We have investigated trap loss in a dual-species magneto-optical trap (MOT) comprised of ${}^{85}\mathrm{Rb}$ and metastable ${}^{40}\mathrm{Ar}$. We measure the trap loss rate coefficients for each species due to the presence of the other as a function of trap light intensity. We clearly identify both Penning ionization of Rb by ${\mathrm{Ar}}^{*}$ and associative ionization to form the molecular ion $\mathrm{Rb}{\mathrm{Ar}}^{+}$ as two of the trap loss channels. We have also measured the trap loss rate coefficient for the ${\mathrm{Ar}}^{*}$ MOT alone and observe production of ${\mathrm{Ar}}^{+}$ and $\mathrm{Ar}_2{}^{+}$ ions.

We introduce two- and one-dimensional (1D) models of a binary BEC (Bose-Einstein condensate) in a periodic potential, with repulsive interactions. We chiefly consider the most fundamental case of the interspecies repulsion with zero intraspecies interactions. The same system may also model a mixture of two mutually repulsive fermionic species. Existence and stability regions for gap solitons (GSs) supported by the interplay of the interspecies repulsion and periodic potential are identified. Two-component GSs are constructed by means of the variational approximation (VA) and in a numerical form. The VA provides an accurate description for the GS which is a compound state of two tightly bound components, each essentially trapped in one cell of the periodic potential. GSs of this type dominate in the case of intragap solitons, with both components belonging to the first finite bandgap of the linear spectrum (only this type of solitons is possible in a weak lattice). Intergap solitons, with one component residing in the second bandgap, and intragap solitons which have both components in the second gap, are possible in a deeper periodic potential, with the strength essentially exceeding the recoil energy of the atoms. Intergap solitons are, typically, bound states of one tightly and one loosely bound component. In this case, results are obtained in a numerical form. The number of atoms in experimentally relevant situations is estimated to be $\sim 5000$ in the 2D intragap soliton, and $\sim 25000$ in its intergap counterpart; in 1D solitons, it may be up to ${10}^5$. For 2D solitons, the stability is identified in direct simulations, while in the 1D case it is done via eigenfrequencies of small perturbations, and then verified by simulations. In the latter case, if the intragap soliton in the first bandgap is weakly unstable, it evolves into a stable breather, while unstable solitons of other types (in particular, intergap solitons) get completely destroyed. The intragap 2D solitons in the first bandgap are less robust, and in some cases they are completely destroyed by the instability. Addition of intraspecies repulsion to the repulsion between the components leads to further stabilization of the GSs.

Stability and instability of quantum evolution are studied in the interaction between a two-level atom with photon recoil and a quantized field mode in an ideal cavity, the basic model of cavity quantum electrodynamics. It is shown that the Jaynes-Cummings dynamics can be unstable in the regime of chaotic motion of the atomic center of mass in the quantized field of a standing wave in the absence of any kind of interaction with environment. This kind of quantum instability manifests itself in strong variations of reduced quantum purity and entropy, correlating with the respective classical Lyapunov exponent, and in exponential sensitivity of fidelity of quantum states to small variations in the atom-field detuning. The connection between quantum entanglement and fidelity and the center-of-mass motion is clarified analytically and numerically for a few regimes of that motion. The results are illustrated with two specific initial field states: the Fock and coherent ones. Numerical experiments demonstrate various manifestations of the quantum-classical correspondence, including dynamical chaos and fractals, which can be, in principle, observed in real experiments with atoms and photons in high-finesse cavities.

We present theoretical calculations of electronic stopping power, including excitation and loss of both projectile and target electrons. The energy loss due to target valence and inner-shell electrons are separately evaluated: the former in the usual dielectric formalism, and the latter by employing a shell-wise local plasma approximation. Stopping power calculations of He ions in Al and Zn are presented, including the electron-electron contribution mentioned above. The agreement with the experimental data is very good, and let us conclude that, for helium ions, projectile excitation and loss contribute to the stopping power at the most in 1%, and it is reasonable for the theoretical descriptions to neglect it.

