The temporal evolution of two coupled cavities, each containing a single three-level atom, is studied when the cavities exchange two coherent photons. The general state of the system is a linear superposition of symmetric and antisymmetric states with the symmetric states controlled by two of the four eigenfrequencies and the antisymmetric states by the other two. The system undergoes Rabi oscillations between the two symmetric (antisymmetric) states. There is state transfer between the cavities when both atoms are in the ground state and two photons are exchanged. In addition, there is also Rabi “flopping” whereby one atom is in the excited state and the other in the ground state and the roles are reversed in a periodic fashion by the exchange of two photons. The generation of entanglement can be explicitly given as a function of time. Models of coupled cavities are of interest in distributed quantum information and computation.

A study of the effects of absorption on the nonlinear process of parametric down-conversion is presented. Absorption within the nonlinear medium is accounted for by employing the framework of macroscopic QED and the Green tensor quantization of the electromagnetic field. An effective interaction Hamiltonian, which describes the nonlinear interaction of the electric field and the linear noise polarization field, is used to derive the quantum state of the light leaving a nonlinear crystal. The signal and idler modes of this quantum state are found to be a superposition of the electric and noise polarization fields. Using this state, the expression for the coincident count rates for both type-I and type-II down-conversion are found. The nonlinear interaction with the noise polarization field was shown to cause an increase in the rate on the order of ${10}^{-12}$ for absorption of 10% per centimeter. This astonishingly small effect is found to be negligible compared to the decay caused by linear absorption of the propagating modes. From the expressions for the biphoton amplitude, it can be seen that the maximally entangled states can still be produced even in the presence of strong absorption.

The universal transpose of quantum states is an antiunitary transformation that is not allowed in quantum theory. In this work, we investigate approximating the universal transpose of quantum states of two-level systems (qubits) using the method known as the structural physical approximation to positive maps. We also report its experimental implementation in linear optics. The scheme is optimal in that the maximal fidelity is attained and also practical as measurement and preparation of quantum states that are experimentally feasible within current technologies are solely applied.

Large-scale quantum computation will only be achieved if experimentally implementable quantum error correction procedures are devised that can tolerate experimentally achievable error rates. We describe an improved decoding algorithm for the Kitaev surface code, which requires only a two-dimensional square lattice of qubits that can interact with their nearest neighbors, that raises the tolerable quantum gate error rate to over 1%. The precise maximum tolerable error rate depends on the error model, and we calculate values in the range 1.1–1.4% for various physically reasonable models. These values represent a very high threshold error rate calculated in a constrained setting.

Absolute rate coefficients for dielectronic recombination (DR) of H-like ${\mathrm{U}}^{91+}$ ions have been measured. The electron-ion merged-beam technique at a heavy-ion storage ring was employed using a stochastically cooled ion beam. Thereby, the previously accessible electron-ion collision energies could be greatly extended to the range 63–90 keV. High-resolution DR spectra were measured covering all $\mathit{KLL}$ and $\mathit{KLM}$ resonances. For the resonance strengths, excellent agreement between relativistic theory and experiment is found only if the Breit contribution to the electron-electron interaction is included in the calculations. For the $\mathit{KL}$${}_{1/2}$$L$${}_{1/2}$ and $\mathit{KL}$${}_{1/2}$$M$${}_{1/2}$ groups the Breit contribution amounts to 44% of their total resonance strengths.

Additional radio-frequency (rf) potentials applied to linear multipole traps create extra field nodes in the radial plane which allow one to confine single ions, or strings of ions, in totally rf field-free regions. The number of nodes depends on the order of the applied multipole potentials and their relative distance can be easily tuned by the amplitude variation of the applied voltages. Simulations using molecular dynamics show that strings of ions can be laser cooled down to the Doppler limit in all directions of space. Once cooled, organized systems can be moved with very limited heating, even if the cooling process is turned off.

We study an impurity atom in a two-dimensional Fermi gas using variational wave functions for (i) an impurity dressed by particle-hole excitations (polaron) and (ii) a dimer consisting of the impurity and a majority atom. In contrast to three dimensions, where similar calculations predict a sharp transition to a dimer state with increasing interspecies attraction, we show that the polaron Ansatz always gives a lower energy. However, the exact solution for a heavy impurity reveals that both a two-body bound state and distortions of the Fermi sea are crucial. This reflects the importance of particle-hole pairs in lower dimensions and makes simple variational calculations unreliable. We show that the energy of an impurity gives important information about its dressing cloud, for which both Ansätze give inaccurate results.

