We solve the time-dependent Schrödinger equation for the hydrogen atom exposed to strong-field few-cycle laser pulses with the central photon energy corresponding to the $1s\to 2p$ transition. Comparison with results from other recent calculations suggests that the approximations made in those works produced inaccuracies that shed doubt on the physical effects predicted at large photoelectron energies.

We study the dynamics of atoms in optical traps when exposed to laser cooling light that induces light-assisted collisions. We experimentally prepare individual atom pairs and observe their evolution. Due to the simplicity of the system (just two atoms in a microtrap) we can directly simulate the pair's dynamics, thereby revealing detailed insight into it. We find that often only one of the collision partners gets expelled, similar to when using blue detuned light for inducing the collisions. This enhances schemes for using light-assisted collisions to prepare individual atoms and affects other applications as well.

We point out that the Wigner crystal (WC) state of rotating dipolar fermions in the lowest Landau level is strongly correlated. From intra-Landau-level excitations of the fractional quantum Hall effect (FQHE) liquid, we find that the roton minimum of the excitation spectrum at $\nu =1/7$ becomes negative, indicating an absence of the FQHE state, which is supposed to be the ground state of the system. Therefore, we show that the Hartree-Fock theory of the WC does not predict the correct ground-state energy in the lowest Landau level. There exists a strong correlation effect on the Wigner crystallization of rotating dipolar gases.

We provide theoretical and experimental evidence of a classical-optics realization of quantum Zeno dynamics. In contrast to other approaches, the classical implementation is extremely simple and adaptable. Frequently monitoring the transverse spreading of the light emitted by an extended source results in a nontrivial dynamics where spreading and all other transverse effects associated with light propagation as the increase of spatial coherence tends to be inhibited. This is fully equivalent to a quantum Zeno dynamics where monitoring whether a quantum free particle at rest remains in a spatial interval causes inhibition of the quantum-state evolution, including the degree of purity.

We propose a scheme for the entanglement generation of two micromechanical mirrors through two-mode fields generated by a correlated emission laser source in a doubly resonant cavity. The coherence between upper and lower levels is generated in a cascade-driven scheme using two external classical fields. Two input laser fields are both tuned at the Stokes sidebands such that cooling and entanglement of micromechanical mirrors are realizable. We investigate two different conditions for entanglement generation. We show controllable entanglement of both two-mode fields and two mirrors in the steady state. The proposed scheme for the macroscopic entanglement generation is robust against environmental thermalization and in favor of a large cavity dissipation rate.

Electron-core interactions play a key role in strong-field ionization and the formation of photoelectron spectra. We analyze the temporal dynamics of strong-field ionization associated with these interactions using the time-dependent analytical $R$-matrix (ARM) method, developed in our previous work [Phys. Rev. A 88, 013421 (2013)]. The approach is fully quantum but includes the concept of trajectories. However, the trajectories are not classical in the sense that they have both real and imaginary components all the way to the detector. We show that the imaginary parts of these trajectories, which are usually ignored, have a clear physical meaning and are crucial for the correct description of electron-core interactions after ionization. In particular, they give rise to electron deceleration, as well as dynamics associated with electron recapture and release. Our approach is analytical and time dependent and allows one to gain access to the electron energy distribution and ionization yield as a function of time. Thus we can also rigorously answer the question: when is ionization completed?

We theoretically investigate the wavelength dependence of the HHG yield in spatially inhomogeneous fields with few-cycle laser pulses. The results show that the HHG yield first decreases with a slower scaling ${\lambda }^{-4}$ as the increase of the laser wavelength compared with ${\lambda }^{-5}$ in homogeneous fields. However, with the laser wavelength further increased, an inflection of wavelength appears and then the HHG yield presents an upward trend. Based on the electron trajectories analysis, we demonstrate that the decrease in the scaling law ${\lambda }^{-4}$ mainly results from the cutoff extension in inhomogeneous fields. And the uptrend in HHG yield is attributed to the increasing wavelength-dependent ionization rate of the additional quantum path that dominates the harmonic generation. In addition, the carrier-envelope phase dependence of the wavelength-dependent HHG yield is also investigated.

