scheme can be implemented with the current technology.

We have carried out a comprehensive study of electron impact on an open shell NCO molecule by using the $R$-matrix method. Elastic (integrated and differential), momentum-transfer, excitation, and ionization cross sections, along with effective collision frequencies over a wide electron temperature range $(500–30000\text{K})$ have been presented. The target states are represented by including correlations via a configuration interaction formalism. The results of the static-exchange, correlated 1-state and 13-state close-coupling approximation are presented. Our study has detected five core excited shape resonances in the 13-state model. All calculations are done in the $C_{2v}$ point group. We also detect a stable bound state of ${\text{NCO}}^{-}$ of ${}^1{A}_1$ symmetry having configuration $\cdots 7a_1^2,1b_1^{2}2b_1^2,1b_2^{2}2b_2^2$ with a vertical electronic affinity value of $3.035\text{eV}$ which is in good agreement with the experimental (adiabatic) value of $3.609\pm 0.0005\text{eV}$. The ionization cross sections are calculated using the binary-encounter-Bethe model in which molecular orbitals at self-consistent Hartree-Fock level are used to calculate kinetic and binding energies of the occupied molecular orbitals. For scattering calculations we have used partial waves up to $l=4$ to represent the continuum electron in our $R$-matrix approach. A Born top-up procedure is invoked for dipole allowed transitions to account for the contribution of partial waves higher than $l=4$ to obtain converged cross sections.

A close coupling quantum-mechanical calculation is performed for rotational energy transfer in a HD+HD collision at very low energy, down to the ultracold temperatures: $T\sim {10}^{-8}$ K. A global six-dimensional H${}_2$-H${}_2$ potential-energy surface is adopted from a previous work [Boothroyd et al., J. Chem. Phys. 116, 666 (2002)]. State-resolved integral cross sections ${\sigma }_{ij\to i^{\prime }{j}^{\prime }}\left({\varepsilon }_{\text{kin}}\right)$ of different quantum-mechanical rotational transitions $ij\to i^{\prime }{j}^{\prime }$ in the HD molecules and corresponding state-resolved thermal rate coefficients $k_{ij\to i^{\prime }{j}^{\prime }}\left(T\right)$ have been computed. Additionally, for comparison, H${}_2$+H${}_2$ calculations for a few selected rotational transitions have also been performed. The hydrogen and deuterated hydrogen molecules are treated as rigid rotors in this work. A pronounced isotope effect is identified in the cross sections of these collisions at low and ultracold temperatures.

The recently developed weak-field asymptotic theory [Phys. Rev. A 84, 053423 (2011)] is applied to the analysis of tunneling ionization of a molecular ion (H${}_2^{+}$), several homonuclear (H${}_2$, N${}_2$, O${}_2$) and heteronuclear (CO, HF) diatomic molecules, and a linear triatomic molecule (CO${}_2$) in a static electric field. The dependence of the ionization rate on the angle between the molecular axis and the field is determined by a structure factor for the highest occupied molecular orbital. This factor is calculated using a virtually exact discrete variable representation wave function for H${}_2^{+}$, very accurate Hartree-Fock wave functions for the diatomics, and a Hartree-Fock quantum chemistry wave function for CO${}_2$. The structure factors are expanded in terms of standard functions and the associated structure coefficients, allowing the determination of the ionization rate for any orientation of the molecule with respect to the field, are tabulated. Our results, which are exact in the weak-field limit for H${}_2^{+}$ and, in addition, under the Hartree-Fock approximation for the diatomics, are compared with results from the recent literature.

We study the time evolution of ultracold atoms in an accelerated optical lattice. For a Bose-Einstein condensate with a narrow quasimomentum distribution in a shallow optical lattice the decay of the survival probability in the ground band has a steplike structure. In this regime we establish a connection between the wave-function-renormalization parameter $Z$ introduced by P. Facchi, H. Nakazato, and S. Pascazio [Phys. Rev. Lett. 86, 2699 (2001)] to characterize nonexponential decay and the phenomenon of resonantly enhanced tunneling, where the decay rate is peaked for particular values of the lattice depth and the accelerating force.

In this paper we present a scheme to simulate the four-level stimulated Raman adiabatic passage in a four-waveguide array. The engineered tripod-well optical-waveguide structure provides a classic analog of coherent transport in cold atoms coupled with optical traps. We show that a non-Abelian gauge structure emerges in this adiabatic passage. Furthermore, we demonstrate that this waveguide structure may help us to see “directly” the noncommutative feature of the non-Abelian geometric phases.

We demonstrate a quantum regime of dissipative nonlinear oscillators where the creation of Fock states as well as the superpositions of Fock states are realized for time intervals exceeding the characteristic decoherence time. The preparation of quantum states is conditioned by strong Kerr nonlinearity as well as by excitation of resolved lower oscillatory energy levels with a specific train of Gaussian pulses. This provides practical signatures to look for in experiments with cooled nonlinear oscillators.

