In the scattering of relativistic spin-polarized electrons from point nuclei, two types of polarization correlations are compared: those of a left- or right-circular bremsstrahlung photon at the short-wavelength limit (when the outgoing electron is not observed) and those of an elastically scattered, left- or right–handed electron. Bremsstrahlung is calculated from the Dirac–Sommerfeld-Maue model, and elastic electron scattering is obtained from a partial-wave analysis. By considering a gold target and electron energies $E_i$ up to 20 MeV, a striking similarity of the respective polarization correlations is found to develop when the collision energy is increased beyond 5 MeV. From analytical Born results for light targets it is shown that only for a longitudinally spin-polarized electron do the respective polarization correlations agree in the limit $E_i\to \infty$. In the general case, a very high nuclear charge is needed in addition, leading to a sum rule for bremsstrahlung well known from elastic electron scattering.

We report an experimental and theoretical study of the evolution of vibrational coherence in a thermal ensemble of nitrogen molecules. Rotational dephasing and rephasing of the vibrational coherence is detected by coherent anti-Stokes Raman scattering. The existence of rovibrational coupling and the discrete energy spectrum of the rotational bath lead to a whole new class of full and fractional rovibrational revivals. Following the rich rovibrational dynamics on a nanosecond time scale with subpicosecond time resolution enables us to determine the second-order rovibrational constant ${\gamma }_e$ and assess new possibilities of controlling decoherence.

Multichannel quantum defect theory is used to calculate photoionization cross sections for vibrationally excited diatomic hydrogen. The calculations are based on the state-of-the-art clamped-nuclei potential energy curves and electronic dipole transition moments of Wolniewicz. The calculations indicate that, in contrast to what had been assumed previously, autoionization resonances dominate the cross section.

Sympathetic cooling of trapped ions has become an indispensable tool for quantum-information processing and precision spectroscopy. In the simplest situation a single Doppler-cooled ion sympathetically cools another ion which typically has a different mass. We analytically investigate the effect of the mass ratio of such an ion crystal on the achievable temperature limit in the presence of external heating. As an example, we show that cooling of a single Al${}^{+}$ with Be${}^{+}$, Mg${}^{+}$, and Ca${}^{+}$ ions provides similar results for heating rates typically observed in ion traps, whereas cooling ions with a larger mass perform worse. Furthermore, we present numerical simulation results of the rethermalization dynamics after a background gas collision for the Al${}^{+}$-Ca${}^{+}$ crystal for different cooling laser configurations.

We study the quantum phase transition of the Dicke model in the classical oscillator limit, where it occurs already for finite spin length. In contrast to the classical spin limit, for which spin-oscillator entanglement diverges at the transition, entanglement in the classical oscillator limit remains small. We derive the quantum phase transition with identical critical behavior in the two classical limits and explain the differences with respect to quantum fluctuations around the mean-field ground state through an effective model for the oscillator degrees of freedom. With numerical data for the full quantum model we study convergence to the classical limits. We contrast the classical oscillator limit with the dual limit of a high-frequency oscillator, where the spin degrees of freedom are described by the Lipkin-Meshkov-Glick model. An alternative limit can be defined for the Rabi case of spin length one-half, in which spin frequency renormalization replaces the quantum phase transition.

In a recent paper [Phys. Rev. A 83, 032106 (2011)], Mackenroth and Di Piazza studied photoemission spectra of an electron driven by intense ultrashort laser pulses. Using kinematic principles, they argued that an electron experiences no mass dressing in an ultrashort pulse. They also proposed a method by which one might experimentally verify their claim. We argue that the scattering kinematics do not imply this conclusion nor do they justify the proposed experiment.

We clearly agree with the Comment by Corson and Peatross that experimental tests based on Eq. (2) of the Comment cannot discriminate between the presence or absence of mass dressing. At the same time, we insist that the conclusions of our paper [Phys. Rev. A 83, 032106 (2011)] remain valid and further clarify our interpretation of the concept of mass dressing in a laser field and its experimental measurability.

We formulate the concept of dominant interaction Hamiltonians to obtain an integrable approximation to the dynamics of an electron exposed to a strong laser field and an atomic potential leading to high-order-harmonic generation. The concept relies on local information in phase space to switch between the interactions. This information is provided by classical integrable trajectories from which we construct a semiclassical wave function. The high-order-harmonic spectrum obtained is in excellent agreement with the accurate quantum spectrum. The separation in the atomic potential and laser coupling interactions should facilitate the calculation of high-order-harmonic spectra in complex systems.

We investigate the photon-induced tunneling phenomena in a photonic crystal cavity containing a strongly coupled quantum dot and describe how this tunneling can be used to generate photon states consisting mainly of a particular Fock state. Additionally, we study experimentally the second-order autocorrelation $g^2\left(0\right)$ in the photon-induced tunneling regime as a function of the frequency and the power of the probe laser and observe signs of higher manifolds of the Jaynes-Cummings Hamiltonian in the frequency-dependent photon statistics of the transmitted light, as well as the strong power dependence of $g^2\left(0\right)$, distinguishing this effect clearly from bunching occurring in a thermal light source.

A protocol is provided to reconstruct the Wigner function for the motional state of a trapped ion via fluorescence detection on another ion in the same trap. This “sympathetic tomography” of a dark ion without optical transitions suitable for state measurements is based on the mapping of its motional state onto one of the collective modes of the ion pair. The quantum state of this vibrational eigenmode is subsequently measured through sideband excitation of the bright ion. Physical processes to implement the desired state transfer are derived and the accomplishment of the scheme is evaluated for different mass ratios.

