25 research outputs found
Observables suitable for restricting the fidelity to multipartite maximally entangled states
We present a class of observables which are suitable for determining the
fidelity of a state to the multipartite Greenberger-Horne-Zeilinger (GHZ)
state. Given an expectation value of an observable belonging to the class, we
give a simple formula that gives a lower bound and an upper bound for the
fidelity. Applying the formula to the GHZ-state preparation experiment by Pan
{\it et al}. {[Nature (London) {\bf 403}, 515 (2000)]}, we show that the
observed state lies outside of the class of biseparable mixed three-qubit
states. We also show that for this class of operators, adopting the principle
of minimum variance {[Phys. Rev. A {\bf 60}, 4338 (1999)]} in the state
estimation always results in the state with the minimum fidelity.Comment: 6 page
Efficient and robust entanglement generation in a many-particle system with resonant dipole-dipole interactions
We propose and discuss a scheme for robust and efficient generation of
many-particle entanglement in an ensemble of Rydberg atoms with resonant
dipole-dipole interactions. It is shown that in the limit of complete dipole
blocking, the system is isomorphic to a multimode Jaynes-Cummings model. While
dark-state population transfer is not capable of creating entanglement, other
adiabatic processes are identified that lead to complex, maximally entangled
states, such as the N-particle analog of the GHZ state in a few steps. The
process is robust, works for even and odd particle numbers and the
characteristic time for entanglement generation scales with N^a, with a being
less than unity.Comment: 4 figure
Measurement-based quantum foundations
I show that quantum theory is the only probabilistic framework that permits
arbitrary processes to be emulated by sequences of local measurements. This
supports the view that, contrary to conventional wisdom, measurement should not
be regarded as a complex phenomenon in need of a dynamical explanation but
rather as a primitive -- and perhaps the only primitive -- operation of the
theory.Comment: 8 pages, version to appear in Found. Phy
Anthropogenic Space Weather
Anthropogenic effects on the space environment started in the late 19th
century and reached their peak in the 1960s when high-altitude nuclear
explosions were carried out by the USA and the Soviet Union. These explosions
created artificial radiation belts near Earth that resulted in major damages to
several satellites. Another, unexpected impact of the high-altitude nuclear
tests was the electromagnetic pulse (EMP) that can have devastating effects
over a large geographic area (as large as the continental United States). Other
anthropogenic impacts on the space environment include chemical release ex-
periments, high-frequency wave heating of the ionosphere and the interaction of
VLF waves with the radiation belts. This paper reviews the fundamental physical
process behind these phenomena and discusses the observations of their impacts.Comment: 71 pages, 35 figure