341 research outputs found
Spin-polarized transport and Andreev reflection in semiconductor/superconductor hybrid structures
We show that spin-polarized electron transmission across
semiconductor/superconductor (Sm/S) hybrid structures depends sensitively on
the degree of spin polarization as well as the strengths of potential and
spin-flip scattering at the interface. We demonstrate that increasing the Fermi
velocity mismatch in the Sm and S regions can lead to enhanced junction
transparency in the presence of spin polarization. We find that the Andreev
reflection amplitude at the superconducting gap energy is a robust measure of
the spin polarization magnitude, being independent of the strengths of
potential and spin-flip scattering and the Fermi velocity of the
superconductor.Comment: 4 pages, 2 figure
A Generalization of Quantum Stein's Lemma
We present a generalization of quantum Stein's Lemma to the situation in
which the alternative hypothesis is formed by a family of states, which can
moreover be non-i.i.d.. We consider sets of states which satisfy a few natural
properties, the most important being the closedness under permutations of the
copies. We then determine the error rate function in a very similar fashion to
quantum Stein's Lemma, in terms of the quantum relative entropy.
Our result has two applications to entanglement theory. First it gives an
operational meaning to an entanglement measure known as regularized relative
entropy of entanglement. Second, it shows that this measure is faithful, being
strictly positive on every entangled state. This implies, in particular, that
whenever a multipartite state can be asymptotically converted into another
entangled state by local operations and classical communication, the rate of
conversion must be non-zero. Therefore, the operational definition of
multipartite entanglement is equivalent to its mathematical definition.Comment: 30 pages. (see posting by M. Piani arXiv:0904.2705 for a different
proof of the strict positiveness of the regularized relative entropy of
entanglement on every entangled state). published version
Looking for a Heavy Higgsino LSP in Collider and Dark Matter Experiments
A large part of the mSUGRA parameter space satisfying the WMAP constraint on
the dark matter relic density corresponds to a higgsino LSP of mass
TeV. We find a promising signal for this LSP at CLIC, particularly with
polarized electron and positron beams. One also expects a viable monochromatic
-ray signal from its pair annihilation at the galactic center at least
for cuspy DM halo profiles. All these results hold equally for the higgsino LSP
of other SUSY models like the non-universal scalar or gaugino mass models and
the so-called inverted hierarchy and more minimal supersymmetry models.Comment: LaTeX, 18 pages with 5 embedded postscript figures. Added a couple of
references (version to appear in Phys. Lett. B
The STAR Photon Multiplicity Detector
Details concerning the design, fabrication and performance of STAR Photon
Multiplicity Detector (PMD) are presented. The PMD will cover the forward
region, within the pseudorapidity range 2.3--3.5, behind the forward time
projection chamber. It will measure the spatial distribution of photons in
order to study collective flow, fluctuation and chiral symmetry restoration.Comment: 15 pages, including 11 figures; to appear in a special NIM volume
dedicated to the accelerator and detectors at RHI
A Self Assembled Nanoelectronic Quantum Computer Based on the Rashba Effect in Quantum Dots
Quantum computers promise vastly enhanced computational power and an uncanny
ability to solve classically intractable problems. However, few proposals exist
for robust, solid state implementation of such computers where the quantum
gates are sufficiently miniaturized to have nanometer-scale dimensions. Here I
present a new approach whereby a complete computer with nanoscale gates might
be self-assembled using chemical synthesis. Specifically, I demonstrate how to
self-assemble the fundamental unit of this quantum computer - a 2-qubit
universal quantum controlled-NOT gate - based on two exchange coupled
multilayered quantum dots. Then I show how these gates can be wired using
thiolated conjugated molecules as electrical connectors. A qubit is encoded in
the ground state of a quantum dot spin-split by the Rashba interaction.
Arbitrary qubit rotations are effected by bringing the spin splitting energy in
a target quantum dot in resonance with a global ac magnetic field by applying a
potential pulse of appropriate amplitude and duration to the dot. The
controlled dynamics of the 2-qubit controlled-NOT operation (XOR) can be
realized by exploiting the exchange coupling with the nearest neighboring dot.
