We study binary and the recently discovered process of ternary He-assisted recombination of $\text{H}_3{}^{+}$ ions with electrons in a low-temperature afterglow plasma. The experiments are carried out over a broad range of pressures and temperatures of an afterglow plasma in a helium buffer gas. Binary and He-assisted ternary recombination are observed and the corresponding recombination rate coefficients are extracted for temperatures from 77 to 330 K. We describe the observed ternary recombination as a two-step mechanism: first, a rotationally excited long-lived neutral molecule $\text{H}_3{}^{\ast }$ is formed in electron-$\text{H}_3{}^{+}$ collisions. Second, the $\text{H}_3{}^{\ast }$ molecule collides with a helium atom that leads to the formation of a very long-lived Rydberg state with high orbital momentum. We present calculations of the lifetimes of $\text{H}_3{}^{\ast }$ and of the ternary recombination rate coefficients for para- and ortho-$\text{H}_3{}^{+}$. The calculations show a large difference between the ternary recombination rate coefficients of ortho- and para-$\text{H}_3{}^{+}$ at temperatures below 300 K. The measured binary and ternary rate coefficients are in reasonable agreement with the calculated values.

We report on high-resolution measurements of the $L_1$- and $L_3$-subshell x-ray lines of Pd induced by 0.5–3.0 MeV proton bombardment. The experimental results for the relative intensities of the $L{\beta }_3$ $\left(L_1\text{−}{M}_3\right)$, $L{\beta }_4$ $\left(L_1\text{−}{M}_2\right)$, and $L{\beta }_6$ $\left(L_3\text{−}{N}_1\right)$ lines are compared to the theoretically predicted values employing $L$-subshell ionization cross sections calculated within the semiclassical approximation and the energy-loss Coulomb-repulsion perturbed-stationary-state relativistic theory. A structure of the $L_1$-subshell cross section at low proton energies generated by the nodal structure of the $2s$ wave function, analogous to the well-known Cooper minimum in the photoabsorption cross section, has been observed also for the case of charged particle ionization. In addition, the influence of the multiple ionization on the intensity ratio of the two most intense $L_1$-subshell x-ray transitions is studied by comparing the proton-induced intensities with the corresponding photoinduced value measured in an experiment employing monochromatic synchrotron radiation with an energy tuned just above the $L_1$ edge.

A $B$-spline $R$-matrix (close-coupling) method has been used to perform a systematic study of angle-differential cross sections for electron scattering from neutral magnesium. The calculations cover elastic scattering and excitation of the five excited states $\left(3s3p\right){{}^{3,1}P}^o$, $\left(3s3d\right){}^{1}D$, $\left(3s4s\right){}^{1}S$, and $\left(3s4p\right){{}^{1}P}^o$. A multiconfiguration Hartree-Fock method with nonorthogonal orbitals was employed for an accurate representation of the target wave functions. The close-coupling expansion for the collision problem included 37 bound states of neutral magnesium. Angle-differential cross sections are presented for incident electron energies from 10 to 100 eV. These results, as well as the corresponding angle-integrated cross sections, are compared with various experimental data and predictions from other close-coupling and distorted-wave calculations. In spite of a few remaining discrepancies, the overall agreement between our results and the experimental data is very satisfactory.

We provide the quantum-mechanical description of the excitation of long-range surface-plasmon polaritons (LRSPPs) on thin metallic strips. The excitation process consists of an attenuated-reflection setup, where efficient photon-to-LRSPP wave-packet transfer is shown to be achievable. For calculating the coupling, we derive the first quantization of LRSPPs in the polaritonic regime. We study quantum statistics during propagation and characterize the performance of photon-to-LRSPP quantum state transfer for single-photons, photon-number states, and photonic coherent superposition states.