This paper was published online on 24 January 2006 with an incorrect electronic address in the first author’s byline footnote. The electronic address for the first author should read “sabrina.maniscalco@utu.fi.” The byline footnote has been corrected as of 26 January 2006. The byline footnote is correct in the printed version of the journal.

We experimentally implement a fault-tolerant quantum key distribution protocol with two photons in a decoherence-free subspace [Phys. Rev. A 72, 050304(R) (2005)]. It is demonstrated that our protocol can yield a good key rate even with a large bit-flip error rate caused by collective rotation, while the usual realization of the Bennett-Brassard 1984 protocol cannot produce any secure final key given the same channel. Since the experiment is performed in polarization space and does not need the calibration of a reference frame, important applications in free-space quantum communication are expected. Moreover, our method can also be used to robustly transmit an arbitrary two-level quantum state in a type of decoherence-free subspace.

Both symmetric and symmetry breaking analytic solutions to the one-dimensional nonlinear Schrödinger equation with a double square well potential are known, but not straightforward to obtain numerically. The former generalize solutions to the linear equations, the latter owe their very existence to the nonlinearity. These include, for example, solutions corresponding to the wave function localized almost entirely in one of the wells. Here we propose a systematic method for generating these solutions starting from the linear limit. In particular we find a simple exact formula giving the bifurcation point in terms of the parameters of the symmetric solution. This bifurcation point is then reproduced to a surprising degree of accuracy by a simple variational method.

Geometric phases may arise in a linear molecule interacting via its anisotropic polarizability with a strong nonresonant laser field. When the polarization vector of laser radiation undergoes cyclic evolution, the molecule returns to its original state but may acquire a Berry or Aharonov-Anandan phase factor.

Experiments searching for parity- and time-reversal-invariance-violating effects that rely on measuring the magnetization of a condensed-matter sample induced by application of an electric field are considered. A limit on statistical sensitivity arises due to random fluctuations of the spins in the sample. The scaling of this limit with the number of spins and their relaxation time is derived. Application to an experiment searching for nuclear Schiff moment in a ferroelectric is discussed.

We show deterministic generation of Werner states as a steady state of the collective decay dynamics of a pair of neutral atoms coupled to a leaky cavity and strong coherent drive. We also show how the scheme can be extended to generate a $2N$-particle analogue of the bipartite Werner states.

The triple-differential cross section for the electron-impact ionization of the $3s$ orbital of argon has been measured over the complete backward scattering direction of the ejected electron, using a magnetic angle changer in a conventional $(e,2e)$ spectrometer. The results reveal the position and relative magnitude of the recoil scattering peak in the cross section for this target, and enable a detailed comparison with recently published calculations of argon $3s$ ionization which have been performed using a hybrid distorted-wave Born approximation—$R$-matrix approach. These theoretical calculations show dramatically improved agreement with the experimental results, in comparison with earlier approaches, but puzzling discrepancies still remain.

Using three precisely timed laser pulses, we show experimentally that subvibrational period, nonresonant laser radiation can control a vibrational wave packet. One pulse launches the wave packet and the second modifies the potential energy surfaces as the wave packet moves and the third probes its time evolution by Coulomb explosion imaging. First, we observe $\mathrm{D}_2{}^{+}$ wave packets up to the first half revival without a control pulse. Next, we apply the control pulse to split the wave packet when the wave packet is near the outer turning point. We observe that one piece remains in the bound state and the other propagates to the dissociation continuum. By varying the delay between the pump and control pulses, we control the branching ratios. By a quantum mechanical calculation, we also show that if carrier-envelope phases of subvibrational period pulses are controlled, it can break left-right symmetry during dissociation, freezing the electron on one of the two dissociating fragments during dissociation.

We introduce an interference measure which allows to quantify the amount of interference present in any physical process that maps an initial density matrix to a final density matrix. In particular, the interference measure enables one to monitor the amount of interference generated in each step of a quantum algorithm. We show that a Hadamard gate acting on a single qubit is a basic building block for interference generation and realizes one bit of interference, an “ibit.” We use the interference measure to quantify interference for various examples, including Grover’s search algorithm and Shor’s factorization algorithm. We distinguish between “potentially available” and “actually used” interference, and show that for both algorithms the potentially available interference is exponentially large. However, the amount of interference actually used in Grover’s algorithm is only about $3\mathrm{ibits}$ and asymptotically independent of the number of qubits, while Shor’s algorithm indeed uses an exponential amount of interference.

