Ground States for Exponential Random Graphs

We propose a perturbative method to estimate the normalization constant in exponential random graph models as the weighting parameters approach infinity. As an application, we give evidence of discontinuity in natural parametrization along the critical directions of the edge-triangle model.Comment: 12 pages, 3 figures, 1 tabl

On Size and Shape of the Average Meson Fields in the Semibosonized Nambu & Jona-Lasinio Model

We consider a two-flavor Nambu \& Jona-Lasinio model in Hartree approximation involving scalar-isoscalar and pseudoscalar-isovector quark-quark interactions. Average meson fields are defined by minimizing the effective Euklidean action. The fermionic part of the action, which contains the full Dirac sea, is regularized within Schwinger's proper-time scheme. The meson fields are restricted to the chiral circle and to hedgehog configurations. The only parameter of the model is the constituent quark mass $M$ which simultaneously controls the regularization. We evaluate meson and quark fields self-consistently in dependence on the constituent quark mass. It is shown that the self-consistent fields do practically not depend on the constituent quark mass. This allows us to define a properly parameterized reference field which for physically relevant constituent masses can be used as a good approximation to the exactly calculated one. The reference field is chosen to have correct behaviour for small and large radii. To test the agreement between self-consistent and reference fields we calculate several observables like nucleon energy, mean square radius, axial-vector constant and delta-nucleon mass splitting in dependence on the constituent quark mass. The agreement is found to be very well. Figures available on request.Comment: 12 pages (LATEX), 3 figures available on request, report FZR 93-1

Regulation of Membrane Targeting of the G Protein-coupled Receptor Kinase 2 by Protein Kinase A and Its Anchoring Protein AKAP79

The beta 2 adrenergic receptor (beta 2AR) undergoes desensitization by a process involving its phosphorylation by both protein kinase A (PKA) and G protein-coupled receptor kinases (GRKs). The protein kinase A-anchoring protein AKAP79 influences beta 2AR phosphorylation by complexing PKA with the receptor at the membrane. Here we show that AKAP79 also regulates the ability of GRK2 to phosphorylate agonist-occupied receptors. In human embryonic kidney 293 cells, overexpression of AKAP79 enhances agonist-induced phosphorylation of both the beta 2AR and a mutant of the receptor that cannot be phosphorylated by PKA (beta 2AR/PKA-). Mutants of AKAP79 that do not bind PKA or target to the beta 2AR markedly inhibit phosphorylation of beta 2AR/PKA-. We show that PKA directly phosphorylates GRK2 on serine 685. This modification increases Gbeta gamma subunit binding to GRK2 and thus enhances the ability of the kinase to translocate to the membrane and phosphorylate the receptor. Abrogation of the phosphorylation of serine 685 on GRK2 by mutagenesis (S685A) or by expression of a dominant negative AKAP79 mutant reduces GRK2-mediated translocation to beta 2AR and phosphorylation of agonist-occupied beta 2AR, thus reducing subsequent receptor internalization. Agonist-stimulated PKA-mediated phosphorylation of GRK2 may represent a mechanism for enhancing receptor phosphorylation and desensitization

Quantum computers based on electron spins controlled by ultra-fast, off-resonant, single optical pulses

We describe a fast quantum computer based on optically controlled electron spins in charged quantum dots that are coupled to microcavities. This scheme uses broad-band optical pulses to rotate electron spins and provide the clock signal to the system. Non-local two-qubit gates are performed by phase shifts induced by electron spins on laser pulses propagating along a shared waveguide. Numerical simulations of this scheme demonstrate high-fidelity single-qubit and two-qubit gates with operation times comparable to the inverse Zeeman frequency.Comment: 4 pages, 4 figures, introduction is clarified, the section on two-qubit gates was expanded and much more detail about gate fidelities is given, figures were modified, one figure replaced with a figure showing gate fidelities for relevant parameter