We calculate the isotope shift of $2{}^1{P}_0-2{}^1{S}_0$, $3{}^1{S}_0-2{}^1{S}_0$ transitions and of the $2{}^1{S}_0$ ionization potential in the four-electron beryllium atom. The achieved precision is high enough to make possible the accurate determination of the nuclear charge radii from the experimental isotope shifts. This calls for corresponding measurements and opens the window for the determination of charge radii of heavier nuclei.

The two-center approach to positron-impact ionization of atomic hydrogen is shown to follow from the exact post form of the breakup amplitude [Kadyrov, Bray, Mukhamedzhanov, and Stelbovics, Phys. Rev. Lett. 101, 230405 (2008)]. In such approaches distinct ionization amplitudes arise from each center for the same ionization process. The fully differential cross section for the positron-impact breakup of atomic hydrogen is calculated including direct ionization of the target and electron capture into the positronium continuum. We show that the coherent combination of the amplitudes leads to oscillations in the differential cross sections, whereas the incoherent combination does not. The latter has also the advantage of being consistent with the unitary close-coupling formalism.

Electron-transfer processes in interaction between highly charged ions and multiply protonated proteins have been studied. Collisions between Xe${}^{8+}$ at 96 keV and protonated cytochrome-C at selected charge state ($q$ from 15+ to 19+) result in mass spectra composed mainly of intact molecular ions. From the spectra, single and double electron capture processes by Xe${}^{8+}$ from the protonated molecular ions were identified and the relative cross sections were measured. An unexpected process, the deprotonation process, was also observed. It is tentatively attributed to the loss of a proton induced by the strong electric field carried by the projectile ion in long-distance collisions. Upon charge variation of the molecular target from 15 to 19, the single and double electron capture cross sections remain nearly constant, while the relative cross section of the deprotonation process increases dramatically from 0.8% (±0.1%) to 17% (±1%). This strong charge dependency is explained by the decrease of the proton affinities with the charge. This proton removal process has not been observed previously. It seems to be specific to the long-distance Coulomb interactions between protons bound along the protein chain and the highly charged atomic ions.

We present kinematically complete measurements of the photo-double-ionization of ethylene (double CC bond) and acetylene (triple CC bond) hydrocarbons just above the double-ionization threshold. We discuss the results in terms of the coincident kinetic energy of the photoelectrons and the nuclear kinetic-energy release of the recoiling ions. We have incorporated quantum chemistry calculations to interpret which of the electronic states of the dication have been populated and trace the various subsequent fragmentation channels. We suggest pathways that involve the electronic ground and excited states of the precursor ethylene dication and explore the strong influence of the conical intersections between the different electronic states. The nondissociative ionization yield is small in ethylene and high in acetylene when compared with the dissociative ionization channels. The reason for such a striking difference is explained in part on the basis of a propensity rule that influences the population of states in the photo-double-ionization of a centrosymmetric closed-shell molecule by favoring singlet ungerade and triplet gerade final states. This propensity rule and the calculated potential-energy surfaces clarify a picture of the dynamics leading to the observed dication dissociation products.

We theoretically study time delays obtained using the attosecond-streaking technique. To this end, we compute time delays by numerically solving the corresponding time-dependent Schrödinger equation and analyze the delays using two classical methods, namely, a perturbative approach and a full numerical solution of Newton's equation describing the motion of the photoelectron in the continuum. A good agreement between the quantum streaking results and those from the full classical solution is found. This indicates that the streaking time delay arises from the continuum dynamics of the electron in the coupled potential of the Coulomb and streaking fields, while the transition of the photoelectron from the bound state to the continuum occurs instantaneously upon absorption of the photon. We further analyze the variation of the time delay with respect to the delay between the ionizing XUV pulse and a long streaking pulse, its dependence on the polarization direction of the streaking pulse, and the influence of the shape of the streaking pulse and/or additional static electric fields on the numerically obtained time delays. The results are interpreted based on the previously revealed property that the attosecond-streaking time delay depends on the finite region in space over which the electron propagates between its instant of transition into the continuum and the end of the streaking pulse.

We have investigated the orientation-selective molecular tunneling ionization of carbonyl sulfide (OCS) molecules induced by linearly polarized double-frequency phase-controlled laser fields consisting of a fundamental and a second-harmonic light with a pulse duration of 130 fs and an intensity of 5 × 10${}^{13}$ W/cm${}^2$. We performed simultaneous measurements using gas mixtures of OCS and carbon monoxide to calibrate the relative phase difference of the phase-controlled fields and to verify the mechanism of the tunnel ionization. It is demonstrated that there is a definite correlation between the orientation of ionized molecules and the structure of the highest occupied molecular orbital. We have discussed the experimental results by means of the weak field asymptotic theory. In addition, we have analyzed the quantum dynamics of photoelectrons in simultaneous ion-electron detection. The experimental results can be explained by a two-step model including the interaction with the parent ion. The recollision process plays a minor role for determining the preferable directions of polar molecules in the tunneling ionization at the experimental laser intensity used.

