197 research outputs found
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
Relativistic entanglement of two massive particles
We describe the spin and momentum degrees of freedom of a system of two
massive spin-- particles as a 4 qubit system. Then we explicitly
show how the entanglement changes between different partitions of the qubits,
when considered by different inertial observers. Although the two particle
entanglement corresponding to a partition into Alice's and Bob's subsystems is,
as often stated in the literature, invariant under Lorentz boosts, the
entanglement with respect to other partitions of the Hilbert space on the other
hand, is not. It certainly does depend on the chosen inertial frame and on the
initial state considered. The change of entanglement arises, because a Lorentz
boost on the momenta of the particles causes a Wigner rotation of the spin,
which in certain cases entangles the spin- with the momentum states. We
systematically investigate the situation for different classes of initial spin
states and different partitions of the 4 qubit space.
Furthermore, we study the behavior of Bell inequalities for different
observers and demonstrate how the maximally possible degree of violation, using
the Pauli-Lubanski spin observable, can be recovered by any inertial observer.Comment: 17 pages, 4 figure
Two computable sets of multipartite entanglement measures
We present two sets of computable entanglement measures for multipartite
systems where each subsystem can have different degrees of freedom (so-called
qudits). One set, called 'separability' measure, reveals which of the
subsystems are separable/entangled. For that we have to extend the concept of
k-separability for multipartite systems to a novel unambiguous separability
concept which we call \gamma_k-separability. The second set of entanglement
measures reveals the 'kind' of entanglement, i.e. if it is bipartite,
tripartite, ..., n-partite entangled and is denoted as the 'physical' measure.
We show how lower bounds on both sets of measures can be obtained by the
observation that any entropy may be rewritten via operational expressions known
as m-concurrences. Moreover, for different classes of bipartite or multipartite
qudit systems we compute the bounds explicitly and discover that they are often
tight or equivalent to positive partial transposition (PPT).Comment: 3 figures, 21 page
Revealing Bell's Nonlocality for Unstable Systems in High Energy Physics
Entanglement and its consequences - in particular the violation of Bell
inequalities, which defies our concepts of realism and locality - have been
proven to play key roles in Nature by many experiments for various quantum
systems. Entanglement can also be found in systems not consisting of ordinary
matter and light, i.e. in massive meson--antimeson systems. Bell inequalities
have been discussed for these systems, but up to date no direct experimental
test to conclusively exclude local realism was found. This mainly stems from
the fact that one only has access to a restricted class of observables and that
these systems are also decaying. In this Letter we put forward a Bell
inequality for unstable systems which can be tested at accelerator facilities
with current technology. Herewith, the long awaited proof that such systems at
different energy scales can reveal the sophisticated "dynamical" nonlocal
feature of Nature in a direct experiment gets feasible. Moreover, the role of
entanglement and CP violation, an asymmetry between matter and antimatter, is
explored, a special feature offered only by these meson-antimeson systems.Comment: 6 pages, 3 figure
Heisenberg's Uncertainty Relation and Bell Inequalities in High Energy Physics
An effective formalism is developed to handle decaying two-state systems.
Herewith, observables of such systems can be described by a single operator in
the Heisenberg picture. This allows for using the usual framework in quantum
information theory and, hence, to enlighten the quantum feature of such systems
compared to non-decaying systems. We apply it to systems in high energy
physics, i.e. to oscillating meson-antimeson systems. In particular, we discuss
the entropic Heisenberg uncertainty relation for observables measured at
different times at accelerator facilities including the effect of CP violation,
i.e. the imbalance of matter and antimatter. An operator-form of Bell
inequalities for systems in high energy physics is presented, i.e. a
Bell-witness operator, which allows for simple analysis of unstable systems.Comment: 17 page
Finite-Temperature Scaling of Magnetic Susceptibility and Geometric Phase in the XY Spin Chain
We study the magnetic susceptibility of 1D quantum XY model, and show that
when the temperature approaches zero, the magnetic susceptibility exhibits the
finite-temperature scaling behavior. This scaling behavior of the magnetic
susceptibility in 1D quantum XY model, due to the quantum-classical mapping,
can be easily experimentally tested. Furthermore, the universality in the
critical properties of the magnetic susceptibility in quantum XY model is
verified. Our study also reveals the close relation between the magnetic
susceptibility and the geometric phase in some spin systems, where the quantum
phase transitions are driven by an external magnetic field.Comment: 6 pages, 4 figures, get accepted for publication by J. Phys. A: Math.
Theo
Multi-distributed Entanglement in Finitely Correlated Chains
The entanglement-sharing properties of an infinite spin-chain are studied
when the state of the chain is a pure, translation-invariant state with a
matrix-product structure. We study the entanglement properties of such states
by means of their finitely correlated structure. These states are recursively
constructed by means of an auxiliary density matrix \rho on a matrix algebra B
and a completely positive map E: A \otimes B -> B, where A is the spin 2\times
2 matrix algebra. General structural results for the infinite chain are
therefore obtained by explicit calculations in (finite) matrix algebras. In
particular, we study not only the entanglement shared by nearest-neighbours,
but also, differently from previous works, the entanglement shared between
connected regions of the spin-chain. This range of possible applications is
illustrated and the maximal concurrence C=1/\sqrt{2} for the entanglement of
connected regions can actually be reached.Comment: 7 pages, 2 figures, to be published in Eur.Phys.Let
Berry phase in entangled systems: a proposed experiment with single neutrons
The influence of the geometric phase, in particular the Berry phase, on an
entangled spin-1/2 system is studied. We discuss in detail the case, where the
geometric phase is generated only by one part of the Hilbert space. We are able
to cancel the effects of the dynamical phase by using the ``spin-echo'' method.
We analyze how the Berry phase affects the Bell angles and the maximal
violation of a Bell inequality. Furthermore we suggest an experimental
realization of our setup within neutron interferometry.Comment: 10 pages, 6 figures, Introduction extended, References adde
Experimental GHZ Entanglement beyond Qubits
The Greenberger-Horne-Zeilinger (GHZ) argument provides an all-or-nothing
contradiction between quantum mechanics and local-realistic theories. In its
original formulation, GHZ investigated three and four particles entangled in
two dimensions only. Very recently, higher dimensional contradictions
especially in three dimensions and three particles have been discovered but it
has remained unclear how to produce such states. In this article we
experimentally show how to generate a three-dimensional GHZ state from
two-photon orbital-angular-momentum entanglement. The first suggestion for a
setup which generates three-dimensional GHZ entanglement from these entangled
pairs came from using the computer algorithm Melvin. The procedure employs
novel concepts significantly beyond the qubit case. Our experiment opens up the
possibility of a truly high-dimensional test of the GHZ-contradiction which,
interestingly, employs non-Hermitian operators.Comment: 6+6 pages, 8 figure
- …