537 research outputs found
Quantum dynamics and transport in a double well system
The simplest one-dimensional model for the studying of non-trivial
geometrical effects is a ring shaped device which is formed by joining two
arms. We explore the possibility to model such a system as a two level system
(TLS). Of particular interest is the analysis of quantum stirring, where it is
not evident that the topology is properly reflected within the framework of the
TLS modeling. On the technical side we provide a practical "neighboring level"
approximation for the analysis of such quantum devices, which remains valid
even if the TLS modeling does not apply.Comment: 10 pages, 4 figures, version to be published in PR
Operating a quantum pump in a closed circuit
During an adiabatic pumping cycle a conventional two barrier quantum device
takes an electron from the left lead and ejects it to the right lead. Hence the
pumped charge per cycle is naively expected to be . This zero order
adiabatic point of view is in fact misleading. For a closed device we can get
and even . In this paper a detailed analysis of the
quantum pump operation is presented. Using the Kubo formula for the geometric
conductance, and applying the Dirac chains picture, we derive practical
estimates for~.Comment: 19 pages, 8 figs, minor textual corretions, to be published in JP
Quantum Stirring in low dimensional devices
A circulating current can be induced in the Fermi sea by displacing a
scatterer, or more generally by integrating a quantum pump into a closed
circuit. The induced current may have either the same or the opposite sense
with respect to the "pushing" direction of the pump. We work out explicit
expressions for the associated geometric conductance using the Kubo-Dirac
monopoles picture, and illuminate the connection with the theory of adiabatic
passage in multiple path geometry.Comment: 6 pages, 5 figures, improved versio
Optimal rewards in contests
We study all-pay contests under incomplete information where the reward is a function of the contestant's type and effort. We analyze the optimal reward for the designer when the reward is either multiplicatively separable or additively separable in effort and type. In the multiplicatively separable environment, the optimal reward is always positive while in the additively separable environment it may also be negative. In both environments, depending on the designer's utility, the optimal reward may either increase or decrease in the contestants' effort. Finally, in both environments, the designer's payoff depends only upon the expected value of the effort-dependent rewards and not the number of rewards.Leverhulme Foundation (Grant RF/7/2006/0325
Anomalous decay of a prepared state due to non-Ohmic coupling to the continuum
We study the decay of a prepared state into a continuum {E_k} in the
case of non-Ohmic models. This means that the coupling is with . We find that irrespective of model details
there is a universal generalized Wigner time that characterizes the
evolution of the survival probability . The generic decay behavior
which is implied by rate equation phenomenology is a slowing down stretched
exponential, reflecting the gradual resolution of the bandprofile. But
depending on non-universal features of the model a power-law decay might take
over: it is only for an Ohmic coupling to the continuum that we get a robust
exponential decay that is insensitive to the nature of the intra-continuum
couplings. The analysis highlights the co-existence of perturbative and
non-perturbative features in the dynamics. It turns out that there are special
circumstances in which is reflected in the spreading process and not only
in the survival probability, contrary to the naive linear response theory
expectation.Comment: 13 pages, 11 figure
Quantum anomalies and linear response theory
The analysis of diffusive energy spreading in quantized chaotic driven
systems, leads to a universal paradigm for the emergence of a quantum anomaly.
In the classical approximation a driven chaotic system exhibits stochastic-like
diffusion in energy space with a coefficient that is proportional to the
intensity of the driving. In the corresponding quantized problem
the coherent transitions are characterized by a generalized Wigner time
, and a self-generated (intrinsic) dephasing process leads to
non-linear dependence of on .Comment: 8 pages, 2 figures, textual improvements (as in published version
Non-adiabatic pumping in an oscillating-piston model
We consider the prototypical "piston pump" operating on a ring, where a
circulating current is induced by means of an AC driving. This can be regarded
as a generalized Fermi-Ulam model, incorporating a finite-height moving wall
(piston) and non trivial topology (ring). The amount of particles transported
per cycle is determined by a layered structure of phase-space. Each layer is
characterized by a different drift velocity. We discuss the differences
compared with the adiabatic and Boltzmann pictures, and highlight the
significance of the "diabatic" contribution that might lead to a
counter-stirring effect.Comment: 6 pages, 4 figures, improved versio
Dislocation-mediated melting of one-dimensional Rydberg crystals
We consider cold Rydberg atoms in a one-dimensional optical lattice in the
Mott regime with a single atom per site at zero temperature. An external laser
drive with Rabi frequency \Omega and laser detuning \Delta, creates Rydberg
excitations whose dynamics is governed by an effective spin-chain model with
(quasi) long-range interactions. This system possesses intrinsically a large
degree of frustration resulting in a ground-state phase diagram in the
(\Delta,\Omega) plane with a rich topology. As a function of \Delta, the
Rydberg blockade effect gives rise to a series of crystalline phases
commensurate with the optical lattice that form a so-called devil's staircase.
The Rabi frequency, \Omega, on the other hand, creates quantum fluctuations
that eventually lead to a quantum melting of the crystalline states. Upon
increasing \Omega, we find that generically a commensurate-incommensurate
transition to a floating Rydberg crystal occurs first, that supports gapless
phonon excitations. For even larger \Omega, dislocations within the floating
Rydberg crystal start to proliferate and a second,
Kosterlitz-Thouless-Nelson-Halperin-Young dislocation-mediated melting
transition finally destroys the crystalline arrangement of Rydberg excitations.
This latter melting transition is generic for one-dimensional Rydberg crystals
and persists even in the absence of an optical lattice. The floating phase and
the concomitant transitions can, in principle, be detected by Bragg scattering
of light.Comment: 21 pages, 9 figures; minor changes, published versio
Counting statistics in multiple path geometries and the fluctuations of the integrated current in a quantum stirring device
The amount of particles that are transported via a path of motion is
characterized by its expectation value and by its variance . We
analyze what happens if a particle has two optional paths available to get from
one site to another site, and in particular what is for the current
which is induced in a quantum stirring device. It turns out that coherent
splitting and the stirring effect are intimately related and cannot be
understood within the framework of the prevailing probabilistic theory.Comment: 11 pages, 2 figures, published version, Latex Eq# correcte
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