36 research outputs found
Resonance states in a cylindrical quantum dot with an external magnetic field
Bound and resonance states of quantum dots play a significant role in
photo-absorption processes. In this work, we analyze a cylindrical quantum dot,
its spectrum and, in particular, the behaviour of the lowest resonance state
when a magnetic field is applied along the symmetry axis of the cylinder. To
obtain the energy and width of the resonance we use the complex rotation
method. As it is expected the structure of the spectrum is strongly influenced
by the Landau levels associated to the magnetic field. We show how this
structure affects the behaviour of the resonance state and that the binding of
the resonance has a clear interpretation in terms of the Landau levels and the
probability of localization of the resonance state. The localization
probability and the fidelity of the lowest energy state allows to identify two
different physical regimes, a large field-small quantum dot radius regime and a
small field-large quantum dot radius, where the binding of the resonance is
dominated by the field strength or the potential well, respectively
Near-threshold properties of the electronic density of layered quantum-dots
We present a way to manipulate an electron trapped in a layered quantum dot
based on near-threshold properties of one-body potentials. We show that
potentials with a simple global parameter allows the manipulation of the wave
function changing its spatial extent. This phenomenon seems to be fairly
general and could be implemented using current quantum-dot quantum wells
technologies and materials if a proper layered quantum dot is designed. The
layered quantum dot under consideration is similar to a quantum-dot quantum
well device, i.e. consists of a spherical core surrounded by successive layers
of different materials. The number of layers and the constituent material are
chosen to highlight the near-threshold properties.
In particular we show that the near-threshold phenomena can be observed using
an effective mass approximation model that describes the layered quantum dot
which is consistent with actual experimental parameters.Comment: 15 pages, 6 figures, regular articl
Quantum control of a model qubit based on a multi-layered quantum dot
In this work we present a model qubit whose basis states are eigenstates of a
multi-layered quantum dot. We show that the proper design of the quantum dot
results in qubit states that have excellent dynamical properties when a
time-dependent driving is applied to it. In particular, it is shown that a
simple sinusoidal driving is sufficient to obtain good quality Rabi
oscillations between the qubit states. Moreover, the switching between states
can be performed with very low leakage, even under off-resonance conditions. In
this sense, the quantum control of the qubit is robust under some perturbations
and achieved with simple means.Comment: 19 pages, 8 figure
Study of the transition from resonance to bound states in quantum dots embedded on a nanowire using the method
We study the band structure of semiconductor nanowires with quantum dots
embedded in them. The band structure is calculated using the Rayleigh-Ritz
variational method. We consider quantum dots of two different types, one type
is defined by electrostatic potentials applied to the nanowire, while the other
one is defined by adding materials with band offsets with respect to the band
parameters of the nanowire. We are particularly interested in the appearance of
discrete energy levels in the gap between the conduction band and the valence
band of the nanostructure, and in the dependence of the energy of these levels
with the intensity of a magnetic field applied along the wire. It is shown that
several scenarios are possible, being of particular interest the possibility of
transforming states of the discrete into resonances and vice versa
Exact finite reduced density matrix and von Neumann entropy for the Calogero model
The information content of continuous quantum variables systems is usually
studied using a number of well known approximation methods. The approximations
are made to obtain the spectrum, eigenfunctions or the reduced density matrices
that are essential to calculate the entropy-like quantities that quantify the
information. Even in the sparse cases where the spectrum and eigenfunctions are
exactly known the entanglement spectrum, {\em i.e.} the spectrum of the reduced
density matrices that characterize the problem, must be obtained in an
approximate fashion. In this work, we obtain analytically a finite
representation of the reduced density matrices of the fundamental state of the
N-particle Calogero model for a discrete set of values of the interaction
parameter. As a consequence, the exact entanglement spectrum and von Neumann
entropy is worked out.Comment: Journal of Physics A (in press
Quantum state transfer in disordered spin chains: How much engineering is reasonable?
The transmission of quantum states through spin chains is an important
element in the implementation of quantum information technologies. Speed and
fidelity of transfer are the main objectives which have to be achieved by the
devices even in the presence of imperfections which are unavoidable in any
manufacturing process. To reach these goals, several kinds of spin chains have
been suggested, which differ in the degree of fine-tuning, or engineering, of
the system parameters. In this work we present a systematic study of two
important classes of such chains. In one class only the spin couplings at the
ends of the chain have to be adjusted to a value different from the bulk
coupling constant, while in the other class every coupling has to have a
specific value. We demonstrate that configurations from the two different
classes may perform similarly when subjected to the same kind of disorder in
spite of the large difference in the engineering effort necessary to prepare
the system. We identify the system features responsible for these similarities
and we perform a detailed study of the transfer fidelity as a function of chain
length and disorder strength, yielding empirical scaling laws for the fidelity
which are similar for all kinds of chain and all disorder models. These results
are helpful in identifying the optimal spin chain for a given quantum
information transfer task. In particular, they help in judging whether it is
worthwhile to engineer all couplings in the chain as compared to adjusting only
the boundary couplings.Comment: 20 pages, 13 figures. Revised version, title changed, accepted by
Quantum Information & Computatio
Robustness of spin-chain state-transfer schemes
This is a shortened and slightly edited version of a chapter in the
collection "Quantum State Transfer and Network Engineering", edited by G.M.
Nikolopoulos and I. Jex, where we review our own research about the robustness
of spin-chain state-transfer schemes along with other approaches to the topic.
Since our own research is documented elsewhere to a large extent we here
restrict ourselves to a review of other approaches which might be useful to
other researchers in the field