Adiabatic transport of information is a widely invoked resource in connection with quantum information processing and distribution. The study of adiabatic transport via spin-half chains or clusters is standard in the literature, while in practice the true realization of a completely isolated two-level quantum system is not achievable. We explore here, theoretically, the extension of spin-half chain models to higher spins. Considering arrangements of three spin-one particles, we show that adiabatic transport, specifically a generalization of the dark-state adiabatic passage procedure, is applicable to spin-one systems. We thus demonstrate a qutrit state transfer protocol. We discuss possible ways to physically implement this protocol, considering quantum dot and nitrogen-vacancy implementations.

We study the dynamics of a system comprised of a single qubit interacting equally with $N$ qubits (a “spin star” system). Although this model can be solved exactly, the exact solution does not give much intuition for the dynamics of the model. Here, we find an approximation that gives some insight into the dynamics for a particular class of initial spin-coherent states of the $N$ qubits. We find an effective Hamiltonian for the system that is a finite Kerr (one-axis twisting) Hamiltonian for the $N+1$ qubits. The initial spin-coherent state evolves to spin-squeezed states on short time scales, and to “multiple-Schrödinger-cat” states (superpositions of many spin-coherent states) on longer time scales, a manifestation of the phenomenon of fractional revivals of the initial state. The evolution of the system is visualized with phase-space plots ($Q$ functions) that, when plotted against time, reveal a “quantum carpet” pattern. Of particular interest is the fact that our approximation captures the qualitative features of the model even for small values of $N$. This suggests the possibility of observing the phenomenon of fractional revival in this model for systems of few qubits.

Understanding how the effectiveness of natural photosynthetic energy-harvesting systems arises from the interplay between quantum coherence and environmental noise represents a significant challenge for quantum theory. Recently it has begun to be appreciated that discrete molecular vibrational modes may play an important role in the dynamics of such systems. Here we present a microscopic mechanism by which intramolecular vibrations may be able to contribute to the efficiency and directionality of energy transfer. Excited vibrational states create resonant pathways through the system, supporting fast and efficient energy transport. Vibrational damping together with the natural downhill arrangement of molecular energy levels gives intrinsic directionality to the energy flow. Analytical and numerical results demonstrate a significant enhancement of the efficiency and directionality of energy transport that can be directly related to the existence of resonances between vibrational and excitonic levels.

On the basis of general theoretical results developed previously [I. M. Sokolov et al., J. Exp. Theor. Phys. 112, 246 (2011)], we analyze spontaneous decay of a single atom inside cold atomic clouds under conditions when the averaged interatomic separation is less than or comparable with the wavelength of quasiresonant radiation. Beyond the decay dynamics we analyze shifts of resonance as well as distortion of the spectral shape of the atomic transition.

Quantum interference arising from spontaneous emission, or cross-damping, is an important yet frequently overlooked systematic in precision spectroscopy experiments which aim to determine a transition frequency with an uncertainty smaller than the natural linewidth. Here, we calculate the effects of such interference in two-photon frequency-comb spectroscopy using a perturbative approach and by integration of the density matrix equations. We then apply these techniques to the two-photon spectroscopy of the hydrogen $1S-3S$ transition currently being performed in our group. Depending on the detection geometry, we find distortions of the line shapes which can lead to systematic errors of $\sim$1 kHz if such interference effects are ignored in the data analysis. This result is independent of whether a cw laser or frequency comb is used for the excitation. Finally, we propose a time-dependent detection scheme which, when used in conjunction with frequency-comb excitation, can mitigate the line distortions arising from such interference.

