Table of contents

The possibility of performing the C-NOT gate operation at the ground and the first excited states of two harmonic oscillators interacting via a two-level system subject to complete control is demonstrated. The system resembles Turing machine, where the result of interaction between oscillators and the two-level system is restricted to a certain fixed unitary transformation matrix, while all the control required for the implementation of the gate is provided via manipulations with the two-level system, which remains the only fully-controllable part of the entire system. Each gate operation requires a 'Turing programming', which can be realized as a series of $\gtrsim 63$ elementary unitary operations. The result shows a way how one can construct a quantum processor in a multimode microwave cavity equipped with a fully controlled two-level system, such as a Josephson-junction chip. Parameters of already existing experimental devises could allow one to perform up to 15 gate operations in an ensemble of about 10 qubits.

The ability to modify light−matter coupling in time (e.g. using external pulses) opens up the exciting possibility of generating and probing new aspects of quantum correlations in many-body light–matter systems. Here we study the impact of such a pulsed coupling on the light–matter entanglement in the Dicke model as well as the respective subsystem quantum dynamics. Our dynamical many-body analysis exploits the natural partition between the radiation and matter degrees of freedom, allowing us to explore time-dependent intra-subsystem quantum correlations by means of squeezing parameters, and the inter-subsystem Schmidt gap for different pulse duration (i.e. ramping velocity) regimes—from the near adiabatic to the sudden quench limits. Our results reveal that both types of quantities indicate the emergence of the superradiant phase when crossing the quantum critical point. In addition, at the end of the pulse light and matter remain entangled even though they become uncoupled, which could be exploited to generate entangled states in non-interacting systems.

We use the concept of two-particle probability amplitude to derive the stochastic evolution equation for two-particle four-point correlations in tight-binding networks affected by diagonal dynamic disorder. It is predicted that in the presence of dynamic disorder, the average spatial wavefunction of indistinguishable particle pairs delocalizes and populates all network sites including those which are weakly coupled in the absence of disorder. Interestingly, our findings reveal that correlation elements accounting for particle indistinguishability are immune to the impact of dynamic disorder.

We present a new derivation of the proton–electron mass ratio from the hydrogen molecular ion, HD⁺. The derivation entails the adjustment of the mass ratio in highly precise theory so as to reproduce accurately measured ro-vibrational frequencies. This work is motivated by recent improvements of the theory, as well as the more accurate value of the electron mass in the recently published CODATA-14 set of fundamental constants, which justifies using it as input data in the adjustment, rather than the proton mass value as done in previous works. This leads to significantly different sensitivity coefficients and, consequently, a different value and larger uncertainty margin of the proton–electron mass ratio as obtained from HD⁺.

In this work we perform a Green's function analysis of giant-dipole systems. First, we derive the Green's functions of different magnetically field-dressed systems, in particular of electronically highly excited atomic species in crossed electric and magnetic fields—so-called giant-dipole states. We determine the dynamical polarizability of atomic giant-dipole states as well as the adiabatic potential energy surfaces of giant-dipole molecules in the framework of the Green's function approach. Furthermore, we perform an comparative analysis of the latter to an exact diagonalization scheme and show the general divergence behavior of the widely applied Fermi-pseudopotential approach. Finally, we derive the giant-dipole's regularized Green's function representation.

Using the theory of imaging with partially coherent light, we derive general expressions for different kinds of interferometric setups such as double slit, shift and mirror interference. We show that in all cases the interference patterns depend not only on the point spread function of the imaging setup but also strongly on the spatial emission pattern of the sample. Taking typical experimentally observed spatial emission patterns into account, we can reproduce, at least qualitatively, all the observed interference structures, which have been interpreted as signatures for spontaneous long range coherence of excitons, already for incoherent emitters (Butov et al 2002 Nature418 751; Dubin et al 2006 Nat. Phys.2 32). This requires a critical reexamination of the previous work.

The extreme ultraviolet plasma emission from liquid microsized argon droplets exposed to intense near-infrared laser pulses has been investigated. Emission from the warm dense matter targets is recorded in a spectral range in between 16 and 30 nm at laser intensities of 10¹⁴ W cm⁻². Above the emission threshold, soft x-ray radiation exponentially increases with the pulse energy whereby a strong dependence of the yields on the pulse duration is observed, which points at an effective electron collisional heating of the microplasma by inverse bremsstrahlung. Accompanying hydrodynamic simulations reveal the temporal and spatial development of the microplasma conditions. The good agreement in between the measured and calculated emission spectra as well as the extracted electron temperatures confirm that hydrodynamic simulations can be applied in the analysis of strongly excited droplets.

