Uhrig’s dynamical decoupling pulse sequence has emerged as a universal and highly promising approach to decoherence suppression. So far, both the theoretical and experimental studies have examined single-qubit decoherence only. This work extends Uhrig’s universal dynamical decoupling from one-qubit to two-qubit systems and even to general multilevel quantum systems. In particular, we show that by designing appropriate control Hamiltonians for a two-qubit or a multilevel system, Uhrig’s pulse sequence can also preserve a generalized quantum coherence measure to the order of $1+O(T^{N+1})$ with only $N$ pulses. Our results lead to a very useful scheme for efficiently locking two-qubit entangled states. Future important applications of Uhrig’s pulse sequence in preserving the quantum coherence of multilevel quantum systems can also be anticipated.

We consider the problem of determining the weights of a quantum ensemble. That is to say, given a quantum system that is in a set of possible known states according to an unknown probability law, we give strategies to estimate the individual probabilities, weights, or mixing proportions. Such strategies can be used to estimate the frequencies at which different independent signals are emitted by a source. They can also be used to estimate the weights of particular terms in a canonical decomposition of a quantum channel. The quality of these strategies is quantified by a covariance-type error matrix. According with this cost function, we give optimal strategies in both the single-shot and multiple-copy scenarios. The latter is also analyzed in the asymptotic limit of large number of copies. We give closed expressions of the error matrix for two-component quantum mixtures of qubit systems. The Fisher information plays an unusual role in the problem at hand, providing exact expressions of the minimum covariance matrix for any number of copies.

We present a comprehensive investigation of nonideal continuous-variable quantum teleportation implemented with entangled non-Gaussian resources. We discuss in a unified framework the main decoherence mechanisms, including imperfect Bell measurements and propagation of optical fields in lossy fibers, applying the formalism of the characteristic function. By exploiting appropriate displacement strategies, we compute analytically the success probability of teleportation for input coherent states and two classes of non-Gaussian entangled resources: two-mode squeezed Bell-like states (that include as particular cases photon-added and photon-subtracted de-Gaussified states), and two-mode squeezed catlike states. We discuss the optimization procedure on the free parameters of the non-Gaussian resources at fixed values of the squeezing and of the experimental quantities determining the inefficiencies of the nonideal protocol. It is found that non-Gaussian resources enhance significantly the efficiency of teleportation and are more robust against decoherence than the corresponding Gaussian ones. Partial information on the alphabet of input states allows further significant improvement in the performance of the nonideal teleportation protocol.

Remote state preparation (RSP) is the act of preparing a quantum state at a remote location without actually transmitting the state itself. Using at most two classical bits and a single shared maximally entangled state, one can in theory remotely prepare any qubit state with certainty and with perfect fidelity. However, in any experimental implementation the average fidelity between the target and output states cannot be perfect. In order for an RSP experiment to demonstrate genuine quantum advantages, it must surpass the optimal threshold of a comparable classical protocol. Here we study the fidelity achievable by RSP protocols lacking shared entanglement and determine the optimal value for the average fidelity in several different cases. We implement an experimental scheme for deterministic remote preparation of arbitrary photon polarization qubits, preparing $178$ different pure and mixed qubit states with an average fidelity of $0.995$. Our experimentally achieved average fidelities surpass our derived classical thresholds whenever the classical protocol does not trivially allow for perfect RSP.

Time-resolved Fourier-transform infrared spectroscopy was applied for observations of the emission arising after irradiation of a Au target with a pulsed nanosecond ArF ($\lambda =193$ nm) laser. A high-repetition-rate laser ($1.0$ kHz) with an output energy of $15$ mJ was focused on a rotating metal target inside a vacuum chamber (average pressure ${10}^{-2}$ Torr). The infrared emission spectra were observed in the $1800–4000{\mathrm{cm}}^{-1}$ spectral region with a time profile showing maximum emission intensity in $9–11$ $\mu$s after a laser shot. Seven prominent lines are reported; most of them are identified as being due to transitions between low Rydberg $\mathit{nl}$ states of Au with $n=5–10$ and $l=0–3$. An alternative identification ($6f_{5/2}\to 7d_{3/2}$) is given for the $2518.489{\mathit{cm}}^{-1}$ line previously classified as $8d_{5/2}\to 5f_{5/2}$. On the basis of the measured lines, three $f$ levels of the Au atom are reported and revised values are given for six other levels.