Two-photon ionization of ground-state strontium is investigated experimentally in the 360–370-nm spectral range with dye laser pulses of long (∼ns) duration and low $\mathrm{(}\mathrm{\sim }10{}^{10}W\mathrm{\hspace{0.17em}cm}{}^{-2}$) intensity. The ${\mathrm{Sr}}^{+}$ spectra recorded with linear laser polarization are dominated by the presence of the highly correlated $5p^2$ ${}^1{S}_0$ state and by the even parity $[4d$$6d]$]${}_J$${}_{=0,2}$ autoionizing states. The partial $J=0$ and $J=2$ two-photon ionization cross sections are recovered from these data along with the spectra obtained with circular polarization. For linear polarization, the observed wavelength dependence of ${\mathrm{Sr}}^{+}$ production is compared with earlier theoretical work based on the $R$-matrix–multichannel-quantum-defect-theory approach and the agreement is generally found to be quite satisfactory. There are, however, a few, albeit noticeable, discrepancies. The latter motivated us to proceed to a more comprehensive experimental characterization of the examined energy range, by also measuring the kinetic energy and angular distribution of the ejected photoelectrons under linear laser polarization. The shapes of the angular distributions vary rapidly in the neighborhood of each resonance, while these distributions are found to be almost spherically symmetric on the maxima of the sharp $[4d$$6d]$]${}_J$${}_{=0}$ lines. On these, and only these, maxima, electron energy spectra reveal the absorption of a third photon, resulting in production of ${\mathrm{Sr}}^{+}$ into its excited $4d_j$ and $5p_j$ states. The non-negligible $5p_j$ branching ratio reflects the mixing between the $[4d$$6d]$]${}_J$${}_{=0}$ states and the $5p^2$ ${}^1{S}_0$ perturber, found to be rather underestimated by the aforementioned theoretical work. Thus, the presently acquired ion and electron spectra as well as the asymmetry parameters obtained by fitting the photoelectron angular distributions provide a stringent test of future theoretical efforts, addressing structure as well as excitation and ionization issues in an improved manner.

When atoms are exposed to intense laser or microwave pulses, $\sim 10\%$ of the atoms are found in Rydberg states subsequent to the pulse, even if it is far more intense than required for static field ionization. The optical spectra of the surviving Li atoms in the presence of a 38-GHz microwave field suggest how atoms survive an intense pulse. The spectra exhibit a periodic train of peaks 38 GHz apart. One peak is just below the limit, and with a 90-V/cm field amplitude the train extends from 300 GHz above the limit to 3000 GHz below it. The spectra and quantum-mechanical calculations imply that the atoms survive in quasistable states in which the Rydberg electron is in a weakly bound orbit infrequently returning to the ionic core during the intense pulse.

Helium atoms in Rydberg states have been manipulated coherently with microwave radiation pulses near a gold surface and near a superconducting NbTiN surface at a temperature of $3\mathrm{K}$. The experiments were carried out with a skimmed supersonic beam of metastable ${\left(1s\right)}^1{\left(2s\right)}^1{}^1{S}_0$ helium atoms excited with laser radiation to $np$ Rydberg levels with principal quantum number $n$ between 30 and 40. The separation between the cold surface and the center of the collimated beam is adjustable down to $250\mu \mathrm{m}$. Short-lived $np$ Rydberg states were coherently transferred to the long-lived $ns$ state to avoid radiative decay of the Rydberg atoms between the photoexcitation region and the region above the cold surfaces. Further coherent manipulation of the $ns$ Rydberg states with pulsed microwave radiation above the surfaces enabled measurements of stray electric fields and allowed us to study the decoherence of the atomic ensemble. Adsorption of residual gas onto the surfaces and the resulting slow buildup of stray fields was minimized by controlling the temperature of the surface and monitoring the partial pressures of ${\mathrm{H}}_2\mathrm{O}$, ${\mathrm{N}}_2$, ${\mathrm{O}}_2$, and $\mathrm{C}{\mathrm{O}}_2$ in the experimental chamber during the cool-down procedure. Compensation of the stray electric fields to levels below $100\mathrm{mV}/\mathrm{cm}$ was achieved over a region of $6\mathrm{mm}$ along the beam-propagation direction which, for the $1770\text{-m}/\mathrm{s}$ beam velocity, implies the possibility to preserve the coherence of the atomic sample for several microseconds above the cold surfaces.

Motivated by recent experimental progress in the realization of synthetic gauge fields in systems of ultracold atoms, we consider interacting bosons on the dice lattice with half flux per plaquette. All bands of the noninteracting spectrum of this system were previously found to have the remarkable property of being completely dispersionless. We show that degeneracies remain when interactions are treated at the level of mean-field theory, and the ground state exhibits vortex lattice configurations already established in the simpler $XY$ model in the same geometry. We argue that including quantum and thermal fluctuations will select a unique vortex lattice up to overall symmetries based on the order-by-disorder mechanism. We verify the stability of the selected state by analyzing the condensate depletion. The latter is shown to exhibit an unusual nonmonotonic behavior as a function of the interaction parameters which can be understood as a consequence of the dispersionless nature of the noninteracting spectrum. Finally, we comment on the role of domain walls which have interactions mediated through fluctuations.