By adding an imaginary interacting term proportional to $i{p}_1{p}_2$ to the Hamiltonian of a free anisotropic planar oscillator, we construct a model which is described by the $PT$-pseudo-Hermitian Hamiltonian with the permutation symmetry of two dimensions. We prove that our model is equivalent to the Pais-Uhlenbeck oscillator and thus establish a relationship between our $PT$-pseudo-Hermitian system and the fourth-order derivative oscillator model. We also point out the spontaneous breaking of permutation symmetry which plays a crucial role in giving a real spectrum free of interchange of positive and negative energy levels in our model. Moreover, we find that the permutation symmetry of two dimensions in our Hamiltonian corresponds to the identity (not in magnitude but in attribute) of two different frequencies in the Pais-Uhlenbeck oscillator, and reveal that the unequal-frequency condition imposed as a prerequisite upon the Pais-Uhlenbeck oscillator can reasonably be explained as the spontaneous breaking of this identity.

Communication between a free-falling observer and an observer hovering above the Schwarzschild horizon of a black hole suffers from Unruh-Hawking noise, which degrades communication channels. Ignoring time dilation, which affects all channels equally, we show that for bosonic communication using single- and dual-rail encoding, the classical channel capacity reaches a finite value and the quantum coherent information tends to zero. We conclude that classical correlations still exist at infinite acceleration, whereas the quantum coherence is fully removed.

A theoretical investigation of double capture into autoionizing states are presented for collisions between fully stripped low-charge ions and dihydrogen molecules at impact energies ranging from 0.05 to 25 keV/u. The state-to-state cross sections stem from a nonperturbative semiclassical treatment, using asymptotic states expansions with proper translational conditions and taking into account statically and dynamically interelectronic correlation in a configuration interaction approach. The results are presented for three projectiles (He${}^{2+}$, Li${}^{3+}$, and B${}^{5+}$) and discussed in the context of future interferometric collision experimental investigations as well as further improvements in the coherent description of the decay of the autoionizing states populated in the scattering stage.

We have measured electron impact ionization (EII) of the beryllium-like ion ${\mathrm{S}}^{12+}$. The use of an isotopically pure $A=33$ beam of ${\mathrm{S}}^{12+}$ ions is established as a method to eliminate all metastable levels, including the extremely long-lived metastable level $2s2p{}^3{P}_0$, which, for this isotope, decays by hyperfine-induced radiative transitions. The energy dependence and absolute size of the EII cross section were measured from the resulting pure ground-state population at energies from below the threshold at $\approx$ 652.2 eV up to 3000 eV. These data provide an experimental benchmark for theory.

Time-resolved high-harmonic spectroscopy is an emerging approach to measuring coupled electronic and nuclear dynamics in photochemical reactions. A general conceptual and theoretical model for the technique is derived from first principles and applied to study the sensitivity of the technique to nonadiabatic dynamics. By comparing the model with detailed experimental data on photoexcited NO${}_2$ molecules, we find that time-resolved high-harmonic spectroscopy is primarily sensitive to electronic population dynamics. The coordinate dependence of the vertical ionization potential and photorecombination matrix elements contribute also, but much less significantly, to the observed dynamics because of the rapid spreading of the wave packet in the excited state. We discuss the extension of the method to larger polyatomic molecules in light of this insight.

The absolute time-dependent nonlinear response of O${}_2$, N${}_2$, N${}_2$O, and Ar to intense nonionizing, ultrashort optical pump pulses is measured with single-shot spectral interferometry. The instantaneous and delayed rotational responses are distinguished as a function of pump-pulse duration and probe central wavelength. Our measurements are central to the modeling and understanding of nonlinear propagation of intense ultrashort laser pulses in gases.

We study the validity range of the semiclassical Boltzmann equation in describing the dynamics of two-dimensional Fermi gas. Here we numerically solve the Boltzmann equation based on the relaxation-time approximation. By comparing the numerically extracted damping of the quadrupole mode with the experimental data reported in Vogt et al. [Phys. Rev. Lett. 108, 070404 (2012)], we demonstrate that the Boltzmann equation is applicable at least when the temperature is larger than $0.47T_F$, up to the strong coupling regime, where $T_F$ is the Fermi temperature.

The chemical compass model, based on radical pair reactions, is an idea to explain avian magnetoreception. At present, to the best of our knowledge, questions concerning the key ingredients responsible for the high sensitivity of a chemical compass and the possible role of quantum coherence and decoherence remain unsolved. Here, we investigate the optimized hyperfine coupling for a chemical compass in order to achieve the best magnetic field sensitivity. We show that its magnetic sensitivity limit can be further extended by simple quantum control and may benefit from additional decoherence. With this, we clearly demonstrate how quantum coherence can be exploited in the functioning of a chemical compass. The present results also provide routes toward the design of a biomimetic weak magnetic field sensor.

Motivated by experiments on Josephson junction arrays, and cold atoms in an optical lattice in a synthetic magnetic field, we study the “fully frustrated” Bose-Hubbard model with half a magnetic flux quantum per plaquette. We obtain the phase diagram of this model on a two-leg ladder at integer filling via the density matrix renormalization group approach, complemented by Monte Carlo simulations on an effective classical $XY$ model. The ground state at intermediate correlations is consistently shown to be a chiral Mott insulator (CMI) with a gap to all excitations and staggered loop currents which spontaneously break time-reversal symmetry. We characterize the CMI state as a vortex supersolid or an indirect exciton condensate, and discuss various experimental implications.