1,925 research outputs found
On the Opening of Branes
We relate, in 10 and 11 dimensional supergravities, configurations of
intersecting closed branes with vanishing binding energy to configurations
where one of the branes opens and has its boundaries attached to the other.
These boundaries are charged with respect to fields living on the closed brane.
The latter hosts electric and magnetic charges stemming from dual pairs of open
branes terminating on it. We show that charge conservation, gauge invariance
and supersymmetry entirely determine these charges and these fields, which can
be seen as Goldstone fields of broken supersymmetry. Open brane boundary
charges can annihilate, restoring the zero binding energy configuration. This
suggests emission of closed branes by branes, a generalization of closed string
emission by D-branes. We comment on the relation of the Goldstone fields to
matrix models approaches to M-theory.Comment: 13 pages, LaTeX, no figure
A simple minimax estimator for quantum states
Quantum tomography requires repeated measurements of many copies of the
physical system, all prepared by a source in the unknown state. In the limit of
very many copies measured, the often-used maximum-likelihood (ML) method for
converting the gathered data into an estimate of the state works very well. For
smaller data sets, however, it often suffers from problems of rank deficiency
in the estimated state. For many systems of relevance for quantum information
processing, the preparation of a very large number of copies of the same
quantum state is still a technological challenge, which motivates us to look
for estimation strategies that perform well even when there is not much data.
In this article, we review the concept of minimax state estimation, and use
minimax ideas to construct a simple estimator for quantum states. We
demonstrate that, for the case of tomography of a single qubit, our estimator
significantly outperforms the ML estimator for small number of copies of the
state measured. Our estimator is always full-rank, and furthermore, has a
natural dependence on the number of copies measured, which is missing in the ML
estimator.Comment: 26 pages, 3 figures. v2 contains minor improvements to the text, and
an additional appendix on symmetric measurement
Kac-Moody Symmetries of Ten-dimensional Non-maximal Supergravity Theories
A description of the bosonic sector of ten-dimensional N=1 supergravity as a
non-linear realisation is given. We show that if a suitable extension of this
theory were invariant under a Kac-Moody algebra, then this algebra would have
to contain a rank eleven Kac-Moody algebra, that can be identified to be a
particular real form of very-extended D_8. We also describe the extension of
N=1 supergravity coupled to an abelian vector gauge field as a non-linear
realisation, and find the Kac-Moody algebra governing the symmetries of this
theory to be very-extended B_8. Finally, we discuss the related points for the
N=1 supergravity coupled to an arbitrary number of abelian vector gauge fields
G+++ Invariant Formulation of Gravity and M-Theories: Exact BPS Solutions
We present a tentative formulation of theories of gravity with suitable
matter content, including in particular pure gravity in D dimensions, the
bosonic effective actions of M-theory and of the bosonic string, in terms of
actions invariant under very-extended Kac-Moody algebras G+++. We conjecture
that they host additional degrees of freedom not contained in the conventional
theories. The actions are constructed in a recursive way from a level expansion
for all very-extended algebras G+++. They constitute non-linear realisations on
cosets, a priori unrelated to space-time, obtained from a modified Chevalley
involution. Exact solutions are found for all G+++. They describe the algebraic
properties of BPS extremal branes, Kaluza-Klein waves and Kaluza-Klein
monopoles. They illustrate the generalisation to all G+++ invariant theories of
the well-known duality properties of string theories by expressing duality as
Weyl invariance in G+++. Space-time is expected to be generated dynamically. In
the level decomposition of E8+++ = E11, one may indeed select an A10
representation of generators Pa which appears to engender space-time
translations by inducing infinite towers of fields interpretable as field
derivatives in space and time.Comment: Latex 45 pages, 1 figure. Discussion on pages 19 and 20 altered.
Appendix B amplified. 4 footnotes added. 2 references added. Acknowledgments
updated. Additional minor correction
Quantum Cryptography: Security Criteria Reexamined
We find that the generally accepted security criteria are flawed for a whole
class of protocols for quantum cryptography. This is so because a standard
assumption of the security analysis, namely that the so-called square-root
measurement is optimal for eavesdropping purposes, is not true in general.
There are rather large parameter regimes in which the optimal measurement
extracts substantially more information than the square-root measurement.Comment: 4 pages, 4 figures. No substantial changes. Updated version as
published in PR
Interplanar binding in graphite studied with the Englert-Schwinger equation
A model of a graphite crystal is used which consists of a set of parallel slabs of positive charge immersed in an electron sea. The density of electrons in the region between slabs is calculated from the Englert-Schwinger equation. That equation improves Thomas-Fermi theory by including exchange and inhomogeneity corrections to the kinetic energy. The results are in semiquantitative agreement with empirical data and are slightly better than previous calculations of the interplanar binding of graphite
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