308 research outputs found
Global-to-local incompatibility, monogamy of entanglement, and ground-state dimerization: Theory and observability of quantum frustration in systems with competing interactions
Frustration in quantum many body systems is quantified by the degree of
incompatibility between the local and global orders associated, respectively,
to the ground states of the local interaction terms and the global ground state
of the total many-body Hamiltonian. This universal measure is bounded from
below by the ground-state bipartite block entanglement. For many-body
Hamiltonians that are sums of two-body interaction terms, a further inequality
relates quantum frustration to the pairwise entanglement between the
constituents of the local interaction terms. This additional bound is a
consequence of the limits imposed by monogamy on entanglement shareability. We
investigate the behavior of local pair frustration in quantum spin models with
competing interactions on different length scales and show that valence bond
solids associated to exact ground-state dimerization correspond to a transition
from generic frustration, i.e. geometric, common to classical and quantum
systems alike, to genuine quantum frustration, i.e. solely due to the
non-commutativity of the different local interaction terms. We discuss how such
frustration transitions separating genuinely quantum orders from classical-like
ones are detected by observable quantities such as the static structure factor
and the interferometric visibility.Comment: 11 pages, 7 figures. Matches published versio
Four-photon orbital angular momentum entanglement
Quantum entanglement shared between more than two particles is essential to
foundational questions in quantum mechanics, and upcoming quantum information
technologies. So far, up to 14 two-dimensional qubits have been entangled, and
an open question remains if one can also demonstrate entanglement of
higher-dimensional discrete properties of more than two particles. A promising
route is the use of the photon orbital angular momentum (OAM), which enables
implementation of novel quantum information protocols, and the study of
fundamentally new quantum states. To date, only two of such multidimensional
particles have been entangled albeit with ever increasing dimensionality. Here
we use pulsed spontaneous parametric downconversion (SPDC) to produce photon
quadruplets that are entangled in their OAM, or transverse-mode degrees of
freedom; and witness genuine multipartite Dicke-type entanglement. Apart from
addressing foundational questions, this could find applications in quantum
metrology, imaging, and secret sharing.Comment: 5 pages, 4 figure
The state space for two qutrits has a phase space structure in its core
We investigate the state space of bipartite qutrits. For states which are
locally maximally mixed we obtain an analog of the ``magic'' tetrahedron for
bipartite qubits--a magic simplex W. This is obtained via the Weyl group which
is a kind of ``quantization'' of classical phase space. We analyze how this
simplex W is embedded in the whole state space of two qutrits and discuss
symmetries and equivalences inside the simplex W. Because we are explicitly
able to construct optimal entanglement witnesses we obtain the border between
separable and entangled states. With our method we find also the total area of
bound entangled states of the parameter subspace under intervestigation. Our
considerations can also be applied to higher dimensions.Comment: 3 figure
A simplex of bound entangled multipartite qubit states
We construct a simplex for multipartite qubit states of even number n of
qubits, which has the same geometry concerning separability, mixedness, kind of
entanglement, amount of entanglement and nonlocality as the bipartite qubit
states. We derive the entanglement of the class of states which can be
described by only three real parameters with the help of a multipartite measure
for all discrete systems. We prove that the bounds on this measure are optimal
for the whole class of states and that it reveals that the states possess only
n-partite entanglement and not e.g. bipartite entanglement. We then show that
this n-partite entanglement can be increased by stochastic local operations and
classical communication to the purest maximal entangled states. However, pure
n-partite entanglement cannot be distilled, consequently all entangled states
in the simplex are n-partite bound entangled. We study also Bell inequalities
and find the same geometry as for bipartite qubits. Moreover, we show how the
(hidden) nonlocality for all n-partite bound entangled states can be revealed.Comment: 11 pages, 4 figures; 2nd version changed considerably and a detailed
derivation of the multipartite measure is include
Are collapse models testable with quantum oscillating systems? The case of neutrinos, kaons, chiral molecules
Collapse models provide a theoretical framework for understanding how
classical world emerges from quantum mechanics. Their dynamics preserves
(practically) quantum linearity for microscopic systems, while it becomes
strongly nonlinear when moving towards macroscopic scale. The conventional
approach to test collapse models is to create spatial superpositions of
mesoscopic systems and then examine the loss of interference, while
environmental noises are engineered carefully. Here we investigate a different
approach: We study systems that naturally oscillate --creating quantum
superpositions-- and thus represent a natural case-study for testing quantum
linearity: neutrinos, neutral mesons, and chiral molecules. We will show how
spontaneous collapses affect their oscillatory behavior, and will compare them
with environmental decoherence effects. We will show that, contrary to what
previously predicted, collapse models cannot be tested with neutrinos. The
effect is stronger for neutral mesons, but still beyond experimental reach.
Instead, chiral molecules can offer promising candidates for testing collapse
models.Comment: accepted by NATURE Scientific Reports, 12 pages, 1 figures, 2 table
Macroscopic Observables Detecting Genuine Multipartite Entanglement and Partial Inseparability in Many-Body Systems
We show a general approach for detecting genuine multipartite entanglement
(GME) and partial inseparability in many-body-systems by means of macroscopic
observables (such as the energy) only. We show that the obtained criteria, the
"GME gap" and "the k-entanglement gap", detect large areas of genuine
multipartite entanglement and partial entanglement in typical many body states,
which are not detected by other criteria. As genuine multipartite entanglement
is a necessary property for several quantum information theoretic applications
such as e.g. secret sharing or certain kinds of quantum computation, our
methods can be used to select or design appropriate condensed matter systems.Comment: 4 pages, 3 figures, published version, title extende
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