Ultrafast all-optical shutter based on two-photon absorption

An ultrafast all-optical shutter is presented, based on a simple two-color, two-photon absorption technique. For time-resolved luminescence measurements, this shutter is an interesting alternative to the optical Kerr gate. The rejection efficiency is 99%; the switching-off and switching-on speeds are limited by the pulse length only; the rejection time is determined by the crystal slab thickness; and the bandwidth spans the entire visible spectrum. We show that our shutter can also be used for accurate measurement of group velocity inside a transparent material.     

Optimal teleportation with a mixed state of two qubits

We consider a single copy of a mixed state of two qubits and derive the optimal trace-preserving local operations assisted by classical communication such as to maximize the fidelity of teleportation that can be achieved with this state. These optimal local operations turn out to be implementable by one-way communication and always yield a teleportation fidelity larger than 2/3 if the original state is entangled. This maximal achievable fidelity is an entanglement measure and turns out quantifying the minimal amount of mixing required to destroy the entanglement in a quantum state.     

General analysis ofn-loop corrections in the 2PPI variational approach to λφ4

We present the general analysis of then-loop contribution in a new variational approach to quantum field theory, applied to the scalar model λφ⁴, which is based on the use of an effective action for the local composite operator \(\hat \phi ^2 \) . In a previous paper [1] the three-loop analysis was performed and it was found that the differences with the one loop calculation were very small. We have now generalised this argument for any arbitrary but finite loop calculation. In particular, we examine stability of the precarious phase.     

A modified Poisson-Boltzmann model including charge regulation for the adsorption of ionizable polyelectrolytes to charged interfaces, applied to lysozyme adsorption on silica

The equilibrium adsorption of polyelectrolytes with multiple types of ionizable groups is described using a modified Poisson-Boltzmann equation including charge regulation of both the polymer and the interface. A one-dimensional mean-field model is used in which the electrostatic potential is assumed constant in the lateral direction parallel to the surface. The electrostatic potential and ionization degrees of the different ionizable groups are calculated as function of the distance from the surface after which the electric and chemical contributions to the free energy are obtained. The various interactions between small ions, surface and polyelectrolyte are self-consistently considered in the model, such as the increase in charge of polyelectrolyte and surface upon adsorption as well as the displacement of small ions and the decrease of permittivity. These interactions may lead to complex dependencies of the adsorbed amount of polyelectrolyte on pH, ionic strength, and properties of the polymer (volume, permittivity, number, and type of ionizable groups) and of the surface (number of ionizable groups, pK, Stern capacity). For the adsorption of lysozyme on silica, the model qualitatively describes the gradual increase of adsorbed amount with pH up to a maximum value at pHc, which is below the iso-electric point, as well as the sharp decrease of adsorbed amount beyond pHc. With increasing ionic strength the adsorbed amount decreases (for pH > pHc), and pHc shifts to lower values.     

Comparison of nearly free electron and tight binding limits in one-dimensional solids

We introduce a new one-dimensional crystal model that interpolates smoothly between the (nearly) free electron and δ-well models used before. In those two limits the energy band structure can be determined analytically. In the general case an efficient numerical scheme is established. We investigate the effects of truncating the infinite determinant arising in this context and compare our results with previous work. For a sufficiently strong potential the lowest lying energy levels are very narrow. The transition to this tight binding limit is discussed.     

A technique for generating Feynman diagrams

We present a simple technique that allows to generate Feynman diagrams for vector models with interactions of order2n and similar models (Gross-Neveu, Thirring model) using a bootstrap equation that uses only the free field value of the energy as an input. The method allows to find the diagrams to, in principle, arbitrarily high order and applies to both energy and correlation functions. It automatically generates the correct symmetry factor (as a function of the number of components of the field) and the correct sign for any diagram in the case of fermion loops. We briefly discuss the possibility of treating QED as a Thirring model with non-local interaction.     

Non-perturbative calculation of the mass-gap in the Gross-Neveu model

We calculate a fully renormalizable effective action for local composite operators in the U(N) Gross-Neveu model, recently introduced by one of us, to two loop order. We obtain results for the massgap in one and two loops which are optimized using the Grunberg method of effective charges. At one loop the accuracy is of the order of 2% or less (except for N = 2). At two loops the accuracy is significantly improved for N ≥ 5.     

Soliton quantization in gauge theories

We present a simple quantization method for solitons in gauge theories based on a straight-forward application of pathintegral techniques. There is no zero mode problem and the intricacies of the canonical approach (the Dirac theory of constrained systems) are avoided.