We propose a set of linear quantum entanglement witnesses constituted by local quantum-mechanical observables with each two possible measurement outcomes. These witnesses detect all the entangled resources which give rise to a better fidelity than separable states in quantum teleportation and present both sufficient and necessary conditions in experimentally detecting the useful resources for quantum teleportation.

Methods which use time-dependent orbitals for the description of quantum dynamics of electrons, like (multiconfiguration) time-dependent Hartree-Fock or time-dependent density-functional theory, produce an effectively time-dependent electronic structure. This effect is strongest if only a single determinant represents the system. Focusing on this case, we show how features of the time-dependent electronic structure show up in optimized laser pulses used for the coherent control task of a population inversion.

We propose a method to simulate a Dirac or Majorana equation evolving under particular potentials with the use of the corresponding free evolution, while the potential dynamics is encoded in a static transformation upon the initial state. We extend our results to interacting two-body systems.

The premise of density-functional theory is that knowledge of the ground-state density uniquely determines the Hamiltonian, and thereby, via solution of the corresponding time-independent Schrödinger equation, all the properties of the system. The density therefore constitutes a basic variable of quantum mechanics. There are at present two paths from the density to the Hamiltonian: the Hohenberg and Kohn proof of the bijectivity between the external potential and the basic variable, and the Percus, Levy, and Lieb constrained-search proof. We argue the Hohenberg- and Kohn-type proof to be the more fundamental, and that this is the case in general when both external electrostatic and magnetostatic fields are present, and the basic variables are the ground-state density and physical current density.

It is known that the strong-field approximations commonly used in models for computing high-order-harmonic generation invalidate the Ehrenfest theorem. Therefore, the time derivative of the atomic dipole or of the dipole velocity does not correspond to the dipole acceleration. We study the consequences of this invalidation for the quantitative evaluation of high-order-harmonic spectra in hydrogen at different wavelengths and intensities. As a main result, we propose a form for the time derivative of the kinetic momentum that allows a quantitatively accurate computation of the acceleration spectra using the dipole-velocity matrix elements.

We theoretically study lattice modulation experiments with ultracold fermions in optical lattices. We focus on the regime relevant to current experiments when interaction strength is larger than the bandwidth and temperature is higher than magnetic superexchange energy. We obtain analytical expressions for the rate of doublon production as a function of modulation frequency, filling factor, and temperature. We use local density approximation to average over inhomogeneous density for atoms in a parabolic trap and find excellent agreement with experimentally measured values. Our results suggest that lattice modulation experiments can be used for thermometry of strongly interacting fermionic ensembles in optical lattices.

We investigate quantitatively the wave-particle duality in a general Mach-Zehnder interferometer setup with an asymmetric beam splitter. The asymmetric beam splitter introduces additional a priori which-path knowledge, and the additional knowledge is different for particles detected at one or the other output port. Accordingly, the interference patterns appearing at the two output ports are also different. Hence, in sharp contrast with the symmetric case, in the present case we should treat the two output ports separately. It turns out that two nonorthogonal unsharp observables are measured jointly in this setup. We apply the condition for joint measurability of these unsharp observables to obtain a trade-off relation between the fringe visibility of the interference pattern and the which-path distinguishability.

The thermodynamic properties of the unitary Fermi gas (UFG) have recently been measured to unprecedented accuracy at the MIT. In particular, these measurements provide an improved understanding of the regime below $T/{\epsilon }_F^{}\simeq 0.20$, where a transition into a superfluid phase occurs. In light of this development, we present an overview of state-of-the-art auxiliary field quantum Monte Carlo (AFQMC) results for the UFG at finite temperature and compare them with the MIT data for the energy, chemical potential, and density. These AFQMC results have been obtained using methods based on the hybrid Monte Carlo (HMC) algorithm, which was first introduced within the context of lattice QCD.

We study the Casimir interaction in the plane-sphere geometry in the classical limit of high temperatures. In this limit, the finite conductivity of the metallic plates needs to be taken into account. For the Drude model, the classical Casimir interaction is nevertheless found to be independent of the conductivity so that it can be described by a single universal function depending only on the aspect ratio $x=L/R$, where $L$ is the interplate distance and $R$ is the sphere radius. This universal function differs from the one found for perfect reflectors and is in principle amenable to experimental tests. The asymptotic approach of the exact result to the proximity force approximation appears to be well fitted by polynomial expansions in $\ln x$.

We report two novel techniques for cooling and rotating Bose-Einstein condensates in a dilute rubidium vapor that highlight the control and versatility afforded over cold atom systems by time-averaged adiabatic potentials (TAAPs). The intrinsic loss channel of the TAAP has been successfully employed to evaporatively cool a sample of trapped atoms to quantum degeneracy. The speed and efficiency of this process compares well with that of conventional forced rf evaporation. In an independent experiment, we imparted angular momentum to a cloud of atoms forming a Bose-Einstein condensate by introducing a rotating elliptical deformation to the TAAP geometry. Triangular lattices of up to 60 vortices were created. All findings reported herein result from straightforward adjustments of the magnetic fields that give rise to the TAAP.