We study the use of exciton-polaritons in semiconductor microcavities to generate radiation spanning the infrared to terahertz regions of the spectrum by exploiting transitions between upper and lower polariton branches. The process, which is analogous to difference-frequency generation (DFG), relies on the use of semiconductors with a nonvanishing second-order susceptibility. For an organic microcavity composed of a nonlinear optical polymer, we predict a DFG irradiance enhancement of $2.8\times {10}^2$, as compared to a bare nonlinear polymer film, when triple resonance with the fundamental cavity mode is satisfied. In the case of an inorganic microcavity composed of (111) GaAs, an enhancement of $8.8\times {10}^3$ is found, as compared to a bare GaAs slab. Both structures show high wavelength tunability and relaxed design constraints due to the high modal overlap of polariton modes.

We present the normal-mode splitting and optomechanically induced transparency or absorption phenomena in the strongly tunnel-coupled optomechanical cavities. In the probe output spectrum, there appear central transparency windows or absorption peaks around which two broad sidebands are symmetrically located. It has been confirmed by the quantitative findings that two broad sidebands, which include the distorted absorption peaks, indicate the normal-mode splitting of the two hybridized cavities, and central transparency windows or absorption peaks character the interference induced by the optomechanical interactions. Additionally, the switching from absorption to amplification can be realized by only adjusting the tunnel interaction. These spectrum properties can be used for the coherent control of light pulses via microfabricated optomechanical arrays.

A successful approach to understand field theories is to resolve the physics into different length or energy scales using the renormalization group framework. We propose a quantum simulation of quantum field theory which encodes field degrees of freedom in a wavelet basis—a multiscale description of the theory. Since wavelet families can be constructed to have compact support at all resolutions, this encoding allows for quantum simulations to create particle excitations which are local at some chosen scale and provides a natural way to associate observables in the theory to finite-resolution detectors.

Absolutely maximally entangled (AME) states are those multipartite quantum states that carry absolute maximum entanglement in all possible bipartitions. AME states are known to play a relevant role in multipartite teleportation, in quantum secret sharing, and they provide the basis novel tensor networks related to holography. We present alternative constructions of AME states and show their link with combinatorial designs. We also analyze a key property of AME states, namely, their relation to tensors, which can be understood as unitary transformations in all of their bipartitions. We call this property multiunitarity.

We report the results of our theoretical study and analysis of earlier experimental data for the $g$-factor tensor components of the ground ${}^2{\mathrm{\Pi }}_{1/2}$ state of the free PbF radical. The values were obtained both within the relativistic coupled-cluster method combined with the generalized relativistic effective core potential approach and with our fit of the experimental data from [R. J. Mawhorter, B. S. Murphy, A. L. Baum, T. J. Sears, T. Yang, P. M. Rupasinghe, C. P. McRaven, N. E. Shafer-Ray, L. D. Alphei, and J.-U. Grabow, Phys. Rev. A 84, 022508 (2011); A. L. Baum, B.A. thesis, Pomona College, 2011]. The obtained results agree very well with each other but contradict the previous fit performed in the cited works. Our final prediction for $g$ factors is $G_{\parallel }=0.081\left(5\right),G_{\perp }=-0.27\left(1\right)$.

A Feshbach-optimized photoassociation for preparing ultracold ${}^{40}\mathrm{K}{}^{87}\mathrm{Rb}$ molecules with short pulses is investigated theoretically. The total probability density of the coupled hyperfine states is greatly enhanced at short interatomic separations in the vicinity of magnetically tuned Feshbach resonances. Due to the coupling of the free hyperfine state with different bound hyperfine states, the maximal probability density distributes at different interatomic regions and determines the choice of the intermediate state. By using the spin-orbit coupling of the excited states, the population is successfully transferred to the singlet ground state $X{}^1{\mathrm{\Sigma }}^{+}$ from the initial state mostly having triplet state character. By comparing the final population in the vicinity of Feshbach resonances at $B_0=546.80$ and 659.34 G, our results show that a Feshbach resonance corresponding to a more deeply bound vibrational level can enhance the photoassociation efficiency of ultracold atoms to some degree.

As one of the most important physical processes of strong-field laser-matter interaction, laser-driven electron-ion recollision depends sensitively on the polarization of the laser field and can be effectively manipulated via an orthogonally polarized two-color (OTC) laser field. Here we present an extension of recollision manipulation in an OTC field to the midinfrared laser field regime, and demonstrate that only a few recollisions occur in a multicycle $1800/900\mathrm{nm}$ OTC laser field. The number of recollisions can be controlled by simply tuning the relative delay of the two-color laser pulses, and this manipulation process can be visualized by measuring the high-order harmonic spectra. When the intensity of a midinfrared OTC field is further increased, a single recollision can be correlated to one hump in the harmonic spectrum, which will help to increase the measurement accuracy of time-resolved dynamics in atoms and molecules.

