13,146 research outputs found
On exchangeable continuous variable systems
We investigate permutation-invariant continuous variable quantum states and their covariance matrices. We provide a complete characterization of the latter with respect to permutation invariance and exchangeability and representing convex combinations of tensor power states. On the level of the respective density operators this leads to necessary criteria for all these properties which become necessary and sufficient for Gaussian states. For these we use the derived results to provide de Finetti-type theorems for various distance measures
The complex Busemann-Petty problem on sections of convex bodies
The complex Busemann-Petty problem asks whether origin symmetric convex
bodies in \C^n with smaller central hyperplane sections necessarily have
smaller volume. We prove that the answer is affirmative if and
negative if Comment: 18 page
Dipole operator constraints on composite Higgs models
Flavour- and CP-violating electromagnetic or chromomagnetic dipole operators
in the quark sector are generated in a large class of new physics models and
are strongly constrained by measurements of the neutron electric dipole moment
and observables sensitive to flavour-changing neutral currents, such as the
branching ratio and . After a
model-independent discussion of the relevant constraints, we analyze these
effects in models with partial compositeness, where the quarks get their masses
by mixing with vector-like composite fermions. These scenarios can be seen as
the low-energy limit of composite Higgs or warped extra dimensional models. We
study different choices for the electroweak representations of the composite
fermions motivated by electroweak precision tests as well as different flavour
structures, including flavour anarchy and or flavour
symmetries in the strong sector. In models with "wrong-chirality" Yukawa
couplings, we find a strong bound from the neutron electric dipole moment,
irrespective of the flavour structure. In the case of flavour anarchy, we also
find strong bounds from flavour-violating dipoles, while these constraints are
mild in the flavour-symmetric models.Comment: 30 pages, 2 figures, 11 tables. v3: Misprints in table 8 corrected.
Numerics and conclusions unchange
Super-poissonian noise, negative differential conductance, and relaxation effects in transport through molecules, quantum dots and nanotubes
We consider charge transport through a nanoscopic object, e.g. single
molecules, short nanotubes, or quantum dots, that is weakly coupled to metallic
electrodes. We account for several levels of the molecule/quantum dot with
level-dependent coupling strengths, and allow for relaxation of the excited
states. The current-voltage characteristics as well as the current noise are
calculated within first-order perturbation expansion in the coupling strengths.
For the case of asymmetric coupling to the leads we predict
negative-differential-conductance accompanied with super-poissonian noise. Both
effects are destroyed by fast relaxation processes. The non-monotonic behavior
of the shot noise as a function of bias and relaxation rate reflects the
details of the electronic structure and level-dependent coupling strengths.Comment: 8 pages, 7 figures, submitted to Phys. Rev. B, added reference
Violation of the Wiedemann-Franz Law in a Single-Electron Transistor
We study the influence of Coulomb interaction on the thermoelectric transport
coefficients for a metallic single-electron transistor. By performing a
perturbation expansion up to second order in the tunnel-barrier conductance, we
include sequential and cotunneling processes as well as quantum fluctuations
that renormalize the charging energy and the tunnel conductance. We find that
Coulomb interaction leads to a strong violation of the Wiedemann-Franz law: the
Lorenz ratio becomes gate-voltage dependent for sequential tunneling, and is
increased by a factor 9/5 in the cotunneling regime. Finally, we suggest a
measurement scheme for an experimental realization.Comment: published version, minor changes; 4 pages, 3 figure
Non--Heisenberg Spin Dynamics of Double-Exchange Ferromagnets with Coulomb Repulsion
With a variational three--body calculation we study the role of the interplay
between the onsite Coulomb, Hund's rule, and superexchange interactions on the
spinwave excitation spectrum of itinerant ferromagnets. We show that
correlations between a Fermi sea electron--hole pair and a magnon result in a
very pronounced zone boundary softening and strong deviations from the
Heisenberg spinwave dispersion. We show that this spin dynamics depends
sensitively on the Coulomb and exchange interactions and discuss its possible
relevance to experiments in the manganites.Comment: 4 pages, 4 figures, published in Physical Review B as rapid
communication
Classification of topologically protected gates for local stabilizer codes
Given a quantum error correcting code, an important task is to find encoded
operations that can be implemented efficiently and fault-tolerantly. In this
Letter we focus on topological stabilizer codes and encoded unitary gates that
can be implemented by a constant-depth quantum circuit. Such gates have a
certain degree of protection since propagation of errors in a constant-depth
circuit is limited by a constant size light cone. For the 2D geometry we show
that constant-depth circuits can only implement a finite group of encoded gates
known as the Clifford group. This implies that topological protection must be
"turned off" for at least some steps in the computation in order to achieve
universality. For the 3D geometry we show that an encoded gate U is
implementable by a constant-depth circuit only if the image of any Pauli
operator under conjugation by U belongs to the Clifford group. This class of
gates includes some non-Clifford gates such as the \pi/8 rotation. Our
classification applies to any stabilizer code with geometrically local
stabilizers and sufficiently large code distance.Comment: 6 pages, 2 figure
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