We investigate, from a theoretical perspective, photoemission of electrons induced by ultra-short infrared pulses covering only a few photon cycles. In particular, we investigate the impact of the carrier envelope phase of the laser pulse which plays an increasingly large role for decreasing pulse length. As key observable we look at the asymmetry of the angular distribution as function of kinetic energy of the emitted electrons. The focus of the present study lies on the system dependence of the reaction. To this end, we study two very different systems in comparison, an Ar atom and the Na${}_{9}^{+}$ cluster. The study employs a fully quantum–mechanical description of electron dynamics at the level of time-dependent density functional theory. We find a sensitive dependence on the system which can be related to the different spectral response properties. Results can be understood from an interplay of the ponderomotive motion driven by the external photon field and dynamical polarization of the system.

Spectroscopic properties, useful for plasma diagnostics and astrophysics, of a few rubidium-like ions are studied here. We choose one of the simplest, but correlationally challenging series where d- and f-orbitals are present in the core and/or valence shells with $4d$${}^{2}{D}_{3/2}$ as the ground state. We study different correlation characteristics of this series and make precise calculations of electronic structure and rates of electromagnetic transitions. Our calculated lifetimes and transition rates are compared with other available experimental and theoretical values. Radiative rates of vacuum ultraviolet electromagnetic transitions of the long lived Tc⁶⁺ ion, useful in several areas of physics and chemistry, are estimated. To the best of our knowledge, there is no literature for most of these transitions.

A many-mode Floquet theory (MMFT) formalism is applied to study the interaction of a polychromatic rf field with cold atoms trapped in a quadrupole magnetic trap. The validity of the MMFT approach is first established by comparing its results with those of the previously used formalisms for the cases of single- and two-frequency rf fields. The effect of truncation of the infinite matrices has been studied and a suitable truncation order is obtained for the numerical implementations. The results obtained for the composite atom–field system has shown some exquisite features such as lattice-like periodic variation in the eigen-energies and large two-photon transition probabilities between the atomic states. Dependence of the trapping parameters such as trap position, depth and width on the rf field parameters like field strength and mode separation has also been studied. This work predicts the generation of lattice type atom-trapping geometries which can be controlled by varying the rf field parameters of a polychromatic rf field.

We present a theoretical study of energetic electron-beam excitation of 'six-level helium atom' embedded in a laser field. The angular distribution of the scattered electron-beam, which contains all information about the combined interactions, is analyzed and exhibits a resonant peak in the vicinity of the $1{sns}\mbox{--}1{snp}$ (n = 2, 3) transitions. Here, we show that this resonant feature can be stimulated by means of the transferred momentum between the collision partners during the scattering process. By manipulating the latter parameter, one can enhance the resonant peak or lead to its attenuation and even to its cancellation. Our findings are supported by additional calculations based on the time-dependent perturbation theory, where a spurious divergence is expected at a resonant region.

The spectroscopic properties of the fundamental and several excited states of Sr⁺Ar and SrAr, Van der Waals systems are investigated by employing an ab initio method in a pseudo-potential approach. The potential energy curves and the spectroscopic parameters are displayed for the 1–10 ²Σ⁺, 1–6 ²Π and 1–3 ²Δ electronic states of the Sr⁺Ar molecule and for the 1–6 ¹Σ⁺, 1–4 ³Σ⁺, 1–3 ^1,3Π and 1–3 ^1,3Δ states of the neutral molecule SrAr. In addition, from these curves, the vibrational levels and their energy spacing are deduced for Σ⁺, Π and Δ symmetries. The spectra of the permanent and transition dipole moments are studied for the ^1,3Σ⁺ states of SrAr, which are considered to be two-electron systems and ²Σ⁺ states of the single electron Sr⁺Ar ion. The spectroscopic parameters obtained for each molecular system are compared with previous theoretical and experimental works. A significant correlation revealed the accuracy of our results.

A new heuristic model of interaction of an atomic system with a gravitational wave (GW) is proposed. In it, the GW alters the local electromagnetic field of the atomic nucleus, as perceived by the electron, changing the state of the system. The spectral decomposition of the wave function is calculated, from which the energy is obtained. The results suggest a shift in the difference of the atomic energy levels, which will induce a small detuning to a resonant transition. The detuning increases with the quantum numbers of the levels, making the effect more prominent for Rydberg states. We performed calculations on the Rabi oscillations of atomic transitions, estimating how they would vary as a result of the proposed effect.

