We study the scattering of guided light from a multilevel cesium atom with the transitions between the hyperfine levels $6S_{1/2}F=4$ and $6P_{3/2}{F}^{\prime }=5$ of the $D_2$ line into the guided modes of a nanofiber. We show that the rate of scattering of guided light from the atom in the steady-state regime into the guided modes is asymmetric with respect to the forward and backward directions and depends on the polarization of the probe field. The asymmetry between the forward and backward scattering is a result of the complex transition structure of the atom and the existence of a longitudinal component of the guided-mode profile function. In the case of a two-level atom, the rates of spontaneous emission (and consequently the rates of scattering) into the forward and backward guided modes differ from each other when the atomic dipole matrix-element vector is a complex vector in the plane that contains the fiber axis and the atomic position.

We present a generic quantum master equation whose dissipative dynamics autonomously stabilizes a harmonic oscillator in the $n=1$ Fock state. A multimode optomechanical system is analyzed and shown to be an example of a physical system obeying this model. We show that the optomechanical setup enables preparation of a mechanical oscillator in a nonclassical steady state, and that this state indeed approaches a single phonon Fock state in the ideal parameter regime. The generic model may be useful in other settings, such as cavity or circuit quantum electrodynamics or trapped ion physics.

It is shown that the Hamiltonian version of the Duffin-Kemmer-Petiau theory with electromagnetic coupling brings about a source term at the current. It is also shown that such a source term disappears from the scenario if one uses the correct physical form for the Duffin-Kemmer-Petiau field, regardless of the choice for representing the Duffin-Kemmer-Petiau matrices. This result is used to fix the ambiguity in the electromagnetic coupling in the Duffin-Kemmer-Petiau theory. Moreover, some widespread misconceptions about the Hermiticity in the Duffin-Kemmer-Petiau theory are discussed.

The evolution of an electron beam colliding head on with a strong plane-wave field is investigated in the framework of strong-field QED including radiation-reaction effects due to photon emission. Employing a kinetic approach to describe the electron and the photon distribution it is shown that at a given total laser fluence the final electron distribution depends on the shape of the laser envelope and on the pulse duration, in contrast to the classical predictions of radiation reaction based on the Landau-Lifshitz equation. Finally, it is investigated how the pair-creation process leads to a nonlinear coupled evolution of the electrons in the beam, of the produced charged particles, and of the emitted photons.

We investigate quantum-computational complexity of calculating partition functions of Ising models. We construct a quantum algorithm for an additive approximation of Ising partition functions on square lattices. To this end, we utilize the overlap mapping developed by M. Van den Nest, W. Dür, and H. J. Briegel [Phys. Rev. Lett. 98, 117207 (2007)] and its interpretation through measurement-based quantum computation (MBQC). We specify an algorithmic domain, on which the proposed algorithm works, and an approximation scale, which determines the accuracy of the approximation. We show that the proposed algorithm performs a nontrivial task, which would be intractable on any classical computer, by showing that the problem that is solvable by the proposed quantum algorithm is BQP-complete. In the construction of the BQP-complete problem coupling strengths and magnetic fields take complex values. However, the Ising models that are of central interest in statistical physics and computer science consist of real coupling strengths and magnetic fields. Thus we extend the algorithmic domain of the proposed algorithm to such a real physical parameter region and calculate the approximation scale explicitly. We found that the overlap mapping and its MBQC interpretation improve the approximation scale exponentially compared to a straightforward constant-depth quantum algorithm. On the other hand, the proposed quantum algorithm also provides partial evidence that there exist no efficient classical algorithm for a multiplicative approximation of the Ising partition functions even on the square lattice. This result supports the observation that the proposed quantum algorithm also performs a nontrivial task in the physical parameter region.

We theoretically investigate the mechanism of enhanced ionization in two-electron molecules by analyzing Bohmian trajectories for a one-dimensional ${\mathrm{H}}_2$ in an intense laser field. We identify both types of ionizing trajectories corresponding to the ejection from the up-field and down-field cores. The trajectories of the two electrons are correlated with each other in the former, while correlation is negligible in the latter. The contributions from the two ionization types, though dependent on laser intensity and internuclear distance, are comparable to each other.

We compute the zero-temperature equation of state of a mixture of two fermionic atomic species with repulsive interspecies interactions using second-order perturbation theory. We vary the interaction strength, the population, and the mass imbalance, and we analyze the competition between different states: homogeneous, partially separated, and fully separated. The canonical phase diagrams are determined for various mass ratios, including the experimentally relevant case of the ${}^6\mathrm{Li}$-${}^{40}\mathrm{K}$ mixture. We find substantial differences with respect to the equal-mass case: phase separation occurs at weaker interaction strength, and the partially separated state can be stable even in the limit of a large majority of heavy atoms. We highlight the effects due to correlations by making comparisons with previous mean-field results.

