We examine the relationship between nonseparable transformations and nonlinear optical processes, surpassing the Heisenberg limit in precision measurements.

We report the results of a study of the effects of combined electric and magnetic fields on dielectronic recombination (DR) from a continuum of finite bandwidth. Specifically, we have examined the process $\mathrm{Ba}6p_{3∕2}8g\to \mathrm{Ba}6p_{1∕2}nk\to \mathrm{Ba}6s_{1∕2}nk+h\nu$ in the presence of electric fields from $0\text{to}7\mathrm{V}∕\mathrm{cm}$ and magnetic fields from $0\text{to}250\mathrm{G}$. Our observations elucidate the requirements for magnetic field enhancement of the DR rate. In particular, they demonstrate that the magnetic coupling must be comparable not only to the electric field splitting of the intermediate autoionizing Rydberg states, but also to their decay rates.

We present a general treatment of a bosonic dissipative network: a chain of coupled dissipative harmonic oscillators whatever its topology—i.e., whichever the way the oscillators are coupled together, the strength of their couplings, and their natural frequencies. Starting with a general more realistic scenario where each oscillator is coupled to its own reservoir, we also discuss the case where all the network oscillators are coupled to a common reservoir. We obtain the master equation governing the dynamic of the network states and the associated evolution equation of the Glauber-Sudarshan $P$ function. With these instruments we briefly show how to analyze the decoherence and the evolution of the linear entropy of general states of the network. We also show how to obtain the master equation for the case of distinct reservoirs from that of a common one.

We propose an experimental setup that is capable of unambiguously discriminating any pair of linearly independent single photon polarization qubits, about which we do not have any knowledge except that an extra pair of these unknown states are provided as the reference. This setup, which is constructed with optical controlled-NOT (CNOT) gates, weak cross Kerr nonlinearities, Bell state analyzers, and other linear optical elements, transforms the unknown triple photon input states to the corresponding single photon states to be deterministically processed by a linear optics circuit. The optimal discrimination of the unknown states is achieved by this setup.

A two-electron system confined in two coupled semiconductor quantum dots is investigated as a candidate for performing quantum logic operations with spin qubits. We study different processes of swapping the electron spins by a controlled switching on and off of the exchange interaction. The resulting spin swap corresponds to an elementary operation in quantum-information processing. We perform direct simulations of the time evolution of the two-electron system. Our results show that, in order to obtain the full interchange of spins, the exchange interaction should change smoothly in time. The presence of jumps and spikes in the time characteristics of the confinement potential leads to a considerable increase of the spin-swap time. We propose several mechanisms to modify the exchange interaction by changing the confinement potential profile and discuss their advantages and disadvantages.

Previous work on the classical information capacities of bosonic channels has established the capacity of the single-user pure-loss channel, bounded the capacity of the single-user thermal-noise channel, and bounded the capacity region of the multiple-access channel. The latter is a multiple-user scenario in which several transmitters seek to simultaneously and independently communicate to a single receiver. We study the capacity region of the bosonic broadcast channel, in which a single transmitter seeks to simultaneously and independently communicate to two different receivers. It is known that the tightest available lower bound on the capacity of the single-user thermal-noise channel is that channel’s capacity if, as conjectured, the minimum von Neumann entropy at the output of a bosonic channel with additive thermal noise occurs for coherent-state inputs. Evidence in support of this minimum output entropy conjecture has been accumulated, but a rigorous proof has not been obtained. We propose a minimum output entropy conjecture that, if proved to be correct, will establish that the capacity region of the bosonic broadcast channel equals the inner bound achieved using a coherent-state encoding and optimum detection. We provide some evidence that supports this conjecture, but again a full proof is not available.