A rigorous quantum mechanical calculation is carried out for negative muon capture by atomic hydrogen (${\mu }^{-}+\mathrm{H}\to {\mu }^{-}p+e$) by using the $R$-matrix method. The total and final-state selected capture cross sections are calculated at low collision energies ranging from 0.001 to 1 eV. The total capture cross section can, on average, be explained in terms of a previously obtained empirical formula [K. Sakimoto, Phys. Rev. A 66, 032506 (2002)]. However, the present result exhibits additional undulation and cusp structures, which stem from quantum phenomena. The muons are predominantly captured into the highest energetically possible state of ${\mu }^{-}p$ in the present energy region. However, the ${\mu }^{-}p$ products having high angular momenta cannot be formed unless the collision energy becomes high.

The radiative self-energy correction to the bound-electron $g$ factor of $2P_{1/2}$ and $2P_{3/2}$ states in one-electron ions is evaluated to order $\alpha (Z\alpha ){}^2$. The contribution of high-energy virtual photons is treated by means of an effective Dirac equation, and the result is verified by an approach based on long-wavelength quantum electrodynamics. The contribution of low-energy virtual photons is calculated both in the velocity and in the length gauge, and gauge invariance is verified explicitly. The results compare favorably to recently available numerical data for hydrogenlike systems with low nuclear charge numbers.

We report vibrational excitation cross sections for C–C and C–H symmetric stretch modes of acetylene by positron impact. The contribution of these infrared inactive modes to the annihilation parameter is also addressed. The Feshbach projection operator approach was employed to vibrationally resolve $e^{+}$-acetylene scattering phase shifts obtained with the Schwinger multichannel method. The present results point out a virtual state pole at the equilibrium geometry of acetylene that becomes a bound state as either bond is stretched, in qualitative agreement with previous calculations for small hydrocarbons. The vibrational couplings are stronger for the C–C mode, giving rise to a bound state pole within the Franck-Condon region of the vibrational ground state. These bound and virtual states give rise to sharp threshold structures (vibrational resonances) in both the vibrational excitation cross sections and the annihilation parameter ($Z_{\mathrm{eff}}$). We found fair agreement between the present calculations and previously reported $e^{+}$-acetylene vibrational excitation cross sections.

We report on measurements of inelastic loss processes in ultracold ${}^{85}\mathrm{Rb}$ $|F=2\rangle$ atoms. Our apparatus creates ultracold ${}^{85}\mathrm{Rb}$ clouds by sympathetic cooling with a ${}^{87}\mathrm{Rb}$ reservoir in a quadrupole-Ioffe magnetic trap and subsequently in a weak, large-volume optical dipole trap. We demonstrate strong sympathetic cooling of ${}^{85}\mathrm{Rb}$ in the magnetic trap, increasing its phase-space density by three orders of magnitude with no detectable loss in number. Ultracold samples created in this way are used to observe the variation of inelastic loss in ${}^{85}\mathrm{Rb}$ $|F=2,\hspace{0.5em}{m}_F=-2\rangle$ clouds as a function of magnetic field near the $155$-G Feshbach resonance and to measure the decay due to inelastic losses in all five Zeeman sublevels of the $F=2$ manifold, finding a particularly high three-body recombination rate in the lowest energy state. We have also observed and characterized a previously unobserved loss feature at $219.9(1)$ G with a width of $0.28(6)$ G, which we associate with a narrow Feshbach resonance predicted by theory.

We report experimental angular differential cross sections for nonradiative single-electron capture in $p$-He collisions ($p+$ He $\to$ H $+$ He${}^{+}$) with a separate peak at the $0.47$ mrad Thomas scattering angle for energies in the $1.3–12.5$ MeV range. We find that the intensity of this peak scales with the projectile velocity as $v_{\mathrm{P}}^{-11}$. This constitutes the first experimental test of the prediction from 1927 by L. H. Thomas [Proc. R. Soc. 114, 561 (1927)]. At our highest energy, the peak at the Thomas angle contributes with $13.5\%$ to the total integrated nonradiative single-electron capture cross section.