We investigate the properties of an impurity immersed in a dilute Bose gas at zero temperature using quantum Monte Carlo methods. The interactions between bosons are modeled by a hard-sphere potential with scattering length $a$, whereas the interactions between the impurity and the bosons are modeled by a short-range, square-well potential where both the sign and the strength of the scattering length $b$ can be varied by adjusting the well depth. We characterize the attractive and the repulsive polaron branch by calculating the binding energy and the effective mass of the impurity. Furthermore, we investigate the structural properties of the bath, such as the impurity-boson contact parameter and the change of the density profile around the impurity. At the unitary limit of the impurity-boson interaction, we find that the effective mass of the impurity remains smaller than twice its bare mass, while the binding energy scales with ${\hslash }^2{n}^{2/3}/m$, where $n$ is the density of the bath and $m$ is the common mass of the impurity and the bosons in the bath. The implications for the phase diagram of binary Bose-Bose mixtures at low concentrations are also discussed.

We investigate vortex shedding from a moving penetrable obstacle in a highly oblate Bose-Einstein condensate. The penetrable obstacle is formed by a repulsive Gaussian laser beam that has the potential barrier height lower than the chemical potential of the condensate. The moving obstacle periodically generates vortex dipoles and the vortex shedding frequency $f_v$ linearly increases with the obstacle velocity $v$ as $f_v=a(v-v_c)$, where $v_c$ is a critical velocity. Based on the periodic shedding behavior, we demonstrate deterministic generation of a single vortex dipole by applying a short linear sweep of a laser beam. This method will allow further controlled vortex experiments such as dipole-dipole collisions.

Quantum mechanics predicts superpositions of position states even for macroscopic objects. Recently, the use of a quasifreely suspended mirror combined with a laser was proposed to prepare such states [H. Müller-Ebhardt et al., Phys. Rev. Lett. 100, 013601 (2008)]. One of the key milestones towards this goal is the preparation of the mechanical oscillator mainly driven by measurement-induced backaction in the quantum regime. Here we describe the observation of backaction acting on a suspended 5-mg mirror in the classical regime. Furthermore, its quantum component is estimated to be larger than the thermal fluctuating force due to internal damping of the suspension, by a factor of $1.4\pm 0.2$ at 325 Hz.

In the absence of decay, the conditional dynamics for an open system is often describable by a non-Hermitian Hamiltonian. This study investigates spin squeezing (SS) in a non-Hermitian one-axis twisting (OAT) model. Somewhat surprisingly, SS close to the limit of Hermitian two-axis counter twisting (TACT) Hamiltonian is achievable for some parameters, which significantly improves upon the optimal value realizable by the Hermitian OAT model. The drawback is like with all conditional schemes, it takes on average longer time to evolve into a steady state, and the probability of no decay or success decreases as the number of atoms (spins) increases. The result above for the steady-state SS in a non-Hermitian OAT Hamiltonian is thus limited to small systems. For other parameter regimes, however, desirable SS arrives dynamically before a steady state is achieved, with a greatly shortened evolution time and enhanced probability of success, while still remains significantly improved over the limit of Hermitian OAT.

There is no doubt about the fact that entanglement and nonlocality are distinct resources. It is acknowledged that a clear illustration of this point is the difference between maximally entangled states and states that maximally violate a Bell inequality. We give strong evidence that this anomaly may be an artifact of the measures that have been used to quantify nonlocality. By reasoning that the numeric value of a Bell function is akin to a witness rather than a quantifier, we define a measure of nonlocality and show that, for pairs of qutrits and of four-level systems, maximal entanglement does correspond to maximal nonlocality in the same scenario that gave rise to the discrepancy.

We present magic wavelengths for the $n{S}_{1/2}\text{−}n{P}_{1/2,3/2},$ and $n{S}_{1/2}\text{−}m{D}_{3/2,5/2}$ transitions in ${\mathrm{Mg}}^{+},$ ${\mathrm{Ca}}^{+},$ ${\mathrm{Sr}}^{+},$ and ${\mathrm{Ba}}^{+}$ alkaline-earth-metal ions, for the principal quantum numbers $n$ and $m$ for the ground and first excited $D$ states, respectively. These wavelengths are presented for linearly polarized light by plotting the dynamic polarizabilities of the $nS,n{P}_{1/2,3/2},$ and $m{D}_{3/2,5/2}$ states of the considered ions. Required dynamic polarizabilities are evaluated by employing a relativistic all-order perturbative method and their accuracies are ratified by comparing their static values with available high-precision experimental or other theoretical results. Moreover, some of the magic wavelengths identified by us in ${\mathrm{Ca}}^{+}$ concur with the recent measurements reported by Liu et al. [Phys. Rev. Lett. 114, 223001 (2015)]. Knowledge of these magic wavelengths is propitious to carry out many proposed high-precision measurements trapping the above ions in electric fields with the corresponding frequencies.