We study quantum systems with even numbers $N$ of levels that are completely state controlled by unitary transformations generated by Lie algebras isomorphic to $\mathrm{sp}\left(N\right)$ of dimension $N(N+1)∕2$ as discussed by Albertini and D’Allesandro [IEEE Trans. Autom. Control 48, 1399 (2003)]. These Lie algebras are smaller than the corresponding $\mathrm{su}\left(N\right)$ with dimension $N^2-1$. We show that this reduction constrains the field-free Hamiltonian to have symmetric energy levels. An example of such a system is an $n$-qubit system with state-independent interaction terms. Using Clifford’s geometric algebra to represent the quantum wave function of a finite system, we present an explicit example of a two-qubit system that can be controlled by the elements of the Lie algebra $\mathrm{sp}\left(4\right)$ [isomorphic to $\mathrm{spin}\left(5\right)$ and $\mathrm{so}\left(5\right)$] with dimension 10 rather than $\mathrm{su}\left(4\right)$ with dimension 15, but only if its field-free energy levels are symmetrically distributed about an average. These results enable one to envision more efficient algorithms for the design of fields for quantum-state engineering in certain quantum-computing applications, and provide more insight into the fundamental structure of quantum control.

We present a theoretical framework for the electronic dynamics of arbitrarily oriented molecular hydrogen in strong and short electromagnetic fields. The ground state of ${\mathrm{H}}_2$ is obtained by propagating the time-dependent Schrödinger equation in imaginary time by assuming the Hartree-Fock ansatz for the interaction between the electrons. The interaction of ${\mathrm{H}}_2$ with the radiation field is considered in the single-active-electron approximation, with the continuum electron subject to Hartree-Fock radial potentials. We propagate the wave function by a split-operator scheme projected on a spherical harmonics basis. Alignment-dependent yields and angular distributions for one- and two-photon ionization induced by an external femtosecond light source are presented and compared with available theoretical results.

Based on the quantitative complementarity relations, we analyze thoroughly the properties of multipartite quantum correlation and entanglement in four-qubit pure states. It is found that, unlike the three-qubit case, the single residual correlation and the genuine correlations of three and four qubits are unable to quantify entanglement appropriately. More interestingly, from our qualitative and numerical analysis, it is conjectured that the sum of all residual correlations is a good quantity for characterizing the multipartite entanglement in the system.

We operated a high-current hollow cathode discharge in pure neon at pressures from 93 to $173\mathrm{Pa}$ and measured the branching fractions of Ne II emission lines originating from several $2p^5$ $3p$, $3d$, and $4f$ levels, for which lifetime data were available in the literature. This allowed the determination of transition probabilities for all downward transitions, which included a number of weak intersystem lines. We focused our study on these weak lines, using care to assure correct identifications and accurate intensity measurements, in spite of the presence of much stronger other lines in their vicinity. In contrast to an earlier experiment, we achieved close agreement with a recent multiconfiguration Hartree-Fock calculation.

Experimental measurements of electron impact excitation of the $6p7s{}^{3}P_{0,1}$ states of Pb atoms have been made at incident electron energies $E_0=10$, 20, 40, 60, 80, and $100\mathrm{eV}$ and scattering angles from 10° to 150°. In addition, relativistic distorted-wave calculations have been carried out at these energies. The data obtained include the differential (DCS), integral $\left(Q_I\right)$, momentum transfer $\left(Q_M\right)$, and viscosity $\left(Q_V\right)$ cross sections. Absolute values for the differential cross sections have been obtained by normalizing the relative DCSs at 10° to the experimental DCS values of [S. Milisavljević, M. S. Rabasović, D. Šević, V. Pejčev, D. M. Filipović, L. Sharma, R. Srivastava, A. D. Stauffer, and B. P. Marinković, Phys. Rev. A 75, 052713 (2007)]. The integrated cross sections were determined by numerical integration of the absolute DCSs. The experimental results have been compared with the corresponding calculations and good agreement is obtained.

We discuss the general problem of generating confining potentials for neutral matter by combining different kinds of static forces. The interactions taken into account are those having a linear or quadratic dependence on the modulus of an irrotational and solenoidal field. Particular attention is devoted to the combination of electric and magnetic forces for paramagnetic species. In this case, tight confinement can be achieved by enhancing and balancing the field gradients. Combined potentials turn out to be conveniently generated by configurations based on high permeability materials. Feasibility of combined electric-magnetic traps and guides for neutral alkali-metal atoms is discussed.

