Self-steepening of optical pulses arises due to the dispersive contribution of the effective Kerr nonlinearity. In typical structures this response is on the order of a few femtoseconds with a fixed frequency response. In contrast, the effective Kerr nonlinearity in photonic crystal waveguides (PhCWGs) is largely determined by the geometrical parameters of the structure and is consequently tunable over a wide range. Here we show group-velocity (group-index) modulation leads to a previously unexplored physical mechanism for generating self-steepening. Further, we demonstrate that periodic media such as PhCWGs can exhibit self-steepening coefficients two orders of magnitude larger than typical systems. At these magnitudes the self-steepening strongly affects the nonlinear pulse dynamics even for picosecond pulses. Due to interaction with additional higher-order nonlinearities in the semiconductor materials under consideration, we employ a generalized nonlinear Schrödinger equation numerical model to describe the impact of self-steepening on the temporal and spectral properties of the optical pulses in practical systems, and define appropriate figures of merit. These results provide a theoretical description for recent experimental results presented by C. A. Husko et al. [Sci. Rep. 3, 1100 (2013)] and F. Raineri et al. [Phys. Rev. A 87, 041802 (2013)]. More generally, these observations apply to all periodic media due to the rapid group-velocity variation characteristic of these structures.

Dipole-dipole interactions between helium atoms in Rydberg-Stark states with principal quantum number $n=53$ and approximately linear Stark energy shifts, resulting from induced electric dipole moments of approximately 7900 D, have been investigated experimentally. The experiments were performed in pulsed supersonic metastable helium beams, with particle number densities of up to $\sim {10}^9{\mathrm{cm}}^{-3}$. In the presence of amplitude-modulated, radio-frequency electric fields, changes in the spectral intensity distributions associated with the transitions to these states that are attributed to dipole-dipole interactions within the ensembles of excited atoms have been observed. The experimental results are in excellent agreement with calculations of the Rydberg energy level structure carried out using Floquet methods, and excitations shared by up to four atoms. The use of these Rydberg-Stark states as sensors for nonresonant broadband radio-frequency electrical noise is also discussed.

In this work, we consider the dissipative dynamics of a quantum Brownian magneto-oscillator in contact with different types of environment at zero temperature as well as at high temperatures. We derive closed-form expressions of decay rate and heating rate for the magneto-oscillator in contact with Ohmic, sub-Ohmic, and super-Ohmic environments with non-Markovian characteristics. The present study is useful in determining the relationship between the thermalization dynamics of a magneto-oscillator and different forms of environmental spectrum and external magnetic field. Although our study is limited to the short-time (Non-Markovian) and long-time (Markovian) regimes, it manifests that one can actually tune the effective coupling of the system reservoir either by tuning oscillator frequency through reservoir engineering techniques or changing external magnetic field. Thus the present work establishes the essential role of non-Markovian dissipation and the crucial role of magnetic field in the environment induced decoherence and decay processes of the dissipative magneto-oscillator.

Individual atoms in optical cavities can provide an efficient interface between stationary qubits and flying qubits (photons), which is an essential building block for quantum communication. Furthermore, cavity-assisted controlled-not (cnot) gates can be used for swapping entanglement to long distances in a quantum repeater setup. Nonetheless, dissipation introduced by the cavity during the cnot may increase the experimental difficulty in obtaining long-distance entanglement distribution using these systems. We analyze and compare a number of cavity-based repeater schemes combining various entanglement generation schemes and cavity-assisted cnot gates. We find that a scheme, where high-fidelity entanglement is first generated in a two-photon detection scheme and then swapped to long distances using a recently proposed heralded controlled-$Z$ (cz) gate, exhibits superior performance compared to the other schemes. The heralded gate moves the effect of dissipation from the fidelity to the success probability of the gate thereby enabling high-fidelity entanglement swapping. As a result, high-rate entanglement distribution can be achieved over long distances even for low cooperativities of the atom-cavity systems. This high-fidelity repeater is shown to outperform the other cavity-based schemes by up to two orders of magnitude in the rate for realistic parameters and large distances (1000 km).

Constructing a universal operation scheme for Majorana qubits remains a central issue for the topological quantum computation. We study the Landau-Zener-Stückelberg interference in a Majorana qubit and show that this interference can be used to achieve controllable operations. The Majorana qubit consists of an rf superconducting quantum interference device (SQUID) with a topological nanowire Josephson junction which hosts Majorana bound states. In the SQUID, a magnetic flux pulse can drive the quantum evolution of the Majorana qubit. The qubit experiences two Landau-Zener transitions when the amplitude of the pulse is tuned around the superconducting flux quanta $h/2e$. The Landau-Zener-Stückelberg interference between the two transitions rotates the Majorana qubit, with the angle controlled by the time scale of the pulse. This rotation operation implements a high-speed single-qubit gate on the Majorana qubit, which is a necessary ingredient for the topological quantum computation.