We present an electromagnetic theory describing the resonant enhancement of the spontaneous-decay (SD) rate of quantum emitters in the vicinity of single plasmonic nanoparticles. Relying on the correlation between spectral singularities of the scattering matrix and optical resonances in the nanoparticles, the theory provides a rigorous and closed-form expression for the SD rate enhancement. We reveal that the SD rate enhancement is fundamentally constrained by energy conservation considerations. Furthermore, we find that the maximum SD rate enhancement in plasmonic nanoparticles can be independent of the quality factor of the resonances, in contrast to the conventional Purcell effect in optical resonators. Our results could facilitate physical understanding of the resonant SD rate enhancement in single plasmonic nanoparticles.

The radiative transfer equation is fundamental to multiple physical applications based on light propagation. There have been three different formulations of the radiative transfer equation for media with spatially varying refractive index. Because the radiative transfer equation demonstrates macroscopic phenomena of radiation, the intensity law from geometric optics has been used to validate these formulations of radiative transfer equations. We review the different formulations and compute the steady-state intensity for each of the radiative transfer equations in a nonabsorption and nonscattering medium with a spatially variant refraction index without light sources. By checking each one of the intensities with the intensity law from geometric optics, we find that there is only one formulation that is consistent with the intensity law of geometric optics.

The interaction of molecules with light may lead to electronic transitions and simultaneous vibrational excitations. Franck-Condon factors (FCFs) play an important role in quantifying the intensities of such vibronic transitions occurring during molecular photoexcitations. In this article, we describe a general method for estimating FCFs using a quantum information processor. The method involves the application of a translation operator followed by the measurement of certain projections. We also illustrate the method by experimentally estimating FCFs with the help of a three-qubit nuclear magnetic resonance quantum information processor. We describe two methods for the measurement of projections: (i) using the Moussa protocol and (ii) using diagonal tomography. The experimental results agree fairly well with the theory.

We present results for $p$+He single electron capture and transfer with target excitation using the first Born approximation. The effect of approximating the center of mass of the helium atom and outgoing hydrogen atom at the respective nuclei is explored. Semianalytical results are compared for the calculations with and without the approximation, and it is shown that one must properly account for the center of mass of the atoms. It is also shown that this approximation is the result of the apparent $v^4$ scaling that was previously observed with the four-body transfer with target excitation model.

We report the sympathetic cooling and Coulomb crystallization of molecular ions above the surface of an ion-trap chip. ${\mathrm{N}}_2^{+}$ and ${\mathrm{CaH}}^{+}$ ions were confined in a surface-electrode radio-frequency (rf) ion trap and cooled by the interaction with laser-cooled ${\mathrm{Ca}}^{+}$ ions to secular translational temperatures in the millikelvin range. The configuration of trapping potentials generated by the surface electrodes enabled the formation of planar bicomponent Coulomb crystals and the spatial separation of the molecular from the atomic ions on the chip. The structural and thermal properties of the Coulomb crystals were characterized using molecular dynamics simulations. The present study extends chip-based trapping techniques to cold molecular ions with potential applications in mass spectrometry, chemistry, quantum information science, and spectroscopy.

Resonance-like enhancements of generalized three-photon cross sections for extreme ultraviolet ionization of Ar, Kr, and Xe are demonstrated and analyzed within a single-active-electron, central-potential model. The resonant-like behavior is shown to originate from the potential barriers experienced by intermediate- and final-state photoelectron wave packets corresponding to absorption of one, two, or three photons. The resonance-like profiles in the generalized three-photon-ionization cross sections are shown to be similar to those found in the generalized two-photon-ionization cross sections [Phys. Rev. A 82, 053414 (2010)]. The complexity of Cooper minima in multiphoton-ionization processes is also discussed. Owing to the similar resonance-like profiles found in both two- and three-photon generalized cross sections, we expect such potential-barrier effects to be general features of multiphoton-ionization processes in most atoms with occupied $p$ and $d$ subshells.

We examine a classically chaotic system consisting of two reservoirs of particles connected by a channel containing oscillating potential-energy barriers. We investigate whether such a system can preferentially pump particles from one reservoir to the other, a process often called “quantum pumping.” We show how to make a “particle diode” which under specified conditions permits net particle pumping in only one direction. Then we examine systems having symmetric barriers. We find that if all initial particle energies are considered, a system with symmetric barriers cannot preferentially pump particles. However, if only finite initial energy bands are considered, the system can create net particle transport in either direction. We study the system classically, semiclassically, and quantum mechanically, and find that the quantum description cannot be fully understood without the insight gained from classical and semiclassical analysis.

