875 research outputs found
Spin Decay in a Quantum Dot Coupled to a Quantum Point Contact
We consider a mechanism of spin decay for an electron spin in a quantum dot
due to coupling to a nearby quantum point contact (QPC) with and without an
applied bias voltage. The coupling of spin to charge is induced by the
spin-orbit interaction in the presence of a magnetic field. We perform a
microscopic calculation of the effective Hamiltonian coupling constants to
obtain the QPC-induced spin relaxation and decoherence rates in a realistic
system. This rate is shown to be proportional to the shot noise of the QPC in
the regime of large bias voltage and scales as where is the
distance between the quantum dot and the QPC. We find that, for some specific
orientations of the setup with respect to the crystallographic axes, the
QPC-induced spin relaxation and decoherence rates vanish, while the charge
sensitivity of the QPC is not changed. This result can be used in experiments
to minimize QPC-induced spin decay in read-out schemes.Comment: 10 pages, 2 figures, 2 table
Discrete Fourier Transform in Nanostructures using Scattering
In this paper we show that the discrete Fourier transform can be performed by
scattering a coherent particle or laser beam off a two-dimensional potential
that has the shape of rings or peaks. After encoding the initial vector into
the two-dimensional potential, the Fourier-transformed vector can be read out
by detectors surrounding the potential. The wavelength of the laser beam
determines the necessary accuracy of the 2D potential, which makes our method
very fault-tolerant.Comment: 6 pages, 5 EPS figures, REVTe
Phonon Bottleneck Effect Leads to Observation of Quantum Tunneling of the Magnetization and Butterfly Hysteresis Loops in (Et4N)3Fe2F9
A detailed investigation of the unusual dynamics of the magnetization of
(Et4N)3Fe2F9 (Fe2), containing isolated [Fe2F9]3- dimers, is presented and
discussed. Fe2 possesses an S=5 ground state with an energy barrier of 2.40 K
due to an axial anisotropy. Poor thermal contact between sample and bath leads
to a phonon bottleneck situation, giving rise to butterfly-shaped hysteresis
loops below 5 K concomitant with slow decay of the magnetization for magnetic
fields Hz applied along the Fe--Fe axis. The butterfly curves are reproduced
using a microscopic model based on the interaction of the spins with resonant
phonons. The phonon bottleneck allows for the observation of resonant quantum
tunneling of the magnetization at 1.8 K, far above the blocking temperature for
spin-phonon relaxation. The latter relaxation is probed by AC magnetic
susceptibility experiments at various temperatures and bias fields. At H=0, no
out-of-phase signal is detected, indicating that at T smaller than 1.8 K Fe2
does not behave as a single-molecule magnet. At 1 kG, relaxation is observed,
occurring over the barrier of the thermally accessible S=4 first excited state
that forms a combined system with the S=5 state.Comment: 10 pages, 10 figure
Direct Measurement of the Spin-Orbit Interaction in a Two-Electron InAs Nanowire Quantum Dot
We demonstrate control of the electron number down to the last electron in
tunable few-electron quantum dots defined in catalytically grown InAs
nanowires. Using low temperature transport spectroscopy in the Coulomb blockade
regime we propose a simple method to directly determine the magnitude of the
spin-orbit interaction in a two-electron artificial atom with strong spin-orbit
coupling. Due to a large effective g-factor |g*|=8+/-1 the transition from
singlet S to triplet T+ groundstate with increasing magnetic field is dominated
by the Zeeman energy rather than by orbital effects. We find that the
spin-orbit coupling mixes the T+ and S states and thus induces an avoided
crossing with magnitude =0.25+/-0.05 meV. This allows us to
calculate the spin-orbit length 127 nm in such systems
using a simple model.Comment: 21 pages, 7 figures, including supplementary note
Transport through a double quantum dot in the sequential- and co- tunneling regimes
We study transport through a double quantum dot, both in the sequential
tunneling and cotunneling regimes. Using a master equation approach, we find
that, in the sequential tunneling regime, the differential conductance
as a function of the bias voltage has a number of satellite
peaks with respect to the main peak of the Coulomb blockade diamond. The
position of these peaks is related to the interdot tunnel splitting and the
singlet-triplet splitting. We find satellite peaks with both {\em positive} and
{\em negative} values of differential conductance for realistic parameter
regimes. Relating our theory to a microscopic (Hund-Mulliken) model for the
double dot, we find a temperature regime for which the Hubbard ratio (=tunnel
coupling over on-site Coulomb repulsion) can be extracted from
in the cotunneling regime. In addition, we consider a combined effect of
cotunneling and sequential tunneling, which leads to new peaks (dips) in
inside the Coulomb blockade diamond below some temperature
scales, which we specify.Comment: 16 pages, 10 figure
Reply to the comment of Chudnovsky&Garanin on "Spin relaxation in Mn12-acetate"
Reply to the comment of E.M. Chudnovsky and D.A. Garanin on Europhys. Lett.