Photon bunching effect and stimulated emission of single photons is investigated and observed in the parametric amplification process. It is found that the two-photon interference is responsible for the photon bunching effect and stimulated emission, which is discussed in the temporal degree of freedom and in the presence of photon losses. Moreover, the direct measurement of the indistinguishability of two signal photons without any filtering is accomplished via the correlation between the two idler photons from two cascaded parametric down-conversion processes. This result shows that photon indistinguishability plays an essential role in stimulated emission.

We demonstrate the capability of complete thresholdless lasing operation between dressed states of a two-level atom located inside a microscopic cavity engineered in a photonic band-gap material. We distinguish between threshold and thresholdless behaviors by analyzing Mandel’s $Q$ parameter for the cavity field. We find that the threshold behavior depends on whether the spontaneous emission is or is not present on the lasing transition. In the presence of the spontaneous emission, the mean photon number of the cavity field exhibits threshold behavior, indicating that the system may operate as an ordinary laser. When the spontaneous emission is eliminated on the lasing transition, no threshold is observed for all values of the pumping rate, indicating the system becomes a thresholdless laser. Moreover, we find that, under a thresholdless operation, the mean photon number can increase nonlinearly with the pumping rate, and this process is accompanied by a sub-Poissonian statistics of the field. This suggests that the nonclassical statistics can be used to distinguish a nonlinear operation of the dressed-atom laser.

Motivated by recent experiments, which demonstrated lasing and cooling of the electromagnetic field in an electrical resonator coupled to a superconducting qubit, we study the phase coherence and diffusion of the system in the lasing state. We also discuss phase locking and synchronization induced by an additional ac driving of the resonator. We extend earlier work to account for the strong qubit-resonator coupling and to include the effects of low-frequency qubit’s noise. We show that the strong coupling may lead to a double-peak structure of the spectrum, while the shape and width are determined to the low-frequency noise.

We demonstrate sympathetic cooling of a ${{}^{43}\text{C}\text{a}}^{+}$ trapped-ion “memory” qubit by a ${{}^{40}\text{C}\text{a}}^{+}$ “coolant” ion sufficiently near the ground state of motion for fault-tolerant quantum logic, while maintaining coherence of the qubit. This is an essential ingredient in trapped-ion quantum computers. The isotope shifts are sufficient to suppress decoherence and phase shifts of the memory qubit due to the cooling light which illuminates both ions. We measure the qubit coherence during ten cycles of sideband cooling, finding a coherence loss of 3.3% per cooling cycle. The natural limit of the method is $O\left({10}^{-4}\right)$ infidelity per cooling cycle.

We discuss the concept of generalized concatenated quantum codes. This generalized concatenation method provides a systematical way for constructing good quantum codes, both stabilizer codes and nonadditive codes. Using this method, we construct families of single-error-correcting nonadditive quantum codes, in both binary and nonbinary cases, which not only outperform any stabilizer codes for finite block length but also asymptotically meet the quantum Hamming bound for large block length.

Conditions for the validity of the quantum adiabatic approximation are analyzed. For the case of linear Hamiltonians, a simple and general sufficient condition is derived, which is valid for arbitrary spectra and any kind of time variation. It is shown that in some cases the found condition is necessary and sufficient. The adiabatic theorem is generalized for the case of nonlinear Hamiltonians.

Physical systems must fulfill a number of conditions to qualify as useful quantum bits (qubits) for quantum-information processing, including ease of manipulation, long decoherence times, and high fidelity readout operations. Since these conditions are hard to satisfy with a single system, it may be necessary to combine different degrees of freedom. Here we discuss a possible system based on electronic and nuclear spin degrees of freedom in trapped ions. The nuclear spin yields long decoherence times, while the electronic spin, in a magnetic field gradient, provides efficient manipulation, and the optical transitions of the ions assure a selective and efficient initialization and readout.

We study a natural generalization of the additivity problem in quantum information theory: given a pair of quantum channels, then what is the set of convex trace functions that attain their maximum on unentangled inputs if they are applied to the corresponding output state? We prove several results on the structure of the set of those convex functions that are “additive” in this more general sense. In particular, we show that all operator convex functions are additive for the Werner-Holevo channel in $3\times 3$ dimensions, which contains the well-known additivity results for this channel as special cases.

