162 research outputs found
Decoherence and Measurement in Open Quantum Systems
We review results of a recently developed model of a microscopic quantum
system interacting with the macroscopic world components which are modeled by
collections of bosonic modes. The interaction is via a general operator
of the system, coupled to the creation and annihilation operators of
the environment modes. We assume that in the process of a nearly instantaneous
quantum measurement, the function of the environment involves two distinct
parts: the pointer and the bath. Interaction of the system with the bath leads
to decoherence such that the system and the pointer both evolve into a
statistical mixture state described by the density matrix such that the system
is in one of the eigenstates of with the correct quantum mechanical
probability, whereas the expectation values of pointer operators retain
amplified information on that eigenstate. We argue that this process represents
the initial step of a quantum measurement. Calculation of the elements of the
reduced density matrix of the system and pointer is carried out exactly, and
time dependence of decoherence is identified. We discuss general implications
of our model of energy-conserving coupling to a heat bath for processes of
adiabatic quantum decoherence. We also evaluate changes in the expectation
values of certain pointer operators and suggest that these can be interpreted
as macroscopic indicators of the measurement outcome.Comment: 12 pages in LaTeX, requires the spie.sty style fil
Chirality waves in two-dimensional magnets
We theoretically show that moderate interaction between electrons confined to
move in a plane and localized magnetic moments leads to formation of a
noncoplanar magnetic state. The state is similar to the skyrmion crystal
recently observed in cubic systems with the Dzyaloshinskii-Moriya interaction;
however, it does not require spin-orbit interaction. The non-coplanar magnetism
is accompanied by the ground-state electrical and spin currents, generated via
the real-space Berry phase mechanism. We examine the stability of the state
with respect to lattice discreteness effects and the magnitude of magnetic
exchange interaction. The state can be realized in a number of transition metal
and magnetic semiconductor systems
Coherent Interaction of Spins Induced by Thermal Bosonic Environment
We obtain and analyze the indirect exchange interaction between two two-state
systems, e.g., spins, in a formulation that also incorporates the quantum noise
that they experience, due to a bosonic environment, for instance, phonons. We
utilize a perturbative approach to obtain a quantum evolution equation for the
two-spin dynamics. A non-perturbative approach is used to study the onset of
the induced interaction, which is calculated exactly. We predict that for low
enough temperatures the interaction is coherent over time scales sufficient to
create entanglement, dominated by the zero-point quantum fluctuations of the
environment. We identify the time scales for which the spins develop
entanglement for various spatial separations.Comment: 10 pages, 3 figures; typos correcte
- …