137 research outputs found
Theory of Single Electron Spin Relaxation in Si/SiGe Lateral Coupled Quantum Dots
We investigate the spin relaxation induced by acoustic phonons in the
presence of spin-orbit interactions in single electron Si/SiGe lateral coupled
quantum dots. The relaxation rates are computed numerically in single and
double quantum dots, in in-plane and perpendicular magnetic fields. The
deformation potential of acoustic phonons is taken into account for both
transverse and longitudinal polarizations and their contributions to the total
relaxation rate are discussed with respect to the dilatation and shear
potential constants. We find that in single dots the spin relaxation rate
scales approximately with the seventh power of the magnetic field, in line with
a recent experiment. In double dots the relaxation rate is much more sensitive
to the dot spectrum structure, as it is often dominated by a spin hot spot. The
anisotropy of the spin-orbit interactions gives rise to easy passages, special
directions of the magnetic field for which the relaxation is strongly
suppressed. Quantitatively, the spin relaxation rates in Si are typically 2
orders of magnitude smaller than in GaAs due to the absence of the
piezoelectric phonon potential and generally weaker spin-orbit interactions.Comment: 10 pages, 9 figure
Theory of anisotropic exchange in laterally coupled quantum dots
The effects of spin-orbit coupling on the two-electron spectra in lateral
coupled quantum dots are investigated analytically and numerically. It is
demonstrated that in the absence of magnetic field the exchange interaction is
practically unaffected by spin-orbit coupling, for any interdot coupling,
boosting prospects for spin-based quantum computing. The anisotropic exchange
appears at finite magnetic fields. A numerically accurate effective spin
Hamiltonian for modeling spin-orbit-induced two-electron spin dynamics in the
presence of magnetic field is proposed.Comment: 4 pages, 3 figures; paper rewritte
Helical nuclear spin order in a strip of stripes in the Quantum Hall regime
We investigate nuclear spin effects in a two-dimensional electron gas in the
quantum Hall regime modeled by a weakly coupled array of interacting quantum
wires. We show that the presence of hyperfine interaction between electron and
nuclear spins in such wires can induce a phase transition, ordering electrons
and nuclear spins into a helix in each wire. Electron-electron interaction
effects, pronounced within the one-dimensional stripes, boost the transition
temperature up to tens to hundreds of millikelvins in GaAs. We predict specific
experimental signatures of the existence of nuclear spin order, for instance
for the resistivity of the system at transitions between different quantum Hall
plateaus.Comment: 16+ pages, 6 figures, updated reference
Circuit QED with Hole-Spin Qubits in Ge/Si Nanowire Quantum Dots
We propose a setup for universal and electrically controlled quantum
information processing with hole spins in Ge/Si core/shell nanowire quantum
dots (NW QDs). Single-qubit gates can be driven through electric-dipole-induced
spin resonance, with spin-flip times shorter than 100 ps. Long-distance
qubit-qubit coupling can be mediated by the cavity electric field of a
superconducting transmission line resonator, where we show that operation times
below 20 ns seem feasible for the entangling square-root-of-iSWAP gate. The
absence of Dresselhaus spin-orbit interaction (SOI) and the presence of an
unusually strong Rashba-type SOI enable precise control over the transverse
qubit coupling via an externally applied, perpendicular electric field. The
latter serves as an on-off switch for quantum gates and also provides control
over the g factor, so single- and two-qubit gates can be operated
independently. Remarkably, we find that idle qubits are insensitive to charge
noise and phonons, and we discuss strategies for enhancing noise-limited gate
fidelities.Comment: 6 pages main article + 12 pages supplement, 4 figure
Majorana bound states in magnetic skyrmions
Magnetic skyrmions are highly mobile nanoscale topological spin textures. We
show, both analytically and numerically, that a magnetic skyrmion of an even
azimuthal winding number placed in proximity to an s-wave superconductor hosts
a zero-energy Majorana bound state in its core, when the exchange coupling
between the itinerant electrons and the skyrmion is strong. This Majorana bound
state is stabilized by the presence of a spin-orbit interaction. We propose the
use of a superconducting tri-junction to realize non-Abelian statistics of such
Majorana bound states.Comment: published versio
Theory of Spin Relaxation in Two-Electron Lateral Coupled Quantum Dots
A global quantitative picture of the phonon-induced two-electron spin
relaxation in GaAs double quantum dots is presented using highly accurate
numerical calculations. Wide regimes of interdot coupling, magnetic field
magnitude and orientation, and detuning are explored in the presence of a
nuclear bath. Most important, the unusually strong magnetic anisotropy of the
singlet-triplet relaxation can be controlled by detuning switching the
principal anisotropy axes: a protected state becomes unprotected upon detuning,
and vice versa. It is also established that nuclear spins can dominate spin
relaxation for unpolarized triplets even at high magnetic fields, contrary to
common belief. These findings are central to designing quantum dots geometries
for spin-based quantum information processing with minimal environmental
impact.Comment: 8 pages, 8 figure
Local Spin Susceptibilities of Low-Dimensional Electron Systems
We investigate, assess, and suggest possibilities for a measurement of the
local spin susceptibility of a conducting low-dimensional electron system. The
basic setup of the experiment we envisage is a source-probe one. Locally
induced spin density (e.g. by a magnetized atomic force microscope tip) extends
in the medium according to its spin susceptibility. The induced magnetization
can be detected as a dipolar magnetic field, for instance, by an
ultra-sensitive nitrogen-vacancy center based detector, from which the spatial
structure of the spin susceptibility can be deduced. We find that
one-dimensional systems, such as semiconducting nanowires or carbon nanotubes,
are expected to yield a measurable signal. The signal in a two-dimensional
electron gas is weaker, though materials with high enough -factor (such as
InGaAs) seem promising for successful measurements.Comment: 11 pages, 12 figure
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