7,448 research outputs found
Tensor Representation of Spin States
We propose a generalization of the Bloch sphere representation for arbitrary
spin states. It provides a compact and elegant representation of spin density
matrices in terms of tensors that share the most important properties of Bloch
vectors. Our representation, based on covariant matrices introduced by Weinberg
in the context of quantum field theory, allows for a simple parametrization of
coherent spin states, and a straightforward transformation of density matrices
under local unitary and partial tracing operations. It enables us to provide a
criterion for anticoherence, relevant in a broader context such as quantum
polarization of light.Comment: 5 pages + 7 pages of supplementary informatio
Anticoherence of spin states with point group symmetries
We investigate multiqubit permutation-symmetric states with maximal entropy
of entanglement. Such states can be viewed as particular spin states, namely
anticoherent spin states. Using the Majorana representation of spin states in
terms of points on the unit sphere, we analyze the consequences of a
point-group symmetry in their arrangement on the quantum properties of the
corresponding state. We focus on the identification of anticoherent states (for
which all reduced density matrices in the symmetric subspace are maximally
mixed) associated with point-group symmetric sets of points. We provide three
different characterizations of anticoherence, and establish a link between
point symmetries, anticoherence and classes of states equivalent through
stochastic local operations with classical communication (SLOCC). We then
investigate in detail the case of small numbers of qubits, and construct
infinite families of anticoherent states with point-group symmetry of their
Majorana points, showing that anticoherent states do exist to arbitrary order.Comment: 15 pages, 5 figure
Tunneling of magnetization versus spin-phonon and spin-spin transitions in LiY_0.998Ho_0.002F_4
Strong hyperfine coupling in a 0.2% Holmium doped LiYF_4 single crystal
induces staircaselike hysteresis loops of the magnetization at very low
temperatures. The field sweep rate dependence of hysteresis loops allows the
study of two different regimes in the magnetic relaxation of these weakly
coupled magnetic moments. At slow field sweep rates, quantum tunneling of the
magnetization occurs at avoided level crossings in the low-energy scheme of a
single ion Ho^3+. At faster sweep rates, nonequilibrated spin-phonon and
spin-spin transitions, mediated by weak dipolar interactions, lead to
magnetization oscillations and additional steps.Comment: 4 figures, Conference proceeding: 1st Joint European Magnetic
Symposia (JEMS01'), Grenoble (France), 28th August - 1st September, 200
The Ongoing Gender Gap in Art Museum Directorships
In a 2014 report, AAMD and the National Center for Arts Research (NCAR) found that a gender gap existed in art museum directorships. We found that women held less than half of directorships, that the average female director's salary lagged behind that of the average male director, and that these phenomena were most persistent in the largest museums. Three years later, despite press attention and field-wide dialogue on the topic, the gender gap persists, although trends showing incremental gains in some areas of pay and employment representation deserve recognition
Quasi-gaussian fixed points and factorial cumulants in nuclear multifragmentation
We re-analyze the conditions for the phenomenon of intermittency
(self-similar fluctuations) to occur in models of multifragmentation. Analyzing
two different mechanisms, the bond-percolation and the ERW (Elattari, Richert
and Wagner) statistical fragmentation models, we point out a common
quasi-gaussian shape of the total multiplicity distribution in the critical
range. The fixed-point property is also observed for the multiplicity of the
second bin. Fluctuations are studied using scaled factorial cumulants instead
of scaled factorial moments. The second-order cumulant displays the
intermittency signal while higher order cumulants are equal to zero, revealing
a large information redundancy in scaled factorial moments. A practical
criterion is proposed to identify the gaussian feature of light-fragment
production, distinguishing between a self-similarity mechanism (ERW) and the
superposition of independent sources (percolation).Comment: 20 pages, uuencoded .tex file including 16 figure
Eigenmodes of Decay and Discrete Fragmentation Processes
Linear rate equations are used to describe the cascading decay of an initial
heavy cluster into fragments. This representation is based upon a triangular
matrix of transition rates. We expand the state vector of mass multiplicities,
which describes the process, into the biorthonormal basis of eigenmodes
provided by the triangular matrix. When the transition rates have a scaling
property in terms of mass ratios at binary fragmentation vertices, we obtain
solvable models with explicit mathematical properties for the eigenmodes. A
suitable continuous limit provides an interpolation between the solvable
models. It gives a general relationship between the decay products and the
elementary transition rates.Comment: 6 pages, plain TEX, 2 figures available from the author
Direct probing of band-structure Berry phase in diluted magnetic semiconductors
We report on experimental evidence of the Berry phase accumulated by the
charge carrier wave function in single-domain nanowires made from a
(Ga,Mn)(As,P) diluted ferromagnetic semiconductor layer. Its signature on the
mesoscopic transport measurements is revealed as unusual patterns in the
magnetoconductance, that are clearly distinguished from the universal
conductance fluctuations. We show that these patterns appear in a magnetic
field region where the magnetization rotates coherently and are related to a
change in the band-structure Berry phase as the magnetization direction
changes. They should be thus considered as a band structure Berry phase
fingerprint of the effective magnetic monopoles in the momentum space. We argue
that this is an efficient method to vary the band structure in a controlled way
and to probe it directly. Hence, (Ga,Mn)As appears to be a very interesting
test bench for new concepts based on this geometrical phase.Comment: 7 pages, 6 figure
Quantum computing of delocalization in small-world networks
We study a quantum small-world network with disorder and show that the system
exhibits a delocalization transition. A quantum algorithm is built up which
simulates the evolution operator of the model in a polynomial number of gates
for exponential number of vertices in the network. The total computational gain
is shown to depend on the parameters of the network and a larger than quadratic
speed-up can be reached.
We also investigate the robustness of the algorithm in presence of
imperfections.Comment: 4 pages, 5 figures, research done at
http://www.quantware.ups-tlse.fr
Optimal measurement strategies for fast entanglement detection
With the advance of quantum information technology, the question of how to
most efficiently test quantum circuits is becoming of increasing relevance.
Here we introduce the statistics of lengths of measurement sequences that
allows one to certify entanglement across a given bi-partition of a multi-qubit
system over the possible sequence of measurements of random unknown states, and
identify the best measurement strategies in the sense of the (on average)
shortest measurement sequence of (multi-qubit) Pauli measurements. The approach
is based on the algorithm of truncated moment sequences that allows one to deal
naturally with incomplete information, i.e. information that does not fully
specify the quantum state. We find that the set of measurements corresponding
to diagonal matrix elements of the moment matrix of the state are particularly
efficient. For symmetric states their number grows only like the third power of
the number of qubits. Their efficiency grows rapidly with , leaving
already for less than a fraction of randomly chosen entangled
states undetected.Comment: 12 pages, 9 figure
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