214 research outputs found
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
L-Drawings of Directed Graphs
We introduce L-drawings, a novel paradigm for representing directed graphs
aiming at combining the readability features of orthogonal drawings with the
expressive power of matrix representations. In an L-drawing, vertices have
exclusive - and -coordinates and edges consist of two segments, one
exiting the source vertically and one entering the destination horizontally.
We study the problem of computing L-drawings using minimum ink. We prove its
NP-completeness and provide a heuristics based on a polynomial-time algorithm
that adds a vertex to a drawing using the minimum additional ink. We performed
an experimental analysis of the heuristics which confirms its effectiveness.Comment: 11 pages, 7 figure
Parametrized Complexity of Weak Odd Domination Problems
Given a graph , a subset of vertices is a weak odd
dominated (WOD) set if there exists such that
every vertex in has an odd number of neighbours in . denotes
the size of the largest WOD set, and the size of the smallest
non-WOD set. The maximum of and , denoted
, plays a crucial role in quantum cryptography. In particular
deciding, given a graph and , whether is of
practical interest in the design of graph-based quantum secret sharing schemes.
The decision problems associated with the quantities , and
are known to be NP-Complete. In this paper, we consider the
approximation of these quantities and the parameterized complexity of the
corresponding problems. We mainly prove the fixed-parameter intractability
(W-hardness) of these problems. Regarding the approximation, we show that
, and admit a constant factor approximation
algorithm, and that and have no polynomial approximation
scheme unless P=NP.Comment: 16 pages, 5 figure
Dynamic response of 1D bosons in a trap
We calculate the dynamic structure factor S(q,omega) of a one-dimensional
(1D) interacting Bose gas confined in a harmonic trap. The effective
interaction depends on the strength of the confinement enforcing the 1D motion
of atoms; interaction may be further enhanced by superimposing an optical
lattice on the trap potential. In the compressible state, we find that the
smooth variation of the gas density around the trap center leads to softening
of the singular behavior of S(q,omega) at Lieb-1 mode compared to the behavior
predicted for homogeneous 1D systems. Nevertheless, the density-averaged
response remains a non-analytic function of q and omega at Lieb-1 mode in the
limit of weak trap confinement. The exponent of the power-law non-analyticity
is modified due to the inhomogeneity in a universal way, and thus, bears
unambiguously the information about the (homogeneous) Lieb-Liniger model. A
strong optical lattice causes formation of Mott phases. Deep in the Mott
regime, we predict a semi-circular peak in S(q,\omega) centered at the on-site
repulsion energy, omega=U. Similar peaks of smaller amplitudes exist at
multiples of U as well. We explain the suppression of the dynamic response with
entering into the Mott regime, observed recently by D. Clement et al., Phys.
Rev. Lett. v. 102, p. 155301 (2009), based on an f-sum rule for the
Bose-Hubbard model.Comment: 24 pages, 11 figure
All-Electrical Quantum Computation with Mobile Spin Qubits
We describe and discuss a solid state proposal for quantum computation with
mobile spin qubits in one-dimensional systems, based on recent advances in
spintronics. Static electric fields are used to implement a universal set of
quantum gates, via the spin-orbit and exchange couplings. Initialization and
measurement can be performed either by spin injection from/to ferromagnets, or
by using spin filters and mesoscopic spin polarizing beam-splitters. The
vulnerability of this proposal to various sources of error is estimated by
numerical simulations. We also assess the suitability of various materials
currently used in nanotechnology for an actual implementation of our model.Comment: 10 pages, 6 figs, RevTeX
Measurement efficiency and n-shot read out of spin qubits
We consider electron spin qubits in quantum dots and define a measurement
efficiency e to characterize reliable measurements via n-shot read outs. We
propose various implementations based on a double dot and quantum point contact
(QPC) and show that the associated efficiencies e vary between 50% and 100%,
allowing single-shot read out in the latter case. We model the read out
microscopically and derive its time dynamics in terms of a generalized master
equation, calculate the QPC current and show that it allows spin read out under
realistic conditions.Comment: 5 pages, 1 figur
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
Theory of Spin Hall Magnetoresistance from a Microscopic Perspective
We present a theory of the spin Hall magnetoresistance of metals in contact
with magnetic insulators. We express the spin-mixing conductances, which govern
the phenomenology of the effect, in terms of the microscopic parameters of the
interface and the spin-spin correlation functions of the local moments on the
surface of the magnetic insulator. The magnetic field and temperature
dependence of the spin-mixing conductances leads to a rich behaviour of the
resistance due to an interplay between the Hanle effect and spin mixing at the
interface. Our theory provides a useful tool for understanding the experiments
on heavy metals in contact with magnetic insulators of different kinds, and it
predicts striking behaviours of magnetoresistance.Comment: 8 pages, four figure
Electron Spins in Artificial Atoms and Molecules for Quantum Computing
Achieving control over the electron spin in quantum dots (artificial atoms)
or real atoms promises access to new technologies in conventional and in
quantum information processing. Here we review our proposal for quantum
computing with spins of electrons confined to quantum dots. We discuss the
basic requirements for implementing spin-qubits, and describe a complete set of
quantum gates for single- and two-qubit operations. We show how a quantum dot
attached to leads can be used for spin filtering and spin read-out, and as a
spin-memory device. Finally, we focus on the experimental characterization of
the quantum dot systems, and discuss transport properties of a double-dot and
show how Kondo correlations can be used to measure the Heisenberg exchange
interaction between the spins of two dots.Comment: 13 pages, 8 figures, Invited Review (Semiconductor Spintronics,
Special Issue of SST
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