We study the performance of dynamical decoupling in suppressing decoherence caused by soft-cutoff Gaussian noise, using short-time expansion of the noise correlations and numerical optimization. For the noise with soft cutoff at high frequencies, there exists no dynamical decoupling scheme to eliminate the decoherence to arbitrary orders of the short time, regardless of the timing or pulse shaping of the control under the population conserving condition. We formulate the equations for optimizing pulse sequences that minimize decoherence up to the highest possible order of the short time for the noise correlations with odd power terms in the short-time expansion. In particular, we show that the Carr-Purcell-Meiboom-Gill sequence is optimal in the short-time limit for the noise correlations with a linear order term in the time expansion.

The Casimir force between metal-dielectric composite slabs containing nonspherical particles is investigated. The composite slab may have the symmetric (the nonspherical metal particles and spherical dielectric particles are randomly distributed) or asymmetric microstructure (the nonspherical metal particles are randomly embedded in the dielectric host medium), and the corresponding effective permittivity is described by the generalized Bruggeman effective medium approximation or generalized Maxwell-Garnett approximation. As a consequence, the Casimir force can be controlled by the volume fraction and the particles’ shape. It is found that the Casimir force achieves a minimal value for spherical particles, and the magnitude of Casimir force can become strong for nonspherical particles. In addition, the Casimir force for the metal-dielectric composites with the symmetric microstructure shows a fast change at the shape-dependent percolation threshold, above (below) which the composite slab is metallic (dielectric). Our study may be of great interest for making precise comparisons between theoretical and experimental results on the Casimir force.

For the last 50 years, there has been considerable interest in the possibility of observing the equivalence of a Young's double-slit wave interference at the quantum level for diatomic molecules. For electron-impact ionization of diatomic molecules, indirect evidence for this type of interference has been found by changing the energy (wavelength) of the ejected electron while keeping the incident projectile scattering angle fixed. The present work represents an experimental and theoretical collaboration to better understand the physics of this type of interference. In addition to examining the effect of changing the ejected electron energy for a fixed scattered projectile angle, we have also examined the effect of keeping the ejected electron energy fixed while varying the projectile scattering angle. Model calculations are performed for three different types of possible two-center interference effects, and it is found that the most important one is diffraction of the projectile off two scattering centers.

We apply two different monotonically convergent optimization algorithms to the control of molecular rotational dynamics by laser pulses. This example represents a quantum control problem where the interaction of the system with the external field is nonlinear. We test the validity and accuracy of the two methods on the key control targets of producing molecular orientation and planar delocalization at zero temperature, and maximizing permanent alignment at nonzero temperature.

We use a high-temperature expansion to explore spin correlations around a single hole in a two-dimensional lattice filled with a hard-core two-component bose gas. We find that the spins around the hole develop ferromagnetic order and quantify the degree of polarization at temperatures of the order of the hopping energy, finding a measurably nonzero polarization. We also discuss the effect of fixing the overall magnetization of the system for finite-sized systems.

Multimodal nonlinear propagation dominates the evolution of intense laser pulses propagating in high-pressure gas-filled capillaries used for high harmonic generation. A fully multimodal nonlinear propagation model is used to predict pulse evolution along such a capillary, and the length and pressure distribution are optimized to produce the shortest pulses at the capillary output. This optimization is shown theoretically to result in self-compression of the pulse from $\sim$53 to $\sim$7 fs, and is shown experimentally to increase the flux of high harmonic radiation from the capillary by an order of magnitude over comparable capillary and gas jet designs.

Recently, we proposed a method for the local detection of quantum correlations on the basis of local measurements and state tomography at different instances in time [Phys. Rev. Lett. 107, 180402 (2011)]. The method allows for the detection of quantum discord in bipartite systems when access is restricted to only one of the subsystems. Here, we elaborate the details of this method and provide applications to specific physical models. In particular, we discuss the performance of the scheme for generic complex systems by investigating thermal equilibrium states corresponding to randomly generated Hamiltonians. Moreover, we formulate an ergodicity-type hypothesis which links the time average to the analytically obtained average over the group of unitary operators equipped with the Haar measure.

Amplitude damping changes entangled pure states into usually less-entangled mixed states. We show, however, that even local amplitude damping of one or two qubits can result in mixed states more entangled than pure states if one compares the relative entropy of entanglement (REE) for a given degree of the Bell–Clauser-Horne-Shimony-Holt inequality violation (referred to as nonlocality). By applying Monte Carlo simulations, we find the maximally entangled mixed states and show that they are likely to be optimal by checking the Karush-Kuhn-Tucker conditions, which generalize the method of Lagrange multipliers for this nonlinear optimization problem. We show that the REE for mixed states can exceed that of pure states if the nonlocality is in the range (0,0.82) and the maximal difference between these REEs is 0.4. A former comparison [Phys. Rev. A 78, 052308 (2008)] of the REE for a given negativity showed analogous property but the corresponding maximal difference in the REEs is one order smaller (i.e., 0.039) and the negativity range is (0,0.53) only. For appropriate comparison, we normalized the nonlocality measure to be equal to the standard entanglement measures, including the negativity, for arbitrary two-qubit pure states. We also analyze the influence of the phase-damping channel on the entanglement of the initially pure states. We show that the minimum of the REE for a given nonlocality can be achieved by this channel, contrary to the amplitude-damping channel.

We report an experimental self-calibrating tomography scheme for entanglement characterization in high-dimensional quantum systems using Schmidt decomposition techniques. The self-tomography technique based on maximal likelihood estimation was developed for characterizing nonideal measurements in the Schmidt basis, allowing us to infer both Schmidt eigenvalues and detecting efficiencies.

