2,327 research outputs found
Phase Transitions in Generalised Spin-Boson (Dicke) Models
We consider a class of generalised single mode Dicke Hamiltonians with
arbitrary boson coupling in the pseudo-spin - plane. We find exact
solutions in the thermodynamic, large-spin limit as a function of the coupling
angle, which allows us to continuously move between the simple dephasing and
the original Dicke Hamiltonians. Only in the latter case (orthogonal static and
fluctuating couplings), does the parity-symmetry induced quantum phase
transition occur.Comment: 6 pages, 5 figue
Non-equilibrium correlations and entanglement in a semiconductor hybrid circuit-QED system
We present a theoretical study of a hybrid circuit-QED system composed of two
semiconducting charge-qubits confined in a microwave resonator. The qubits are
defined in terms of the charge states of two spatially separated double quantum
dots (DQDs) which are coupled to the same photon mode in the microwave
resonator. We analyze a transport setup where each DQD is attached to
electronic reservoirs and biased out-of-equilibrium by a large voltage, and
study how electron transport across each DQD is modified by the coupling to the
common resonator. In particular, we show that the inelastic current through
each DQD reflects an indirect qubit-qubit interaction mediated by off-resonant
photons in the microwave resonator. As a result of this interaction, both
charge qubits stay entangled in the steady (dissipative) state. Finite shot
noise cross-correlations between currents across distant DQDs are another
manifestation of this nontrivial steady-state entanglement.Comment: Final versio
Three-level mixing and dark states in transport through quantum dots
We consider theoretically the transport through the double quantum dot
structure of the recent experiment of C. Payette {\it et al.} [Phys. Rev. Lett.
{\bf 102}, 026808 (2009)] and calculate stationary current and shotnoise.
Three-level mixing gives rise to a pronounced current suppression effect, the
character of which charges markedly with bias direction. We discuss these
results in connexion with the dark states of coherent population trapping in
quantum dots.Comment: 6 pages, 5 fig
Truncation method for Green's functions in time-dependent fields
We investigate the influence of a time dependent, homogeneous electric field
on scattering properties of non-interacting electrons in an arbitrary static
potential. We develop a method to calculate the (Keldysh) Green's function in
two complementary approaches. Starting from a plane wave basis, a formally
exact solution is given in terms of the inverse of a matrix containing
infinitely many 'photoblocks' which can be evaluated approximately by
truncation. In the exact eigenstate basis of the scattering potential, we
obtain a version of the Floquet state theory in the Green's functions language.
The formalism is checked for cases such as a simple model of a double barrier
in a strong electric field. Furthermore, an exact relation between the
inelastic scattering rate due to the microwave and the AC conductivity of the
system is derived which in particular holds near or at a metal-insulator
transition in disordered systems.Comment: to appear in Phys. Rev. B., 21 pages, 3 figures (ps-files
Universal Conductance and Conductivity at Critical Points in Integer Quantum Hall Systems
The sample averaged longitudinal two-terminal conductance and the respective
Kubo-conductivity are calculated at quantum critical points in the integer
quantum Hall regime. In the limit of large system size, both transport
quantities are found to be the same within numerical uncertainty in the lowest
Landau band, and , respectively. In
the 2nd lowest Landau band, a critical conductance is
obtained which indeed supports the notion of universality. However, these
numbers are significantly at variance with the hitherto commonly believed value
. We argue that this difference is due to the multifractal structure
of critical wavefunctions, a property that should generically show up in the
conductance at quantum critical points.Comment: 4 pages, 3 figure
Two-Particle Dark State in the Transport through a Triple Quantum Dot
We study transport through a triple quantum dot in a triangular geometry with
applied bias such that both singly- and doubly- charged states participate. We
describe the formation of electronic dark states -- coherent superpositions
that block current flow -- in the system, and focus on the formation of a
two-electron dark state. We discuss the conditions under which such a state
forms and describe the signatures that it leaves in transport properties such
as the differential conductance and shotnoise.Comment: (9 pages, 7 figures), we now consider two different sets of charging
energie
Dynamics of interacting transport qubits
We investigate the electronic transport through two parallel double quantum
dots coupled both capacitively and via a perpendicularly aligned charge qubit.
The presence of the qubit leads to a modification of the coherent tunnel
amplitudes of each double quantum dot. We study the influence of the qubit on
the electronic steady state currents through the system, the entanglement
between the transport double quantum dots, and the back action on the charge
qubit. We use a Born-Markov-Secular quantum master equation for the system. The
obtained currents show signatures of the qubit. The stationary qubit state may
be tuned and even rendered pure by applying suitable voltages. In the Coulomb
diamonds it is also possible to stabilize pure entangled states of the
transport double quantum dots
Modeling Disordered Quantum Systems with Dynamical Networks
It is the purpose of the present article to show that so-called network
models, originally designed to describe static properties of disordered
electronic systems, can be easily generalized to quantum-{\em dynamical}
models, which then allow for an investigation of dynamical and spectral
aspects. This concept is exemplified by the Chalker-Coddington model for the
Quantum Hall effect and a three-dimensional generalization of it. We simulate
phase coherent diffusion of wave packets and consider spatial and spectral
correlations of network eigenstates as well as the distribution of
(quasi-)energy levels. Apart from that it is demonstrated how network models
can be used to determine two-point conductances. Our numerical calculations for
the three-dimensional model at the Metal-Insulator transition point delivers
among others an anomalous diffusion exponent of .
The methods presented here in detail have been used partially in earlier work.Comment: 16 pages, Rev-TeX. to appear in Int. J. Mod. Phys.
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