178 research outputs found
Strengthened Bell inequalities for orthogonal spin directions
We strengthen the bound on the correlations of two spin-1/2 particles
(qubits) in separable (non-entangled) states for locally orthogonal spin
directions by much tighter bounds than the well-known Bell inequality. This
provides a sharper criterion for the experimental distinction between entangled
and separable states, and even one which is a necessary and sufficient
condition for separability. However, these improved bounds do not apply to
local hidden-variable theories, and hence they provide a criterion to test the
correlations allowed by local hidden-variable theories against those allowed by
separable quantum states. Furthermore, these bounds are stronger than some
recent alternative experimentally accessible entanglement criteria. We also
address the issue of finding a finite subset of these inequalities that would
already form a necessary and sufficient condition for non-entanglement. For
mixed state we have not been able to resolve this, but for pure states a set of
six inequalities using only three sets of orthogonal observables is shown to be
already necessary and sufficient for separability.Comment: v2: Considerably changed, many new and stronger results v3: Published
version; To appear in Physics Letters A. Online available from publishers
websit
Tracing Moments
The interactive art system +-now captures moments in the past and present for dreamy, reflective play. It is composed of sand, imagery and interaction. This paper traces the creative process from initial landscape studies to museum installation in 2008
Partial separability and entanglement criteria for multiqubit quantum states
We explore the subtle relationships between partial separability and
entanglement of subsystems in multiqubit quantum states and give experimentally
accessible conditions that distinguish between various classes and levels of
partial separability in a hierarchical order. These conditions take the form of
bounds on the correlations of locally orthogonal observables. Violations of
such inequalities give strong sufficient criteria for various forms of partial
inseparability and multiqubit entanglement. The strength of these criteria is
illustrated by showing that they are stronger than several other well-known
entanglement criteria (the fidelity criterion, violation of Mermin-type
separability inequalities, the Laskowski-\.Zukowski criterion and the
D\"ur-Cirac criterion), and also by showing their great noise robustness for a
variety of multiqubit states, including N-qubit GHZ states and Dicke states.
Furthermore, for N greater than or equal to 3 they can detect bound entangled
states. For all these states, the required number of measurement settings for
implementation of the entanglement criteria is shown to be only N+1. If one
chooses the familiar Pauli matrices as single-qubit observables, the
inequalities take the form of bounds on the anti-diagonal matrix elements of a
state in terms of its diagonal matrix elements.Comment: 25 pages, 3 figures. v4: published versio
A CMOS OTA for HF filters with programmable transfer function
A CMOS operational transconductance amplifier (OTA) for programmable HF filters is presented. When used in an OTA-C integrator, the unity-gain frequency phase error remains less than 0.3° for frequencies up to more than one tenth of the OTA bandwidth. The OTA has built-in phase compensation, which allows tuning of the transconductance of the OTA (Gm) while maintaining a small phase error. Since this phase error is preserved over the Gm range of the OTA, the OTA is suitable for filters with a programmable transfer function. Using stacked square-law transistors improves the linearity, DC-gain, and power-consumption properties of the OT
Addendum to "Sufficient conditions for three-particle entanglement and their tests in recent experiments"
A recent paper [M. Seevinck and J. Uffink, Phys. Rev. A 65, 012107 (2002)]
presented a bound for the three-qubit Mermin inequality such that the violation
of this bound indicates genuine three-qubit entanglement. We show that this
bound can be improved for a specific choice of observables. In particular, if
spin observables corresponding to orthogonal directions are measured at the
qubits (e.g., X and Y spin coordinates) then the bound is the same as the bound
for states with a local hidden variable model. As a consequence, it can
straightforwardly be shown that in the experiment described by J.-W. Pan et al.
[Nature 403, 515 (2000)] genuine three-qubit entanglement was detected.Comment: Two pages, no figures, revtex4; minor changes before publicatio
Monogamy of Correlations vs. Monogamy of Entanglement
A fruitful way of studying physical theories is via the question whether the
possible physical states and different kinds of correlations in each theory can
be shared to different parties. Over the past few years it has become clear
that both quantum entanglement and non-locality (i.e., correlations that
violate Bell-type inequalities) have limited shareability properties and can
sometimes even be monogamous. We give a self-contained review of these results
as well as present new results on the shareability of different kinds of
correlations, including local, quantum and no-signalling correlations. This
includes an alternative simpler proof of the Toner-Verstraete monogamy
inequality for quantum correlations, as well as a strengthening thereof.
Further, the relationship between sharing non-local quantum correlations and
sharing mixed entangled states is investigated, and already for the simplest
case of bi-partite correlations and qubits this is shown to be non-trivial.
Also, a recently proposed new interpretation of Bell's theorem by Schumacher in
terms of shareability of correlations is critically assessed. Finally, the
relevance of monogamy of non-local correlations for secure quantum key
distribution is pointed out, although, and importantly, it is stressed that not
all non-local correlations are monogamous.Comment: 12 pages, 2 figures. Invited submission to a special issue of Quantum
Information Processing. v2: Published version. Open acces
Entanglement Detection in the Stabilizer Formalism
We investigate how stabilizer theory can be used for constructing sufficient
conditions for entanglement. First, we show how entanglement witnesses can be
derived for a given state, provided some stabilizing operators of the state are
known. These witnesses require only a small effort for an experimental
implementation and are robust against noise. Second, we demonstrate that also
nonlinear criteria based on uncertainty relations can be derived from
stabilizing operators. These criteria can sometimes improve the witnesses by
adding nonlinear correction terms. All our criteria detect states close to
Greenberger-Horne-Zeilinger states, cluster and graph states. We show that
similar ideas can be used to derive entanglement conditions for states which do
not fit the stabilizer formalism, such as the three-qubit W state. We also
discuss connections between the witnesses and some Bell inequalities.Comment: 15 pages including 2 figures, revtex4; typos corrected, presentation
improved; to appear in PR
Quadratic Bell inequalities as tests for multipartite entanglement
This letter presents quantum mechanical inequalities which distinguish, for
systems of spin-\half particles (), between fully entangled states
and states in which at most particles are entangled. These inequalities
are stronger than those obtained by Gisin and Bechmann-Pasquinucci [Phys.\
Lett. A {\bf 246}, 1 (1998)] and by Seevinck and Svetlichny [quant-ph/0201046].Comment: 4 pages, including 1 figure. Typo's removed and one proof simplified
in revised versio
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