46, 692 (1999).Comment: 2 pages, Latex (europhys.sty
Spin dynamics in InAs-nanowire quantum-dots coupled to a transmission line
We study theoretically electron spins in nanowire quantum dots placed inside
a transmission line resonator. Because of the spin-orbit interaction, the spins
couple to the electric component of the resonator electromagnetic field and
enable coherent manipulation, storage, and read-out of quantum information in
an all-electrical fashion. Coupling between distant quantum-dot spins, in one
and the same or different nanowires, can be efficiently performed via the
resonator mode either in real time or through virtual processes. For the latter
case we derive an effective spin-entangling interaction and suggest means to
turn it on and off. We consider both transverse and longitudinal types of
nanowire quantum-dots and compare their manipulation timescales against the
spin relaxation times. For this, we evaluate the rates for spin relaxation
induced by the nanowire vibrations (phonons) and show that, as a result of
phonon confinement in the nanowire, this rate is a strongly varying function of
the spin operation frequency and thus can be drastically reduced compared to
lateral quantum dots in GaAs. Our scheme is a step forward to the formation of
hybrid structures where qubits of different nature can be integrated in a
single device
Phonon-induced decay of the electron spin in quantum dots
We study spin relaxation and decoherence in a
GaAs quantum dot due to spin-orbit interaction. We derive an effective
Hamiltonian which couples the electron spin to phonons or any other fluctuation
of the dot potential. We show that the spin decoherence time is as large
as the spin relaxation time , under realistic conditions. For the
Dresselhaus and Rashba spin-orbit couplings, we find that, in leading order,
the effective magnetic field can have only fluctuations transverse to the
applied magnetic field. As a result, for arbitrarily large Zeeman
splittings, in contrast to the naively expected case
. We show that the spin decay is drastically suppressed for
certain magnetic field directions and values of the
Rashba coupling constant. Finally, for the spin coupling to acoustic phonons,
we show that
for all spin-orbit mechanisms in leading order in the
electron-phonon interaction.Comment: 5 pages, 1 figur
Spin electric effects in molecular antiferromagnets
Molecular nanomagnets show clear signatures of coherent behavior and have a
wide variety of effective low-energy spin Hamiltonians suitable for encoding
qubits and implementing spin-based quantum information processing. At the
nanoscale, the preferred mechanism for control of quantum systems is through
application of electric fields, which are strong, can be locally applied, and
rapidly switched. In this work, we provide the theoretical tools for the search
for single molecule magnets suitable for electric control. By group-theoretical
symmetry analysis we find that the spin-electric coupling in triangular
molecules is governed by the modification of the exchange interaction, and is
possible even in the absence of spin-orbit coupling. In pentagonal molecules
the spin-electric coupling can exist only in the presence of spin-orbit
interaction. This kind of coupling is allowed for both and
spins at the magnetic centers. Within the Hubbard model, we find a relation
between the spin-electric coupling and the properties of the chemical bonds in
a molecule, suggesting that the best candidates for strong spin-electric
coupling are molecules with nearly degenerate bond orbitals. We also
investigate the possible experimental signatures of spin-electric coupling in
nuclear magnetic resonance and electron spin resonance spectroscopy, as well as
in the thermodynamic measurements of magnetization, electric polarization, and
specific heat of the molecules.Comment: 31 pages, 24 figure
Topological Quantum Gates with Quantum Dots
We present an idealized model involving interacting quantum dots that can
support both the dynamical and geometrical forms of quantum computation. We
show that by employing a structure similar to the one used in the Aharonov-Bohm
effect we can construct a topological two-qubit phase-gate that is to a large
degree independent of the exact values of the control parameters and therefore
resilient to control errors. The main components of the setup are realizable
with present technology.Comment: 8 pages, 3 figures, submitted to Jour. of Opt. B (special issue on
Quantum Computing
- …