We investigate the quantum capacity of noisy quantum channels which can be represented by coupling a system to an effectively small environment. A capacity formula is derived for all cases where both system and environment are two dimensional—including all extremal qubit channels. Similarly, for channels acting on higher-dimensional systems we show that the capacity can be determined if the channel arises from a sufficiently small coupling to a qubit environment. Extensions to instances of channels with larger environment are provided and it is shown that bounds on the capacity with unconstrained environment can be obtained from decompositions into channels with small environment.

The intensity and angular distribution of the argon $3p^{4}nl$ photoelectron lines have been measured with a high resolution (of about $55\mathrm{meV}$) in the $246.51–246.93\mathrm{eV}$ photon energy region. For these photon energies, the excitation of the $2p_{1∕2}^{5}4s$ and $2p_{3∕2}^5(3d+5s)$ $J=1$ intermediate resonances leads to a remarkable (interference) pattern in the intensity of the observed fine-structure states of the satellite lines, which can be attributed to a coherent interplay between the direct photoionization and various excitation-autoionization channels. Detailed computations show that this behavior in the observed electron intensities can be understood only if both the overlap of the resonances as well as their relative phases are taken into account. For these photon energies, therefore, any accurate description of the photoionization (with excitation) requires going beyond the two-step model, which separates the excitation process of the atoms from the subsequent photoelectron emission. Reasonable agreement between experiment and theory is found for most lines by applying multiconfiguration Dirac-Fock wave functions. Apart from a first quantitative analysis of the $3p^{4}nl$ electron lines, however, the high photon and electron energy resolution of the present experiments may stimulate future investigations on resonance phenomena in atomic photoionization, including rearrangement effects in the bound-state electron density and a refined treatment of the electron continuum.

We measured the double-to-single ionization ratios $R$ of helium bombarded by ${\mathrm{C}}^{q+}$ ions ($q=1–3)$. The velocity of the projectile varies from $v_{\mathit{\text{Bohr}}}$ to $4v_{\mathit{\text{Bohr}}}$, the energy varies from $25\mathrm{keV}∕\mathrm{amu}\text{to}500\mathrm{keV}∕\mathrm{amu}$. Meanwhile, a simple model is presented to estimate the value $R$ varying with the collision velocity. The results calculated by the model are in agreement with the experimental data basically.

$KLL$ dielectronic recombination resonances, where a free electron is captured into the $L$ shell and at the same time a $K$ shell electron is excited into the $L$ shell, have been measured for open shell iodine ions by measuring the detected yield of escaping ions of various charge states and modeling the charge balance in an electron beam ion trap. In the modeling, the escape from the trap and multiple charge exchange were considered. Extracted ions were used to measure the charge balance in the trap. The different charge states were clearly separated, which along with the correction for artifacts connected with ion escape and multiple charge exchange made the open shell highly charged ion measurements of this type possible for the first time. From the measured spectra resonant strengths were obtained. The results were $4.27\left(39\right)\times {10}^{-19}{\mathrm{cm}}^2\mathrm{eV}$, $2.91\left(26\right)\times {10}^{-19}{\mathrm{cm}}^2\mathrm{eV}$, $2.39\left(22\right)\times {10}^{-19}{\mathrm{cm}}^2\mathrm{eV}$, $1.49\left(14\right)\times {10}^{-19}{\mathrm{cm}}^2\mathrm{eV}$ and $7.64\left(76\right)\times {10}^{-20}{\mathrm{cm}}^2\mathrm{eV}$ for the iodine ions from He-like to C-like, respectively.

Orbital dynamics in two-electron alkaline earth atoms in a static electric field are studied using scaled-energy recurrence spectroscopy. Photoabsorption spectra for Ba and Ca Rydberg atoms are measured as continuous functions of static electric field and energy. In both Ba and Ca the laser excitation proceeds through the singly excited $nd$ series which is strongly perturbed by doubly excited valence configurations. Clear signatures of the two-electron dynamics associated with these perturbing configurations are identified.

In recent experiments, highly charged molecular cations have been created by ultrashort high-power laser pulses. In this paper, we theoretically study this process by the sequential model of molecular multiple ionization. Two examples—${\mathrm{C}}_6{\mathrm{F}}_6$ and ${\mathrm{C}}_6{\mathrm{H}}_6$—are investigated. We calculate the product ratios, $M^{2+}∕M^{+}$ and $M^{3+}∕M^{+}$ ($M={\mathrm{C}}_6{\mathrm{F}}_6$ or ${\mathrm{C}}_6{\mathrm{H}}_6$), and compare them with the experimental mass spectra. For ${\mathrm{C}}_6{\mathrm{F}}_6$, the calculated ratio $M^{2+}∕M^{+}$ agrees well with the experiment whereas the calculated $M^{3+}∕M^{+}$ is much greater than that of the measurement. This may suggest that heavy fragmentations take place at ${\mathrm{C}}_6\mathrm{F}_6{}^{3+}$ cations. For ${\mathrm{C}}_6{\mathrm{H}}_6$, we predict a large amount of fragmentations at both ${\mathrm{C}}_6\mathrm{H}_6{}^{2+}$ and ${\mathrm{C}}_6\mathrm{H}_6{}^{3+}$.

