In the propagation of optical pulses through dispersive media, the frequency degree of freedom acts as an effective decohering environment on the polarization state of the pulse. Here we discuss the application of open-loop dynamical-decoupling techniques for suppressing such a polarization decoherence in one-way communication channels. We describe in detail the experimental proof of principle of the “bang-bang” protection technique recently applied to flying qubits in Damodarakurup et al. [Phys. Rev. Lett. 103, 040502 (2009)]. Bang-bang operations are implemented through appropriately oriented wave plates and dynamical decoupling is shown to be potentially useful to contrast a generic decoherence acting on polarization qubits propagating in dispersive media like optical fibers.

We investigate the scaling of decoherence rates and their dynamical suppression with the number $N$ of qubits in various entangled states. Remarkably, for sufficiently large $N$, coherence time is always in the Zeno regime. This changes the scaling to the square root of the Markov-regime scaling. We find that a simple and effective control strategy is to locally modulate the individual qubits and thereby not only suppress the decoherence rate of each qubit, but also reduce the decoherence scaling of entangled states that are particularly fragile, from $N^2$ to $N$, resulting in a dramatic reduction of the decoherence (by orders of magnitude). Surprisingly, the conditions for the effectiveness of such decoherence control are independent of $N$.

For a generic dynamical decoupling sequence employing a single-axis control, we study its efficiency in the presence of small errors in the direction of the controlling pulses. In the case that the corresponding ideal dynamical decoupling sequence produces sufficiently good results, the impact of the errors is found to scale as $\xi {}^2$, with negligible first-order effect, where $\xi$ is the dispersion of the random errors. This analytical prediction is numerically tested in a model in which the environment is modeled by one qubit coupled to a quantum kicked rotator in chaotic motion. In this model, with periodic pulses applied to the qubit in the environment, it is found numerically that periodic bang-bang control may outperform Uhrig dynamical decoupling.

Ab initio study of the density-dependent population and lifetime of the long-lived $(\mu p){}_{2s}$ and the yield of $(\mu p){}_{1s}$ atoms with kinetic energy $0.9$ keV have been performed. The direct Coulomb $2s\to 1s$ de-excitation is proved to be the dominant quenching mechanism of the $2s$ state at kinetic energy below $2p$ threshold and explain the lifetime of the metastable $2s$ state and high-energy $0.9$ keV component of $(\mu p){}_{1S}$ observed at low densities. The cross sections of the elastic, Stark, and Coulomb de-excitation processes have been calculated in the close-coupling approach taking into account both the closed channels and vacuum polarization shifts of the $\mathit{ns}$ states. The cross sections are used as the input data in the detailed study of the atomic cascade kinetics. The theoretical predictions are compared with the known experimental data at low densities. The $47\%$ yield of the 0.9 keV $(\mu p){}_{1s}$ atoms is predicted for liquid-hydrogen density.

The present study deals with the collisional broadening of monatomic magnesium, evolving in a helium buffer gas, in the wavelength and temperature ranges 260–310 nm and 100–3000 K, respectively. The computed emission and absorption spectral profiles are based on the most recent potential-energy curves and transition dipole moments. The required interatomic Mg$(3s^2)+\mathrm{He}(1s^2)$ and Mg$(3s3p)+\mathrm{He}(1s^2)$ potentials are constructed from two different sets. The purpose of this treatment is twofold. First, using the quantum-mechanical Baranger impact approximation, the width and shift of the line-core spectra are determined and their variation law with temperature is examined. Then, the satellite structures in the blue and red wings are analyzed quantum mechanically. The calculations show especially that the free-free transitions contribute most to the MgHe photoabsorption spectra and that a satellite structure is observable beyond the temperature $1800\hspace{0.5em}\mathrm{K}$ around the wavelengths $272$ or $276\hspace{0.5em}\mathrm{nm},$ depending on the used potential set. Weak satellites have also been investigated and, for all cases, the obtained results showed good agreement with those already published.

Rate coefficients for collisional-radiative recombination (CRR) of ${\mathrm{Ar}}^{+}$ ions with electrons have been measured at temperatures from $77\hspace{0.5em}$to $180\hspace{0.5em}\mathrm{K}$ in a helium-buffered flowing-afterglow (Cryo-FALP) experiment at electron densities $n_e$ from ${10}^8$ to ${10}^{10}\hspace{0.5em}{\mathrm{cm}}^{-3}$. The measured ternary rate coefficient $K_{\mathrm{CRR}}$ at $77\hspace{0.5em}\mathrm{K}$ is $(1.1\pm 0.4)\times {10}^{-17}\hspace{0.5em}{\mathrm{cm}}^6{\mathrm{s}}^{-1}$ and the observed variation with temperature agrees well with the theoretical $T^{-4.5}$ dependence.