We measured the cross sections for Au Lα, Lβ, Lγ, Lℓ and Lη x-ray production by the impact of electrons with energies from the L₃ threshold to 100 keV using a thin Au film whose mass thickness was determined by Rutherford Backscattering Spectrometry. The x-ray spectra were acquired with a Si drift detector, which allowed to separate the components of the Lγ multiplet lines. The measured Lα, Lβ, ${\rm{L}}{\gamma }_{1}$, L${\gamma }_{\mathrm{2,3,6}}$, ${\rm{L}}{\gamma }_{\mathrm{4,4}^{\prime} }$, ${\rm{L}}{\gamma }_{5}$, ${\rm{L}}{\ell }$ and Lη x-ray production cross sections were then employed to derive Au L₁, L₂ and L₃ subshell ionization cross sections with relative uncertainties of 8%, 7% and 7%, respectively; these figures include the uncertainties in the atomic relaxation parameters. The correction for the increase in electron path length inside the Au film was estimated by means of Monte Carlo simulations. The experimental ionization cross sections are about 10% above the state-of-the-art distorted-wave calculations.

Competing processes (namely, dissociative recombination, vibrational excitation and vibrational de-excitation) taking place in the collisions between slow electrons and hydroxyl cations have been investigated for electron energies below 1 eV in the framework of the multichannel quantum defect theory. Rydberg states converging to the lowest excited ionic core have been included in some computations reported here.

We investigate effects of three-body contact interactions on a trapped dipolar Bose gas at finite temperature using the Hartree–Fock–Bogoliubov approximation. We analyze numerically the behavior of the transition temperature and the condensed fraction. Effects of the three-body interactions, anomalous pair correlations and temperature on the collective modes are discussed.

Off-diagonal Aubry–André (AA) model has recently attracted a great deal of attention as they provide condensed matter realization of topological phases. We numerically study a generalized off-diagonal AA model with p-wave superfluid pairing in the presence of both commensurate and incommensurate hopping modulations. The phase diagram as functions of the modulation strength of incommensurate hopping and the strength of the p-wave pairing is obtained by using the multifractal analysis. We show that with the appearance of the p-wave pairing, the system exhibits mobility-edge phases and critical phases with various number of topologically-protected zero-energy modes. Predicted topological nature of these exotic phases can be realized in a cold atomic system of incommensurate bichromatic optical lattice with induced p-wave superfluid pairing by using a Raman laser in proximity to a molecular Bose–Einstein condensation.

We investigate the two lowest-lying weakly bound states of $N\leqslant 8$ bosons as functions of the strength of two-body Gaussian interactions. We observe the limit for validity of Efimov physics. We calculate energies and second radial moments as functions of scattering length. For identical bosons we find that two $(N-1)$-body states appear before the N-body ground states become bound. This pattern ceases to exist for $N\geqslant 7$ where the size of the ground state becomes smaller than the range of the two-body potential. All mean-square-radii for $N\geqslant 4$ remain finite at the threshold of zero binding, where they vary as ${(N-1)}^{p}$ with $p=-3/2,-3$ for ground and excited states, respectively. Decreasing the mass of one particle we find stronger binding and smaller radii. The identical particles form a symmetric system, while the lighter particle is further away in the ground states. In the excited states we find the identical bosons either surrounded or surrounding the light particle for few or many bosons, respectively. We demonstrate that the first excited states for all strengths resemble two-body halos of one particle weakly bound to a dense N-body system for N = 3, 4. This structure ceases to exist for $N\geqslant 5$.

We propose a new scheme for highly efficient three-dimensional (3D) atom localization in a coherently driven closed-loop four-level atomic system via measuring the probe absorption of the weak field. Due to the spatially dependent atom–field interaction, the absorption spectra of the weak probe laser field carry the information about the atomic position. By solving the density-matrix equations of motion and properly modulating the system parameters such as the probe detuning, the relative phase of three driving fields, and the intensity of the control and microwave fields, we can realize high-precision and high-resolution 3D atom localization. Furthermore, we can find the atom at a certain position with 100% probability under appropriate conditions, and then we employ the dressed-state analysis to explain qualitatively the reason of high-precision 3D atom localization.