We propose and analyze a quantum gate that transfers an arbitrary state of a qubit into a superposition of two quasiorthogonal coherent states of a cavity mode (qcMAP), with opposite phases. This qcMAP gate is based on conditional qubit and cavity operations exploiting the energy-level dispersive shifts in the regime where they are much stronger than the cavity and qubit linewidths. The generation of multicomponent superpositions of quasiorthogonal coherent states, nonlocal entangled states of two resonators, and multiqubit Greenberger-Horne-Zeilinger states can be efficiently achieved by this gate.

We study a system consisting of a quantum kicked rotor with an additional degree of freedom. We show analytically and numerically that this model is characterized by its quantum resonances with ballistic spreading and by the entanglement between the internal and momentum degrees of freedom. We conclude that the model shows certain interesting similarities with the standard quantum walk on the line.

A quantum expander is a unital quantum channel that is rapidly mixing, has only a few Kraus operators, and can be implemented efficiently on a quantum computer. We consider the problem of estimating the mixing time (i.e., the spectral gap) of a quantum expander. We show that the problem of deciding whether a quantum channel is not rapidly mixing is a complete problem for the quantum Merlin-Arthur complexity class. This has applications to testing randomized constructions of quantum expanders and studying thermalization of open quantum systems.

The Toffoli gate, which has practical applications in the implementation of practical quantum algorithms and quantum error correction, is usually constructed from concatenated two-qubit or qubit-qutrit gates. Here we show that the Toffoli gate for three qubits can be implemented through a single $\mathit{XY}$ interaction plus two single-qubit rotations. This method greatly simplifies the experimental implementation of the Toffoli gate and promises a much higher fidelity compared to those based on elementary gate decomposition. We present an example for the implementation of the required $\mathit{XY}$ interaction in cavity QED.

Casimir-Polder interactions between an atom and a macroscopic body are typically regarded as being due to the exchange of virtual photons. This is strictly true only at zero temperature. At finite temperatures, real-photon exchange can provide a significant contribution to the overall dispersion interaction. Here, we describe a resonant two-photon process between an atom and a planar interface. We derive a second-order effective Hamiltonian to explain how atoms can couple resonantly to the surface polariton modes of the dielectric medium. This leads to second-order energy exchanges which we compare with the standard nonresonant Casimir-Polder energy.

Multiply ionized ground and excited anilines created by x-ray free-electron lasers (XFELs) were analyzed by first-principles calculations. The analyses revealed that redshift of the optical absorption to the visible light region appears dominantly during XFEL illumination owing to the ultrashort lifetime of the core-hole states. The XFEL pulse envelope information is thus transferred from the x-ray to the optical frequency domain, where precise measurement is feasible. This research provides a foundation for a pulse envelope monitor that is indispensable for future envelope-controlled XFEL experiments.

We identified interatomic Coulombic decay (ICD) channels in argon dimers after spectator-type resonant Auger decay $2p^{-1}3d\to 3p^{-2}3d,4d$ in one of the atoms, using momentum-resolved electron-ion-ion coincidence. The results illustrate that the resonant core excitation is a very efficient way of producing slow electrons at a specific site, which may cause localized radiation damage. We find also that ICD rate for $3p^{-2}4d$ is significantly lower than that for $3p^{-2}3d$.

A form of motional sideband excitation is described in which a rotating dipole electric field is applied asymmetrically onto a Penning-type trap in the presence of a mechanism for cooling the axial motion of the trapped particles. In contrast to the traditional motional sideband excitation, which uses an oscillating electric field, the rotating field results in only one active sideband in each sense of rotation and so avoids accidental excitation of the other sideband making it applicable to Penning-type traps with a large degree of anharmonicity. Expressions are derived for the magnetron radius expansion and compression rates attainable, and approximations are made for the case of strong and weak drives. A comparison is made with data, taken using a two-stage positron accumulator presented by Isaac et al. [C. A. Isaac, C. J. Baker, T. Mortensen, D. P. van der Werf, and M. Charlton, Phys. Rev. Lett. 107, 033201 (2011)], showing good agreement between the model and experiment.

We study a quasi-two-dimensional dipolar gas at finite, but ultralow, temperatures using the classical field approximation. The method, already used for a contact interacting gas, is extended here to samples with a weakly interacting long-range interatomic potential. We present statistical properties of the system for the current experiment with chromium [Müller, Billy, Henn, Kadau, Griesmaier, Jona-Lasinio, Santos, and Pfau, Phys. Rev. A 84, 053601 (2011)] and compare them with statistics for atoms with larger magnetic dipole moments. Significant enhancement of the third-order correlation function, relevant for the particle losses, is found.

We present a theoretical model for Bragg scattering from a Bose-Einstein condensate (BEC) in the vicinity of a magnetic Feshbach resonance, using a two-$c$-field formalism, one $c$ field for the atom and the other for a molecule formed of two atoms. We use this model to numerically simulate an experiment [Papp et al., Phys. Rev. Lett. 101, 135301 (2008)] investigating the effects of strong interactions on the Bragg spectrum from a ${}^{85}$Rb BEC. Results from these simulations and from our previous calculations using an atom-molecule Bogoliubov approach are in very good quantitative agreement with the experimental results, confirming the importance of the resonance bound state in the dynamics of the condensate for fast experiments like Bragg scattering.

We demonstrate experimental excitation-manifold-resolved polarization characterization of quantum states of light ranging from the few-photon to the many-photon level. In contrast to the traditional characterization of polarization that is based on the Stokes parameters, we experimentally determine the Stokes vector of each excitation manifold separately. Only for states with a given photon number do the methods coincide. For states with an indeterminate photon number, for example Gaussian states, the employed method gives a richer and more accurate description. We apply the method both in theory and in experiment to some common states to demonstrate its advantages.