Transition of an electron from a free to a bound state is critical in determining the qualitative shape of the spectrum in high-order-harmonic generation (HHG), and in tomographic imaging of orbitals. We calculate and compare the recombination amplitude, from a continuum state described by a plane wave and an outgoing scattering eigenstate, to the bound state for the noble gases that are commonly used in HHG. These calculations are based on the single active electron model and the Hartree-Fock-Slater method, using both the length form and the acceleration form of the dipole matrix element. We confirm that the recombination amplitude versus emitted photon energy strongly depends upon the wave function used to describe the free electron. Depending on the choice of the wave function and the dipole form, the square of the absolute value of the recombination amplitude can differ by almost two orders of magnitude near the experimentally measured Cooper minima. Moreover, only the outgoing scattering eigenstates with the length form roughly predict the experimentally observed Cooper minimum for Ar ($\sim 50\mathrm{eV}$) and Kr ($\sim 85\mathrm{eV}$). We provide a detailed derivation of the photorecombination cross sections from photoionization cross sections (PICSs) calculated by the relativistic random phase approximation (RRPA). For Ar, Kr, and Xe, we compare the total PICSs calculated using our recombination amplitudes with that obtained from RRPA. We find that PICS calculated using the outgoing scattering eigenstates with the length form is in better agreement with the RRPA calculations than the acceleration form.

We use variational methods to study a spin impurity in a one-dimensional Bose lattice gas. Both in the strongly interacting superfluid regime and in the Mott regime we find that the impurity binds with a hole, forming a polaron. Our calculations for the dispersion of the polaron are consistent with recent experiments by Fukuhara et al. [Nat. Phys. 9, 235 (2013)] and give a better understanding of their numerical simulations. We find that for sufficiently weak interactions there are ranges of momentum for which the polaron is unstable. We propose experimentally studying the stability of the polaron by measuring the correlation between the impurity and the hole. We also study two interacting impurities, finding stable bipolarons for sufficiently strong interactions.

We consider a two-mode model describing scalar bosons with two-body interactions in a single trap, taking into account coherent pair exchange between the modes. It is demonstrated that the resulting fragmented many-body states with continuous (nonsingular) Fock-space distribution amplitudes are robust against perturbations due to occupation number and relative phase fluctuations, Josephson-type tunneling between the modes, and weakly broken parity of orbitals, as well as against perturbations due to interaction with a third mode.

We investigate the weakly- to strongly-fluctuating-coupling transition of two linearly coupled systems under the influence of a phase-fluctuating coupling. In the weakly-fluctuating-coupling regime, the exponential decay of quantum properties is well known. A different scenario occurs in the strongly-fluctuating-coupling regime, namely the inhibition of the dynamics which tends to “freeze” as the ratio between coupling strength and average phase-fluctuation time increases. Exciton-polariton oscillations and the self-trapping phenomenon in the Bose-Einstein condensate qualitatively illustrate the weak and strong regimes, respectively.

We investigate the optomechanical properties of a periodic array of identical scatterers placed inside an optical cavity and extend our previous results [Xuereb, Genes, and Dantan, Phys. Rev. Lett. 109, 223601 (2012)]. We show that operating at the points where the array is transmissive results in linear optomechanical coupling strengths between the cavity field and collective motional modes of the array that may be several orders of magnitude larger than is possible with an equivalent reflective ensemble. We describe and interpret these effects in detail and investigate the nature of the scaling laws of the coupling strengths for the different transmissive points in various regimes.

The energy loss of fast ions at close collision is mainly due to electron-ion collisions. The electrons are approximately stationary and they collide with a fast-moving ion. Here we study the same collision experimentally, in a reference system where the ions (or atoms) are stationary and interacting with keV electrons. Scattering cross sections under these conditions deviate from Rutherford, and we link these deviations, at higher energies, to the $Z^3$ contributions to the electronic stopping and the related Barkas effect and, at lower energies, also to quantum interference. The present measurements are well described by partial-wave calculations of the elastic cross section of electrons scattering from atoms. Encouraged by this agreement we use these calculations to estimate the Barkas factor for all elements and many energies. A universal curve for the Barkas factor due to close collisions is obtained for neutral projectiles and similar curves with smaller magnitude are found for ions.

Recently introduced methods which result in shortcuts to adiabaticity, particularly in the context of frictionless cooling, are rederived and discussed in the framework of an approach based on Ehrenfest dynamics. This construction provides physical insights into the emergence of the Ermakov equation, the choice of its boundary conditions, and the use of minimum uncertainty states as indicators of the efficiency of the procedure. Additionally, it facilitates the extension of frictionless cooling to more general situations of physical relevance, such as optical dipole trapping schemes. In this context, we discuss frictionless cooling in the short-time limit, a complementary case to the one considered in the literature, making explicit the limitations intrinsic to the technique when the full three-dimensional case is analyzed.

