High-order harmonic generation in the model hydrogenlike system He${}^{+}$ prepared in an initial 2$p_0$ state with nonvanishing angular quantum number is investigated by numerically solving the fully three-dimensional time-dependent Schrödinger equation. It is found that isolated attosecond pulses with elliptical polarization can be generated by using linearly polarized driving fields. The ellipticity, the major axis direction, and the helicity of the elliptically polarized attosecond pulses can be easily controlled by changing the polarization direction and the peak intensity of the driving pulses. It is shown that the resulting polarization state is related to the tunnel ionization step in the harmonic-generation process. Moreover, by using a simple analytical model it is demonstrated that the atomic nonvanishing angular momentum of the atomic medium used for harmonic generation plays a crucial role.

We study thermalization of a two-component Bose-Hubbard model by exact diagonalization. Initially, the two components do not interact and are both at equilibrium but with different temperatures. As the onsite intercomponent interaction is turned on, perfect thermalization occurs. Remarkably, not merely those simple “realistic” physical observables thermalize but even the density matrix of the whole system—the time-averaged density matrix of the system can be well approximated by that of a canonical ensemble. We also discuss how symmetry of the quenched Hamiltonian affects the level spacing distribution and thermalization of the system.

We propose an experimental scheme to simulate and detect the properties of time-reversal invariant topological insulators, using cold atoms trapped in one-dimensional bichromatic optical lattices. This system is described by a one-dimensional Aubry-Andre model with an additional SU(2) gauge structure, which captures the essential properties of a two-dimensional $Z_2$ topological insulator. We demonstrate that topologically protected edge states, with opposite spin orientations, can be pumped across the lattice by sweeping a laser phase adiabatically. This process is an elegant way to transfer topologically protected quantum states in a highly controllable environment. We discuss how density measurements could provide clear signatures of the topological phases emanating from our one-dimensional system.

We present a local measurement of gravity combining Bloch oscillations and atom interferometry. With a falling distance of 0.8 mm, we achieve a sensitivity of $2\times {10}^{-7}\text{g}$ with an integration time of 300 s. No bias associated with the Bloch oscillations has been measured. A contrast decay with Bloch oscillations has been observed and attributed to the spatial quality of the laser beams. A simple experimental configuration has been adopted where a single retroreflected laser beam performs atom launches, stimulated Raman transitions, and Bloch oscillations. The combination of Bloch oscillations and atom interferometry can thus be accomplished with an apparatus no more complex than a standard atomic gravimeter.

Pioneering methods in recent optical-lattice experiments allow focusing laser beams down to a spot size that is comparable to the lattice constant. Inspired by this achievement, we examine the resonance fluorescence spectra of two-level atoms positioned in adjacent lattice sites and compare the case where the laser hits only one atom (single-site addressing) with cases where several atoms are illuminated. In contrast to the case where the laser hits several atoms, the spectrum for single-site addressing is no longer symmetric around the laser frequency. The shape of the spectrum of fluorescent light, therefore, can serve as a test for single-site addressing. The effects we find can be attributed to a dipole-dipole interaction between the atoms due to the mutual exchange of photons.

Whispering-gallery microcavity lasers possess ultralow thresholds, whereas convenient free-space optical excitation and collection suffer from low efficiencies due to their rotational symmetries. Here we analytically study a three-dimensional microsphere coupled to a nanosized scatterer in the framework of quantum optics. It is found that the scatterer is capable of coupling light in and out of the whispering-gallery modes (WGMs) without seriously degrading their high-$Q$ properties, while the microsphere itself plays the role of a lens to focus the input beam on the scatterer and vice versa. Our analytical results show that (1) the high-$Q$ WGMs can be excited in free space, and (2) over $50\%$ of the microcavity laser emission can be collected within less than $1^{\circ }$. This coupling system holds great potential for low-threshold microlasers free of external couplers.

We propose to apply the notion of the Schmidt number in order to show that a quantum memory or gate process is capable of maintaining a genuine multilevel quantum coherence. We present a simple criterion in terms of an average gate fidelity with respect to the input states of two mutually unbiased bases and demonstrate the coherence of the gate operation in several existing experiments. The Schmidt-number benchmark naturally includes the quantum benchmark as a specific case.