We have measured the kinetic energy of positronium (Ps) atoms emitted into a vacuum from a porous silica film subsequent to positron bombardment, via the Doppler spread of the linewidth of the Ps $1^{3}S$-$2^{3}P$ transition. We find that the deeper in the target film that positrons are implanted the colder is the emitted Ps, an effect we attribute to cooling via collisions in the pores as the atoms diffuse back to the film surface. We observed a lower limit to the mean Ps kinetic energy associated with motion in the direction of the laser, $E_x=42\pm 3$ meV, that is consistent with conversion of the confinement energy of Ps in the 2.7-nm-diameter pores to kinetic energy in vacuum. An implication is that a porous sample would need to be composed of pores greater than around 10 nm in diameter in order to produce thermal Ps in vacuum with temperatures of less than 100 K. By performing Doppler spectroscopy on intense pulses of Ps we have experimentally demonstrated the production of many excited-state Ps atoms simultaneously, which could have numerous applications, including laser cooling and fundamental spectroscopic studies of Ps and the production of antihydrogen.

We describe the dissociation of a diatomic Feshbach molecule due to a time-varying external magnetic field in a realistic trap and guide setting. An analytic expression for the asymptotic state of the two ultracold atoms is derived, which can serve as a basis for the analysis of dissociation protocols to generate motionally entangled states. For instance, the gradual dissociation by sequences of magnetic field pulses may delocalize the atoms into macroscopically distinct wave packets, whose motional entanglement can be addressed interferometrically. The established relation between the applied magnetic field pulse and the generated dissociation state reveals that square-shaped magnetic field pulses minimize the momentum spread of the atoms. This is required to control the detrimental influence of dispersion in a recently proposed experiment to perform a Bell test in the motion of the two atoms [C. Gneiting and K. Hornberger, Phys. Rev. Lett. 101, 260503 (2008)].

We present a quantum electrodynamical procedure to demonstrate the all-optical control of molecular fluorescence. The effect is achieved on passage of an off-resonant laser beam through an optically activated system; the presence of a surface is not required. Following the derivation and analysis of the all-optical control mechanism, calculations are given to quantify the significant modification of spontaneous fluorescent emission with input laser irradiance. Specific results are given for molecules whose electronic spectra are dominated by transitions between three electronic levels, and suitable laser experimental methods are proposed. It is also shown that the phenomenon is sensitive to the handedness of circularly polarized throughput, producing a conferred form of optical activity.

We explore atom-laser-like transport processes of ultracold Bose-condensed atomic vapors in mesoscopic waveguide structures beyond the Gross-Pitaevskii mean-field theory. Based on a microscopic description of the transport process in the presence of a coherent source that models the outcoupling from a reservoir of perfectly Bose-Einstein condensed atoms, we derive a system of coupled quantum evolution equations that describe the dynamics of a dilute condensed Bose gas in the framework of the Hartree-Fock–Bogoliubov approximation. We apply this method to study the transport of dilute Bose gases through an atomic quantum dot and through waveguides with disorder. Our numerical simulations reveal that the onset of an explicitly time-dependent flow corresponds to the appearance of strong depletion of the condensate on the microscopic level and leads to a loss of global phase coherence.

We generalize the idea of chainwise stimulated Raman adiabatic passage (STIRAP) [Kuznetsova et al., Phys. Rev. A 78, 021402(R) (2008)] to a photoassociation-based chainwise atom-molecule system, with the goal of directly converting two-species atomic Bose-Einstein condensates (BEC) into a ground polar molecular BEC. We pay particular attention to the intermediate Raman laser fields, a control knob inaccessible to the usual three-level model. We find that an appropriate exploration of both the intermediate laser fields and the stability property of the atom-molecule STIRAP can greatly reduce the power demand on the photoassociation laser, a key concern for STIRAPs starting from free atoms due to the small Franck-Condon factor in the free-bound transition.