A complete prescription for initialization of the computer and data
input/output operations is presented.Comment: 22 pages, 4 figure
Probing R-parity violating models of neutrino mass at the Tevatron via top Squark decays
We have estimated the limiting branching ratio of the R-parity violating
(RPV) decay of the lighter top squark, \tilde t_1 \ar l^+ d ( or
and d is a down type quark of any flavor), as a function of top squark
mass(\MST) for an observable signal in the di-lepton plus di-jet channel at
the Tevatron RUN-II experiment with 2 fb luminosity. Our simulations
indicate that the lepton number violating nature of the underlying decay
dynamics can be confirmed via the reconstruction of \MST. The above decay is
interesting in the context of RPV models of neutrino mass where the RPV
couplings () driving the above decay are constrained to be
small (\lsim 10^{-3} - 10^{-4} ). If is the next lightest super
particle - a theoretically well motivated scenario - then the RPV decay can
naturally compete with the R-parity conserving (RPC) modes which also have
suppressed widths. The model independent limiting BR can delineate the
parameter space in specific supersymmetric models, where the dominating RPV
decay is observable and predict the minimum magnitude of the RPV coupling that
will be sensitive to Run-II data. We have found it to be in the same ballpark
value required by models of neutrino mass, for a wide range of \MST. A
comprehensive future strategy for linking top squark decays with models of
neutrino mass is sketched.Comment: 28 pages, 14 Figure
Exponential Decay of Correlations Implies Area Law
We prove that a finite correlation length, i.e. exponential decay of
correlations, implies an area law for the entanglement entropy of quantum
states defined on a line. The entropy bound is exponential in the correlation
length of the state, thus reproducing as a particular case Hastings proof of an
area law for groundstates of 1D gapped Hamiltonians.
As a consequence, we show that 1D quantum states with exponential decay of
correlations have an efficient classical approximate description as a matrix
product state of polynomial bond dimension, thus giving an equivalence between
injective matrix product states and states with a finite correlation length.
The result can be seen as a rigorous justification, in one dimension, of the
intuition that states with exponential decay of correlations, usually
associated with non-critical phases of matter, are simple to describe. It also
has implications for quantum computing: It shows that unless a pure state
quantum computation involves states with long-range correlations, decaying at
most algebraically with the distance, it can be efficiently simulated
classically.
The proof relies on several previous tools from quantum information theory -
including entanglement distillation protocols achieving the hashing bound,
properties of single-shot smooth entropies, and the quantum substate theorem -
and also on some newly developed ones. In particular we derive a new bound on
correlations established by local random measurements, and we give a
generalization to the max-entropy of a result of Hastings concerning the
saturation of mutual information in multiparticle systems. The proof can also
be interpreted as providing a limitation on the phenomenon of data hiding in
quantum states.Comment: 35 pages, 6 figures; v2 minor corrections; v3 published versio
Magnetic Interactions and Transport in (Ga,Cr)As
The magnetic, transport, and structural properties of (Ga,Cr)As are reported.
Zincblende GaCrAs was grown by low-temperature molecular beam
epitaxy (MBE). At low concentrations, x0.1, the materials exhibit unusual
magnetic properties associated with the random magnetism of the alloy. At low
temperatures the magnetization M(B) increases rapidly with increasing field due
to the alignment of ferromagnetic units (polarons or clusters) having large
dipole moments of order 10-10. A standard model of
superparamagnetism is inadequate for describing both the field and temperature
dependence of the magnetization M(B,T). In order to explain M(B) at low
temperatures we employ a distributed magnetic moment (DMM) model in which
polarons or clusters of ions have a distribution of moments. It is also found
that the magnetic susceptibility increases for decreasing temperature but
saturates below T=4 K. The inverse susceptibility follows a linear-T
Curie-Weiss law and extrapolates to a magnetic transition temperature
=10 K. In magnetotransport measurements, a room temperature resistivity
of =0.1 cm and a hole concentration of cm
are found, indicating that Cr can also act as a acceptor similar to Mn. The
resistivity increases rapidly for decreasing temperature below room
temperature, and becomes strongly insulating at low temperatures. The
conductivity follows exp[-(T/T)] over a large range of
conductivity, possible evidence of tunneling between polarons or clusters.Comment: To appear in PRB 15 Mar 200
Probing a Supersymmetric Model for Neutrino Masses at Ultrahigh Energy Neutrino Telescopes
A bilinear R-Parity breaking SUSY model for neutrino mass and mixing predicts
the lightest superparticle to decay mainly into a pair of tau leptons or b
quarks along with a neutrino for relatively light SUSY spectra. This leads to a
distinctive triple bang signature of SUSY events at ultrahigh energy neutrino
telescopes like IceCube or Antares. While the expected signal size is only
marginal at IceCube, it will be promising for a future multi-km^3 size neutrino
telescope.Comment: 11 pages pdf including 2 figures. Discussion added. Final version to
appear in Physics Letters
Spin-based all-optical quantum computation with quantum dots: understanding and suppressing decoherence
We present an all-optical implementation of quantum computation using
semiconductor quantum dots. Quantum memory is represented by the spin of an
excess electron stored in each dot. Two-qubit gates are realized by switching
on trion-trion interactions between different dots. State selectivity is
achieved via conditional laser excitation exploiting Pauli exclusion principle.
Read-out is performed via a quantum-jump technique. We analyze the effect on
our scheme's performance of the main imperfections present in real quantum
dots: exciton decay, hole mixing and phonon decoherence. We introduce an
adiabatic gate procedure that allows one to circumvent these effects, and
evaluate quantitatively its fidelity
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