Motivated by recent advances in the realization of complex two-dimensional optical lattices, we investigate theoretically the quantum transport of ultracold fermions in an optical kagome lattice. In particular, we focus on its extensively degenerate localized states (flat band). By loading fermions in a partial region of the lattice and depleting the mobile atoms at the far boundary of the initially unoccupied region, we find a dynamically generated flat-band insulator, which is also a population-inverted state. We further show that inclusion of weak repulsion leads to a dynamical stripe phase for two-component fermions in a similar setup. Finally, by preparing a topological insulating state in a partially occupied kagome lattice, we find that the topological chiral current decays but exhibits an interesting oscillating dynamics during the nonequilibrium transport. Given the broad variety of lattice geometries supporting localized or topological states, our work suggests new possibilities for using geometrical effects and their dynamics in atomtronic devices.

We study density wave instabilities in a doubly degenerate Fermi-Fermi mixture with $\mathrm{SU}\left(2\right)\times \mathrm{SU}\left(2\right)$ symmetry on a square lattice. For sufficiently large on-site inter-species repulsion, when the two species of fermions are both at half-filling, two conventional ($s$ wave) number density waves are formed with a $\pi$-phase difference between them to minimize the interspecies repulsion. Upon moving one species away from half-filling, an unconventional density wave with $d_{xy}$-wave symmetry emerges. When both species are away from the vicinity of half-filling, superconducting instabilities dominate. We present results of a functional renormalization-group calculation that maps out the phase diagram at weak couplings. Also, we provide a simple explanation for the emergence of the $d_{xy}$-density wave phase based on a four-patch model. We find a robust and general mechanism for $d_{xy}$-density-wave formation that is related to the shape and size of the Fermi surfaces. The density imbalance between the two species of fermions in the vicinity of half-filling leads to a phase-space discrepancy for different interspecies umklapp couplings. Using a phase-space argument for leading corrections in the one-loop renormalization-group approach to fermions, we show that the phase-space discrepancy in our system causes opposite flows for the two leading intraspecies umklapp couplings and that this triggers the $d_{xy}$-density-wave instability.

We show that strongly interacting multicomponent gases in one dimension realize an effective spin chain, offering an alternative simple scenario for the study of one-dimensional (1D) quantum magnetism in cold gases in the absence of an optical lattice. The spin-chain model allows for an intuitive understanding of recent experiments and for a simple calculation of relevant observables. We analyze the adiabatic preparation of antiferromagnetic and ferromagnetic ground states, and show that many-body spin states may be efficiently probed in tunneling experiments. The spin-chain model is valid for more than two components, opening the possibility of realizing SU(N) quantum magnetism in strongly interacting 1D alkaline-earth-metal or ytterbium Fermi gases.

Localization and dynamics of the one-dimensional biharmonic nonlinear Schrödinger (NLS) equation in the presence of an external periodic potential is studied. The band-gap structure is determined using the Floquet-Bloch theory and the shape of its dispersion curves as a function of the fourth-order dispersion coupling constant $\beta$ is discussed. Contrary to the classical NLS equation ($\beta =0$) with an external periodic potential for which a gap in the spectrum opens for any nonzero potential, here it is found that for certain negative $\beta ,$ there exists a nonzero threshold value of potential strength below which there is no gap. For increasing values of potential amplitudes, the shape of the dispersion curves change drastically leading to the formation of localized nonlinear modes that have no counterpart in the classical NLS limit. A higher-order two-band tight-binding model is introduced that captures and intuitively explains most of the numerical results related to the spectral bands. Lattice solitons corresponding to spectral eigenvalues lying in the semi-infinite and first band gaps are constructed. In the anomalous dispersion case, i.e., $\beta <0$ (where for the self-focusing nonlinearity no localized nonradiating solitons exist in the absence of an external potential), nonlinear finite-energy stationary modes with eigenvalues residing in the first band gap are found and their properties are discussed. The stability of various localized lattice modes is studied via linear stability analysis and direct numerical simulation.

The mode characteristics (dispersion relation and $Q$ factor) of metal-coated microcylinders are achieved from the analytical resonance eigenequation. Among the resonance modes, two surface plasmon polariton (SPP) modes have completely different characteristics compared to the hybrid plasmonic whispering gallery modes (WGMs). In the metal-coated microcylinder, the $Q$ factor of the asymmetrical SPP mode can be greater than the symmetric mode, and the hybrid plasmonic WGMs have a greater mode index range, extending down to zero. Also, the mode indexes and $Q$ factors of the resonance modes are measured by the attenuated total reflection and resonance linewidth method, respectively. The experimental results agree well with the theoretical.

