We report on an experimental and numerical study of chaotic behavior in random lasers. The complex emission spectra from a disordered amplifying material with static disorder are investigated in a configuration with controlled, stable experimental conditions. It is found that, upon repeated identical excitation, the emission spectra are distinct and uncorrelated. This behavior can be understood in terms of strongly coupled modes that are triggered by spontaneous emission, and is expected to play an important role in most pulsed random lasers.

First order Doppler effects are usually ignored in laser driven trapped ions when the recoil frequency is much smaller than the trapping frequency (Lamb-Dicke regime). This means that the central, carrier excitation band is supposed to be unaffected by vibronic transitions in which the vibrational number changes. While this is strictly true in the Lamb-Dicke limit (infinitely tight confinement), the vibronic transitions do play a role in the Lamb-Dicke regime. In this paper we quantify the asymptotic behavior of their effect with respect to the Lamb-Dicke parameter. In particular, we give analytical expressions for the frequency shift, “pulling” or “pushing,” produced in the carrier absorption band by the vibronic transitions both for Rabi and Ramsey schemes. This shift is shown to be independent of the initial vibrational state.

We describe a numerical method to compute Casimir forces in arbitrary geometries, for arbitrary dielectric and metallic materials, with arbitrary accuracy (given sufficient computational resources). Our approach, based on well-established integration of the mean stress tensor evaluated via the fluctuation-dissipation theorem, is designed to directly exploit fast methods developed for classical computational electromagnetism, since it only involves repeated evaluation of the Green’s function for imaginary frequencies (equivalently, real frequencies in imaginary time). We develop the approach by systematically examining various formulations of Casimir forces from the previous decades and evaluating them according to their suitability for numerical computation. We illustrate our approach with a simple finite-difference frequency-domain implementation, test it for known geometries such as a cylinder and a plate, and apply it to new geometries. In particular, we show that a pistonlike geometry of two squares sliding between metal walls, in both two and three dimensions with both perfect and realistic metallic materials, exhibits a surprising nonmonotonic “lateral” force from the walls.

We consider quantum fluctuations of the Casimir-Polder force between a neutral atom and a perfectly conducting wall in the ground state of the system. In order to obtain the atom-wall force fluctuation we first define an operator directly associated with the force experienced by the atom considered as a polarizable body in an electromagnetic field and we use a time-averaged force operator in order to avoid ultraviolet divergences appearing in the fluctuation of the force. This time-averaged force operator takes into account that any measurement involves a finite time. We also calculate the Casimir-Polder force fluctuation for an atom between two conducting walls. Experimental observability of these Casimir-Polder force fluctuations is also discussed, as well as the dependence of the relative force fluctuation on the duration of the measurement.

We demonstrate a sequence of two quantum teleportations of optical coherent states, combining two high-fidelity teleporters for continuous variables. In our experiment, the individual teleportation fidelities are evaluated as $F_1=0.70\pm 0.02$ and $F_2=0.75\pm 0.02$, while the fidelity between the input and the sequentially teleported states is determined as $F^{\left(2\right)}=0.57\pm 0.02$. This still exceeds the optimal fidelity of one half for classical teleportation of arbitrary coherent states and almost attains the value of the first (unsequential) quantum teleportation experiment with optical coherent states.

By means of photon emission spectroscopy, state selective electron capture cross section for low energy $(0.1–7.5\mathrm{keV}∕\mathrm{amu})$ collisions of ${\mathrm{O}}^{6+}$ on ${\mathrm{H}}_2\mathrm{O}$ molecules have been measured. Over the range of interaction energies the state selective cross sections change strongly, i.e., by factors up to 5, while the total one-electron capture cross section turns out to increase by approximately 40%. Possibilities of using $\mathrm{O}\mathrm{VI}$ line emission ratios as a remote diagnostics of astrophysical plasma are indicated.

