Using scaled-energy spectroscopy techniques (recurrence spectroscopy) we investigate the influence of collisionally induced decoherence on semiclassical orbits in the Stark-Rydberg system. This experimental investigation represents the first application of recurrence spectroscopy to an atomic system perturbed by collisions with external gas partners. We present high-resolution recurrence spectra of potassium, revealing core-scattered combination orbits, and with minor modifications to the data acquisition process, recurrence spectra were also obtained with the system perturbed by 10 torr of krypton gas. A simple decoherence model is applied to semiclassical theory to explain the loss of recurrence strength at high actions. In several cases we also observe the loss of coherent interference between orbits. The loss of coherent destructive interference causes an increase in the recurrence strength of several core-scattered combination orbits.

We investigate numerically with Hylleraas coordinates the frequency dependence of the carrier-envelope phase (CEP) effect on bound-bound transitions of helium induced by an ultrashort laser pulse of a few cycles. We find that the CEP effect is very sensitive to the carrier frequency of the laser pulse, occurring regularly even at far-off-resonance frequencies. By analyzing a two-level model, we find that the CEP effect can be attributed to the quantum interference between neighboring multiphoton transition pathways, which is made possible by the broadened spectrum of the ultrashort laser pulse. A general picture is developed along this line to understand the sensitivity of the CEP effect to the laser’s carrier frequency. Multilevel influence on the CEP effect is also discussed.

We study fundamental and compound gap solitons (GSs) of matter waves in one-dimensional (1D) optical lattices (OLs) in a three-dimensional (3D) weak-radial-confinement regime, which corresponds to realistic experimental conditions in Bose-Einstein condensates (BECs). In this regime GSs exhibit nontrivial radial structures. Associated with each 3D linear spectral band exists a family of fundamental gap solitons that share a similar transverse structure with the Bloch waves of the corresponding linear band. GSs with embedded vorticity $m$ may exist inside bands corresponding to other values of $m$. Stable GSs, both fundamental and compound ones (including vortex solitons), are those which originate from the bands with lowest axial and radial quantum numbers. These findings suggest a scenario for the experimental generation of robust GSs in 3D settings.

The excitation of room-temperature four-level ${}^{87}\mathrm{Rb}$ atoms by a train of ultrashort pulses is investigated theoretically in the conditions when the pulse repetition period is shorter than the characteristic atomic relaxation times. It is shown that coherent accumulation of excitation leads to coherent population trapping and electromagnetically induced transparency of the excitation pulses when the pulse repetition rate is a subharmonic of the ground-state hyperfine splitting. It is illustrated how the judicious choice for the frequency comb parameters can provide a means to effectively control the degree of coherence between the ground-state hyperfine levels for selective atomic groups, and even transfer the whole atomic distribution to the dark state with up to $95\%$ efficiency.

We investigate the high-energy above-threshold ionization (HATI) plateau for Xe in a strong laser field at peak intensity of about $0.85\times {10}^{14}\hspace{0.5em}{\mathrm{W}/\mathrm{cm}}^2$ with wavelengths of 800, 1250, 1500, and 2000 nm. Numerical simulations are performed using a recently developed quantitative rescattering (QRS) model. According to the QRS model, the two-dimensional (2D) photoelectron momentum distribution can be treated as a product of the returning electron wave packet and the elastic differential cross section (DCS) for free electrons scattered with the parent ion. From the 2D momentum distributions, the HATI plateau can be obtained. By using different single-active electron potentials in the DCS calculations, we test the potential dependence of the high-energy plateau spectra. Good agreement between the simulated results and the experimental data confirms again the validity of the QRS model.

We investigate how to estimate from atom-position measurements the relative phase of two Bose-Einstein condensates released from a double-well potential. We demonstrate that the phase-estimation sensitivity via the fit of the average density to the interference pattern is fundamentally bounded by shot noise. The shot noise can be overcome by estimating the phase from the measurement of order $\sqrt{N}$ (or higher) correlation functions. The optimal estimation strategy, saturating the quantum Fisher information, requires the measurement of the $N$th-order correlation function. We demonstrate that a different estimation method—based on the detection of the center of mass of the interference pattern—also provides sub-shot-noise sensitivity. The implementation of both protocols, however, might be experimentally challenging.

