We have measured the branching ratios between ${\mathrm{C}}_2$ evaporation, asymmetrical fission, and multifragmentation of ${\mathrm{C}}_{60}^{r+}$ $(r=3\mathrm{}–9)$ in ${\mathrm{Xe}}^{30+}{-\mathrm{C}}_{60}$ collisions at 300 keV impact energy $(v=0.3\mathrm{}\mathrm{a}.\mathrm{u}.\mathrm{}).\mathrm{}$ By multifragmentation we mean here a complete destruction of the ${\mathrm{C}}_{60}$ cage leading to observation of rings and linear chains. The fragmentation ratio, equal to the number of unstable ${\mathrm{C}}_{60}^{r+}$ divided by the sum of stable and unstable parent ions, has been found to increase from 0.3% to 97% for ${\mathrm{C}}_{60}^{3+}$ to ${\mathrm{C}}_{60}^{9+}$ ions. From these measurements, the internal excitation energy has been estimated to be about 75 eV, slightly increasing from ${\mathrm{C}}_{60}^{3+}$ to ${\mathrm{C}}_{60}^{5+}$ ion. The multifragmentation process has been found dominant for ${\mathrm{C}}_{60}^{(7\mathrm{}–9)+}.$ The asymmetrical fission process has been observed for fullerene ions up to ${\mathrm{C}}_{60}^{9+}$ showing that, in these collisions, ${\mathrm{C}}_{60}$ ions can be produced at very low-internal energy. The “phase transition” associated with the multifragmentation process is observed starting with ${\mathrm{C}}_{60}^{6+}$ ions.

Absolute dielectronic recombination (DR) rates for lithiumlike ${\mathrm{Ni}}^{25+}{(1s}^{2}2s)$ ions were measured at high-energy resolution at the Heidelberg heavy-ion storage ring TSR. We studied the center-of-mass energy range 0–130 eV which covers all $\Delta n=0\mathrm{}$ core excitations. The influence of external crossed electric (0–300 V/cm) and magnetic (41.8–80.1 mT) fields was investigated. For the measurement at near-zero electric field, resonance energies and strengths are given for Rydberg levels up to $n=32;$ also Maxwellian plasma rate coefficients for the $\Delta n=0\mathrm{}$ DR at electron temperatures between 0.5 and 200 eV are provided. For increasing electric-field strength we find that for both the ${2p}_{1/2}$ and ${2p}_{3/2}$ series of ${\mathrm{Ni}}^{24+}{(1s}^2{2p}_{j}nl)$ Rydberg resonances with $n>\mathrm{}30,$ the DR rate coefficient increases approximately linearly by up to a factor of 1.5. The relative increase due to the applied electric field for ${\mathrm{Ni}}^{25+}$ is remarkably lower than that found in previous measurements with lighter isoelectronic ${\mathrm{Si}}^{11+},$ ${\mathrm{Cl}}^{14+},$ and ${\mathrm{Ti}}^{19+}$ ions [T. Bartsch et al., Phys. Rev. Lett. 79, 2233 (1997); 82, 3779 (1999); J. Phys. B 33, L453 (2000)], and in contrast to the results for lighter ions no clear dependence of the electric-field enhancement on the magnetic-field strength is found. The Maxwellian plasma rate coefficients for $\Delta n=0\mathrm{}$ DR of ${\mathrm{Ni}}^{25+}$ are enhanced by at most $11\%$ in the presence of the strongest experimentally applied fields.

We calculate the effect of light-induced forces on the fluorescence line shape of a two-level atom crossing at right angles two counterpropagating light beams of parallel linear polarizations $\left(\mathrm{lin}\Vert \mathrm{lin}\right)$ in a common configuration for ultrahigh-precision optical spectroscopy. For an incident atomic beam with a narrow spread of transverse velocities the dipole force induces a redshift of the fluorescence maximum, while in the reverse case of a wide spread of transverse velocities the radiation-pressure force induces a blueshift of the saturation dip minimum. We then use our theory to explain the blueshift of the saturation line-center dip occurring for the closed transition $2{}^3{S}_1\to 2{}^3{P}_2$ of a ${}^4\mathrm{He}$ beam. The observed shift, which is in quite good agreement with the theory, can be of the order of 1/10 of the transition natural linewidth and hence quite important for ultrahigh-precision spectroscopy measurements.

Nonsequential double ionization of several molecules such as ${\mathrm{N}}_2,{\mathrm{CO}}_2,{\mathrm{C}}_2{\mathrm{H}}_2,$ and ${\mathrm{C}}_3{\mathrm{H}}_4$ in strong laser fields has been unambiguously identified from the ion-yield measurements in comparison with a sequential ionization model in the ${10}^{13}–{10}^{15}\hspace{0.5em}\mathrm{W}/{\mathrm{cm}}^2$ laser intensity range. This effect is observed mainly using linearly polarized laser light. In circular polarization, nonsequential double ionization might be present, for instance for the ${\mathrm{C}}_2{\mathrm{H}}_2$ molecule, but with a much lower contribution than in linear polarization. The experimental method allows us to detect all the double-ionization decay channels that are the molecular dication and the two-missing electron fragmentation channels. The validity of a single-ionization tunneling model developed for atoms has been extended to molecules, and a good agreement is observed in laser intensity ranges where single-ionization processes take place.

