4 research outputs found
Quantum State Separation, Unambiguous Discrimination and Exact Cloning
Unambiguous discrimination and exact cloning reduce the square-overlap
between quantum states, exemplifying the more general type of procedure we term
state separation. We obtain the maximum probability with which two equiprobable
quantum states can be separated by an arbitrary degree, and find that the
established bounds on the success probabilities for discrimination and cloning
are special cases of this general bound. The latter also gives the maximum
probability of successfully producing N exact copies of a quantum system whose
state is chosen secretly from a known pair, given M initial realisations of the
state, where N>M. We also discuss the relationship between this bound and that
on unambiguous state discrimination.Comment: RevTeX, 5 pages postscrip
Joint measurements via quantum cloning
We explore the possibility of achieving optimal joint measurements of
noncommuting observables on a single quantum system by performing conventional
measurements of commuting self adjoint operators on optimal clones of the
original quantum system. We consider the case of both finite dimensional and
infinite dimensional Hilbert spaces. In the former we study the joint
measurement of three orthogonal components of a spin 1/2, in the latter we
consider the case of the joint measurements of any pair of noncommuting
quadratures of one mode of the electromagnetic field. We show that universally
covariant cloning is not ideal for joint measurements, and a suitable non
universally covariant cloning is needed.Comment: 8 page
Thermal entanglement in three-qubit Heisenberg models
We study pairwise thermal entanglement in three-qubit Heisenberg models and
obtain analytic expressions for the concurrence. We find that thermal
entanglement is absent from both the antiferromagnetic model, and the
ferromagnetic model with anisotropy parameter . Conditions
for the existence of thermal entanglement are discussed in detail, as is the
role of degeneracy and the effects of magnetic fields on thermal entanglement
and the quantum phase transition. Specifically, we find that the magnetic field
can induce entanglement in the antiferromagnetic model, but cannot induce
entanglement in the ferromagnetic model.Comment: 9 pages, 6 figures, minor revisions, resubmitted to J. Phys.