566 research outputs found
Interferometry in astrophysics as a roadmap for interferometry in multiparticle dynamics
Interferometry is one of the most powerful experimental tools of modern
astrophysics. Some of its methods are considered in view of potential
applicability to studies of correlations in multiparticle dynamics.Comment: 6 pages, 2 figures in ps, talk given at XXXI International Symposium
on Multiparticle Dynamics, Sept 1-7, 2001, Datong China. URL
http://ismd31.ccnu.edu.cn
Separable balls around the maximally mixed multipartite quantum states
We show that for an m-partite quantum system, there is a ball of radius
2^{-(m/2-1)} in Frobenius norm, centered at the identity matrix, of separable
(unentangled) positive semidefinite matrices. This can be used to derive an
epsilon below which mixtures of epsilon of any density matrix with 1 - epsilon
of the maximally mixed state will be separable. The epsilon thus obtained is
exponentially better (in the number of systems) than existing results. This
gives a number of qubits below which NMR with standard pseudopure-state
preparation techniques can access only unentangled states; with parameters
realistic for current experiments, this is 23 qubits (compared to 13 qubits via
earlier results). A ball of radius 1 is obtained for multipartite states
separable over the reals.Comment: 8 pages, LaTe
Separability in terms of a single entanglement witness
The separability problem is formulated in terms of a characterization of a
single entanglement witness. More specifically, we show that any (in general
multipartite) state \varrho is separable if and only if a specially constructed
entanglement witness W_{\varrho} is weakly optimal, i.e., its expectation value
vanishes on at least one product vector. Interestingly, the witness can always
be chosen to be decomposable. Our result changes the conceptual aspect of the
separability problem and rises some questions about properties of positive
maps.Comment: 4.4 pages, 1 figure, published versio
Multi-frequency investigation of the parsec- and kilo-parsec-scale radio structures in high-redshift quasar PKS 1402+044
We investigate the frequency-dependent radio properties of the jet of the
luminous high-redshift (z = 3.2) radio quasar PKS 1402+044 (J1405+0415) by
means of radio interferometric observations. The observational data were
obtained with the VLBI Space Observatory Programme (VSOP) at 1.6 and 5 GHz,
supplemented by other multi-frequency observations with the Very Long Baseline
Array (VLBA; 2.3, 8.4, and 15 GHz) and the Very Large Array (VLA; 1.4, 5, 15,
and 43 GHz). The observations span a period of 7 years. We find that the
luminous high-redshift quasar PKS 1402+044 has a pronounced "core-jet"
morphology from the parsec to the kilo-parsec scales. The jet shows a steeper
spectral index and lower brightness temperature with increasing distance from
the jet core. The variation of brightness temperature agrees well with the
shock-in-jet model. Assuming that the jet is collimated by the ambient magnetic
field, we estimate the mass of the central object as ~10^9 M_sun. The upper
limit of the jet proper motion of PKS 1402+044 is 0.03 mas/yr (~3c) in the
east-west direction.Comment: 9 pages, 6 figures
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