We use a semiclassical two-fluid model to study the momentum distribution of a Bose-Einstein condensed gas with repulsive interactions inside a harmonic trap at finite temperature, with specific focus on atomic hydrogen. We give particular attention to the average kinetic energy, which is almost entirely associated with the thermal cloud. A nonlinear dependence of the kinetic energy on temperature is displayed, affording a precise way to assess the temperature of the gas. We also show that the kinetic energy increases with the strength of the interactions, reflecting an enhanced rate of depletion of the condensate with increasing temperature.

We use an operational approach to discuss ways to measure the higher-order cross correlations between optical and matter-wave fields. We pay particular attention to the fact that atomic fields actually consist of composite particles that can easily be separated into their basic constituents by a detection process such as photoionization. In the case of bosonic fields, that we specifically consider here, this leads to the appearance in the detection signal of exchange contributions due to both the composite bosonic field and its individual fermionic constituents. We also show how time-gated counting schemes allow to isolate specific contributions to the signal, in particular involving different orderings of the Schrödinger and Maxwell fields.

Temporal light modulation methods which are of great practical importance in optical technology, are emulated with matter waves. This includes generation and tailoring of matter-wave sidebands, using amplitude and phase modulation of an atomic beam. In the experiments atoms are Bragg diffracted at standing light fields, which are periodically modulated in intensity or frequency. This gives rise to a generalized Bragg situation under which the atomic matter waves are both diffracted and coherently shifted in their de Broglie frequency. In particular, we demonstrate creation of complex and non-Hermitian matter-wave modulations. One interesting case is a potential with a time-dependent complex helicity $\left[V\propto \exp \right(i\omega t\left)\right],$ which produces a purely lopsided energy transfer between the atoms and the photons, and thus violates the usual symmetry between absorption and stimulated emission of energy quanta. Possible applications range from atom cooling over advanced atomic interferometers to a new type of mass spectrometer.

Nonlinear behavior of weakly anisotropic (vector) lasers with fast material dynamics (class-A lasers) containing an anisotropic Kerr-type nonlinear material inside the cavity has been theoretically explored. Among interesting periodic amplitude-polarization dynamics, such vector nonlinear systems exhibit important features which are promising for progress in the elaboration of simple, inexpensive, and highly sensitive methods of measurement. In particular, it is shown that under stationary laser operation, the turning angle of the polarization plane of the emitted field is a unique measure of the field-independent anisotropy of the Kerr material. In contrast, dynamical features of the system behavior reflect solely influence of the third-order Kerr nonlinearity. As a consequence, the modulus and sign of the coefficient of this nonlinearity can be determined when a linearly polarized laser mode is destabilized at the corresponding Hopf bifurcation point. Accuracy and sensitivity of the measurements in this system can be enhanced by the bistability phenomenon, which is an inherent feature of vector lasers. The dynamics of this system subject to the action of a longitudinal magnetic field is also investigated.

For a wide set of quantum systems it is demonstrated that the quantum regime can be considered as the transient phase, while the final classical statistical regime is a permanent state. A basis where exact matrix decoherence appears for these final states is found. The relation with the decoherence of histories formalism is studied. A set of final intrinsically consistent histories is found.

In the conventional picture of quantum feedback control, sensors perform measurements on the system, a classical controller processes the results of the measurements, and actuators supply semiclassical potentials to alter the behavior of the quantum system. In this picture, the sensors tend to destroy coherence in the process of making measurements, and although the controller can use the actuators to act coherently on the quantum system, it is processing and feeding back classical information. This paper proposes an alternative method for quantum feedback control, in which the sensors, controller, and actuators are quantum systems that interact coherently with the system to be controlled. In this picture, the controller gets, processes, and feeds back quantum information. Controllers that operate using such quantum feedback loops can perform tasks such as entanglement transfer that are not possible using classical feedback. Necessary and sufficient conditions are presented for Hamiltonian quantum systems to be controllable and observable using both classical and quantum feedback.

We consider the teleportation of entangled two-particle and multiparticle states and present a scheme for the teleportation that may be suitable for both entangled atomic states or field states inside high-Q cavities.

A high-resolution measurement of the electron affinity of potassium has been made using the laser photodetachment threshold technique. A sensitive collinear laser–negative-ion beam apparatus was used in the experiment. We investigated the threshold for photodetachment of ${\mathrm{K}}^{-}$ leading to the formation of residual K atoms in the ${4p}_{3/2}$ state. These excited atoms were selectively detected by use of resonance ionization. The result, an electron affinity of 501.459(12) meV for K, is significantly more accurate than previous measurements and current theory.

