The Lueders Postulate and the Distinguishability of Observables

The Lueders postulate is reviewed and implications for the distinguishability of observables are discussed. As an example the distinguishability of two similar observables for spin-1/2 particles is described. Implementation issues are briefly analyzed.Comment: Submitted to the proceedings of ICFNCS, Hong Kong, 200

A Test of CPT Symmetry in K^0 vs \bar{K}^0 to \pi^+\pi^-\pi^0 Decays

I show that the CP-violating asymmetry in K^0 vs \bar{K}^0 \to \pi^+\pi^-\pi^0 decays differs from that in K_L \to \pi^+\pi^-, K_L \to \pi^0\pi^0 or the semileptonic K_L transitions, if there exists CPT violation in K^0-\bar{K}^0 mixing. A delicate measurement of this difference at a super flavor factory (e.g., the \phi factory) will provide us with a robust test of CPT symmetry in the neutral kaon system.Comment: 4 pages, 1 figure. To appear in the Proceedings of the International PHIPSI09 Workshop, October 2009, Beijing, Chin

Chains of Quasi-Classical Informations for Bipartite Correlations and the Role of Twin Observables

Having the quantum correlations in a general bipartite state in mind, the information accessible by simultaneous measurement on both subsystems is shown never to exceed the information accessible by measurement on one subsystem, which, in turn is proved not to exceed the von Neumann mutual information. A particular pair of (opposite- subsystem) observables are shown to be responsible both for the amount of quasi-classical correlations and for that of the purely quantum entanglement in the pure-state case: the former via simultaneous subsystem measurements, and the latter through the entropy of coherence or of incompatibility, which is defined for the general case. The observables at issue are so-called twin observables. A general definition of the latter is given in terms of their detailed properties.Comment: 7 pages, Latex2e, selected for the December 2002 issue of the Virtual Journal of Quantum Informatio

Passage-time distributions from a spin-boson detector model

The passage-time distribution for a spread-out quantum particle to traverse a specific region is calculated using a detailed quantum model for the detector involved. That model, developed and investigated in earlier works, is based on the detected particle's enhancement of the coupling between a collection of spins (in a metastable state) and their environment. We treat the continuum limit of the model, under the assumption of the Markov property, and calculate the particle state immediately after the first detection. An explicit example with 15 boson modes shows excellent agreement between the discrete model and the continuum limit. Analytical expressions for the passage-time distribution as well as numerical examples are presented. The precision of the measurement scheme is estimated and its optimization discussed. For slow particles, the precision goes like $E^{-3/4}$, which improves previous $E^{-1}$ estimates, obtained with a quantum clock model.Comment: 11 pages, 6 figures; minor changes, references corrected; accepted for publication in Phys. Rev.

Entangling atoms and ions in dissipative environments

Peer reviewe

Enhanced magnetic moment and conductive behavior in NiFe2O4 spinel ultrathin films

Bulk NiFe2O4 is an insulating ferrimagnet. Here, we report on the epitaxial growth of spinel NiFe2O4 ultrathin films onto SrTiO3 single-crystals. We will show that - under appropriate growth conditions - epitaxial stabilization leads to the formation of a spinel phase with magnetic and electrical properties that radically differ from those of the bulk material : an enhanced magnetic moment (Ms) - about 250% larger - and a metallic character. A systematic study of the thickness dependence of Ms allows to conclude that its enhanced value is due to an anomalous distribution of the Fe and Ni cations among the A and B sites of the spinel structure resulting from the off-equilibrium growth conditions and to interface effects. The relevance of these findings for spinel- and, more generally, oxide-based heterostructures is discussed. We will argue that this novel material could be an alternative ferromagetic-metallic electrode in magnetic tunnel junctions.Comment: accepted for publication in Phys. Rev.

Quantum Nondemolition Monitoring of Universal Quantum Computers

The halt scheme for quantum Turing machines, originally proposed by Deutsch, is reformulated precisely and is proved to work without spoiling the computation. The ``conflict'' pointed out recently by Myers in the definition of a universal quantum computer is shown to be only apparent. In the context of quantum nondemolition (QND) measurement, it is also shown that the output observable, an observable representing the output of the computation, is a QND observable and that the halt scheme is equivalent to the QND monitoring of the output observable.Comment: 5 pages, RevTeX, no figures, revised, to appear in Phys. Rev. Let

Charge Modulation at the Surface of High-T_c Superconductors

It is shown here that surfaces of high-temperature superconductors are covered by dipole layers. The charge density modulation is induced by the local suppression of the gap function at the surface. This effect is studied in the framework of the Ginzburg-Landau theory and crucially depends on the appropriate boundary conditions. Those are derived from Gor'kov's equations for a d-wave pairing symmetry. Within this framework the structure of the surface dipole layer is determined. The contribution of this charging to a lens-effect of superconducting films with holes, which has been studied in recent experiments, is discussed.Comment: 10 pages, RevTeX, 5 postscript figure

Supersymmetry and Lorentz Violation

Supersymmetric field theories can be constructed that violate Lorentz and CPT symmetry. We illustrate this with some simple examples related to the original Wess-Zumino model.Comment: 4 page

Fixed points of commutative L\"uders operations

This paper verifies a conjecture posed in a pair of papers on the fixed point sets for a class of quantum operations. Specifically, it is proved that if a quantum operation has mutually commuting operation elements that are effects forming a resolution of the identity, then the fixed points set of the quantum operation is exactly the commutant of the operation elements