499 research outputs found
Time-Energy coherent states and adiabatic scattering
Coherent states in the time-energy plane provide a natural basis to study
adiabatic scattering. We relate the (diagonal) matrix elements of the
scattering matrix in this basis with the frozen on-shell scattering data. We
describe an exactly solvable model, and show that the error in the frozen data
cannot be estimated by the Wigner time delay alone. We introduce the notion of
energy shift, a conjugate of Wigner time delay, and show that for incoming
state the energy shift determines the outgoing state.Comment: 11 pages, 1 figur
Transport and Dissipation in Quantum Pumps
This paper is about adiabatic transport in quantum pumps. The notion of
``energy shift'', a self-adjoint operator dual to the Wigner time delay, plays
a role in our approach: It determines the current, the dissipation, the noise
and the entropy currents in quantum pumps. We discuss the geometric and
topological content of adiabatic transport and show that the mechanism of
Thouless and Niu for quantized transport via Chern numbers cannot be realized
in quantum pumps where Chern numbers necessarily vanish.Comment: 31 pages, 10 figure
Smooth adiabatic evolutions with leaky power tails
Adiabatic evolutions with a gap condition have, under a range of
circumstances, exponentially small tails that describe the leaking out of the
spectral subspace. Adiabatic evolutions without a gap condition do not seem to
have this feature in general. This is a known fact for eigenvalue crossing. We
show that this is also the case for eigenvalues at the threshold of the
continuous spectrum by considering the Friedrichs model.Comment: Final form, to appear in J. Phys. A; 11 pages, no figure
Distilling entanglement from cascades with partial "Which Path" ambiguity
We develop a framework to calculate the density matrix of a pair of photons
emitted in a decay cascade with partial "which path" ambiguity. We describe an
appropriate entanglement distillation scheme which works also for certain
random cascades. The qualitative features of the distilled entanglement are
presented in a two dimensional "phase diagram". The theory is applied to the
quantum tomography of the decay cascade of a biexciton in a semiconductor
quantum dot. Agreement with experiment is obtained
One-Dimensional Discrete Stark Hamiltonian and Resonance Scattering by Impurities
A one-dimensional discrete Stark Hamiltonian with a continuous electric field
is constructed by extension theory methods. In absence of the impurities the
model is proved to be exactly solvable, the spectrum is shown to be simple,
continuous, filling the real axis; the eigenfunctions, the resolvent and the
spectral measure are constructed explicitly. For this (unperturbed) system the
resonance spectrum is shown to be empty. The model considering impurity in a
single node is also constructed using the operator extension theory methods.
The spectral analysis is performed and the dispersion equation for the
resolvent singularities is obtained. The resonance spectrum is shown to contain
infinite discrete set of resonances. One-to-one correspondence of the
constructed Hamiltonian to some Lee-Friedrichs model is established.Comment: 20 pages, Latex, no figure
Quantum Transport in Molecular Rings and Chains
We study charge transport driven by deformations in molecular rings and
chains. Level crossings and the associated Longuet-Higgins phase play a central
role in this theory. In molecular rings a vanishing cycle of shears pinching a
gap closure leads, generically, to diverging charge transport around the ring.
We call such behavior homeopathic. In an infinite chain such a cycle leads to
integral charge transport which is independent of the strength of deformation.
In the Jahn-Teller model of a planar molecular ring there is a distinguished
cycle in the space of uniform shears which keeps the molecule in its manifold
of ground states and pinches level crossing. The charge transport in this cycle
gives information on the derivative of the hopping amplitudes.Comment: Final version. 26 pages, 8 fig
A geometric theory of swimming: Purcell's swimmer and its symmetrized cousin
We develop a qualitative geometric approach to swimming at low Reynolds
number which avoids solving differential equations and uses instead landscape
figures of two notions of curvatures: The swimming curvature and the curvature
derived from dissipation. This approach gives complete information for swimmers
that swim on a line without rotations and gives the main qualitative features
for general swimmers that can also rotate. We illustrate this approach for a
symmetric version of Purcell's swimmer which we solve by elementary analytical
means within slender body theory. We then apply the theory to derive the basic
qualitative properties of Purcell's swimmer.Comment: 24 pages, 12 figure
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