The behavior of the dipole photoelectron angular distribution parameter ${\beta }_{nl}\left(\omega \right)$ in the vicinity of autoionizing resonances is discussed. It is shown that from this behavior, surprisingly, many photoionization parameters that cannot be measured experimentally can be extracted. These are the energy positions and ordering of autoionizing resonance minima in the partial photoionization cross sections ${\sigma }_{l+1}$ and ${\sigma }_{l-1}$, the energies at which these two cross sections intersect, and signs and magnitudes of the $\cos ({\delta }_{l+1}-{\delta }_{l-1})$ (${\delta }_{l\pm 1}$ being the phase shifts of the dipole photoionization amplitudes $D_{l\pm 1}$, respectively) through the autoionizing resonance energy region. Based on this, a deeper interpretation of such effects as the width-narrowing, width-fluctuating, and $q$-reversal in the ${\beta }_{nl}$ parameter spectrum in the autoionizing resonance energy region is given. As an example, calculated data for partial photoionization cross sections ${\sigma }_{3d\to f}$ and ${\sigma }_{3d\to p}$, and ${\beta }_{3d}$ parameters for $3d$ photoelectrons from ${\mathrm{Cr}}^{+}$ are presented.

We propose a simplified scheme to teleport a superposition of coherent states from one mode to another of the same bimodal lossy cavity. Based on current experimental capabilities, we present a calculation of the fidelity that can be achieved, demonstrating accurate teleportation if the mean photon number of each mode is at most $1.5$. Our scheme applies as well for teleportation of coherent states from one mode of a cavity to another mode of a second cavity, when both cavities are embedded in a common reservoir.

We show that the usefulness of a state as an activator in teleportation protocols is equivalent to the robustness of its entanglement to noise. The robustness of entanglement of a bipartite state $\sigma$ is linked to the maximum increase in the fidelity of teleportation of any other state when $\sigma$ is used as an extra resource. On the one hand, this connection gives an operational meaning to the robustness of entanglement. On the other hand, it shows that the activation capability—which has a central role as an operational way of quantifying bound entangled states—can be estimated experimentally by measuring entanglement witnesses.

We show how discrete squeezed states in an $N^2$-dimensional phase space can be properly constructed out of the finite-dimensional context. Such discrete extensions are then applied to the framework of quantum tomography and quantum information theory with the aim of establishing an initial study on the interference effects between discrete variables in a finite phase space. Moreover, the interpretation of the squeezing effects is seen to be direct in the present approach, and has some potential applications in different branches of physics.

Photon splitting due to vacuum polarization in a laser field is considered. Using an operator technique, we derive the amplitudes for arbitrary strength, spectral content, and polarization of the laser field. The case of a monochromatic circularly polarized laser field is studied in detail, and the amplitudes are obtained as threefold integrals. The asymptotic behavior of the amplitudes for various limits of interest is investigated also in the case of a linearly polarized laser field. Using the results obtained, the possibility of experimental observation of the process is discussed.

One of the broadest concepts of measurement in quantum theory is the generalized measurement. Another paradigm of measurement—arising naturally in quantum optics, among other fields—is that of continuous-time measurements, which can be seen as the limit of a consecutive sequence of weak measurements. They are naturally described in terms of stochastic processes, or time-dependent random variables. We show that any generalized measurement can be decomposed as a sequence of weak measurements with a mathematical limit as a continuous stochastic process. We give an explicit construction for any generalized measurement, and prove that the resulting continuous evolution, in the long-time limit, collapses the state of the quantum system to one of the final states generated by the generalized measurement, being decomposed, with the correct probabilities. A prominent feature of the construction is the presence of a feedback mechanism—the instantaneous choice weak measurement at a given time depends on the outcomes of earlier measurements. For a generalized measurement with $n$ outcomes, this information is captured by a real $n$-vector on an $n$-simplex, which obeys a simple classical stochastic evolution.

We introduce a marginal picture of the evolution of quantum systems, in which the representation vectors are the quantities that evolve and operators and wave packets remain static. The representation vectors can be seen as probe functions that are the evolution of a $\delta$ function with initial support on $q=X$ in coordinate space. This picture of the dynamics is suited for the determination of intrinsic arrival distributions for quantum systems, providing a clear physical meaning to the “time eigenstates” used in these calculations. We also analyze Galapon et al.’s “confined time eigenstates” [Phys. Rev. Lett. 93, 180406 (2004)] from this point of view, and propose alternative probe functions for confined systems without the need of a quantized time.