Nonlinear spectroscopy employs a series of laser pulses to interrogate dynamics in large interacting many-body systems, and it has become a highly successful method for experiments in chemical physics. Current quantum optical experiments approach system sizes and levels of complexity that require the development of efficient techniques to assess spectral and dynamical features with scalable experimental overhead. However, established methods from optical spectroscopy of macroscopic ensembles cannot be applied straightforwardly to few-atom systems. Based on the ideas proposed in M. Gessner et al., (arXiv:1312.3365), we develop a diagrammatic approach to construct nonlinear measurement protocols for controlled quantum systems, and we discuss experimental implementations with trapped ion technology in detail. These methods, in combination with distinct features of ultracold-matter systems, allow us to monitor and analyze excitation dynamics in both the electronic and vibrational degrees of freedom. They are independent of system size, and they can therefore reliably probe systems in which, e.g., quantum state tomography becomes prohibitively expensive. We propose signals that can probe steady-state currents, detect the influence of anharmonicities on phonon transport, and identify signatures of chaotic dynamics near a quantum phase transition in an Ising-type spin chain.

Using a numerical implementation of the truncated Wigner approximation, we simulate the experiment reported by Ramanathan et al. in Phys. Rev. Lett. 106, 130401 (2011), in which a Bose-Einstein condensate is created in a toroidal trap and set into rotation via a phase imprinting technique. A potential barrier is then placed in the trap to study the decay of the superflow. We find that the current decays via thermally activated phase slips, which can also be visualized as vortices crossing the barrier region in the radial direction. Adopting the notion of critical velocity used in the experiment, we determine it to be lower than the local speed of sound at the barrier, in contradiction to the predictions of the zero-temperature Gross-Pitaevskii equation. We map out the superfluid decay rate and critical velocity as a function of temperature and observe a strong dependence. Thermal fluctuations offer a partial explanation of the experimentally observed reduction of the critical velocity from the phonon velocity.

The influence of thermal noise on bipartite and tripartite quantum steering induced by a short laser pulse in a hybrid three-mode optomechanical system is investigated. The calculation is carried out under the bad cavity limit, the adiabatic approximation of a slowly varying amplitude of the cavity mode, and with the assumption of driving the cavity mode with a blue detuned strong laser pulse. Under such conditions, explicit expressions of the bipartite and tripartite steering parameters are obtained, and the concept of collective tripartite quantum steering, recently introduced by He and Reid [Phys. Rev. Lett. 111, 250403 (2013)], is clearly explored. It is found that both bipartite and tripartite steering parameters are sensitive functions of the initial state of the modes and distinctly different steering behavior could be observed depending on whether the modes were initially in a thermal state or not. For the modes initially in a vacuum state, the bipartite and tripartite steering occur simultaneously over the entire interaction time. This indicates that collective tripartite steering cannot be achieved. The collective steering can be achieved for the modes initially prepared in a thermal state. We find that the initial thermal noise is more effective in destroying the bipartite rather than the tripartite steering which, on the other hand, can persist even for a large thermal noise. For the initial vacuum state of a steered mode, the tripartite steering exists over the entire interaction time even if the steering modes are in very noisy thermal states. When the steered mode is initially in a thermal state, it can be collectively steered by the other modes. There are thresholds for the average number of the thermal photons above which the existing tripartite steering appears as the collective steering. Finally, we point out that the collective steering may provide a resource in a hybrid quantum network for quantum secret sharing protocol.

We show that a strong short-range repulsive (contact) interaction between two cold bosonic atoms may change the statistics of two-particle tunneling. For a resonance of a width $\Gamma$, the effect would be observed if the time between the impacts of the two atoms is of order of the resonance's lifetime $\hslash /\Gamma$. The absence of the effect in nonresonance tunneling across a broad barrier suggests that there is no appreciable delay in the classically forbidden region.

We extend the definition of entanglement witnesses based on spin structure factors to the case of scatterers with quantum mechanical motion. We show that this allows for hybrid entanglement detection and specialize the witness for a chain of trapped ions. Within this framework, we also show how the collective vibronic state of the chain can act as an undesired quantum environment affecting the spin–spin-entanglement detection. Furthermore, we investigate several specific cases where these witness operators allow us to detect hybrid entanglement.