Using kinetic theory, we calculate the shear viscosity and the spin-diffusion coefficient as well as the associated relaxation times for a two-component Fermi gas in two dimensions, as a function of temperature, coupling strength, polarization, and mass ratio of the two components. It is demonstrated that the minimum value of the viscosity decreases with the mass ratio, since Fermi blocking becomes less efficient. We furthermore analyze recent experimental results for the quadrupole mode of a two-dimensional gas in terms of viscous damping, obtaining a qualitative agreement using no fitting parameters.

The transmission of a probe field experiencing electromagnetically induced transparency and optical switching in an atomic medium enclosed in an optical cavity is investigated. Using a semiclassical input-output theory for the interaction between an ensemble of four-level atoms and three optical cavity fields coupled to the same spatial cavity mode, we derive the steady-state transmission spectra of the probe field and discuss the dynamics of the intracavity field buildup. The analytical and numerical results are in good agreement with recent experiments with ion Coulomb crystals [M. Albert et al., Nature Photon. 5, 633 (2011)].

Coherent states play an important role in quantum mechanics because of their unique properties under time evolution. Here we explore this concept for one-dimensional repulsive nonlinear Schrödinger equations, which describe weakly interacting Bose-Einstein condensates or light propagation in a nonlinear medium. It is shown that the dynamics of phase-space translations of the ground state of a harmonic potential is quite simple: The center follows a classical trajectory whereas its shape does not vary in time. The parabolic potential is the only one that satisfies this property. We study the time evolution of these nonlinear coherent states under perturbations of their shape or of the confining potential. A rich variety of effects emerges. In particular, in the presence of anharmonicities, we observe that the packet splits into two distinct components. A fraction of the wavepacket is transferred toward incoherent high-energy modes, while the amplitude of oscillation of the remaining coherent component is damped toward the bottom of the well.

We report on the observation of enhanced high harmonics from a carbon plasma using sub-5-fs laser pulses. The efficiency of harmonic generation in the range of 14–25 eV was up to five times higher in the case of a plasma medium (graphite ablation) compared with gas (argon) under similar experimental conditions. The harmonic enhancement can be attributed to the presence of carbon nanoparticles in the ablation plumes.

The effect of a number of mechanisms designed to suppress decoherence in open quantum systems are studied with respect to their effectiveness at slowing down the loss of entanglement. The effect of photonic band-gap materials and frequency modulation of the system-bath coupling are along expected lines in this regard. However, other control schemes, like resonance fluorescence, achieve quite the contrary: increasing the strength of the control leads to a faster decay of entanglement. The effect of dynamic decoupling schemes on two qualitatively different system-bath interactions are studied in depth. Dynamic decoupling control has the expected effect of slowing down the decay of entanglement in a two-qubit system coupled to a harmonic oscillator bath under a nondemolition interaction. However, nontrivial phenomena are observed when a Josephson charge qubit, strongly coupled to a random telegraph noise bath, is subject to decoupling pulses. The most striking of these reflects the resonance fluorescence scenario, in that an increase in the pulse strength decreases decoherence but also speeds up the sudden death of entanglement. This demonstrates that the behavior of decoherence and entanglement in time can be qualitatively different in the strong-coupling non-Markovian regime.

We present a scalable method for generation of a cluster state for measurement-based quantum computing using van der Waals or dipole-dipole interactions between neutral atoms or polar molecules in an optical lattice. Nearest neighbor entanglement is accomplished by performing a phase gate using interaction of atoms in Rydberg states or molecules in large dipole moment states. All nearest neighbors are sequentially entangled in a finite number of operations, independent of the number of qubits, producing a one-dimensional (1D) cluster state. A universal two-dimensional (2D) cluster state can be generated in several milliseconds in a 2D optical lattice by producing a series of 1D cluster states in one lattice direction, followed by application of the entangling operations in another lattice direction. We discuss the viability of the scheme with Rb Rydberg atoms.