We investigate the Mott transitions of three-component (colors) repulsive fermionic atoms in optical lattices using the dynamical mean-field theory. We find that for SU(3) symmetry-breaking interactions, the Mott transition occurs at incommensurate half filling. As a result, a characteristic Mott insulating state appears, where paired atoms with two different colors and atoms with the third color are localized, and either of these states is randomly distributed in each site. We also find another Mott state, where atoms with two different colors are randomly localized at different sites, and atoms with the third color remain itinerant. We demonstrate that double occupancy measurements show characteristics of these exotic Mott phases.

We investigate the minimum achievable bit error rate of the discrimination of binary coherent states with the help of arbitrary ancillary states. We adopt homodyne measurement with a common phase of the local oscillator and classical feedforward control. After one ancillary state is measured, its outcome is referred to the preparation of the next ancillary state and the tuning of the next mixing with the signal. It is shown that the minimum bit error rate of the system is invariant under the following operations: feedforward control, deformations, and introduction of any ancillary state. We also discuss the possible generalization of the homodyne detection scheme.

We present a theoretical proposal for the implementation of geometric quantum computing based on a Hamiltonian which has a doubly degenerate ground state. Thus the system which is steered adiabatically, remains in the ground-state. The proposed physical implementation relies on a superconducting circuit composed of three SQUIDs and two superconducting islands with the charge states encoding the logical states. We obtain a universal set of single-qubit gates and implement a nontrivial two-qubit gate exploiting the mutual inductance between two neighboring circuits, allowing us to realize a fully geometric ground-state quantum computing. The introduced paradigm for the implementation of geometric quantum computing is expected to be robust against environmental effects.

The time evolution of angular distributions of 7-keV Ne${}^{7+}$ ions transmitted through SiO${}_2$ nanocapillaries is studied starting with already-charged capillaries, as well as with fully discharged capillaries, under well-defined conditions. We deduce the rearrangement of charge patches for the charged capillaries from the time evolution of the angular distributions. The rearrangement time is found to be orders of magnitude shorter than the discharge time. Combined with a model calculation, we derive quantitative information on the formation of charge patches downstream of the entrance patch during charging-up, starting with the fully discharged capillaries. A pattern of a small number of charge patches guiding ions is deduced in the stationary state of transmission. The model predicts the width and broadening of the transmitted intensity in accord with measurements. We find strong evidence that the deposited charge patches, formed during charge-up, remain localized in the steady state of ion transmission.

Guiding 3-keV Ne${}^{7+}$ ions through nanocapillaries in insulating polyethylene terephthalate was investigated as function of the capillary diameter. Highly parallel capillaries with a density of $4\times {10}^6$ cm${}^{-2}$ and diameters of 100, 150, 200, 300, and 400 nm were utilized. The characteristic charges governing the charge evolution of the total ion yield were found to be independent of the capillary diameter. However, certain dynamic properties were found to change significantly with this diameter: The transmission profiles of the 100 nm capillaries are broad and structureless, whereas those for 300 nm are composed of three peaks and the mean angle oscillates with deposited charge. At equilibrium the total ion yield was studied as a function of the tilt angle. The results were used to evaluate guiding angles, which were found to be almost constant near $5.5{}^{°}$ with varying capillary diameter similarly as the characteristic charge governing the charge evolution. The width of the transmission profiles are found to decrease from $3.3{}^{°}$ to $1.7{}^{°}$ with increasing capillary diameter from 100 to 400 nm. Model considerations were applied to interpret the results.

We study for the time-dependent harmonic oscillator the transient energy excitation in speed-up processes (“shortcuts to adiabaticity”) designed to reproduce the initial populations at some predetermined final frequency and time. We provide lower bounds and examples. Implications for the limits imposed to the process times and for the principle of unattainability of the absolute zero, in a single expansion or in quantum refrigerator cycles, are drawn.

We extend the molecular strong-field approximation for ionization, in the tunneling limit, to include systematically the linear and quadratic static Stark shifts of the ionizing molecular orbital. This approach, simple to implement, is capable of describing the essential physics of the process of strong-field ionization of oriented polar molecules by circularly polarized laser pulses. The modification of the molecular orbitals in strong fields is also discussed and in cases of extreme polarization of inner electrons a simple model is devised. We find very good agreement between the results from our model and those obtained in the recent experiment on ionizaton of oriented carbonyl sulphide and benzonitrile molecules [L. Holmegaard et al., Nature Phys. 6, 428 (2010)].