Quantum routers with a routing rate of much more than 0.5 are important for quantum networks. Here we provide a scheme to significantly improve quantum routing performance. The proposal extends a recent scheme proposed by Zhou et al 2013 (Phys. Rev. Lett.111 103604) by locating another atom in the input channel to generate an effective reflection potential. Our numerical results show that the routing capability from the input channel to another channel can be remarkably enhanced, and even in the presence of moderately low cavity loss, the maximum transfer rate from one channel to another can be more than 0.9. Moreover, we find that there exists an optimum waveguide length to achieve the maximum transfer rate. Our scheme could feasibly be implemented experimentally for quantum routing with a high transfer rate.

The x-ray free electron lasers can enable diffractive structural determination of protein nanocrystals and single molecules that are too small and radiation-sensitive for conventional x-ray diffraction. However the electronic form factor may be modified during the ultrashort x-ray pulse due to photoionization and electron cascade caused by the intense x-ray pulse. For general x-ray imaging techniques, the minimization of the effects of radiation damage is of major concern to ensure reliable reconstruction of molecular structure. Here we show that radiation damage free diffraction can be achieved with atomic spatial resolution by using x-ray parametric down-conversion and ghost diffraction with entangled photons of x-ray and optical frequencies. We show that the formation of the diffraction patterns satisfies a condition analogous to the Bragg equation, with a resolution that can be as fine as the crystal lattice length scale of several Ångstrom. Since the samples are illuminated by low energy optical photons, they can be free of radiation damage.

We investigate the formation and propagation of the Thirring spatial optical solitons in asymmetric quantum wells (QWs). By virtue of the quantum interference effect, the cross-phase modulation (XPM) can be enhanced greatly while the self-phase modulation and linear absorption are well suppressed with proper system parameters. Diffraction of the probe fields can be well balanced by the enhanced XPM alone, which results in their stationary propagation and the formation of Thirring vector optical solitons. In comparison with traditional vector optical solitons, such Thirring spatial optical solitons generated in the QWs can be of various widths and peak powers, implying their more prospective applications in optical communications and optical information processing.

We present a theoretical analysis of the effect of the attochirp on the streaking time delay, intrinsic to photoionization of an atom by an attosecond laser pulse at extreme ultraviolet wavelengths superposed by a femtosecond streaking pulse. To this end, we determine the expectation value of the delay in a chirped pulse using a recently developed model formula. Results of our calculations show that the attochirp can be relevant for photoemission from the $3p$ shell in argon atom at frequencies near the Cooper minimum, while it is negligible if the photoionization cross section as a function of frequency varies smoothly.

The method of molecular dynamics is used to study the behavior of an ultracold non-ideal ion–electron Be⁺ plasma in a uniform magnetic field. Our simulations yield an estimate for the rate of electron–ion collisions which is non-monotonically dependent on the magnetic field magnitude. Also they explicitly show that there are two types of diffusion: a classical one, corresponding to Brownian motion of particles, and Bohm diffusion when the trajectory of particles (guiding centers) includes substantial lengths of drift motion.

The source of the broad radiation of fast hydrogen atoms in plasmas containing noble gases remains one of the most discussed problems relating to plasma–solid interface. In this paper, we present a detailed study of Balmer lines emission generated by fast hydrogen and deuterium atoms in an energy range between 40 and 300 eV in a linear magnetised plasma. The experiments were performed in gas mixtures containing hydrogen or deuterium and one of the noble gases (He, Ne, Ar, Kr or Xe). In the low-pressure regime (0.01–0.1 Pa) of plasma operation emission is detected by using high spectral and spatial resolution spectrometers at different lines-of-sight for different target materials (C, Fe, Rh, Pd, Ag and W). We observed the spatial evolution for H_α, H_β and H_γ lines with a resolution of 50 μm in front of the targets, proving that emission is induced by reflected atoms only. The strongest radiation of fast atoms was observed in the case of Ar–D or Ar–H discharges. It is a factor of five less in Kr–D plasma and an order of magnitude less in other rare gas mixture plasmas. First, the present work shows that the maximum of emission is achieved for the kinetic energy of 70–120 eV/amu of fast atoms. Second, the emission profile depends on the target material as well as surface characteristics such as the particle reflection, e.g. angular and energy distribution, and the photon reflectivity. Finally, the source of emission of fast atoms is narrowed down to two processes: excitation caused by collisions with noble gas atoms in the ground state, and excitation transfer between the metastable levels of argon and the excited levels of hydrogen or deuterium.