We study an experimentally realizable system containing stable black hole–white hole acoustic horizons in toroidally trapped Bose-Einstein condensates—the quantum de Laval nozzle. We numerically obtain stationary flow configurations and assess their stability using Bogoliubov theory, finding both in hydrodynamic and nonhydrodynamic regimes there exist dynamically unstable regions associated with the creation of positive and negative energy quasiparticle pairs in analogy with the gravitational Hawking effect. The dynamical instability takes the form of a two mode squeezing interaction between resonant pairs of Bogoliubov modes. We study the evolution of dynamically unstable flows using the truncated Wigner method, which confirms the two mode squeezed state picture of the analogue Hawking effect for low winding number.

Dielectric microcavities are widely used as laser resonators and characterizations of their spectra are of interest for various applications. We experimentally investigate microlasers of simple shapes (Fabry-Perot, square, pentagon, and disk). Their lasing spectra consist mainly of almost equidistant peaks and the distance between peaks reveals the length of a quantized periodic orbit. To measure this length with a good precision, it is necessary to take into account different sources of refractive index dispersion. Our experimental and numerical results agree with the superscar model describing the formation of long-lived states in polygonal cavities. The limitations of the two-dimensional approximation are briefly discussed in connection with microdisks.

We demonstrate the generation of the 101st harmonic (wavelength $\lambda =7.9\mathrm{nm}$) from manganese plasma, which is generated using the plasma high-order harmonic technique. We also investigate the variation of the high-order harmonic distribution from the manganese plasma plume as a function of the prepulse and main pulse intensities. The harmonic cutoff was at the 31st order, for pump intensities below the barrier suppression intensity for singly charged Mn ions. However, by using higher prepulse and main pulse intensities, the harmonic cutoff was extended to the 101st order, with an appearance of second plateau for harmonics higher than the 31st order. In this case, the low-order harmonics decreased in intensity, or completely disappeared, and the spectrum for wavelengths longer than $27.6\mathrm{nm}$ was dominated by plasma line emission. The origin of the harmonics appearing at this short wavelength plateau is attributed to the interaction of the intense main pump laser field with doubly charged manganese ions.

Various plasmas prepared by laser ablation of the surfaces of solid targets were examined by the narrow-bandwidth radiation of different chirp and pulse durations. The high-order harmonics generated during laser-plasma interaction showed different brightness, wavelength shift, harmonic cutoff, and efficiency by using variable chirps of pump radiation. An analysis of harmonic optimization at these conditions is presented. The blueshifted and redshifted harmonics observed in this case were analyzed and attributed to the abundance of free electrons and self-phase modulation of the driving pulse. The resonance-induced enhancement of the 15th harmonic from GaN-nanoparticle-containing plasma caused by the tuning of harmonic wavelength close to the ionic transition was demonstrated.

A classical wave-optics analog of topological (Aharonov-Bohm) suppression of tunneling in a double-well potential on a ring threaded by a magnetic flux is proposed. The optical system consists of a uniformly twisted optical fiber with a structured annular core, in which the fiber twist mimics the role of the magnetic flux in the corresponding quantum-mechanical problem. Light waves trapped in the annular core of the fiber experience an additional topological (Aharonov-Bohm) phase, which may lead to the destruction of optical tunneling at certain values of the twist rate.

Since in quantum optics light is represented in terms of photons, light propagation through a linear medium is discussed quantum mechanically in this paper by following the multiple-scattering process of one incident photon from the medium. To treat the photon and the medium on the same quantum footing, the medium is assumed to be an ensemble of uniformly distributed identical two-level atoms. It is found that inside the medium the incident photon follows the same propagation rules as a plane wave does in the classical domain, and has a possibility to become entangled with the atoms. It is also found that when interacting with a two-level test atom outside the medium, the output photon appears to be formally in a single mode identical to that of the incident photon.