Two random-phase-approximation correlation methods are introduced that take into account exchange interactions. The first one, termed RPAX, is obtained from a simple modification of the ring coupled-cluster doubles amplitude equation, while the second, termed RPAX2, is based on the first method using a slightly modified update equation for the amplitudes. It is shown that this second RPAX2 method can be implemented with a computational algorithm that scales only with the fifth power of the molecular size with the aid of density fitting or the Cholesky decomposition of two-electron integrals. It is thus not much more costly than standard second-order perturbation theory methods and can be applied to quite large molecular systems. Moreover, numerical tests for chemical reaction energies and intermolecular interaction energies have shown that the RPAX2 method, if based on a Perdew-Burke-Ernzerhof exchange Kohn-Sham reference determinant, yields results which are very close to coupled-cluster with single, double, and perturbative triple excitations reference results.

In this paper, we present detailed high-resolution spectroscopy of rubidium (Rb) vapor confined within a hollow-core photonic crystal fiber (HC-PCF). We find a very low level of additional frequency broadening associated with this confinement, with spectral features being only 1 MHz broader than the natural linewidth of the excited state. We show that this additional broadening is consistent solely with the atoms' transit across the fiber's optical mode. This low level of decoherence opens the door to a wide variety of applications including compact frequency standards and new types of quantum optical devices based on alkali-metal-loaded HC-PCFs. We highlight the low level of decoherence through observation of electromagnetically induced transparency in the confined vapor.

Using single ions in a precision cryogenic Penning trap and by measuring the small shifts in their cyclotron frequencies due to their electric polarizability, the ground-state electric dipole moments of HCO${}^{+}$ and NH${}^{+}$ have been determined. The results are μ(HCO${}^{+}$) $=$ 1.543(12) $e{a}_0$ and μ(NH${}^{+}$) $=$ 0.782(11) $e{a}_0$, which agree with theoretical predictions.

We report on a nonperturbative $R$-matrix with PseudoStates (RMPS) calculation for the electron-impact ionization cross section of the ground state of ${\mathrm{Al}}^{2+}$. We include both the direct ionization of the $3s$ and $2p$ subshells and the indirect ionization from the $2p$ subshell. This calculation, thus, includes extra decay channels for the indirect-ionization process not included in previous RMPS calculations. This lowers the total-ionization cross section, resulting in closer agreement with the most recent experimental measurements. This calculation also shows better agreement with the position and height of the resonant-excitation double autoionization features seen in the experiment.

A theory for time-domain attosecond pump–attosecond probe photoabsorption spectroscopy is formulated and related to the atomic response. The theory is illustrated through a study of attosecond absorption spectroscopy in krypton. The atomic parameters entering the formulation such as energies and Auger widths, as well as wave functions and dipole coupling matrix elements, are determined by accurate many-body structure calculations. We create a hole in a valence shell by an attosecond pump, couple an inner-shell electron to the hole by an attosecond probe, and then monitor the formation of the hole in this manner. In a second example, a hole is created in an inner shell by the first pulse, and the second probe pulse couples an even more tightly bound state to that hole. The hole decays in this example by Auger electron emission, and the absorption spectroscopy follows the decay of the hole and the associated coherences in real time.

We derive the dynamical structure factor for an inhomogeneous Tomonaga-Luttinger liquid (TLL) as can be formed in a confined strongly interacting one-dimensional gas. In view of the current experimental progress in the field, we provide a simple analytic expression for the light-scattering cross section, requiring only the knowledge of the density dependence of the ground-state energy as it can be extracted, e.g., from exact or quantum Monte Carlo (QMC) techniques and a Thomas-Fermi description. We apply the result to the case of one-dimensional quantum bosonic gases with dipolar interaction in a harmonic trap, using an energy functional deduced from QMC computations. We find a universal scaling behavior peculiar to the TLL, a signature that can be probed eventually by Bragg spectroscopy in experimental realizations of such systems.

We analyze the collective modes of a harmonically trapped, strongly interacting Bose gas in an optical lattice in the vicinity of the Mott-insulator transition. For that aim we employ the dynamical Gutzwiller mean-field method, by performing real-time evolution and by solving the equations in linear response. We find a strong dependence on the spatial dimension of the system: while in one dimension the frequency of the dipole mode vanishes at the Mott transition, in higher dimensions the dominant dipole mode is featureless and we find a signature only in the breathing mode. We discuss implications for experiments with bosonic and fermionic atoms.