We study the effect of discrete breathers (DBs) on the transfer of a Bose-Einstein condensate (BEC) in an optical lattice using the discrete nonlinear Schrödinger equation. In previous theoretical (primarily numerical) investigations of the dynamics of BECs in leaking optical lattices, collisions between a DB and a lattice excitation, e.g., a moving breather (MB) or phonon, were studied. These collisions led to the transmission of a fraction of the incident (atomic) norm of the MB through the DB, while the DB can be shifted in the direction of the incident lattice excitation. Here we develop an analytic understanding of this phenomenon, based on the study of a highly localized system—namely, a nonlinear trimer—which predicts that there exists a total energy threshold of the trimer, above which the lattice excitation can trigger the destabilization of the DB and that this is the mechanism leading to the movement of the DB. Furthermore, we give an analytic estimate of upper bound to the norm that is transmitted through the DB. We then show numerically that a qualitatively similar threshold exists in extended lattices. Our analysis explains the results of the earlier numerical studies and may help to clarify functional operations with BECs in optical lattices such as blocking and filtering coherent (atomic) beams.

We propose a model which includes a nearest-neighbor intrinsic spin-orbit coupling and a trimerized Hamiltonian in the kagome lattice and promises to host the transition from the quantum spin Hall insulator to the normal insulator. In addition, we design an experimental scheme to simulate and detect this transition in the ultracold atom system. The lattice intrinsic spin-orbit coupling is generated via the laser-induced-gauge-field method. Furthermore, we establish the connection between the spin Chern number and the spin-atomic density which enables us to detect the quantum spin Hall insulator directly by the standard density-profile technique used in atomic systems.

We present a theoretical comparison of the signal-to-noise characteristics of quantum versus thermal ghost imaging. We first calculate the signal-to-noise ratio of each process in terms of its controllable experimental conditions. We show that a key distinction is that a thermal ghost image always resides on top of a large background; the fluctuations in this background constitutes an intrinsic noise source for thermal ghost imaging. In contrast, there is a negligible intrinsic background to a quantum ghost image. However, for practical reasons involving achievable illumination levels, acquisition times for thermal ghost images are often much shorter than those for quantum ghost images. We provide quantitative predictions for the conditions under which each process provides superior performance. Our conclusion is that each process can provide useful functionality, although under complementary conditions.

We find the optimal scheme for quantum phase estimation in the presence of loss when no a priori knowledge on the estimated phase is available. We prove analytically an explicit lower bound on estimation uncertainty, which shows that, as a function of the number of probes, quantum precision enhancement amounts at most to a constant factor improvement over classical strategies.

Precision spectroscopy on cold molecules can potentially enable novel tests of fundamental laws of physics and alternative determination of some fundamental constants. Realizing this potential requires a thorough understanding of the systematic effects that shift the energy levels of molecules. We have performed a complete ab initio calculation of the magnetic field effects for a particular system, the molecular hydrogen ion HD${}^{+}$. Different spectroscopic schemes have been considered, and several transitions, all accessible by modern radiation sources and exhibiting well controllable or negligible Zeeman shift, have been identified. Thus, HD${}^{+}$ is a candidate for the determination of the ratio of electron-to-nuclear reduced mass, and for tests of its time independence.

We study the quantum interference in spontaneous emission with the inclusion of counterrotating terms and energy shifts. The energy shifts come from the emission and then reabsorption of virtual photons as well as the real photon emission. We show that the quantum interference resulting from the energy shifts has significant influence on the effective decay rates of the two levels, even when the transition dipole elements are the same and the energy separation of the two levels is small. We also show that the energy shift has substantial influence on the spectrum emitted by the atom. The result is valid in the long time limit. The effect of the energy shift can be observed at the time scale of 1 over the atomic decay rate. We suggest an experiment to test the effect of quantum interference because of energy shifts on the emission spectrum.

An ensemble of nuclear spin pairs under certain conditions is known to exhibit singlet state lifetimes much longer than other nonequilibrium states. This property of singlet states can be exploited in quantum information processing for efficient initialization of quantum registers. Here we describe a general method of initialization and experimentally demonstrate it with two-, three-, and four-qubit nuclear spin registers.