Cross sections for positron scattering from krypton have been measured with an energy resolution of ∼60 meV over the energy range 0.5–60 eV. Absolute values of the grand total (${\sigma }_{\mathrm{GT}}$), positronium formation (${\sigma }_{\mathrm{Ps}}$), and grand total minus positronium formation (${\sigma }_{\mathrm{GT}}-{\sigma }_{\mathrm{Ps}}$,) cross sections are presented. Theoretical estimations of ${\sigma }_{\mathrm{GT}}$ and ${\sigma }_{\mathrm{GT}}-{\sigma }_{\mathrm{Ps}}$ are also performed for this target using the convergent close-coupling method and the relativistic optical potential approach. We also provide experimental and theoretical results for elastic differential cross sections, for selected energies both below and above the Ps threshold. Where available, the present results are compared to both experimental and theoretical values from the literature.

In this paper we suggest an approach for the multichannel Coulomb scattering problem. The Schrödinger equation for the problem is reformulated in the form of a set of inhomogeneous equations with a finite-range driving term. The boundary conditions at infinity for this set of equations have been proven to be purely outgoing waves. The formulation presented here is based on splitting the interaction potential into a finite-range core part and a long-range tail part. The conventional matching procedure coupled with the integral Lippmann-Schwinger equations technique is used in the formal theoretical basis of this approach. The reformulated scattering problem is suitable for application in the exterior complex scaling technique: the practical advantage is that after complex scaling, the problem is reduced to a boundary problem with zero boundary conditions. The Coulomb wave functions are used only at a single point; if this point is chosen to be at a sufficiently large distance, on using the asymptotic expansion of Coulomb functions, one may completely avoid the Coulomb functions in the calculations. The theoretical results are illustrated with numerical calculations for two models.

Energy- and angle-resolved electron emission spectra produced by incidence of ultrashort electromagnetic pulses on a LiF(001) surface are studied by employing a distorted-wave method named the crystal surface–Volkov (CSV) approximation. The theory makes use of the Volkov phase to describe the action of the external electric field on the emitted electron, while the electron-surface interaction is represented within the tight-binding model. The CSV approach is applied to investigate the effects introduced by the crystal lattice when the electric field is oriented parallel to the surface plane. These effects are essentially governed by the vector potential of the external field, while the influence of the crystal orientation was found to be negligible.

We investigated sympathetic Coulomb crystallization of CaH${}^{+}$ ions produced by a laser-induced reaction through an excited state according to ${}^{40}\mathrm{Ca}$${}^{+}$($4p\hspace{0.5em}{}^2{P}_{1/2}$) $+$ ${\mathrm{H}}_2$ $\to$ ${}^{40}\mathrm{Ca}$H${}^{+}$ $+$ H. ${}^{40}\mathrm{Ca}$H${}^{+}$ ions stored in a linear Paul trap were characterized by their secular-motion excitation spectrum and by the modified fluorescence images of crystallized Ca${}^{+}$ ions. The number of two-species crystallized ions, the secular motion temperature, and the structure were determined by molecular dynamics simulations. Both the observed two-species ion crystals and simulation images demonstrate that the CaH${}^{+}$ ions were sympathetically crystallized with their secular-motion temperature being less than 10 mK. We determined reaction rates from fluorescence images of two-species Coulomb crystals by systematically changing the reaction time. In addition a lower limit for the reaction rate coefficient $k=8\times {10}^{-10}$ cm${}^3$/s was obtained.

We study the rotational properties of a dipolar Bose-Einstein condensate confined in a quasi-two-dimensional anisotropic trap for an arbitrary orientation of the dipoles with respect to their plane of motion. Within the mean-field approximation, we find that the lowest-energy state of the system depends strongly on the relative strength between the dipolar and the contact interactions, as well as on the size and the orientation of the dipoles and the size and the orientation of the deformation of the trapping potential.

We discuss the filtering of the vibrational states of a cold atom in an optical trap by chaining this trap with two empty ones and adiabatically controlling the tunneling. Matter-wave filtering is performed by selectively transferring the population of the highest populated vibrational state to the most distant trap while the population of the rest of the states remains in the initial trap. Analytical conditions for two-state filtering are derived and then applied to an arbitrary number of populated bound states. Realistic numerical simulations close to state-of-the-art experimental arrangements are performed by modeling the triple well with time-dependent Pöschl-Teller potentials. In addition to filtering of vibrational states, we discuss applications for quantum tomography of the initial population distribution and engineering of atomic Fock states that, eventually, could be used for tunneling-assisted evaporative cooling.

