A coherent superposition of rotational states in ${\mathrm{D}}_2$ has been excited by nonresonant, ultrafast $\left(12\mathrm{fs}\right)$, intense $(2\times {10}^{14}\mathrm{W}{\mathrm{cm}}^{-2})$ $800\mathrm{nm}$ laser pulses, leading to impulsive dynamic alignment. Field-free evolution of this rotational wave packet has been mapped to high temporal resolution by a time-delayed pulse, initiating rapid double ionization, which is highly sensitive to the angle of orientation of the molecular axis with respect to the polarization direction, $\theta$. The detailed fractional revivals of the neutral ${\mathrm{D}}_2$ wave packet as a function of $\theta$ and evolution time have been observed and modeled theoretically.

We use a tunneling–Monte Carlo model to calculate the dynamics and emitted Larmor radiation from electrons ionized in an ultrashort and ultrastrong pulsed laser focus over the intensity range from ${10}^{17}\text{to}{10}^{20}\mathrm{W}∕{\mathrm{cm}}^2$. We find the spatial variation of a laser field affects the radiation and can no longer be neglected at laser intensities leading to relativistic effects. We identify three regimes for the interaction as a function of the ratio of the single cycle quiver amplitude of the photoelectron to the laser focus waist. Adopting a one-dimensional or plane wave approximation when the laser driven excursion of the photoelectron exceeds the focus waist overestimates the total radiated energy by as much as an order of magnitude. Despite this, the spectral amplitude of the highest-energy photons from ionization in a laser focus is comparable to the plane wave case for excursions up to the beam waist since the laser focus imparts an extra boost of speed for electrons exiting the focus. Full spatial and temporal integration that includes the ionization of charge states before the peak of the pulse do not differ significantly from results that include only the radiation from ionization of the charge state at the peak of the laser field.

We theoretically study the effect of laser pulse durations on the ionization yield of hydrogen atoms and negative hydrogen ions. By numerically solving the time-dependent Schrödinger equation we find that the ionization efficiency by few-cycle pulses is unexpectedly larger than that by many-cycle pulses at low intensities when multiphoton ionization is the dominant ionization mechanism.

We consider a two-photon transition in a specific ladder system driven by a chirped laser pulse. In the weak field limit, we find that the excited state probability amplitude arises due to interference of multiple quantum paths which are weighted by quadratic phase factors. The excited state population has the form of a Gauss sum which plays a prominent role in number theory.

We calculate angle-resolved above-threshold ionization spectra for diatomic molecules in linearly polarized laser fields, employing the strong-field approximation. The interference structure resulting from the individual contributions of the different scattering scenarios is discussed in detail, with respect to the dependence on the internuclear distance and molecular orientation. We show that, in general, the contributions from the processes in which the electron is freed at one center and rescatters off the other obscure the interference maxima and minima obtained from single-center processes. However, around the boundary of the energy regions for which rescattering has a classical counterpart, such processes play a negligible role and very clear interference patterns are observed. In such energy regions, one is able to infer the internuclear distance from the energy difference between adjacent interference minima.

We present a thorough analysis of the nonlinear tunneling of Bose-Einstein condensates in static and accelerating two-dimensional lattices within the framework of the mean-field approximation. We deal with nonseparable lattices, considering different initial atomic distributions in highly symmetric states. For an analytical description of the condensate before instabilities develop, we derive several few-mode models, analyzing essentially both nonlinear and quasilinear regimes of tunneling. By direct numerical simulations, we show that two-mode models provide an accurate description of tunneling when either initially two states are populated or tunneling occurs between two stable states. Otherwise, a two-mode model may give only useful qualitative hints for understanding tunneling, but does not reproduce many features of the phenomenon. This reflects the crucial role of instabilities developed due to two-body interactions resulting in a non-negligible population of the higher bands. This effect becomes even more pronounced in the case of accelerating lattices. In the latter case we show that the direction of the acceleration is a relevant physical parameter which affects the tunneling by changing the atomic rates at different symmetric states and by changing the numbers of bands involved in the atomic transfer.

A method is suggested of efficient mid- to far-infrared (ir) pulse amplification via driving optical pulse energy conversion in waveguiding quantum well (QW) heterostructures at room temperature. It is based on the optical pulse creating transient population inversion at a long-wavelength transition in the three-level scheme formed by QW levels of dimensional quantization. As a result, efficient amplification of a weak mid- to far-ir pulse, propagating simultaneously with the driving one, becomes possible. A waveguide and QW heterostructure design that is optimal for optical-long-wavelength pulse conversion is proposed and shown to be much simpler than that used in quantum cascade lasers. For the typical input peak power of a picosecond driving pulse and mid- to far-ir pulse of $\sim 100\mathrm{W}$ and $100–0.1\mathrm{mW}$, respectively, the present scheme is able to produce output ir pulses with peak powers of at least several tens of watts or several tens of milliwatts in the mid- or far-ir range, respectively, i.e., to convert an appreciable part of the optical pulse energy into the mid-ir signal.

We investigate the generation of a strongly entangled electromagnetic field through laser pumping a collection of $N$ two-level atoms in a two-mode optical resonator. The vacuum-multiparticle interactions enhance the coupling of the atomic sample to the cavity modes facilitating the creation of entangled photons at higher frequencies. In the dispersive atom-cavity limit, one can obtain a steady-state output field consisting of tens of such photons per mode.