A forthcoming challenge in ultracold lattice gases is the simulation of quantum magnetism. That involves both the preparation of the lattice atomic gas in the desired spin state and the probing of the state. Here we demonstrate how a probing scheme based on atom-light interfaces gives access to the order parameters of nontrivial quantum magnetic phases, allowing us to characterize univocally strongly correlated magnetic systems produced in ultracold gases. This method, which is also nondemolishing, yields spatially resolved spin correlations and can be applied to bosons or fermions. As a proof of principle, we apply this method to detect the complete phase diagram displayed by a chain of (rotationally invariant) spin-1 bosons.

We derive a set of Bell-type inequalities for arbitrarily-high-dimensional systems based on the assumption of partial separability in the hybrid local-nonlocal hidden-variable model. Partially entangled states would not violate the inequalities, and thus, upon violation, these Bell-type inequalities are sufficient conditions to detect the full $N$-particle entanglement and invalidity of the hybrid local-nonlocal hidden-variable description.

To demonstrate the structure sensitivity of double inner-shell hole spectroscopy, we have measured energies of ${\mathrm{H}}_2$S${}^{2+}$, SO${{}_2}^{2+}$, and CS${{}_2}^{2+}$ with the two vacancies in the sulfur 2$p$ shell using a multielectron coincidence technique combined with synchrotron radiation. We describe how to extract intrinsic chemical information which is masked by the orbital relaxation effect in conventional core-level photoelectron spectroscopy.

We measured the double-to-single-ionization ratios $R$ of helium impacted by the intermediate velocity ${\mathrm{C}}^{4+}$ ion and found that $R$ decreases as 1/$v$. This type of velocity dependence is consistent with the data for very highly charged ions (N${}^{7+}$ ∼ ${\mathrm{U}}^{92+}$). The charge state and velocity dependences of $R$ are interpreted well by the classical over-barrier-ionization (COBI) model. It is found that the ratio $R$ can be written in a manner of $R=0.28\sqrt{q}/v$ for very high-$q$ projectiles in the strong coupling (q/v > 1) regime. This theoretical prediction is in excellent agreement with extensive experimental data.

The confined variational method is used to generate a set of energy-optimized explicitly correlated Gaussians to describe positrons interacting with H, He and the ${\mathrm{H}}_2$ molecule. These basis functions are then used in a Kohn variational calculation of the scattering lengths and zero-energy annihilation parameters, $Z_{\mathrm{eff}}$. The results for the H and He atoms are consistent with previous high-quality variational calculations while the annihilation parameter for ${\mathrm{H}}_2$ is consistent with experiment. An extensive tabulation of the ${\mathrm{H}}_2$ scattering parameters as a function of internuclear distance confirm the existence of a virtual state at $R\approx 3.4$$a_0$. The scattering length was $-$2.71$a_0$ and $Z_{\mathrm{eff}}$ was 15.7 at the mean ${\mathrm{H}}_2$ internuclear distance of 1.45$a_0$.

The bremsstrahlung produced by 17.5-keV electrons incident on a 2415 μg/cm${}^2$ Ag target has been measured at forward angles in the range 0° to 55° relative to the electron beam. The intensity of the thick-target bremsstrahlung produced is presented on an absolute scale at angles in this range, with corrections for detector efficiency, photon attenuation within the target itself, and other factors. After correction for photon attenuation within the target, the results indicate that bremsstrahlung is essentially isotropic. The experimental data are in approximate agreement with both the predictions of a modified version of the Kramers formula and the Monte Carlo program, penelope.

We give a detailed characterization of a recently developed method to prepare translationally cold, internally state-selected molecular ions in ion traps [X. Tong, A. H. Winney, and S. Willitsch, Phys. Rev. Lett. 105, 143001 (2010)]. The technique relies on the generation of molecular ions in a well-defined rotational-vibrational quantum state using threshold photoionization followed by sympathetic cooling of the translational motion with laser-cooled Ca${}^{+}$ ions. We discuss the experimental requirements for the successful generation and sympathetic cooling of state-selected ions, explore the influence of collisional and radiative processes on the population redistribution dynamics, and give an assessment of the scope of the method.

