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We propose and experimentally demonstrate a near-optimal discrimination scheme for the quadrature phase shift keying (QPSK) protocol. We show in theory that the performance of our hybrid scheme is superior to the standard scheme-heterodyne detection-for all signal amplitudes and underpin the predictions with our experimental results. Furthermore, our scheme provides hitherto the best performance in the domain of highly attenuated signals. The discrimination is composed of a quadrature measurement, a conditional displacement and a threshold detector. [ABSTRACT FROM AUTHOR]

The one-body density matrix of weakly interacting condensed bosons in external potentials is calculated using inhomogeneous Bogoliubov theory. We determine the condensate deformation caused by weak external potentials at the mean-field level. The momentum distribution of quantum fluctuations around the deformed ground state is obtained analytically and finally, the resulting quantum depletion is calculated. The depletion due to the external potential, or potential depletion for short, is a small correction to the homogeneous depletion, validating our inhomogeneous Bogoliubov theory. Analytical results are derived for weak lattices and spatially correlated random potentials, with simple, universal results in the Thomas-Fermi limit of very smooth potentials. [ABSTRACT FROM AUTHOR]

Two aspects of bright matter-wave solitons in weak external potentials are discussed. First, we briefly review recent results on the Anderson localization of an entire soliton in disordered potentials, as a paradigmatic showcase of genuine quantum dynamics beyond simple perturbation theory. Second, we calculate the linear response of the mean-field soliton shape to a weak, but otherwise arbitrary, external potential, with a detailed application to lattice potentials. [ABSTRACT FROM AUTHOR]

We use high-precision spectroscopy and detailed theoretical modeling to determine the form of the coupling between a superconducting phase qubit and a two-level defect. Fitting the experimental data with our theoretical model allows us to determine all relevant system parameters. We observe a strong qubit-defect coupling with a nearly vanishing longitudinal component. We quantitatively compare several existing theoretical models for the microscopic origin of two-level defects. [ABSTRACT FROM AUTHOR]

A novel compact, monolithic memory device based on Y-branched junctions with embedded quantum dots is demonstrated. Up sweeps and down sweeps of the voltage difference between the branches lead to a pronounced threshold hysteresis exceeding 0.5 V. It is concluded that the charge state of the quantum dots and thus the hysteresis is also controlled by the self-gating in an electron Y-branch switch. [ABSTRACT FROM AUTHOR]

We suggest a scheme to implement a universal set of non-Abelian geometric transformations for a single logical qubit composed of three superconducting qubits coupled to a single cavity. The scheme utilizes an adiabatic evolution in a rotating frame induced by the effective tripod Hamiltonian which is achieved by longitudinal driving of the qubits. The proposal is experimentally feasible with the current state of the art and could serve as a first proof of principle for geometric quantum computing. [ABSTRACT FROM AUTHOR]

Different growth techniques and growth strategies are presented to climb up the hierarchy of order in the field of self-assembled semiconductor nanostructures. In a first step we report a significant improvement of the nanostructure size homogeneity by using either a repetitive desorption and regrowth procedure or by applying extremely low growth rates at high growth temperatures. With this approach an InAs/GaAs quantum dot (QD) ensemble with a height distribution of ±5% and a final photoluminescence (PL) peak line width of 19 meV at room temperature was fabricated. After capping the low growth rate QDs with GaAs, well-developed rhombus-shaped structures with holes in their center are observed. The PL of closely stacked InAs QDs exhibits a line width of 16 meV at low temperature. Remarkable lateral alignment into square arrays of self-assembled SiGe islands with a pronounced size and shape homogeneity is achieved by deposition near their thermodynamic equilibrium using liquid phase epitaxy. The combination of self-assembly with conventional pre-patterning leads to long-range lateral order of In(Ga)As QDs on GaAs(0 0 1). A three-dimensional crystal is fabricated by stacking multiple layers and vertically aligned In(Ga)As QDs onto a pre-patterned GaAs(0 0 1) substrate. The structure shows good PL properties at room temperature. [Copyright &y& Elsevier]