116,784 research outputs found
Some formal results for the valence bond basis
In a system with an even number of SU(2) spins, there is an overcomplete set
of states--consisting of all possible pairings of the spins into valence
bonds--that spans the S=0 Hilbert subspace. Operator expectation values in this
basis are related to the properties of the closed loops that are formed by the
overlap of valence bond states. We construct a generating function for spin
correlation functions of arbitrary order and show that all nonvanishing
contributions arise from configurations that are topologically irreducible. We
derive explicit formulas for the correlation functions at second, fourth, and
sixth order. We then extend the valence bond basis to include triplet bonds and
discuss how to compute properties that are related to operators acting outside
the singlet sector. These results are relevant to analytical calculations and
to numerical valence bond simulations using quantum Monte Carlo, variational
wavefunctions, or exact diagonalization.Comment: 22 pages, 14 figure
New Results on Quantum Property Testing
We present several new examples of speed-ups obtainable by quantum algorithms
in the context of property testing. First, motivated by sampling algorithms, we
consider probability distributions given in the form of an oracle
. Here the probability \PP_f(j) of an outcome is the
fraction of its domain that maps to . We give quantum algorithms for
testing whether two such distributions are identical or -far in
-norm. Recently, Bravyi, Hassidim, and Harrow \cite{BHH10} showed that if
\PP_f and \PP_g are both unknown (i.e., given by oracles and ), then
this testing can be done in roughly quantum queries to the
functions. We consider the case where the second distribution is known, and
show that testing can be done with roughly quantum queries, which we
prove to be essentially optimal. In contrast, it is known that classical
testing algorithms need about queries in the unknown-unknown case and
about queries in the known-unknown case. Based on this result, we
also reduce the query complexity of graph isomorphism testers with quantum
oracle access. While those examples provide polynomial quantum speed-ups, our
third example gives a much larger improvement (constant quantum queries vs
polynomial classical queries) for the problem of testing periodicity, based on
Shor's algorithm and a modification of a classical lower bound by Lachish and
Newman \cite{lachish&newman:periodicity}. This provides an alternative to a
recent constant-vs-polynomial speed-up due to Aaronson \cite{aaronson:bqpph}.Comment: 2nd version: updated some references, in particular to Aaronson's
Fourier checking proble
Universal Results for Correlations of Characteristic Polynomials: Riemann-Hilbert Approach
We prove that general correlation functions of both ratios and products of
characteristic polynomials of Hermitian random matrices are governed by
integrable kernels of three different types: a) those constructed from
orthogonal polynomials; b) constructed from Cauchy transforms of the same
orthogonal polynomials and finally c) those constructed from both orthogonal
polynomials and their Cauchy transforms. These kernels are related with the
Riemann-Hilbert problem for orthogonal polynomials. For the correlation
functions we obtain exact expressions in the form of determinants of these
kernels. Derived representations enable us to study asymptotics of correlation
functions of characteristic polynomials via Deift-Zhou
steepest-descent/stationary phase method for Riemann-Hilbert problems, and in
particular to find negative moments of characteristic polynomials. This reveals
the universal parts of the correlation functions and moments of characteristic
polynomials for arbitrary invariant ensemble of symmetry class.Comment: 34page
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