134 research outputs found
The canonical phase measurement is pure
We show that the canonical phase measurement is pure in the sense that the
corresponding positive operator valued measure (POVM) is extremal in the convex
set of all POVMs. This means that the canonical phase measurement cannot be
interpreted as a noisy measurement, even if it is not a projection valued
measure.Comment: 4 page
De Finetti theorem on the CAR algebra
The symmetric states on a quasi local C*-algebra on the infinite set of
indices J are those invariant under the action of the group of the permutations
moving only a finite, but arbitrary, number of elements of J. The celebrated De
Finetti Theorem describes the structure of the symmetric states (i.e.
exchangeable probability measures) in classical probability. In the present
paper we extend De Finetti Theorem to the case of the CAR algebra, that is for
physical systems describing Fermions. Namely, after showing that a symmetric
state is automatically even under the natural action of the parity
automorphism, we prove that the compact convex set of such states is a Choquet
simplex, whose extremal (i.e. ergodic w.r.t. the action of the group of
permutations previously described) are precisely the product states in the
sense of Araki-Moriya. In order to do that, we also prove some ergodic
properties naturally enjoyed by the symmetric states which have a
self--containing interest.Comment: 23 pages, juornal reference: Communications in Mathematical Physics,
to appea
Entanglement witnesses arising from Choi type positive linear maps
We construct optimal PPTES witnesses to detect PPT entangled
edge states of type constructed recently \cite{kye_osaka}. To do this,
we consider positive linear maps which are variants of the Choi type map
involving complex numbers, and examine several notions related to optimality
for those entanglement witnesses. Through the discussion, we suggest a method
to check the optimality of entanglement witnesses without the spanning
property.Comment: 18 pages, 4 figures, 1 tabl
Ochratoxin A: a naturally occurring mycotoxin found in human milk samples from Norway
Fagfellevurdert artikkel publisert i tidsskriftet Acta Paediatrica. Versjonen som er lagt ut her, er forfatterens pre-printversjon.The presence of ochratoxin A (OA) in human milk samples from different regions in Norway has been investigated in order to determine the level of infant exposure to OA from human milk. OA was found in 38 (33%) of 115 human milk samples (range 10-130 ng/l). Two to 26 % of the samples contained more than 40 ng/l OA, which will cause a daily intake of OA from human milk exceeding the suggested tolerable dose of 5 ng/kg bw. Significant regional differences were found
Entanglement purification of unknown quantum states
A concern has been expressed that ``the Jaynes principle can produce fake
entanglement'' [R. Horodecki et al., Phys. Rev. A {\bf 59}, 1799 (1999)]. In
this paper we discuss the general problem of distilling maximally entangled
states from copies of a bipartite quantum system about which only partial
information is known, for instance in the form of a given expectation value. We
point out that there is indeed a problem with applying the Jaynes principle of
maximum entropy to more than one copy of a system, but the nature of this
problem is classical and was discussed extensively by Jaynes. Under the
additional assumption that the state of the copies of the
quantum system is exchangeable, one can write down a simple general expression
for . We show how to modify two standard entanglement purification
protocols, one-way hashing and recurrence, so that they can be applied to
exchangeable states. We thus give an explicit algorithm for distilling
entanglement from an unknown or partially known quantum state.Comment: 20 pages RevTeX 3.0 + 1 figure (encapsulated Postscript) Submitted to
Physical Review
Positive maps, positive polynomials and entanglement witnesses
We link the study of positive quantum maps, block positive operators, and
entanglement witnesses with problems related to multivariate polynomials. For
instance, we show how indecomposable block positive operators relate to
biquadratic forms that are not sums of squares. Although the general problem of
describing the set of positive maps remains open, in some particular cases we
solve the corresponding polynomial inequalities and obtain explicit conditions
for positivity.Comment: 17 pages, 1 figur
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