Double photoionization accompanied by loss of $n$ C atoms ($n=0$, 2, 4, 6) was investigated by merging beams of Xe@C${}_{60}^{+}$ ions and synchrotron radiation and measuring the yields of product ions. The giant $4d$ dipole resonance of the caged Xe atom has a prominent signature in the cross section for these product channels, which together account for 6.2 $\pm$ 1.4 of the total Xe $4d$ oscillator strength of 10. Compared to that for a free Xe atom, the oscillator strength is redistributed in photon energy due to multipath interference of outgoing Xe $4d$ photoelectron waves that may be transmitted or reflected by the spherical C${}_{60}^{+}$ molecular cage, yielding so-called confinement resonances. The data are compared with an earlier measurement and with theoretical predictions for this single-molecule photoelectron interferometer system. Relativistic $R$-matrix calculations for the Xe atom in a spherical potential shell representing the fullerene cage show the sensitivity of the interference pattern to the molecular geometry.

We study the dynamics of a driven Dicke model, where the collective spin is rotated with a constant velocity around a fixed axis. The time evolution of the mean photon number and of the atomic inversion is calculated using, on the one hand, a numerical technique for the quantum dynamics of a small number of two-level atoms and, on the other hand, time-dependent mean-field theory for the limit of a large number of atoms. We observe a reduction in the mean photon number compared to its equilibrium value. This dynamically generated darkness is particularly pronounced slightly above the transition to a superradiant phase. We attribute the effect to a slowing-down of the motion in the classical limit of a large ensemble and to an interplay of dynamic and geometric phases in the quantum case.

We calculate the quantum Cramér-Rao bound for the sensitivity with which one or several parameters, encoded in a general single-mode Gaussian state, can be estimated. This includes in particular the interesting case of mixed Gaussian states. We apply the formula to the problems of estimating phase, purity, loss, amplitude, and squeezing. In the case of the simultaneous measurement of several parameters, we provide the full quantum Fisher information matrix. Our results unify previously known partial results and constitute a complete solution to the problem of knowing the best possible sensitivity of measurements based on a single-mode Gaussian state.

Miniature atomic clocks based on coherent population trapping (CPT) states in thermal atoms are an important component in many field applications, particularly where satellite frequency standards are not accessible. Cold-atom CPT clocks promise improved accuracy and stability over existing commercial technologies. Here we demonstrate a cold-atom CPT clock based on ${}^{87}$Rb using a high-contrast double-$\Lambda$ configuration. Doppler frequency shifts are explained using a simple model and canceled by interrogating the atoms with counterpropagating light beams. We realize a compact cold-atom CPT clock with a fractional frequency stability of $4\times {10}^{-11}{\tau }^{-1/2}$, thus demonstrating the potential of these devices. We also show that the long-term stability is currently limited by the second-order Zeeman shift to $2\times {10}^{-12}$ at 1000 s.

Perfect cloning of an unknown quantum state is impossible. Approximate cloning, which is optimal in various senses, has been found in many cases. Paradigmatic examples are Wootters-Zurek cloning and universal cloning. These cloning machines aim at optimal cloning of the full quantum states. However, in practice, what is important and relevant may only involve partial information in quantum states, rather than quantum states themselves. For example, signals are often encoded as parameters in quantum states, whose information content is well synthesized by quantum Fisher information. This raises the basic issue of evaluating the information transferring capability (e.g., distributing quantum Fisher information) of quantum cloning. We assess and compare Wootters-Zurek cloning and universal cloning from this perspective and show that, on average, Wootters-Zurek cloning performs better than universal cloning for the phase (as well as amplitude) parameter, although they are incomparable individually, and universal cloning has many advantages over Wootters-Zurek cloning in other contexts. Physical insights and related issues are further discussed.

A resolution of the quantum measurement problem(s) using the consistent histories interpretation yields in a rather natural way a restriction on what an observer can know about a quantum system, one that is also consistent with some results in quantum information theory. This analysis provides a quantum mechanical understanding of some recent work that shows that certain kinds of quantum behavior are exhibited by a fully classical model if by hypothesis an observer's knowledge of its state is appropriately limited.