We establish a model to describe any alkali-metal atom interacting with a position- and time-dependent magnetic field. It includes the hyperfine structure of the atom and quantizes its center-of-mass motion. The model is used to characterize a proposed phase-space selector. It consists of a method based on hyperfine magnetic dipole transitions to prepare samples of atoms with well-defined position and velocity. The evolution of the atom is determined analytically and relevant physical quantities such as the probability of transition and the expected values of position and momentum are analyzed. It is concluded that the phase-space selector can lead to samples of atoms whose width in velocity tends to those obtained using velocity-dependent Raman transitions.

We theoretically study the process of photoelectron emission from the helium atom using a high-intensity short-wavelength laser at a resonance condition of the residual singly charged ion. Photoionization followed by strong resonance coupling in the ion leads to a change in the photoelectron spectrum due to Rabi oscillations in the residual ion. Similarly to resonance fluorescence at high laser intensity, the photoelectron spectrum at high intensities evolves into a multipeaked structure. The number of peaks in the photoelectron spectrum is related to the number of Rabi cycles following the photoionization process. Moreover, the strong laser-induced coupling to nonresonant states of the residual ion has an imprint on the photoelectron spectrum, leading to additional, isolated peaks at the lower- or higher-energy sides. The effect should be observable at current seeded extreme ultraviolet (XUV) free-electron lasers and persists after volume integration in a realistic experimental geometry.

It is widely believed that the stationary properties after a quantum quench in integrable systems can be described by a generalized Gibbs ensemble (GGE), even if all of the analytical evidence is based on free theories in which the pre- and postquench modes are linearly related. In contrast, we consider the experimentally relevant quench of the one-dimensional Bose gas from zero to infinite interaction, in which the relation between modes is nonlinear, and consequently Wick's theorem does not hold. We provide exact analytical results for the time evolution of the dynamical density-density correlation function at any time after the quench and we prove that its stationary value is described by a GGE in which Wick's theorem is restored.

Within the capacity of current experiments, we design a composite atom-cavity system with a common bath, in which the decay channels of the atom and the cavity mode interfere with each other. When the direct atom-cavity coupling is absent, the system can be trapped in a quasidark state (the coherent superposition of excited states for the atom and the cavity mode) without decay even in the presence of the bath. When the atom directly couples with the cavity, the largest decay rate of the composite system will surpass the sum of the two subsystems while the smallest decay rate may achieve 0. This is manifested in the transmission spectrum, where the vacuum Rabi splitting shows an obvious asymmetric character.

A combination of electron spin interactions in a magnetic field allows us to control the resonance frequencies of a high-Q whispering-gallery (WG) cavity mode doublet, resulting in precise measurements of an avoided crossing between the two modes comprising the doublet. We show that the resonant photons effectively behave as spin-$\frac{1}{2}$ particles and that the physical origins of the doublet phenomenon arise from an energy splitting between the states of photon spin angular momentum. The exclusive role of the photon spin in splitting the mode frequency is emphasized, and we demonstrate that the gyrotropic and anisotropic properties of the crystalline media supporting the WG mode lead to strong coupling between the bosonic and the spin degrees of freedom of cavity photons. Despite the demonstrated similarities to Jaynes-Cummings-type systems, the mode doublet system exhibits a significant difference due to its linearity. Unlike traditional experiments dealing with interactions between fields and matter, here the crystalline medium plays a role of macroscopic symmetry breaking, assisting in the strong coupling between these photon degrees of freedom. Such a regime is demonstrated experimentally with a method to effectively control the photon spin state. Our experiments demonstrate controllable time-reversal symmetry breaking in a high-$Q$ cavity.

In this article the ability to record, store, and read out the quantum properties of light is studied. The discussion is based on high-speed and adiabatic models of quantum memory in $\lambda$ configuration and in the limit of strong resonance. We show that in this case the equality between efficiency and squeezing ratio, predicted by the simple beam-splitter model, is broken. The requirement of the maximum squeezing in the output pulse should not be accompanied by the requirement of maximum efficiency of memory, as in the beam-splitter model. We have demonstrated that for the same optical depth a high output pulse squeezing can be reached earlier than the high efficiency. Comprehension of this “paradox” is achieved on the basis of mode analysis. The memories eigenmodes, which have an impact on the memory process, are found numerically. Also, the spectral analysis of modes was performed to match the spectral width of the input signal to the capacities of the memories.

