1,120 research outputs found
Orbital and spin relaxation in single and coupled quantum dots
Phonon-induced orbital and spin relaxation rates of single electron states in
lateral single and double quantum dots are obtained numerically for realistic
materials parameters. The rates are calculated as a function of magnetic field
and interdot coupling, at various field and quantum dot orientations. It is
found that orbital relaxation is due to deformation potential phonons at low
magnetic fields, while piezoelectric phonons dominate the relaxation at high
fields. Spin relaxation, which is dominated by piezoelectric phonons, in single
quantum dots is highly anisotropic due to the interplay of the Bychkov-Rashba
and Dresselhaus spin-orbit couplings. Orbital relaxation in double dots varies
strongly with the interdot coupling due to the cyclotron effects on the
tunneling energy. Spin relaxation in double dots has an additional anisotropy
due to anisotropic spin hot spots which otherwise cause giant enhancement of
the rate at useful magnetic fields and interdot couplings. Conditions for the
absence of the spin hot spots in in-plane magnetic fields (easy passages) and
perpendicular magnetic fields (weak passages) are formulated analytically for
different growth directions of the underlying heterostructure. It is shown that
easy passages disappear (spin hot spots reappear) if the double dot system
loses symmetry by an xy-like perturbation.Comment: 13 pages, 9 figure
Using quantum state protection via dissipation in a quantum-dot molecule to solve the Deutsch problem
The wide set of control parameters and reduced size scale make semiconductor
quantum dots attractive candidates to implement solid-state quantum
computation. Considering an asymmetric double quantum dot coupled by tunneling,
we combine the action of a laser field and the spontaneous emission of the
excitonic state to protect an arbitrary superposition state of the indirect
exciton and ground state. As a by-product we show how to use the protected
state to solve the Deutsch problem.Comment: 8 pages, 1 figure, 2 table
Short-timescale Fluctuations in the Difference Light Curves of QSO 0957+561A,B: Microlensing or Noise?
From optical R band data of the double quasar QSO 0957+561A,B, we made two
new difference light curves (about 330 days of overlap between the time-shifted
light curve for the A image and the magnitude-shifted light curve for the B
image). We observed noisy behaviours around the zero line and no
short-timescale events (with a duration of months), where the term event refers
to a prominent feature that may be due to microlensing or another source of
variability. Only one event lasting two weeks and rising - 33 mmag was found.
Measured constraints on the possible microlensing variability can be used to
obtain information on the granularity of the dark matter in the main lensing
galaxy and the size of the source. In addition, one can also test the ability
of the observational noise to cause the rms averages and the local features of
the difference signals. We focused on this last issue. The combined
photometries were related to a process consisting of an intrinsic signal plus a
Gaussian observational noise. The intrinsic signal has been assumed to be
either a smooth function (polynomial) or a smooth function plus a stationary
noise process or a correlated stationary process. Using these three pictures
without microlensing, we derived some models totally consistent with the
observations. We finally discussed the sensitivity of our telescope (at Teide
Observatory) to several classes of microlensing variability.Comment: MNRAS, in press (LaTeX, 14 pages, 22 eps figures
Phonon modulation of the spin-orbit interaction as a spin relaxation mechanism in InSb quantum dots
We calculate the spin relaxation rates in a parabolic InSb quantum dots due
to the spin interaction with acoustical phonons. We considered the deformation
potential mechanism as the dominant electron-phonon coupling in the
Pavlov-Firsov spin-phonon Hamiltonian. By studying suitable choices of magnetic
field and lateral dot size, we determine regions where the spin relaxation
rates can be practically suppressed. We analyze the behavior of the spin
relaxation rates as a function of an external magnetic field and mean quantum
dot radius. Effects of the spin admixture due to Dresselhaus contribution to
spin-orbit interaction are also discussed.Comment: 5 page, 2 figures, accepted in Phonons 2007 proceeding
Virtual Processes and Superradiance in Spin-Boson Models
We consider spin-boson models composed by a single bosonic mode and an
ensemble of identical two-level atoms. The situation where the coupling
between the bosonic mode and the atoms generates real and virtual processes is
studied, where the whole system is in thermal equilibrium with a reservoir at
temperature . Phase transitions from ordinary fluorescence to
superradiant phase in three different models is investigated. First a model
where the coupling between the bosonic mode and the atom is via the
pseudo-spin operator is studied. Second, we investigate the
generalized Dicke model, introducing different coupling constants between the
single mode bosonic field and the environment, and for rotating
and counter-rotating terms, respectively. Finally it is considered a modified
version of the generalized Dicke model with intensity-dependent coupling in the
rotating terms. In the first model the zero mode contributes to render the
canonical entropy a negative quantity for low temperatures. The last two models
presents phase transitions, even when only Hamiltonian terms which generates
virtual processes are considered
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