We calculate the isotope mass shift for several light atoms and ions using Dirac wave functions and the mass shift operator with relativistic corrections of the order of $(\alpha Z{)}^2$. Calculated relativistic corrections to the specific mass shift vary from a fraction of a percent for carbon to 2% for magnesium. Relativistic corrections to the normal mass shift are typically smaller. Interestingly, the final relativistic mass shifts for the levels of one multiplet appear to be even closer to each other than for nonrelativistic operators applied to Dirac functions. That can be important for the astrophysical search for possible $\alpha$ variation, where the isotope shift is a source of important systematic error. Our calculations show that for levels of the same multiplet this systematics is negligible, and they can be used as probes for $\alpha$ variation.

To implement quantum-information processing, microwave fields are often used to manipulate superconducting qubits. We study how the coupling between superconducting charge qubits can be controlled by variable-frequency magnetic fields. We also study the effects of the microwave fields on the readout of the charge-qubit states. The measurement of the charge-qubit states can be used to demonstrate the statistical properties of photons.

The femtosecond stimulated Raman process is a versatile technique to excite rotational states in molecules. We demonstrate control over the rotational state population in a sample of NO molecules by varying the time delay between two identical laser pulses. The product of the rotational state distribution is probed by a $1+1$ resonance-enhanced multiphoton ionization scheme and simulated quantum mechanically. There is good agreement between theoretical and experimental results. The product in selected quantum states shows an oscillatory dependence on the time delay. Spectral analysis reveals rotational transition energies and the presence of multiple Raman steps. We show that the relative strength of these frequency components can be related to excitation pathways with predominant $\Delta J=2$ transitions toward higher rotational states. The initial step from $J=1∕2$ involves either $\Delta J=1$ or $\Delta J=2$. We find that one can discriminate between two excitation ladders. The results demonstrate the coherent effects of tailoring the shape of an ultrashort excitation pulse.

Hydrogen and deuterium atoms in supersonic jet expansions have been decelerated using a multistage Zeeman decelerator. The properties of the decelerator have been completely characterized in a series of experiments in which (i) the initial longitudinal velocities of the decelerated atoms, (ii) the maximum magnetic field strength, and (iii) the duration of zero-field intervals between successive field pulses in neighboring deceleration stages were systematically varied. Experiments using Ar and Kr as carrier gases have clearly revealed that the H atoms are located at the surface of the jet expansion cone in each case. Comparison of the results of these experiments with numerical simulations of the atom trajectories through the decelerator provides a full description of the phase-space distribution of the decelerated atoms. Evidence is presented of transverse guiding of the beam and of a partial redistribution of the H atom population among the $M_F$ components of the $F=1$ manifold at times when the magnetic field strength approaches zero.

We have used laser spectroscopy to measure the rotational level distribution of trapped molecular $\mathrm{H}{\mathrm{D}}^{+}$ ions at translational temperatures in the millikelvin range. The $\mathrm{H}{\mathrm{D}}^{+}$ ions are loaded into an ion trap by electron-impact ionization, and sympathetically cooled using laser-cooled ${\mathrm{Be}}^{+}$ ions which are already stored in the trap. Under our experimental conditions, the internal (rotational) degrees of freedom turn out to be independent of the translational degrees of freedom, and an effective rotational temperature close to room temperature is found. The near absence of background-gas collisions allows the rotational temperature to be related directly to the temperature of the ambient blackbody radiation (BBR). This feature suggests the use of molecular ions for BBR thermometry, which may help to improve the accuracy of frequency standards based on trapped atomic ions. For the spectroscopic measurement of the rotational populations, we propose a nondestructive technique.

By adjusting the tunneling couplings over longer than nearest-neighbor distances, it is possible in discrete lattice models to reproduce the properties of the lowest energy band of a real, continuous periodic potential. We propose to include such terms in problems with interacting particles, and we show that they have significant consequences for scattering and bound states of atom pairs in periodic potentials.

We theoretically investigate the possibility of observing resonant activation in the hopping dynamics of two-mode semiconductor lasers. We present a series of simulations of a rate-equation model under random and periodic modulation of the bias current. In both cases, for an optimal choice of the modulation time scale, the hopping times between the stable lasing modes attain a minimum. The simulation data are understood by means of an effective one-dimensional Langevin equation with multiplicative fluctuations. Our conclusions apply to both edge-emitting and vertical cavity lasers, thus opening the way to several experimental tests in such optical systems.

We introduce the definition of group velocity for a system with a discrete spectrum and apply it to a linear resonator. We show that a positive, negative, or zero group velocity can be obtained for light propagating in the whispering-gallery modes of a microspherical resonator. The associated group delay is practically independent of the ring-down time of the resonator. We demonstrate “stopped light” in an experiment with a fused silica microsphere.

Simple examples are constructed that show the entanglement of two qubits being both increased and decreased by interactions on just one of them. One of the two qubits interacts with a third qubit, a control, that is never entangled or correlated with either of the two entangled qubits, and is never entangled, but becomes correlated, with the system of those two qubits. The two entangled qubits do not interact, but their state can change from maximally entangled to separable or from separable to maximally entangled. Similar changes for the two qubits are made with a SWAP operation between one of the qubits and a control; then there are compensating changes of entanglement that involve the control. When the entanglement increases, the map that describes the change of the state of the two entangled qubits is not completely positive. The combination of two independent interactions that individually give exponential decay of the entanglement can cause the entanglement to not decay exponentially but, instead, go to zero at a finite time.

We study some discrete symmetries of unbiased (Hadamard) and biased quantum walk on a line, which are shown to hold even when the quantum walker is subjected to environmental effects. The noise models considered in order to account for these effects are the phase flip, bit flip, and generalized amplitude damping channels. The numerical solutions are obtained by evolving the density matrix, but the persistence of the symmetries in the presence of noise is proved using the quantum trajectories approach. We also briefly extend these studies to quantum walk on a cycle. These investigations can be relevant to the implementation of quantum walks in various known physical systems. We discuss the implementation in the case of NMR quantum information processor and ultracold atoms.