We analyze the formation and dynamics of bright unstaggered solitons in the disk-shaped dipolar Bose-Einstein condensate, which features the interplay of contact (collisional) and long-range dipole-dipole (DD) interactions between atoms. The condensate is assumed to be trapped in a strong optical-lattice potential in the disk’s plane, hence it may be approximated by a two-dimensional (2D) discrete model, which includes the on-site nonlinearity and cubic long-range (DD) interactions between sites of the lattice. We consider two such models, which differ by the form of the on-site nonlinearity, represented by the usual cubic term, or more accurate nonpolynomial one, derived from the underlying three-dimensional Gross-Pitaevskii equation. Similar results are obtained for both models. The analysis is focused on the effects of the DD interaction on fundamental localized modes in the lattice (2D discrete solitons). The repulsive isotropic DD nonlinearity extends the existence and stability regions of the fundamental solitons. New families of on-site, inter-site, and hybrid solitons, built on top of a finite background, are found as a result of the interplay of the isotropic repulsive DD interaction and attractive contact nonlinearity. By themselves, these solutions are unstable, but they evolve into robust breathers which exist on an oscillating background. In the presence of the repulsive contact interactions, fundamental localized modes exist if the DD interaction (attractive isotropic or anisotropic) is strong enough. They are stable in narrow regions close to the anticontinuum limit, while unstable solitons evolve into breathers. In the latter case, the presence of the background is immaterial.

We report a model that makes it possible to analyze quantitatively the dipole blockade effect on the dynamical evolution of a two two-level atom system driven by an external laser field. The multiple excitations of the atomic sample are taken into account. We find very large concurrence in the dipole blockade regime. We further find that entanglement can be tuned by changing the intensity of the exciting laser. We also report a way to lift the dipole blockade paving the way to manipulate, in a controllable way, the blockade effects. We finally report how a continuous monitoring of the dipole blockade will be possible using photon-photon correlations of the scattered light in a regime where the spontaneous emission will dominate dissipation in the sample.

The motion and position of cavity solitons in a vertical-cavity surface-emitting laser with optical injection are investigated. Spatial variations of the phase of the injected field are considered in the form of sinusoidal and triangular modulations. We show how the velocity, distance traveled, and final position of the cavity solitons can be controlled by varying the slope of the phase modulations and the response time of the semiconductor medium. Numerical simulations demonstrate the feasibility of an all-optical delay line in a cavity soliton laser. Merging of cavity solitons is observed when they collide at modulation maxima and is shown to be beneficial in the operation of the delay line. The merging and consequent emission of pulsed and localized light is explained in terms of violation of energy balance for soliton systems in the presence of injection and dissipation.

The magnetic substate-selective single-electron-capture cross sections in collisions of ${\mathrm{N}}^{5+}$ with He and ${\mathrm{H}}_2$ are calculated using the two-center atomic orbital close-coupling method, and the polarization of emitted radiation from the excited state of ${\mathrm{N}}^{4+}$ is investigated for projectile energies between 1.2 and 7 keV/u. The polarization degrees for the $3p\hspace{0.5em}{}^2{P}_{3/2}–3s\hspace{0.5em}{}^2{S}_{1/2}$ transition of ${\mathrm{N}}^{4+}$($3p\hspace{0.5em}{}^2{P}_{3/2}$) produced in ${\mathrm{N}}^{5+}+\mathrm{He}$ and ${\mathrm{N}}^{5+}+{\mathrm{H}}_2$ electron-capture collisions are in general agreement with the experimental measurements. It is found both experimentally and theoretically that there exists a large difference between the polarization degrees of this radiation resulting from the ${\mathrm{N}}^{5+}+\mathrm{He}$ and ${\mathrm{N}}^{5+}+{\mathrm{H}}_2$ electron-capture collisions, namely, $~$0.25 and $~$0, respectively. By studying the time evolution of electron-capture dynamics in the two systems we have found that this difference is caused mainly by the difference in the interactions in the two systems at relatively small internuclear distances, consistent with the molecular picture of the collision dynamics.

For many implementations of quantum computing, $1/f$ and other types of broad-spectrum noise are important sources of decoherence. An important step forward would be the ability to extract the characteristics of this noise from qubit measurements to see if it leads to new physical effects. For certain types of qubits, the working point of the qubit can be varied. Using a new mathematical method that is suited to treating all working points, we present theoretical results that show how this degree of freedom can be used to extract noise parameters and predict a new effect: noise-induced looping on the Bloch sphere. We analyze data on superconducting qubits to show that they are very near the parameter regime where this looping should be observed.