Noise correlations are studied for systems of hard-core bosons in one-dimensional lattices. We use an exact numerical approach based on the Bose-Fermi mapping and properties of Slater determinants. We focus on the scaling of the noise correlations with system size in superfluid and insulating phases, which are generated in the homogeneous lattice, with period-two superlattices and with uniformly distributed random diagonal disorder. For the superfluid phases, the leading contribution is shown to exhibit a density-independent scaling proportional to the system size, while the first subleading term exhibits a density-dependent power-law exponent.

The thermodynamic approach to density-functional theory (DFT) is used to derive a versatile theoretical framework for the treatment of finite-temperature (and in the limit, zero-temperature) Bose-Einstein condensates (BECs). The simplest application of this framework, using the overall density of bosons alone, would yield the DFT of Nunes [J. Phys. B 32, 4293 (1999)]. It is argued that a significant improvement in accuracy may be obtained by using additional density fields: the condensate amplitude and the anomalous density. Thus, two advanced schemes are suggested, one corresponding to a generalized two-fluid model of condensate systems, and another scheme, which explicitly accounts for anomalous density contributions and anomalous effective potentials. The latter reduces to the Hartree-Fock-Bogoliubov approach in the limit of weak interactions. For stronger interactions, a local-density approximation is suggested, but its implementation requires accurate data for the thermodynamic properties of uniform interacting BEC systems, including fictitious perturbed states of such systems. Provided that such data become available (e.g., from quantum Monte Carlo computation), DFT can be used to obtain high-accuracy theoretical results for the equilibrium states of BECs of various geometries and external potentials.

I give the closed-form analytic expression for the cooperative Lamb shift (CLS) in the scalar photon theory for an ensemble of two-level atoms in the shape of a slab located between two parallel-plate perfect conductors. I compute this quantity for the cases when the atomic system is initially prepared in a phased state and in the uniform state.

In attosecond streaking, an electron is released by a short xuv pulse into a strong near infrared laser field. If the laser couples two states in the target, the streaking technique, which allows for a complete determination of the driving field, also gives an accurate measurement of the relative phase of the atomic or molecular ionization matrix elements from the two states through the interference from the two channels. The interference may change the phase of the photoelectron streaking signal within the envelope of the infrared field, an effect to be accounted for when reconstructing short pulses from the photoelectron signal and in attosecond time-resolved measurements.

The bipartite and tripartite entanglement of a 3-qubit fermionic system when one or two subsystems accelerate are investigated. It is shown that all the one-tangles decrease as the acceleration increases. However, unlike the scalar case, here one-tangles ${\mathcal{N}}_{C_I({\mathrm{AB}}_I)}$ and ${\mathcal{N}}_{C_I(\mathrm{AB})}$ never reduce to zero for any acceleration. It is found that the system has only tripartite entanglement when either one or two subsystems accelerate, which means that the acceleration does not generate bipartite entanglement and does not affect the entanglement structure of the quantum states in this system. It is of interest to note that the $\pi$-tangle of the two-observer-accelerated case decreases much quicker than that of the one-observer-accelerated case and it reduces to a nonzero minimum in the infinite-acceleration limit. Thus we argue that the qutrit systems are better than qubit systems in performing quantum information processing tasks in noninertial systems.

Entangled states of two ions are realized by using an adiabatic process. Based on the proposal by Linington and Vitanov [Phys. Rev. A 77, 010302 (2008)], we have generated Dicke states in optical qubits of two ${}^{40}\mathrm{Ca}$${}^{+}$ ions by applying frequency-chirped optical pulses with time-dependent envelopes to perform rapid adiabatic passage on sideband transitions. One of the biggest advantages of adiabatic approaches is their robustness against variations in experimental parameters, which is verified by performing experiments for different pulse widths or peak Rabi frequencies. Fidelities exceeding 0.5, which is the threshold for inseparable states, are obtained over wide ranges of parameter values.