2,723 research outputs found
Dephasing in sequential tunneling through a double-dot interferometer
We analyze dephasing in a model system where electrons tunnel sequentially
through a symmetric interference setup consisting of two single-level quantum
dots. Depending on the phase difference between the two tunneling paths, this
may result in perfect destructive interference. However, if the dots are
coupled to a bath, it may act as a which-way detector, leading to partial
suppression of the phase-coherence and the reappearance of a finite tunneling
current. In our approach, the tunneling is treated in leading order whereas
coupling to the bath is kept to all orders (using P(E) theory). We discuss the
influence of different bath spectra on the visibility of the interference
pattern, including the distinction between "mere renormalization effects" and
"true dephasing".Comment: 18 pages, 8 figures; For a tutorial introduction to dephasing see
http://iff.physik.unibas.ch/~florian/dephasing/dephasing.htm
Separation quality of a geometric ratchet
We consider an experimentally relevant model of a geometric ratchet in which
particles undergo drift and diffusive motion in a two-dimensional periodic
array of obstacles, and which is used for the continuous separation of
particles subject to different forces. The macroscopic drift velocity and
diffusion tensor are calculated by a Monte-Carlo simulation and by a
master-equation approach, using the correponding microscopic quantities and the
shape of the obstacles as input. We define a measure of separation quality and
investigate its dependence on the applied force and the shape of the obstacles
A single trapped atom in front of an oscillating mirror
We investigate the Wigner-Weisskopf decay of a two level atom in front of an
oscillating mirror. This work builds on and extends previous theoretical and
experimental studies of the effects of a static mirror on spontaneous decay and
resonance fluorescence. The spontaneously emitted field is inherently
non-stationary due to the time-dependent boundary conditions and in order to
study its spectral distribution we employ the operational definition of the
spectrum of non-stationary light due to the seminal work by Eberly and
Wodkiewicz. We find a rich dependence of this spectrum as well as of the
effective decay rates and level shifts on the mirror-atom distance and on the
amplitude and frequency of oscillations of the mirror. The results presented
here provide the basis for future studies of more complex setups, where the
motion of the atom and/or the mirror are included as quantum degrees of
freedom.Comment: 10 pages, 12 figures, contribution to the special issue in Optics
Communications devoted to Krzysztof Wodkiewicz's memor
Dynamics of levitated nanospheres: towards the strong coupling regime
The use of levitated nanospheres represents a new paradigm for the
optomechanical cooling of a small mechanical oscillator, with the prospect of
realising quantum oscillators with unprecedentedly high quality factors. We
investigate the dynamics of this system, especially in the so-called
self-trapping regimes, where one or more optical fields simultaneously trap and
cool the mechanical oscillator. The determining characteristic of this regime
is that both the mechanical frequency and single-photon
optomechanical coupling strength parameters are a function of the optical
field intensities, in contrast to usual set-ups where and are
constant for the given system. We also measure the characteristic transverse
and axial trapping frequencies of different sized silica nanospheres in a
simple optical standing wave potential, for spheres of radii \,nm,
illustrating a protocol for loading single nanospheres into a standing wave
optical trap that would be formed by an optical cavity. We use this data to
confirm the dependence of the effective optomechanical coupling strength on
sphere radius for levitated nanospheres in an optical cavity and discuss the
prospects for reaching regimes of strong light-matter coupling. Theoretical
semiclassical and quantum displacement noise spectra show that for larger
nanospheres with \,nm a range of interesting and novel dynamical
regimes can be accessed. These include simultaneous hybridization of the two
optical modes with the mechanical modes and parameter regimes where the system
is bistable. We show that here, in contrast to typical single-optical mode
optomechanical systems, bistabilities are independent of intracavity intensity
and can occur for very weak laser driving amplitudes
Continuous mode cooling and phonon routers for phononic quantum networks
We study the implementation of quantum state transfer protocols in phonon
networks, where in analogy to optical networks, quantum information is
transmitted through propagating phonons in extended mechanical resonator arrays
or phonon waveguides. We describe how the problem of a non-vanishing thermal
occupation of the phononic quantum channel can be overcome by implementing
optomechanical multi- and continuous mode cooling schemes to create a 'cold'
frequency window for transmitting quantum states. In addition, we discuss the
implementation of phonon circulators and switchable phonon routers, which rely
on strong coherent optomechanical interactions only, and do not require strong
magnetic fields or specific materials. Both techniques can be applied and
adapted to various physical implementations, where phonons coupled to spin or
charge based qubits are used for on-chip networking applications.Comment: 33 pages, 8 figures. Final version, a few minor changes and updated
reference
Ground State Energy Fluctuations of a System Coupled to a Bath
It is often argued that a small non-degenerate quantum system coupled to a
bath has a fixed energy in its ground state since a fluctuation in energy would
require an energy supply from the bath. We consider a simple model of a
harmonic oscillator (the system) coupled to a linear string and determine the
mean squared energy fluctuations. We also analyze the two time correlator of
the energy and discuss its behavior for a finite string.Comment: 5 pages, 2 eps figures, minor change
Application of Resonance Perturbation Theory to Dynamics of Magnetization in Spin Systems Interacting with Local and Collective Bosonic Reservoirs
We apply our recently developed resonance perturbation theory to describe the
dynamics of magnetization in paramagnetic spin systems interacting
simultaneously with local and collective bosonic environments. We derive
explicit expressions for the evolution of the reduced density matrix elements.
This allows us to calculate explicitly the dynamics of the macroscopic
magnetization, including characteristic relaxation and dephasing time-scales.
We demonstrate that collective effects (i) do not influence the character of
the relaxation processes but merely renormalize the relaxation times, and (ii)
significantly modify the dephasing times, leading in some cases to a
complicated (time inhomogeneous) dynamics of the transverse magnetization,
governed by an effective time-dependent magnetic field
Decoherence of Einstein-Podolsky-Rosen pairs in a noisy Andreev entangler
We investigate quantum noise effect on the transportation of nonlocal Cooper
pairs accross the realistic Andreev entangler which consists of an s-wave
superconductor coupled to two small quantum dots at resonance which themselves
are coupled to normal leads. The noise emerges due to voltage fluctuations felt
by the electrons residing on the two dots as a result of the finite resistances
in the gate leads or of any resistive lead capacitively coupled to the dots. In
the ideal noiseless case, the setup provides a trustable source of mobile and
nonlocal spin-entangled electrons and the transport is dominated by a
two-particle Breit-Wigner resonance that allows the injection of two
spin-entangled electrons into different leads at the same energy [P. Recher, E.
V. Sukhorukov, and D. Loss, Phys. Rev. B 63, 165314 (2001)]. We seek to revisit
the transport of those nonlocal Cooper pairs as well as the efficiency of such
an Andreev entangler when including the quantum noise (decoherence).Comment: 15 pages and 6 figures; final version to appear in Physical Review
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