Existence and stability of $\mathcal{PT}$-symmetric gap solitons in a periodic structure with defocusing nonlocal nonlinearity are studied both theoretically and numerically. We find that, for any degree of nonlocality, gap solitons are always unstable in the presence of an imaginary potential. The instability manifests itself as a lateral drift of solitons due to an unbalanced particle flux. We also demonstrate that the perturbation growth rate is proportional to the amount of gain (loss), thus predicting the observability of stable gap solitons for small imaginary potentials.

An intriguing phenomenon of third-harmonic generation under fiber continuum generation is the emission of an anharmonic signal. One popular interpretation of this effect has developed into a general theory of fiber third-harmonic generation. Here we produce “static” third-harmonic lines dictated fully by fiber properties independent of pump parameters, in contrast to the signals of all known phase-matched nonlinear optical processes that vary dynamically with these parameters. We argue that the anharmonic signal is an illusion of the continuum generation, that it is in fact harmonic, and that this theory should be reevaluated.

Lasing without inversion (LWI) in the extreme-ultraviolet (XUV) has been the focus of recent research of our Princeton/Texas group. Because of the restriction imposed on the decay rates associated with the usual LWI schemes, it is difficult to transfer the physics gleaned from previous experiments to the XUV. However, with the advent of tunable-ultrashort-high-power laser pulses, we find that the possibility of transient LWI holds promise for a different LWI paradigm in which the lasing is based on forbidden and/or virtual transitions. It is the purpose of the present paper to present simple but (hopefully) convincing arguments and suggest realistic experiments to stimulate interest in this idea.

We study the thermalization of excitations generated by spontaneous emission events for cold bosons in an optical lattice. Computing the dynamics described by the many-body master equation, we characterize equilibration time scales in different parameter regimes. For simple observables, we find regimes in which the system relaxes rapidly to values in agreement with a thermal distribution, and others where thermalization does not occur on typical experimental time scales. Because spontaneous emissions lead effectively to a local quantum quench, this behavior is strongly dependent on the low-energy spectrum of the Hamiltonian, and undergoes a qualitative change at the Mott insulator–superfluid transition point. These results have important implications for the understanding of thermalization after localized quenches in isolated quantum gases, as well as the characterization of heating in experiments.

We study the security of two-way quantum cryptography at different wavelengths of the electromagnetic spectrum, from the optical down to the microwave range. In particular, we consider a two-way quantum communication protocol where Gaussian-modulated thermal states are subject to random Gaussian displacements and finally homodyned. We show how its security threshold (in reverse reconciliation) is extremely robust with respect to the preparation noise and able to outperform the security thresholds of one-way protocols at any wavelength. As a result, improved security distances are now accessible for implementing quantum key distribution in the very challenging regime of infrared frequencies.

We use the combination of a two-dimensional Hartree-Fock and a diffusion quantum Monte Carlo method, both of which we recently presented in this journal [C. Schimeczek et al., Phys. Rev. A 88, 012509 (2013)], for a thorough investigation of the ground state configurations of all atoms and ions with $Z=2$–10 with the exception of hydrogen-like systems in strong magnetic fields. We obtain the most comprehensive data set of ground state configurations as a function of the magnetic field strength currently available and hence are able to analyze and compare the properties of systems with different core charges and electron numbers in detail.

Parabolic quantum cloaks with unbounded and bounded invisible regions are presented with the method of transformation design. The mass parameters of particles for perfect cloaking are shown to be constant along the parabolic coordinate axes of the cloaking shells. The invisibility performance of the cloaks is inspected from the viewpoints of waves and probability currents. The latter shows the controllable characteristic of a probability current by a quantum cloak. It also provides us with a simpler and more efficient way of exhibiting the performance of a quantum cloak without the solutions of the transformed wave equation. Through quantitative analysis of streamline structures in the cloaking shell, one defines the efficiency of the presented quantum cloak in the situation of oblique incidence. The cloaking models presented here give us more choices for testing and applying quantum cloaking.

The Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) phase, a superconducting state with nonzero total momentum Cooper pairs in a large magnetic field, was first predicted about 50 years ago and has since become an important concept in many branches of physics. Despite an intensive search in various materials, unambiguous experimental evidence for the FFLO phase is still lacking in experiments. In this paper we show that both Fulde-Ferrell (FF) (a uniform order parameter with a plane-wave phase) and Larkin-Ovchinnikov (LO) phases (a spatially varying order parameter amplitude) can be observed using fermionic cold atoms in spin-orbit-coupled optical lattices. The increasing spin-orbit coupling enhances the FF phase over the LO phase. The coexistence of superfluid and magnetic orders is also found in the normal BCS phase. The pairing mechanism for different phases is understood by visualizing superfluid pairing densities in different spin-orbit bands. The possibility of observing similar physics using spin-orbit-coupled superconducting ultrathin films is also discussed.