Positronium (Ps) collisions with molecular hydrogen are investigated theoretically. Elastic and Ps ionization cross sections are calculated. For elastic scattering the pseudopotential method, previously developed for rare-gas atoms, is applied. Ps ionization cross sections are calculated using the binary-encounter approximation. The results agree with swarm measurements at low collision energies and with beam measurements at higher energies. The total Ps-${\mathrm{H}}_2$ cross section when plotted as a function of collision velocity is close to the $e^{-}\text{−}{\mathrm{H}}_2$ cross section at velocities above the Ps ionization threshold, confirming earlier observations [Brawley et al., Science 330, 789 (2010)]. However, below the threshold the two sets of cross sections are different because of the different nature of the long-range interaction between the projectile and the target, the polarization interaction in the case of $e^{-}\text{−}{\mathrm{H}}_2$ collisions and the van der Waals interaction in the case of Ps-${\mathrm{H}}_2$ collisions.

We simulate the transient absorption of attosecond pulses of infrared-laser-dressed atoms by considering a three-level system in the adiabatic approximation. The delay-dependent interference features are investigated from the perspective of the coherent interaction processes between the attosecond pulse and the quasiharmonics. We find that many features of the interference fringes in the absorption spectra of the attosecond pulse can be attributed to the coherence phase difference. However, the modulation signals of laser-induced sidebands of the dark state are found to be related to the population dynamics of the dark state by the dressing field.

The Auger decay processes from the $1s$-core-hole state of Ne have been investigated with a multielectron coincidence method. We have observed the double and triple Auger decays, in which both cascade and direct processes are identified from different ways in energy sharing between the Auger electrons. Branching ratios for the different electronic configurations of the Auger final states are determined from the corresponding electron coincidence yields. Particularly for the double Auger decay, the branching ratios are divided into the individual contributions from direct and cascade paths.

Using the strong-field approximation, a net photoelectron current, generated due to the interaction of an intense two-color laser field consisting of the fundamental and the second harmonic of infrared laser radiation with an atom, is found. The focus is on the case in which neither of the two field components can be treated perturbatively. The dependence of the current on the ratio of the harmonic amplitudes and their relative phase is studied and parameters maximizing the current are specified.

We describe optimized coherent control methods for two-photon transitions in atoms of a ladder-type three-state energy configuration. Our approach is based on the spatial coherent control scheme, which uses counterpropagating ultrashort laser pulses to produce complex excitation patterns in an extended space. Because coherent control requires constructive interference of constituent transition pathways, applying it to an atomic transition with a specific energy configuration requires specially designed laser pulses. We show in an experimental demonstration that two-photon transition with an intermediate resonant energy state can be coherently controlled and retrieved from the resonance-induced background, when phase flipping of the laser spectrum near the resonant intermediate transition is used. A simple reason for this behavior is the fact that the transition amplitude function (added to give an overall two-photon transition) changes its sign at the intermediate resonant frequency and, thus, by proper spectral-phase programing, the excitation patterns (or the position-dependent interference of the transition given as a consequence of the spatial coherent control) can be well isolated in space along the focal region of the counterpropagating pulses.

Electronic excitations in metallic nanoparticles in the optical regime that have been of great importance in surface-enhanced spectroscopy and emerging applications of molecular plasmonics, due to control and confinement of electromagnetic energy, may also be of potential to control the motion of nanoparticles and molecules. Here, we propose a concept for trapping polarizable particles and molecules using toroidal metallic nanoparticles. Specifically, gold nanorings are investigated for their scattering properties and field distribution to computationally show that the response of these optically resonant particles to incident photons permit the formation of a nanoscale trap when proper aspect ratio, photon wavelength, and polarization are considered. However, interestingly the resonant plasmonic response of the nanoring is shown to be detrimental to the trap formation. The results are in good agreement with analytic calculations in the quasistatic limit within the first-order perturbation of the scalar electric potential. The possibility of extending the single nanoring trapping properties to two-dimensional arrays of nanorings is suggested by obtaining the field distribution of nanoring dimers and trimers.

Earlier work has shown that ring dark solitons in two-dimensional Bose-Einstein condensates are generically unstable. In this work, we propose a way of stabilizing the ring dark soliton via a radial Gaussian external potential. We investigate the existence and stability of the ring dark soliton upon variations of the chemical potential and also of the strength of the radial potential. Numerical results show that the ring dark soliton can be stabilized in a suitable interval of external potential strengths and chemical potentials. We also explore different proposed particle pictures considering the ring as a moving particle and find, where appropriate, results in very good qualitative and also reasonable quantitative agreement with the numerical findings.