We compare the collective modes for Bose-condensed systems with two degenerate components with and without intercomponent coherence at finite temperature using the time-dependent Hartree-Fock approximation. We show that the coupling between the condensate and noncondensate in these two cases results in qualitatively different collective excitation spectra. We show that at zero temperature the single-particle excitations of the incoherent Bose condensate can be probed by intercomponent excitations.

We present evidence for a new regime of high-harmonic generation in a waveguide where bright, sub-optical-cycle, quasimonochromatic, extreme ultraviolet (EUV) light is generated via a mechanism that is relatively insensitive to carrier-envelope phase fluctuations. The interplay between the transient plasma which determines the phase matching conditions and the instantaneous laser intensity which drives harmonic generation gives rise to a new nonlinear stabilization mechanism in the waveguide, localizing the phase-matched EUV emission to within sub-optical-cycle duration. The sub-optical-cycle EUV emission generated by this mechanism can also be selectively optimized in the spectral domain by simple tuning of parameters.

We study the entanglement between pairs of qubits in a random antiferromagnetic spin-1/2 chain at zero temperature. We show that some very distant pairs of qubits are highly entangled, being almost pure Bell states. Furthermore, the probability to obtain such spin pairs is proportional to the chain disorder strength and inversely proportional to the square of their separation.

We have modeled the Zeno effect controlled-sign gate of Franson et al. [Phys. Rev. A 70, 062302 (2004)] and shown that high two-photon to one-photon absorption ratios, $\kappa$, are needed for high fidelity free-standing operation. Hence we instead employ this gate for cluster state fusion, where the requirement for $\kappa$ is less restrictive. With the help of partially offline one-photon and two-photon distillations, we can achieve a fusion gate with unity fidelity but nonunit probability of success. We conclude that for $\kappa >2200$, the Zeno fusion gate will out perform the equivalent linear optics gate.

The two-photon decay of highly charged hydrogenlike ions is studied within the framework of second-order perturbation theory, based on Dirac’s equation. Attention has been paid to the angle-angle correlations between the two emitted photons. In particular, the density matrix theory is applied in order to explore how these angle-angle correlations are affected by the alignment of the excited states of the ions. Detailed computations have been carried out for the $3d_{5∕2}\to 1s_{1∕2}$ transition in hydrogenlike uranium ${\mathrm{U}}^{91+}$ ions for different (degrees of) initial alignment. It is shown that the angular correlation function is very sensitive to the alignment of the $3d_{5∕2}$ state, an effect which has to be accounted for in the analysis of all (future) two-photon decay experiments, for which the excited state of the ions is produced by relativistic ion-atom collisions.

Two-loop Bethe logarithms are calculated for excited $P$ and $D$ states in hydrogenlike systems, and estimates are presented for all states with higher angular momenta. These results complete our knowledge of the $P$ and $D$ energy levels in hydrogen at the order of ${\alpha }^8{m}_e{c}^2$, where $m_e$ is the electron mass and $c$ is the speed of light, and scale as $Z^6$, where $Z$ is the nuclear charge number. Our analytic and numerical calculations are consistent with the complete absence of logarithmic terms of order ${(\alpha ∕\pi )}^2{\left(Z\alpha \right)}^6\ln \left[{\left(Z\alpha \right)}^{-2}\right]m_e{c}^2$ for $D$ states and all states with higher angular momenta. For higher excited $P$ and $D$ states, a number of poles from lower-lying levels have to subtracted in the numerical evaluation. We find that, surprisingly, the corrections of the “squared decay-rate type” are the numerically dominant contributions in the order ${(\alpha ∕\pi )}^2{\left(Z\alpha \right)}^6{m}_e{c}^2$ for states with large angular momenta, and provide an estimate of the entire $B_{60}$ coefficient for Rydberg states with high angular momentum quantum numbers. Our results reach the predictive limits of the quantum electrodynamic theory of the Lamb shift.