We find an invariant for $n$ qubits and propose residual entanglement for $n$ qubits by means of the invariant. Thus, we establish a relation between stochastic local operations and classical communication (SLOCC) entanglement and residual entanglement. Invariant and residual entanglement can be used for SLOCC entanglement classification of $n$ qubits.

We present results of high resolution photoelectron spectrometry measurements for the $\sigma \left(2s\right)$ relative partial photoionization cross section between the $2p$ and $3p$ ionization thresholds in atomic beryllium. This includes the $3snp$ autoionizing resonance series converging to the $3s$ threshold, the $3pns$, $nd$ series converging to the $3p$ threshold, and several members of the $3dnp$, $nf$ series. The results for the perturbed $3snp$ series are compared with the ion yield results that give the total photoionization cross section. We found very good agreement with calculations for the $2s$ partial cross section both in the $R$-matrix approximation and a hyperspherical calculation. We confirm the perturbation of the series by the $3p4s$ interloper with a resonance profile that is the same as for the total cross section. We did not find evidence of a second interloper $\left(3d4p\right)$ between the $3s10p$ and $3s11p$ resonances. We also find very good agreement between experiment and theory for the Shore parameters of these resonances. The data for the resonances between the $3s$ and $3p$ thresholds are compared with the results of a $R$-matrix $2s$ cross section calculation. Good agreement is found for the energy positions and overall resonance shapes once the calculated energies are referred to the experimental $3p$ threshold.

Using ab initio calculations for the lowest, spin-stretched, quartet potential energy surfaces for Rb and Cs neutral atoms interacting with the $\text{NH}\left({{}^3\Sigma }^{-}\right)$ polar target, the cold collision quantum dynamics is analyzed at ultralow energies to examine possible propensity rules existing for the relaxation of either spin or rotational target states (or both) via sympathetic cooling induced by the partner atoms. The specific coupling of Hund’s case (b) is adopted for the dynamical treatment of spin and rotational angular momenta and the coupled channel approach is employed to solve the scattering equations. Both rotational and/or spin relaxation cross sections are obtained down to ${10}^{-6}{\text{cm}}^{-1}$ of energy and corresponding cooling rates are analyzed and compared between the two systems.

In the customary formulation of the strong-field approximation (SFA) for laser-induced ionization, the initial bound state is taken as field-free. In the formulation of a length-gauge SFA for ionization of a molecule described by a two-center binding potential with sufficiently large internuclear separation, we argue that the initial state has to be dressed in order to account for the different scalar potentials at the various centers. We propose a “dressed-state” SFA to this end.

Molecules made of two fermionic atoms are “cobosons”—a contraction for “composite bosons.” These molecules “feel” each other not only through interactions between atoms, but also through the Pauli exclusion between their fermionic components. In order to point out the importance of this Pauli exclusion in cold atom physics, we have calculated here the energy change of $N$ identical cobosonic molecules when a similar molecule, made of atoms having possibly different spin states, is added. Due to the difference in the number of fermion exchanges with a molecule having zero, one, or two atoms identical to the ones already present in the system, we may think that this energy change can take three different values, even for spin-independent interatomic interactions. We actually find that the energy change, in the Born approximation, is exactly the same at all orders in density, when the added molecule has two or just one atom identical to the ones already there. In other words, the scattering length is predicted to be the same if the added molecule can have exchanges with one set of fermionic components or with two sets. This unexpected equality, which of course holds for spin-independent interactions only, results from a subtle balance in the fermion exchanges. To prove it, we make use of a recent extension of the composite-exciton many-body theory we have constructed, to any type of cobosons, the physical understanding of the various exchange processes being enlightened by the Shiva diagram representation (so named because of the multiarm structure reminiscent of the Hindu god Shiva) of this many-body theory.

We propose a quantum teleportation scheme for the angular spectrum of a single-photon field, which allows for the transmission of a large amount of information. Our proposal also provides a method to tune the frequencies of spatially entangled fields, which is useful for interactions with stationary qubits.