The parametric model potential developed by Peach for describing the electron interaction with the alkali-metal ion core yields energy levels that are in excellent agreement with the measurements of the spectra. Because of its relative simplicity, the $l$-independent model potential is an attractive choice for studying positron–alkali-metal collisions. In order to test how well the model potential can be used to describe an effective three-body system, we use the Peach model potential to calculate the energy and geometry of the weakly bound $e^{+}\mathrm{Li}$ and $e^{-}\mathrm{Li}$ systems. The binding energy is in good agreement with calculations using the exact Hamiltonian.

We have investigated double $K$-shell vacancy production in x-ray photoionization of silver. Measurements were carried out with photon energies $\left(50\text{–}90\mathrm{keV}\right)$ varying from below threshold to beyond the expected maximum of the double $K$-shell ionization cross section. The limit of asymptotically high energies was deduced from measurements of double $K$-shell ionization following $K$-electron capture by ${}^{109}\mathrm{Cd}$ nuclei. The photon energy dependence of the ratio of double to single $K$ vacancies produced is compared to similar measurements in helium and models thereof. The dependence of that ratio on atomic number is assessed by combining these data with previous experimental and theoretical estimates. The results show a clear growth in the relative importance of the dynamical electron-electron scattering contribution in heavy atoms.

We consider the effects of temperature upon the superfluid phase of ultracold, weakly interacting bosons in a one-dimensional optical lattice. We use a finite-temperature treatment of the Bose-Hubbard model based upon the Hartree-Fock-Bogoliubov formalism, considering both a translationally invariant lattice and one with additional harmonic confinement. In both cases we observe an upward shift in the critical temperature for Bose condensation. For the case with additional harmonic confinement, this is in contrast with results for the uniform gas.

The relative importance of density and phase fluctuations in ultracold one-dimensional atomic Bose gases is investigated. By defining appropriate characteristic temperatures for their respective onset, a broad experimental regime is found, where density fluctuations set in at a lower temperature than phase fluctuations. This is in stark contrast to the usual experimental regime explored up to now, in which phase fluctuations are largely decoupled from density fluctuations, a regime also recovered in this work as a limiting case. Observation of the regime of dominant density fluctuations is shown to be well within current experimental capabilities for both ${}^{23}\mathrm{Na}$ and ${}^{87}\mathrm{Rb}$, requiring relatively low temperatures, small atom numbers, and moderate aspect ratios.

We study the possibility of creating many-particle macroscopic quantum superposition (Schrödinger cat)–like states by using a Feshbach resonance to reverse the sign of the scattering length of a Bose-Einstein condensate trapped in a double-well potential. To address the issue of the experimental verification of coherence in the catlike state, we study the revival of the initial condensate state in the presence of environmentally induced decoherence. As a source of decoherence, we consider the interaction between the atoms and the electromagnetic vacuum, due to the polarization induced by an incident laser field. We find that the resulting decoherence is directly related to the rate at which spontaneously scattered photons carry away sufficient information to distinguish between the two atom distributions which make-up the cat state. We show that for a “perfect” cat state, a single scattered photon will bring about a collapse of the superposition, while a less-than-perfect catlike state can survive multiple scatterings before the collapse occurs. In addition, we study the dephasing effect of atom-atom collisions on the catlike states.

The traversal of an elliptically polarized optical field through a thermal vapor cell can give rise to a rotation of its polarization axis. This process, known as polarization self-rotation (PSR), has been suggested as a mechanism for producing squeezed light at atomic transition wavelengths. We show results of the characterization of PSR in isotopically enhanced rubidium-87 cells, performed in two independent laboratories. We observed that, contrary to earlier work, the presence of atomic noise in the thermal vapor overwhelms the observation of squeezing. We present a theory that contains atomic noise terms and show that a null result in squeezing is consistent with this theory.