Isotope shifts (ISs) of $D_1\left(ns{}^{2}S_{1/2}\to np{}^{2}P_{1/2}\right)$ and $D_2\left(ns{}^{2}S_{1/2}\to np{}^{2}P_{3/2}\right)$ transitions for the first three elements of the K isoelectronic sequence are calculated with all-order correlated relativistic coupled-cluster theory. To get precise ab initio results, all the orbitals of the reference states have been optimized by a linear regression technique. Some of the important IS estimations reported here were not available in literature. Interesting features of odd-even staggering and magic neutron number effects are also observed in the case of volume shift calculations. Energy-level corrections for a few levels due to a change in the nuclear model from a point nucleus to a Fermi nucleus have been studied with interesting correlation features and compared with other theoretical results available in literature.

We present a theoretical study of the influence of resonant enhancement on quantum path dynamics in the generation of harmonics above and below the ionization threshold in helium. By varying the wavelength and intensity of the driving field from 425 to 500 nm and from 30 to 140 $\mathrm{TW}/\mathrm{cm}{}^2$, respectively, we identify enhancements of harmonics 7, 9, and 11 that correspond to multiphoton resonances between the ground state and the Stark-shifted $1s2p,1s3p$, and $1s4p$ excited states. A time-frequency analysis of the emission shows that both the short and the long quantum path contributions to the harmonic yield are enhanced through these bound-state resonances. We analyze the subcycle time structure of the ninth harmonic yield in the vicinity of the resonances and find that on resonance the long trajectory contribution is phase shifted by approximately $\pi /4$. Finally, we compare the single atom and the macroscopic response of a helium gas and find that while the subcycle time profiles are slightly distorted by propagation effects, the phase shift of the long-trajectory contribution is still recognizable.

We study, both theoretically and experimentally, modulational instability in optical fibers that have a longitudinal evolution of their dispersion in the form of a Dirac delta comb. By means of Floquet theory, we obtain an exact expression for the position of the gain bands, and we provide simple analytical estimates of the gain and of the bandwidths of those sidebands. An experimental validation of those results has been carried out in several microstructured fibers specifically manufactured for that purpose. The dispersion landscape of those fibers is a comb of Gaussian pulses having widths much shorter than the period, which therefore approximate the ideal Dirac comb. Experimental spontaneous modulational instability spectra recorded under quasicontinuous wave excitation are in good agreement with the theory and with numerical simulations based on the generalized nonlinear Schrödinger equation.

We demonstrate experimental exploration of the attractor diagram of an optomechanical system where the optical forces compensate for the mechanical losses. In this case stable self-induced oscillations occur but only for specific mirror amplitudes and laser detunings. We demonstrate that we can amplify the mechanical mode to an amplitude 500 times larger than the thermal amplitude at 300 K. The lack of unstable or chaotic motion allows us to manipulate our system into a nontrivial steady state and explore the dynamics of self-induced oscillations in great detail.

We describe the lasing effect in the three-dimensional percolation system, where the percolating cluster is filled by active media composed by light emitters excited noncoherently. We show that, due to the presence of a topologically nontrivial photonic structure, the stimulated emission is modified with respect to both conventional and random lasers. The time dynamics and spectra of the lasing output are studied numerically with finite-difference time-domain approach. The Fermat principle and Monte Carlo approach are applied to characterize the optimal optical path and interconnection between the radiating emitters. The spatial structure of the laser mode is found by a long-time FDTD simulation.

We demonstrate that Freeman resonances have a strong impact on the nonlinear optical response in femtosecond filaments. These resonances decrease the transient refractive index within a narrow intensity window and strongly affect the filamentation dynamics. In particular, we demonstrate that the peak intensity of the filament can be clamped at these resonances, hinting at the existence of new regimes of filamentation with electron densities considerably lower than predicted by the standard model. This sheds further light on the phenomenon of filamentary intensity clamping and the nearly plasmaless filaments predicted by the controversial higher-order Kerr model.

Electromagnetic wavefront portions grazing or nearly grazing the surface of a macroscopic particle contribute to the extinction of the incident radiation through a tunneling process similar to the scenario of barrier penetration in quantum mechanics. The aforesaid tunneling contribution, referred to as the edge effect, is critical to a correct depiction of the physical mechanism of electromagnetic extinction. Although an analytical solution for the edge effect in the case of a sphere has been reported in the literature, the counterparts for nonspherical particles remain unknown. The conventional curvature-based formalism of the edge effect breaks down in the case of faceted particles. This paper reports a method, based on the invariant imbedding principle and the Debye expansion technique, to accurately quantify the edge effect associated with an arbitrarily shaped three-dimensional object. The present method also provides a rigorous capability to facilitate the validation of various empirical approximations for electromagnetic extinction. Canonical results are presented to illustrate optical tunneling for two nonspherical geometries.

