324 research outputs found
Single-Step Distillation Protocol with Generalized Beam Splitters
We develop a distillation protocol for multilevel qubits (qudits) using
generalized beam splitters like in the proposal of Pan et al. for ordinary
qubits. We find an acceleration with respect to the scheme of Bennet et al.
when extended to qudits. It is also possible to distill entangled pairs of
photons carrying orbital angular momenta (OAM) states that conserves the total
angular momenta as those produced in recent experiments.Comment: REVTEX4 file, color figure
Distillation Protocols for Mixed States of Multilevel Qubits and the Quantum Renormalization Group
We study several properties of distillation protocols to purify multilevel
qubit states (qudits) when applied to a certain family of initial mixed
bipartite states. We find that it is possible to use qudits states to increase
the stability region obtained with the flow equations to distill qubits. In
particular, for qutrits we get the phase diagram of the distillation process
with a rich structure of fixed points. We investigate the large- limit of
qudits protocols and find an analytical solution in the continuum limit. The
general solution of the distillation recursion relations is presented in an
appendix. We stress the notion of weight amplification for distillation
protocols as opposed to the quantum amplitude amplification that appears in the
Grover algorithm. Likewise, we investigate the relations between quantum
distillation and quantum renormalization processes.Comment: REVTEX4 file, 12 pages, 3 tables, color figure
A paradox in bosonic energy computations via semidefinite programming relaxations
We show that the recent hierarchy of semidefinite programming relaxations
based on non-commutative polynomial optimization and reduced density matrix
variational methods exhibits an interesting paradox when applied to the bosonic
case: even though it can be rigorously proven that the hierarchy collapses
after the first step, numerical implementations of higher order steps generate
a sequence of improving lower bounds that converges to the optimal solution. We
analyze this effect and compare it with similar behavior observed in
implementations of semidefinite programming relaxations for commutative
polynomial minimization. We conclude that the method converges due to the
rounding errors occurring during the execution of the numerical program, and
show that convergence is lost as soon as computer precision is incremented. We
support this conclusion by proving that for any element p of a Weyl algebra
which is non-negative in the Schrodinger representation there exists another
element p' arbitrarily close to p that admits a sum of squares decomposition.Comment: 22 pages, 4 figure
Cool stars in NGC 2547 and pre main sequence lithium depletion
We present the results of a spectroscopic survey of X-ray selected, low-mass
candidate members of the young open cluster NGC 2547. Using a combination of
photometry, spectroscopic indices and radial velocities we refine our candidate
list and then use our spectroscopy to study the progression of lithium
depletion in low-mass pre main sequence stars. We derive lithium abundances or
upper limits for all our candidate members, which have effective temperatures
in the range 5000>Teff>3200K, and compare these with predictions for lithium
burning and depletion provided by a number of models and also with the lithium
depletion seen in younger and older stars. We find that some models can
reproduce the lithium abundance pattern of NGC 2547 if the cluster has an age
of ~20-35Myr, which is also indicated by fits to low-mass isochrones in the
Hertzsprung-Russell diagram. But the lack of significant further lithium
depletion between NGC 2547 and older clusters argues for an age of at least
50Myr, more in keeping with the lack of lithium observed in even fainter NGC
2547 candidates. We show that reconciliation of these age estimates may require
additions to the physics incorporated in current generations of pre main
sequence models.Comment: Accepted for publication in MNRAS (better version of Fig.1 available
at http://www.astro.keele.ac.uk/~rdj/
Experimental Proof of Quantum Nonlocality without Squeezing
It is shown that the ensemble where is a Gaussian distribution of finite variance and is a
coherent state can be better discriminated with an entangled measurement than
with any local strategy supplemented by classical communication. Although this
ensemble consists of products of quasi-classical states, it exhibits some
quantum nonlocality. This remarkable effect is demonstrated experimentally by
implementing the optimal local strategy together with a joint nonlocal strategy
that yields a higher fidelity.Comment: 4 pages, 2 figure
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