145 research outputs found
Collective luminescence and phonon-induced processes in double quantum dots
We study the evolution of a quantum state of a double quantum dot system
interacting with the electromagnetic environment and with the lattice modes, in
the presence of a coupling between the two dots. We propose a unified approach
to the simulation of the system evolution under joint impact of the two
reservoirs. We discuss the sub- and superradiant radiative decay of the system,
the phonon-induced decay of entanglement between the dots, and the transfer of
excitation between them.Comment: "Jaszowiec 2009" conferenc
Electron localization and optical absorption of polygonal quantum rings
We investigate theoretically polygonal quantum rings and focus mostly on the
triangular geometry where the corner effects are maximal. Such rings can be
seen as short core-shell nanowires, a generation of semiconductor
heterostructures with multiple applications. We show how the geometry of the
sample determines the electronic energy spectrum, and also the localization of
electrons, with effects on the optical absorption. In particular, we show that
irrespective of the ring shape low-energy electrons are always attracted by
corners and are localized in their vicinity. The absorption spectrum in the
presence of a magnetic field shows only two peaks within the corner-localized
state domain, each associated with different circular polarization. This
picture may be changed by an external electric field which allows previously
forbidden transitions, and thus enables the number of corners to be determined.
We show that polygonal quantum rings allow absorption of waves from distant
ranges of the electromagnetic spectrum within one sample.Comment: 10 pages, 12 figure
Theory of nonlinear optical response of ensembles of double quantum dots
We study theoretically the time-resolved four-wave mixing (FWM) response of
an ensemble of pairs of quantum dots undergoing radiative recombination. At
short (picosecond) delay times, the response signal shows beats that may be
dominated by the subensemble of resonant pairs, which gives access to the
information on the interdot coupling. At longer delay times, the decay of the
FWM signal is governed by two rates which result from the collective
interaction between the two dots and the radiation modes. The two rates
correspond to the subradiant and super-radiant components in the radiative
decay. Coupling between the dots enhances the collective effects and makes them
observable even when the average energy mismatch between the dots is relatively
large.Comment: 8 pages, 3 figures; moderately modifie
Collective fluorescence and decoherence of a few nearly identical quantum dots
We study the collective interaction of excitons in closely spaced artificial
molecules and arrays of nearly identical quantum dots with the electromagnetic
modes. We discuss how collective fluorescence builds up in the presence of a
small mismatch of the transition energy. We show that a superradiant state of a
single exciton in a molecule of two dots with realistic energy mismatch
undergoes a two-rate decay. We analyze also the stability of subdecoherent
states for non-identical systems.Comment: 7 pages, 5 figure
Controlled Coulomb effects in core-shell quantum rings
We analyse theoretically the possibilities of contactless control of in-gap
states formed by a pair of electrons confined in a triangular quantum ring. The
in-gap states are corner-localized states associated with two electrons
occupying the same corner area, and thus shifted to much higher energies than
other corner states, but still they are below the energies of
corner-side-localized states. We show how the energies, degeneracy and
splittings between consecutive levels change with the orientation of an
external electric field relatively to the polygonal cross section. We also show
how absorption changes in the presence of external electric and magnetic
fields.Comment: 4 pages, 2 figure
Majorana states in prismatic core-shell nanowires
We consider core-shell nanowires with conductive shell and insulating core,
and with polygonal cross section. We investigate the implications of this
geometry on Majorana states expected in the presence of proximity-induced
superconductivity and an external magnetic field. A typical prismatic nanowire
has a hexagonal profile, but square and triangular shapes can also be obtained.
The low-energy states are localized at the corners of the cross section, i.e.
along the prism edges, and are separated by a gap from higher energy states
localized on the sides. The corner localization depends on the details of the
shell geometry, i.e. thickness, diameter, and sharpness of the corners. We
study systematically the low-energy spectrum of prismatic shells using
numerical methods and derive the topological phase diagram as a function of
magnetic field and chemical potential for triangular, square, and hexagonal
geometries. A strong corner localization enhances the stability of Majorana
modes to various perturbations, including the orbital effect of the magnetic
field, whereas a weaker localization favorizes orbital effects and reduces the
critical magnetic field. The prismatic geometry allows the Majorana zero-energy
modes to be accompanied by low-energy states, which we call pseudo Majorana,
and which converge to real Majoranas in the limit of small shell thickness. We
include the Rashba spin-orbit coupling in a phenomenological manner, assuming a
radial electric field across the shell.Comment: 14 pages, 16 figures, accepted for publication in Phys. Rev.
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