Influence of $\mathcal{PT}$ symmetry on the Goos-Hänchen (GH) shift in the reflected light is presented for an ensemble of atomic medium in a cavity, in the configuration of four-level $N$-type (${}^{87}\mathrm{Rb}$ atoms) systems driving by two copropagating strong laser fields and a weak probe field. The atom-field interaction follows the realization of $\mathcal{PT}$ symmetry by adjusting the coupling field detunings [J. Sheng et al., Phys. Rev. A 88, 041803(R) (2013)]. A giant enhancement for the GH shift in the reflected light is revealed when the $\mathcal{PT}$-symmetry condition is satisfied.

The ac Stark shift in the presence of spontaneous decay is typically considered to induce an effective dephasing with a scattering rate equal to ${\mathrm{\Gamma }}_s{\left|\mathrm{\Omega }\right|}^2/{\mathrm{\Delta }}^2$, where ${\mathrm{\Gamma }}_s$ is the spontaneous decay rate, $\mathrm{\Omega }$ is the laser transition coupling, and $\mathrm{\Delta }$ is the detuning. We show that under realistic circumstances this dephasing rate may be strongly modified due to non-Markovian behavior. The non-Markovian behavior arises due to an effective modification of the light-atom coupling in the presence of the ac-Stark-shift laser. An analytical formula for the non-Markovian ac-Stark-shift-induced dephasing is derived. We obtain that for narrow laser linewidths the effective dephasing rate is suppressed by a factor of $Q^2$, where $Q$ is the quality factor of the laser.

We model atomic motion in a sliding superlattice potential to explore “topological charge pumping” and to find optimal parameters for experimental observation of this phenomenon. We analytically study the band structure, finding how the Wannier states evolve as two sinusoidal lattices are moved relative to one another, and relate this evolution to the center-of-mass motion of an atomic cloud. We pay particular attention to counterintuitive or anomalous regimes, such as when the atomic motion is opposite to that of the lattice. We propose a practical cold-atom experiment to detect this anomalous behavior. Through numerical simulations, we find that a negative adiabatic current and a nontrivial Chern number $\mathcal{C}=-1$ are readily measured.

We present a theoretical study of three-photon electromagnetically induced absorption for a ladder-type three-level atomic system. A probe beam was tuned to the lower line and two counterpropagating, linearly polarized coupling beams were tuned to the upper line. The system can be modeled with a three- (or five-) level scheme when the polarization directions of the coupling beams are parallel (or perpendicular). By calculating the absorption coefficients analytically for the two schemes, we found that the corresponding absorption coefficients were identical except for different transition strengths, and that the primitive scheme embedded in those schemes was a simple four-level scheme.

To explore the spectral and carrier transfer characteristics of $1.55-\mu \mathrm{m}\mathrm{InAs}/\mathrm{InP}$ coupled-quantum-dot lasers (CQDLs), we develop a probabilistically coupled multipopulation rate equation model (PCMPREM) involving intradot and interdot relaxation, inhomogeneous broadening, and homogeneous broadening. After solving the PCMPREM with the fourth-order Runge-Kutta method, a simultaneous quadruple lasing spectrum is observed and explained by both the carrier competition theory and coupled theory. An analysis of the results shows that the coupling strength between different subbands changes with different current injections, giving a systematic understanding of the operation of CQDLs systems. With a lower threshold, the CQDL has a much broader output range of more than $105.3\mathrm{nm}$ around 1.55 $\mu \mathrm{m}$, which is 7.8 times greater than the uncoupled QDL, indicating that CQDLs can be excellent light sources for not only long-haul ultrahigh capacity optical communications, but also on-chip photonics integrated circuits with low power consumption.

A near-resonant rf field pumping the hyperfine transition between the two ground states of a $\mathrm{\Lambda }$-shaped dark resonance leads to a resonance tripling, each component displaying electromagnetically induced transparency (EIT). We investigate the three resonances under high spectral and temporal resolution. The triplet formation is analogous to that of the Mollow triplet but distinct in that the role played by the spontaneous emission rate is now taken by the one-photon scattering rate of the optical Raman transition. Complex phase relations exist between the three em fields under EIT conditions. We explain our observations using numerical solutions of the quantum master equation as well as a simple analytical dressed-state model.

In the pursuit of speculative new particles, forces, and dimensions with vanishingly small influence on normal matter, understanding the ultimate physical limits of experimental sensitivity is essential. Here, I show that quantum decoherence offers a window into otherwise inaccessible realms. There is a standard quantum limit for diffusion that restricts some entanglement-generating phenomena, such as soft collisions with new particle species, from having appreciable classical influence on normal matter. Such phenomena are classically undetectable but can be revealed by the anomalous decoherence they induce on nonclassical superpositions with long-range coherence in phase space. This gives strong, novel motivation for the construction of matter interferometers and other experimental sources of large superpositions, which recently have seen rapid progress. Decoherence is always at least second order in the coupling strength, so such searches are best suited for soft interactions (e.g., small momentum transfers), but not weak ones (i.e., small coupling constants).