685 research outputs found
Entanglement molecules
We investigate the entanglement properties of multiparticle systems,
concentrating on the case where the entanglement is robust against disposal of
particles. Two qubits -belonging to a multipartite system- are entangled in
this sense iff their reduced density matrix is entangled. We introduce a family
of multiqubit states, for which one can choose for any pair of qubits
independently whether they should be entangled or not as well as the relative
strength of the entanglement, thus providing the possibility to construct all
kinds of ''Entanglement molecules''. For some particular configurations, we
also give the maximal amount of entanglement achievable.Comment: 4 pages, 1 figur
Quantum simulation of classical thermal states
We establish a connection between ground states of local quantum Hamiltonians
and thermal states of classical spin systems. For any discrete classical
statistical mechanical model in any spatial dimension, we find an associated
quantum state such that the reduced density operator behaves as the thermal
state of the classical system. We show that all these quantum states are unique
ground states of a universal 5-body local quantum Hamiltonian acting on a
(polynomially enlarged) system of qubits arranged on a 2D lattice. The only
free parameters of the quantum Hamiltonian are coupling strengthes of two-body
interactions, which allow one to choose the type and dimension of the classical
model as well as the interaction strength and temperature.Comment: 4 pages, 1 figur
Multiparticle entanglement and its experimental detection
We discuss several aspects of multiparticle mixed state entanglement and its
experimental detection. First we consider entanglement between two particles
which is robust against disposals of other particles. To completely detect
these kinds of entanglement, full knowledge of the multiparticle density matrix
(or of all reduced density matrixes) is required. Then we review the relation
of the separability properties of l-partite splittings of a state to its
multipartite entanglement properties. We show that it suffices to determine the
diagonal matrix elements of in a certain basis in order to detect
multiparticle entanglement properties of . We apply these observations to
analyze two recent experiments, where multiparticle entangled states of 3 (4)
particles were produced. Finally, we focus on bound entangled states
(non-separable, non-distillable states) and show that they can be activated by
joint actions of the parties. We also provide several examples which show the
activation of bound entanglement with bound entanglement.Comment: 9 pages, no figures; submitted to The Journal of Physics A:
Mathematical and General, special issue in Quantum Information and
Computatio
Macroscopic superpositions require tremendous measurement devices
We consider fundamental limits on the detectable size of macroscopic quantum
superpositions. We argue that a full quantum mechanical treatment of system
plus measurement device is required, and that a (classical) reference frame for
phase or direction needs to be established to certify the quantum state. When
taking the size of such a classical reference frame into account, we show that
to reliably distinguish a quantum superposition state from an incoherent
mixture requires a measurement device that is quadratically bigger than the
superposition state. Whereas for moderate system sizes such as generated in
previous experiments this is not a stringent restriction, for macroscopic
superpositions of the size of a cat the required effort quickly becomes
intractable, requiring measurement devices of the size of the Earth. We
illustrate our results using macroscopic superposition states of photons,
spins, and position. Finally, we also show how this limitation can be
circumvented by dealing with superpositions in relative degrees of freedom.Comment: 20 pages (including appendices), 1 Figur
Entanglement properties of multipartite entangled states under the influence of decoherence
We investigate entanglement properties of multipartite states under the
influence of decoherence. We show that the lifetime of (distillable)
entanglement for GHZ-type superposition states decreases with the size of the
system, while for a class of other states -namely all graph states with
constant degree- the lifetime is independent of the system size. We show that
these results are largely independent of the specific decoherence model and are
in particular valid for all models which deal with individual couplings of
particles to independent environments, described by some quantum optical master
equation of Lindblad form. For GHZ states, we derive analytic expressions for
the lifetime of distillable entanglement and determine when the state becomes
fully separable. For all graph states, we derive lower and upper bounds on the
lifetime of entanglement. To this aim, we establish a method to calculate the
spectrum of the partial transposition for all mixed states which are diagonal
in a graph state basis. We also consider entanglement between different groups
of particles and determine the corresponding lifetimes as well as the change of
the kind of entanglement with time. This enables us to investigate the behavior
of entanglement under re-scaling and in the limit of large (infinite) number of
particles. Finally we investigate the lifetime of encoded quantum superposition
states and show that one can define an effective time in the encoded system
which can be orders of magnitude smaller than the physical time. This provides
an alternative view on quantum error correction and examples of states whose
lifetime of entanglement (between groups of particles) in fact increases with
the size of the system.Comment: 27 pages, 11 figure
- …