524 research outputs found
On the entanglement structure in quantum cloning
We study the entanglement properties of the output state of a universal
cloning machine. We analyse in particular bipartite and tripartite entanglement
of the clones, and discuss the ``classical limit'' of infinitely many output
copies.Comment: 7 pages, 1 figure, contribution to "David Mermin Festschrift",
Foundations of Physic
Relations between Entanglement Witnesses and Bell Inequalities
Bell inequalities, considered within quantum mechanics, can be regarded as
non-optimal witness operators. We discuss the relationship between such Bell
witnesses and general entanglement witnesses in detail for the Bell inequality
derived by Clauser, Horne, Shimony, and Holt (CHSH). We derive bounds on how
much an optimal witness has to be shifted by adding the identity operator to
make it positive on all states admitting a local hidden variable model. In the
opposite direction, we obtain tight bounds for the maximal proportion of the
identity operator that can be subtracted from such a CHSH witness, while
preserving the witness properties. Finally, we investigate the structure of
CHSH witnesses directly by relating their diagonalized form to optimal
witnesses of two different classes.Comment: 8 pages, 2 figure
Optimal quantum repeaters for qubits and qudits
A class of optimal quantum repeaters for qubits is suggested. The schemes are
minimal, i.e. involve a single additional probe qubit, and optimal, i.e.
provide the maximum information adding the minimum amount of noise. Information
gain and state disturbance are quantified by fidelities which, for our schemes,
saturate the ultimate bound imposed by quantum mechanics for randomly
distributed signals. Special classes of signals are also investigated, in order
to improve the information-disturbance trade-off. Extension to higher
dimensional signals (qudits) is straightforward.Comment: Revised version. To appear in PR
LocalâRegional Similarity in Drylands Increases During Multiyear Wet and Dry Periods and in Response to Extreme Events
Climate change is predicted to impact ecosystems through altered precipitation (PPT) regimes. In the Chihuahuan Desert, multiyear wet and dry periods and extreme PPT pulses are the most influential climatic events for vegetation. Vegetation responses are most frequently studied locally, and regional responses are often unclear. We present an approach to quantify correlation of PPT and vegetation responses (as Normalized Difference Vegetation Index [NDVI]) at the Jornada ARSâLTER site (JRN; 550 km2 area) and the surrounding dryland region (from 0 to 500 km distance; 400,000 km2 study area) as a way to understand regional similarity to locally observed patterns. We focused on fluctuating wet and dry years, multiyear wet or dry periods of 3â4 yr, and multiyear wet periods that contained one or more extreme high PPT pulses or extreme low rainfall. In all but extreme high PPT years, JRN PPT was highly correlated... (See article for full abstract)
Cloning a real d-dimensional quantum state on the edge of the no-signaling condition
We investigate a new class of quantum cloning machines that equally duplicate
all real states in a Hilbert space of arbitrary dimension. By using the
no-signaling condition, namely that cloning cannot make superluminal
communication possible, we derive an upper bound on the fidelity of this class
of quantum cloning machines. Then, for each dimension d, we construct an
optimal symmetric cloner whose fidelity saturates this bound. Similar
calculations can also be performed in order to recover the fidelity of the
optimal universal cloner in d dimensions.Comment: 6 pages RevTex, 1 encapuslated Postscript figur
Optimal estimation of multiple phases
We study the issue of simultaneous estimation of several phase shifts induced
by commuting operators on a quantum state. We derive the optimal positive
operator-valued measure corresponding to the multiple-phase estimation. In
particular, we discuss the explicit case of the optimal detection of double
phase for a system of identical qutrits and generalise these results to optimal
multiple phase detection for d-dimensional quantum states.Comment: 6 page
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