We derive the general frequency shift of a microwave atomic clock due to resonant dipole forces acting on the atomic wave functions during the Ramsey interactions. We explicitly demonstrate that only dressed-state populations in position space contribute significantly to the frequency shift and that the de Broglie phases of the dressed-state wave functions do not contribute. In addition, we show that momentum changes in the second Ramsey interaction normally produce negligible frequency shifts in comparison to those from the first Ramsey interaction.

For systems of an arbitrary dimension, a theory of geometric chained Bell inequalities is presented. The approach is based on chained inequalities derived by Pykacz and Santos. For maximally entangled states, the inequalities lead to a complete $0=1$ contradiction with quantum predictions. Local realism suggests that the probability for the two observers to have identical results is 1 (that is, a perfect correlation is predicted), whereas quantum formalism gives an opposite prediction: the local results always differ. This is so for any dimension. We also show that with the inequalities, one can have a version of Bell's theorem which involves only correlations arbitrarily close to perfect ones.

We study how the resilience of the surface code is affected by the coupling to a non-Markovian environment at zero temperature. The qubits in the surface code experience an effective dynamics due to the coupling to the environment that induces correlations among them. The range of the effective induced qubit-qubit interaction depends on parameters related to the environment and the duration of the quantum error correction cycle. We show numerically that different interaction ranges set different intrinsic bounds on the fidelity of the code. These bounds are unrelated to the error thresholds based on stochastic error models. We introduce a definition of stabilizers based on logical operators that allows us to efficiently implement a Metropolis algorithm to determine upper bounds to the fidelity error threshold.

The field of electron-impact ionization of atoms, or $(e,2e)$, has provided significant detailed information about the physics of collisions. For ionization of hydrogen and helium, essentially exact numerical methods have been developed which can correctly predict what will happen. For larger atoms, we do not have theories of comparable accuracy. Considerable attention has been given to ionization of inert gases and, of the inert gases, argon seems to be the most difficult target for theory. There have been several studies comparing experiment and perturbative theoretical approaches over the last few decades, and generally qualitative but not quantitative agreement is found for intermediate energy incident electrons. Recently a nonperturbative method, the $B$-spline $R$-matrix (BSR) method, was introduced which appears to be very promising for ionization of heavier atoms. We have recently performed an experimental and theoretical investigation for ionization of argon, and we found that, although the BSR gave reasonably good agreement with experiment, there were also some cases of significant disagreement. The previous study was performed for 200-eV incident electrons and ejected electron energies of 15 and 20 eV. The purpose of the present work is to extend this study to a much larger range of ejected electron energies (15–50 eV) to see if theory gets better with increasing energy as would be expected for a perturbative calculation. The experimental results are compared with both the BSR and two different perturbative calculations.

A method is described to measure photoexcitation cross sections, relying on the expected behavior of the signal in the saturated regime, when excitation is provided by a Gaussian light beam. The method is implemented on a negative ion beam, with a single-mode pulsed Nd:YAG laser, to make a laser measurement of the photodetachment cross section of ${\mathrm{H}}^{-}$, at the wavelength 1064 nm. This cross section is of importance both as a photodetachment cross section of the most elementary negative ion and as a key parameter for the production of fast neutral ${\mathrm{H}}^0$ or ${\mathrm{D}}^0$ atoms, by photodetachment of accelerated anions. The obtained value $4.5\left(6\right)\times {10}^{-21}{\mathrm{m}}^2$ is greater than the one known from older measurements and most ab initio calculations.

The fluorescence yield quenching of low-lying doubly excited $2pnd$ ${}^1{P}^o$ states is observed to depend strongly on a dc electric field strength and its orientation with respect to the polarization of the incoming photon beam. The reduction of the yield accompanied by the lifetime shortening is attributed to the Stark mixing with the neighboring $2sns$ ${}^1{S}^e$ states, which redirects the $2pnd$ ${}^1{P}^o$ decay to the prompt autoionization channel. For $n\ge 4$, the lifetimes decrease from several hundred picoseconds down to several tens of picoseconds when an electric field in the kV/cm range is applied parallel to the photon probe polarization. Practically no lifetime change is observed for polarization perpendicular to the electric field direction. The results of the complex-scaling calculations are in a good agreement with the experimental data.