Within the framework of the rigorous continuum-bound transition theory of Davies and Seaton [J. Phys. B 2, 757 (1969)], we have developed a numerical method to calculate low-lying resonance photorecombination by directly evaluating the Cauchy principal value of the integral in scattering matrices. The required dipole matrix was obtained using the close-coupling R-matrix code. The advantage of this method is that radiation damping can be accurately estimated and it can naturally be applied to stronger resonance-resonance interference systems. On the basis of this scheme, photorecombination cross sections of ${\mathrm{C}}^{4+}$ ions in the KLL, KLM; and KLN resonance energy regions were calculated, and compared with synchrotron storage ring experimental data and perturbative and close-coupling theoretical results. The comparisons showed that our damped cross sections reproduce the results of the high-resolution experimental measurements, and are in agreement with the theoretical calculations. However, our undamped cross sections are larger than the close-coupling results in the parametrized method by a factor of up to 3. The discrepancies were interpreted.

The linewidth narrowing observed in Brillouin fiber ring lasers is studied within the framework of the usual three-wave model of stimulated Brillouin scattering. We show that the phase noise of the pump laser is transferred to the emitted Stokes wave after being strongly reduced and smoothed under the combined influence of the acoustic damping and the cavity feedback. We then derive a simple analytical relation connecting the full width at half maximum of the Stokes linewidth to that of the pump laser.

We study the temporal evolution of a three-level system (such as an atom or a molecule), initially prepared in an excited state, bathed in a laser field tuned at the transition frequency of the other level. The features of the spontaneous emission are investigated and the lifetime of the initial state is evaluated: a Fermi “golden rule” still applies, but the on-shell matrix elements depend on the intensity of the laser field. In general, the lifetime is a decreasing function of the laser intensity. The phenomenon we discuss can be viewed as an “inverse” quantum Zeno effect and can be analyzed in terms of dressed states.

Monitoring the fluorescent radiation of an atom unravels the master equation evolution by collapsing the atomic state into a pure state which evolves stochastically. A robust unraveling is one that gives pure states that, on average, are relatively unaffected by the master-equation evolution (which applies once the monitoring ceases). The ensemble of pure states arising from the maximally robust unraveling has been suggested to be the most natural way of representing the system [H.M. Wiseman and J.A. Vaccaro, Phys. Lett. A 250, 241 (1998)]. We find that the maximally robust unraveling of a resonantly driven atom requires an adaptive interferometric measurement proposed by Wiseman and Toombes [Phys. Rev. A 60, 2474 (1999)]. The resultant ensemble consists of just two pure states which, in the high driving limit, are close to the eigenstates of the driving Hamiltonian $\Omega {\sigma }_x/2.\mathrm{}$ This ensemble is the closest thing to a classical limit for a strongly driven atom. We also find that it is possible to reasonably approximate this ensemble using just homodyne detection, an example of a continuous Markovian unraveling. This has implications for other systems, for which it may be necessary in practice to consider only continuous Markovian unravelings.

We study the quantum statistics of stimulated photoassociation, using system-reservoir dynamics. We show that, contrary to “common wisdom,” spontaneous emission is not the only mechanism by which entropy loss from a material system to a radiation field can occur. By formally treating the matter waves as the bath and the stimulating radiation field as the system, we are able to derive a Lindblad-type equation for the time dependence of the reduced radiation density matrix and show that entropy exchange between matter and radiation in stimulated photoassociation is indeed possible.

This paper reports a method to build a semiclassical wave function corresponding to an invariant torus satisfying Einstein-Brillouin-Keller quantization conditions. Instead of calculating the stability matrix of the trajectory at each step, as in the standard method of Keller [Ann. Phys. (N.Y.) 4, 180 (1958)] or the modification of Maslov and Fedoriuk [Semiclassical Approximations in Quantum Mechanics (Reidel, Dordrecht, 1981)], we use the actual density of the trajectory, calculated by running the trajectory and counting passages through cells in coordinate space. The method is tested for a system of coupled Morse oscillators, and found to be comparable in accuracy to the standard method. It may be more useful than the standard method for testing ideas for semiclassical quantization in the chaotic regime.

We construct the exact nonspreading coherent state for the Coulomb problem, the averaged motion of which describes a Kepler ellipse, and the coherent state around the average is a Guassian wave function. The obtained coherent state is expanded with respect to the bound eigenstates of the hydrogen atom in parabolic coordinates.

We propose secure protocols for user authenticated quantum key distribution on jammable public channels between two parties, Alice and Bob. Via an arbitrator, Trent, these protocols provide data integrity and mutual identification of the messenger and recipient. The first three are based on single-photon generation and detection. The first and second require (initially) an unjammable channel between the arbitrator and each party. The third requires one broadcast from the arbitrator, disclosing what type of deterministic modification of the states sent through the quantum channel was done by him. The fourth and fifth protocols are based on two-particle entanglement with a preselection of nonorthogonal superpositions of Bell states. These two protocols also require one broadcast from the arbitrator disclosing the type of entangled state in each sending.