We study the two-qubit entanglement preservation for a system in the presence of independent thermal baths. We use a combination of filtering operations and distillation protocols as a series of frequent measurements on the system. It is shown that a small fraction of the total amount of available copies of the system preserves or even improves its initial entanglement during the evolution.

Size-selected cationic transition-metal-doped silicon clusters have been studied with x-ray absorption spectroscopy at the transition-metal $L_{2,3}$ edges to investigate the local electronic structure of the dopant atoms. For ${\text{VSi}}_{16}^{+}$, the x-ray absorption spectrum is dominated by sharp transitions which directly reveal the formation of a highly symmetric silicon cage around the vanadium atom. In spite of their different number of valence electrons, a nearly identical local electronic structure is found for the dopant atoms in ${\text{TiSi}}_{16}^{+}$, ${\text{VSi}}_{16}^{+}$, and ${\text{CrSi}}_{16}^{+}$. This indicates strongly interlinked electronic and geometric properties: while the transition-metal atom imposes a geometric rearrangement on the silicon cluster, the interaction with the highly symmetric silicon cage determines the local electronic structure of the transition-metal dopant.

We study the effect of the induced interaction on the superfluid transition temperature of a Fermi gas with a Bose-Einstein condensation–Bardeen-Cooper-Schrieffer (BEC-BCS) crossover. The Gorkov-Melik-Barkhudarov theory about the induced interaction is extended from the BCS side to the entire crossover and the pairing fluctuation is treated in the approach by Nozières and Schmitt-Rink. At unitarity, the induced interaction reduces the transition temperature by about 20%. In the BCS limit, the transition temperature is reduced by a factor of about 2.22, as found by Gorkov and Melik-Barkhudarov. Our result shows that the effect of the induced interaction is important both on the BCS side and in the unitary region.

The intrinsic higher-order correlation of intensities which gives a measure of “pure” correlations among photons (corresponding to multiphoton bunching) is investigated with regard to ghost imaging with thermal light. The synchronous detection of the same light field by all reference detectors, which is a necessary condition for achieving an $N\text{th}$-order ghost image based on $N$-photon bunching, is discussed. Furthermore, it is found that the enhanced high visibility of $N\text{th}$-order ghost imaging is a consequence of the contribution of $N$-photon bunching, which is not a small value but is equal to the sum of all contributions from $\left(N-1\right)$-photon bunching. These results differ from those obtained by certain other groups.

We propose a scheme for implementing a partial general quantum cloning machine with superconducting quantum-interference devices coupled to a nonresonant cavity. By regulating the time parameters, our system can perform optimal symmetric (asymmetric) universal quantum cloning, optimal symmetric (asymmetric) phase-covariant cloning, and optimal symmetric economical phase-covariant cloning. In the scheme the cavity is only virtually excited, thus, the cavity decay is suppressed during the cloning operations.

We experimentally demonstrate a programmable quantum gate that applies a sign flip operation to data qubit in an arbitrary basis fully specified by the quantum state of a two-qubit program register. Our linear-optical implementation is inspired by teleportation-based schemes for quantum computing and relies on multiphoton interference and coincidence detection. We confirm the programmability of the gate and its high-fidelity performance by carrying out full quantum process tomography of the resulting operation on data qubit for several different programs.

The so-called fish-eye model, originally investigated by Maxwell in geometrical optics, is studied both in the classical as well as in the quantum formulations. The best agreement between the two approaches is achieved by using a suitably constructed coherent state, which is of the SU(2) type. The perfect quantum-classical correspondence is obtained in the sense that classical rays go exactly over maxima of the corresponding quantum probability distributions. The distributions are made of linear combinations of the $E=0$ bound states of the considered model.