193 research outputs found
Comment on `Detecting non-Abelian geometric phases with three-level systems'
In their recent paper, Yan-Xiong Du et al. [Phys. Rev. A 84, 034103 (2011)]
claim to have found a non-Abelian adiabatic geometric phase associated with the
energy eigenstates of a large-detuned three-level system. They
further propose a test to detect the non-commutative feature of this geometric
phase. On the contrary, we show that the non-Abelian geometric phase picked up
by the energy eigenstates of a system is trivial in the adiabatic
approximation, while, in the exact treatment of the time evolution, this phase
is very small and cannot be separated from the non-Abelian dynamical phase
acquired along the path in parameter space.Comment: Explicit proof that the non-Abelian geometric phase is trivial added,
journal reference adde
Entanglement in Gaussian matrix-product states
Gaussian matrix product states are obtained as the outputs of projection
operations from an ancillary space of M infinitely entangled bonds connecting
neighboring sites, applied at each of N sites of an harmonic chain. Replacing
the projections by associated Gaussian states, the 'building blocks', we show
that the entanglement range in translationally-invariant Gaussian matrix
product states depends on how entangled the building blocks are. In particular,
infinite entanglement in the building blocks produces fully symmetric Gaussian
states with maximum entanglement range. From their peculiar properties of
entanglement sharing, a basic difference with spin chains is revealed: Gaussian
matrix product states can possess unlimited, long-range entanglement even with
minimum number of ancillary bonds (M=1). Finally we discuss how these states
can be experimentally engineered from N copies of a three-mode building block
and N two-mode finitely squeezed states.Comment: 4 pages, 3 figures. Final version to appear as a Rapid Comm. in PR
The Silenced Discourse: Students with Intellectual Disabilities at the Academy of Music in Sweden
In this article, based on a larger research project, the ambition is to critically discuss the first collaboration between students with intellectual disabilities and the Academy of Music in Sweden. The article presents an analysis of video observations of lessons in rhythmics, related to an encounter between the students with intellectual disabilities and a group of student teachers. The theoretical and methodological framework emanates from post-structuralist and social constructionist theories. The results show that the silenced discourse, the unspoken, is constructed from the fact that the students with disabilities both are insufficiently skilled for the task as leaders in rhythmics, and less skilled than the student teachers. Finally, the silenced discourse is discussed, where assumptions of normality and issues of inclusion are addressed as well as a hegemonic discourse in the Swedish politics of education
Global asymmetry of many-qubit correlations: A lattice gauge theory approach
We introduce a novel bridge between the familiar gauge field theory
approaches used in many areas of modern physics such as quantum field theory
and the SLOCC protocols familiar in quantum information. Although the
mathematical methods are the same the meaning of the gauge group will be
different. The measure we introduce, `twist', is constructed as a Wilson loop
from a correlation induced holonomy. The measure can be understood as the
global asymmetry of the bipartite correlations in a loop of three or more
qubits; if the holonomy is trivial (the identity matrix), the bipartite
correlations can be globally untwisted using general local qubit operations,
the gauge group of our theory, which turns out to be the group of Lorentz
transformations familiar from special relativity. If it is not possible to
globally untwist the bipartite correlations in a state globally using local
operations, the twistedness is given by a non-trivial element of the Lorentz
group, the correlation induced holonomy. We provide several analytical examples
of twisted and untwisted states for three qubits, the most elementary
non-trivial loop one can imagine.Comment: 13 pages, 3 figures, title changed, results and content remain
unchange
Measurement of geometric phase for mixed states using single photon interferometry
Geometric phase may enable inherently fault-tolerant quantum computation.
However, due to potential decoherence effects, it is important to understand
how such phases arise for {\it mixed} input states. We report the first
experiment to measure mixed-state geometric phases in optics, using a
Mach-Zehnder interferometer, and polarization mixed states that are produced in
two different ways: decohering pure states with birefringent elements; and
producing a nonmaximally entangled state of two photons and tracing over one of
them, a form of remote state preparation.Comment: To appear in Phys. Rev. Lett. 4 pages, 3 figure
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