The evolution of the vibrational intensity distribution of the singly ionized $A{}^2\Pi$ state in CO is experimentally examined for photon energy detunings below the adiabatic 0-0 transition of the O $1s\to 2\pi$ resonance. We have found a strong suppression of the entire vibrational fine structure of this state, leading to its almost complete quenching for certain excitation energies, followed by a partial restoring for larger values of negative photon energy detuning. Our observation, that cannot be rationalized by the known model of a vibrational collapse for energy detuning, may be explained in terms of a Fano interference between the direct and resonant photoionization channels in the presence of strong lifetime vibrational interference.

Larmor precession of laser-polarized atoms contained in antirelaxation-coated cells, detected via nonlinear magneto-optical rotation (NMOR), is a promising technique for a new generation of ultrasensitive atomic magnetometers. For magnetic fields directed along the light propagation direction, resonances in NMOR appear when linearly polarized light is frequency or amplitude modulated at twice the Larmor frequency. Because the frequency of these resonances depends on the magnitude but not the direction of the field, they are useful for scalar magnetometry. Additional NMOR resonances at the Larmor frequency appear when the magnetic field is tilted away from the light propagation direction in the plane defined by the light propagation and polarization vectors. These resonances, studied both experimentally and with a density matrix calculation in the present work, offer a convenient method of achieving additional information about a direction of the magnetic field.

Atomic cadmium ions are loaded into radiofrequency ion traps by photoionization of atoms in a cadmium vapor with ultrafast laser pulses. The photoionization is driven through an intermediate atomic resonance with a frequency-quadrupled mode-locked Ti:sapphire laser that produces pulses of either $100\text{−}\mathrm{fs}$ or $1\text{−}\mathrm{ps}$ duration at a central wavelength of $229\mathrm{nm}$. The large bandwidth of the pulses photoionizes all velocity classes of the Cd vapor, resulting in a high loading efficiency compared to previous ion trap loading techniques. Measured loading rates are compared with a simple theoretical model, and we conclude that this technique can potentially ionize every atom traversing the laser beam within the trapping volume. This may allow the operation of ion traps with lower levels of background pressures and less trap electrode surface contamination. The technique and laser system reported here should be applicable to loading most laser-cooled ion species.

We present the analytical results at the mean-field level for the asymmetrical fermion system with attractive contact interaction at zero temperature. The results can be expressed in terms of linear combinations of the elliptic integrals of the first and second kinds. In the limit of small gap parameter, we discuss how the asymmetry in fermion species affects the phases of the ground state of the system. In the limit of large gap parameter, we show that two candidate phases are competing for the system’s ground state. The Sarma phase containing a pure Fermi fluid and a mixed condensate is favored at a large degree of asymmetry. The separated phase consisting of a pure Fermi fluid and a boson condensate supports the system at a small degree of asymmetry. The two phases are degenerate in the limit of infinite pairing gap.

We present a theoretical interpretation of a recent experiment presented by Zwierlein et al. [Nature (London) 442, 54 (2006)] on the density profile of Fermi gases with unbalanced spin populations. We show that in the regime of strong interaction, the boundaries of the three phases observed by Zwierlein et al. can be characterized by two dimensionless numbers ${\eta }_{\alpha }$ and ${\eta }_{\beta }$. Using a combination of a variational treatment and a study of the experimental results, we infer rather precise bounds for these two parameters.

Kicked rotators with certain nonanalytic potentials avoid dynamical localization and undergo a metal-insulator transition. We show that typical properties of this transition are still present as the nonanalyticity is progressively smoothed out provided that the smoothing is less than a certain limiting value. We have identified a smoothing-dependent time scale such that full dynamical localization is absent and the quantum momentum distribution develops power-law tails with anomalous decay exponents as in the case of a conductor at the metal-insulator transition. We discuss under what conditions these findings may be verified experimentally by using ultracold atom techniques. It is found that ultracold atoms can indeed be utilized for the experimental investigation of the metal-insulator transition.

In the continuum limit (large number of qubits), adiabatic quantum algorithms display a remarkable similarity to sweeps through quantum phase transitions. We find that transitions of second or higher order are advantageous in comparison to those of first order. With this insight, we propose an adiabatic quantum algorithm for the solution of 3-satisfiability (3-SAT) problems (exact cover), which is significantly faster than previous proposals according to numerical simulations (up to 20 qubits). These findings suggest that adiabatic quantum algorithms can solve NP-complete problems such as 3-SAT much faster than the Grover search routine (yielding a quadratic enhancement), possibly even with an exponential speedup.

The so-called Lindblad equation, a typical master equation describing the dissipative quantum dynamics, is shown to be solvable for finite-level systems in a compact form without resort to writing it down as a set of equations among matrix elements. The solution is then naturally given in an operator form, known as the Kraus representation. Following a few simple examples, the general applicability of the method is clarified.