We investigate the formation of Bose-Einstein condensation and a population imbalance in a three-component Fermi superfluid by increasing the U(3) invariant attractive interaction. We consider the system at zero temperature in three dimensions as well as in two dimensions. Within mean-field theory, we derive explicit formulas for the number densities, the gap order parameter, the condensate density, and the condensate fraction of the uniform system and analyze them in the crossover from the Bardeen-Cooper-Schrieffer (BCS) state of Cooper pairs to the Bose-Einstein condensate (BEC) of strongly bound molecular dimers. In addition we study this Fermi mixture trapped by a harmonic potential: We calculate the density profiles of the three components and the condensate density profile of Cooper pairs in the BCS-BEC crossover.

We show that two-photon absorption (TPA) in rubidium atoms can be greatly enhanced by the use of a hollow-core photonic-band-gap fiber. We investigate off-resonant, degenerate Doppler-free TPA on the $5S_{1/2}\to 5D_{5/2}$ transition and observe $1\%$ absorption of a pump beam with a total power of only 1 mW in the fiber. These results are verified by measuring the amount of emitted blue fluorescence and are consistent with the theoretical predictions which indicate that transit-time effects play an important role in determining the two-photon absorption cross section in a confined geometry.

We demonstrate theoretical conditions for highly efficient degenerate four-wave mixing in triply resonant nonlinear (Kerr) cavities. We employ a general and accurate temporal coupled-mode analysis in which the interaction of light in arbitrary microcavities is expressed in terms of a set of coupling coefficients that we rigorously derive from the full Maxwell equations. Using the coupled-mode theory, we show that light consisting of an input signal of frequency ${\omega }_0-\Delta \omega$ can, in the presence of pump light at ${\omega }_0$, be converted with quantum-limited efficiency into an output shifted signal of frequency ${\omega }_0+\Delta \omega$, and we derive expressions for the critical input powers at which this occurs. We find the critical powers in the order of 10 mW, assuming very conservative cavity parameters (modal volumes $~10$ cubic wavelengths and quality factors $~1000$). The standard Manley-Rowe efficiency limits are obtained from the solution of the classical coupled-mode equations, although we also derive them from simple photon-counting “quantum” arguments. Finally, using a linear stability analysis, we demonstrate that maximal conversion efficiency can be retained even in the presence of self- and cross-phase modulation effects that generally act to disrupt the resonance condition.

We theoretically demonstrate the generation of wavelength-tunable, narrow-bandwidth extreme-ultraviolet (xuv) radiation by high-order harmonic generation (HHG) driven by an orthogonally polarized two-color laser field, which is composed of a 10-fs, 1500-nm laser pulse and a 40-fs, 2400-nm laser pulse. By performing classical analysis, we reveal that the rapid change of electron wave-packet dynamics at a subcycle time scale confines high-order harmonic emission to a small spectral region, leading to the generation of narrow-bandwidth xuv radiation. Furthermore, the central wavelength of the xuv radiation can be continuously tuned over a wide range by changing either the peak intensity of the driver laser or the amplitude ratio between the two laser fields at different wavelengths. It is also verified that the use of driver pulses at longer wavelengths leads to a better spectral confinement of the xuv radiation.

In a recent article Paterek, Dakić, and Brukner [Phys. Rev. A 79, 012109 (2009)] show an algorithm for generating mutually unbiased bases from sets of orthogonal Latin squares. They claim that this algorithm works for every set of orthogonal Latin squares. We show that the algorithm only works for particular sets of orthogonal Latin squares. Furthermore, the algorithm is a more readable version of work previously published [Phys. Rev. A 70, 062101 (2004)].

In this Reply to the preceding Comment by Hall and Rao [Phys. Rev. A 83, 036101 (2011)], we motivate terminology of our original paper and point out that further research is needed in order to (dis)prove the claimed link between every orthogonal Latin square of order being a power of a prime and a mutually unbiased basis.

High-quality quantum sources are of paramount importance for the implementation of quantum technologies. We present here a heralded single-photon source based on commercial-grade polarization-maintaining optical fiber. The heralded photons exhibit a purity of at least 0.84 and an unprecedented heralding efficiency into a single-mode fiber of $85\%$. The birefringent phase-matching condition of the underlying four-wave mixing process can be controlled mechanically to optimize the wavelength tuning needed for interfacing multiple sources, as is required for large-scale entanglement generation.