The decoherence of quantum states defines the transition between the quantum world and classical physics. Decoherence or, correspondingly, quantum mechanical collapse events pose fundamental questions regarding the interpretation of quantum physics.
They are also technologically relevant because they limit the coherent information processing performed by quantum computers. We have discovered that this transition regime enables a novel type of matter transport. Applying this discovery, we present nanoscale devices in which random quantum collapse events produce fundamentally novel phenomena by interrupting the unitary dynamics of electron wave packets. For most of the time, however, the wave packets proceed in coherent superpositions. Geometrically asymmetric conductors with mesoscopic length scales act as rectifiers with unique properties. They function in linear response, so Onsagers reciprocity relations do not apply to transport of this kind. The interface between the quantum and the classical worlds therefore provides a novel transport regime of value for the realization of a new category of mesoscopic electronic devices. These devices provide functions that have been considered impossible until now.
We present the concept of nonreciprocal interferometers. These two-way devices let particles pass in both directions, but in one direction break the phase of the particles wave functions. Such filters can be realized by using, for example, asymmetric
quantum rings. Furthermore, we propose arrangements of these interferometers to obtain larger interferometers which are expected to exhibit a puzzling behavior that resembles Maxwell demon action. We indicate an opportunity to resolve this puzzle experimentally.
We approach multipartite entanglement classification in the symmetric subspace in terms of algebraic geometry, its natural language. We show that the class of symmetric separable states has the structure of a Veronese variety and that its $k$-secant
varieties are SLOCC invariants. Thus SLOCC classes gather naturally into families. This classification presents useful properties such as a linear growth of the number of families with the number of particles, and nesting, i.e. upward consistency of the classification. We attach physical meaning to this classification through the required interaction length of parent Hamiltonians. We show that the states $W_N$ and GHZ$_N$ are in the same secant family and that, effectively, the former can be obtained in a limit from the latter. This limit is understood in terms of tangents, leading to a refinement of the previous families. We compute explicitly the classification of symmetric states with $Nleq 4$ qubits in terms of both secant families and its refinement using tangents. This paves the way to further use of projective varieties in algebraic geometry to solve open problems in entanglement theory.
The two-mode quantum Rabi model with bilinear coupling is studied using extended squeezed states. We derive $G$-functions for each Bargmann index $q$% . They share a common structure with the $G$-function of the one-photon and two-photon quantum Rabi
models. The regular spectrum is given by zeros of the $G$-function while the conditions for the presence of doubly degenerate (exceptional) eigenvalues are obtained in closed form through the lifting property. The simple singularity structure of the $G$-function allows to draw conclusions about the distribution of eigenvalues along the real axis and to understand the spectral collapse phenomenon when the coupling reaches a critical value.
We report on the discovery of a novel triangular phase regime in the system La1-xSrxMnO3 by means of electron spin resonance and magnetic susceptibility measurements. This phase is characterized by the coexistence of ferromagnetic entities within the
globally paramagnetic phase far above the magnetic ordering temperature. The nature of this phase can be understood in terms of Griffiths singularities arising due to the presence of correlated quenched disorder in the orthorhombic phase.
In a recent paper (cond-mat/0009279), Fabricius and McCoy studied the spectrum of the spin 1/2 XXZ-model at Delta = (q+q^{-1})/2 and q^{2N}=1 for integer N >1. They found a certain pattern of degeneracies and linked it to the sl(2)-loop symmetry pres
ent in the commensurable spin sector (N divides S^z). We show that the degeneracies are due to zero-energy, transparent excitations, the cyclic bound states. These exist both in commensurable and incommensurable sectors, indicating a symmetry, of which sl(2)-loop is a partial manifestation. Our approach treats both sectors on even footing and yields an analytical expression for the degeneracies in the case N = 3.
