903 research outputs found
Generation of GHZ entangled states of photons in multiple cavities via a superconducting qutrit or an atom through resonant interaction
We propose an efficient method to generate a GHZ entangled state of n photons
in n microwave cavities (or resonators) via resonant interaction to a single
superconducting qutrit. The deployment of a qutrit, instead of a qubit, as the
coupler enables us to use resonant interactions exclusively for all
qutrit-cavity and qutrit-pulse operations. This unique approach significantly
shortens the time of operation which is advantageous to reducing the adverse
effects of qutrit decoherence and cavity decay on fidelity of the protocol.
Furthermore, the protocol involves no measurement on either the state of qutrit
or cavity photons. We also show that the protocol can be generalized to other
systems by replacing the superconducting qutrit coupler with different types of
physical qutrit, such as an atom in the case of cavity QED, to accomplish the
same task.Comment: 11 pages, 5 figures, accepted by Phys. Rev.
Quantum mechanics and elements of reality inferred from joint measurements
The Einstein-Podolsky-Rosen argument on quantum mechanics incompleteness is
formulated in terms of elements of reality inferred from joint (as opposed to
alternative) measurements, in two examples involving entangled states of three
spin-1/2 particles. The same states allow us to obtain proofs of the
incompatibility between quantum mechanics and elements of reality.Comment: LaTeX, 12 page
Physical model for the generation of ideal resources in multipartite quantum networking
We propose a physical model for generating multipartite entangled states of
spin- particles that have important applications in distributed quantum
information processing. Our protocol is based on a process where mobile spins
induce the interaction among remote scattering centers. As such, a major
advantage lies on the management of stationary and well separated spins. Among
the generable states, there is a class of -qubit singlets allowing for
optimal quantum telecloning in a scalable and controllable way. We also show
how to prepare Aharonov, W and Greenberger-Horne-Zeilinger states.Comment: 5 pages, 2 figures. Format revise
Generating entangled photon pairs from a cavity-QED system
We propose a scheme for the controlled generation of Einstein-Podosky-Rosen
(EPR) entangled photon pairs from an atom coupled to a high Q optical cavity,
extending the prototype system as a source for deterministic single photons. A
thorough theoretical analysis confirms the promising operating conditions of
our scheme as afforded by currently available experimental setups. Our result
demonstrates the cavity QED system as an efficient and effective source for
entangled photon pairs, and shines new light on its important role in quantum
information science.Comment: It has recently come to our attention that the experiment by T. Wilk,
S. C. Webster, A. Kuhn and G. Rempe, published in Science 317, 488 (2007),
exactly realizes what we proposed in this article, which is published in Phy.
Rev. A 040302(R) (2005
Minimal instances for toric code ground states
A decade ago Kitaev's toric code model established the new paradigm of
topological quantum computation. Due to remarkable theoretical and experimental
progress, the quantum simulation of such complex many-body systems is now
within the realms of possibility. Here we consider the question, to which
extent the ground states of small toric code systems differ from LU-equivalent
graph states. We argue that simplistic (though experimentally attractive)
setups obliterate the differences between the toric code and equivalent graph
states; hence we search for the smallest setups on the square- and triangular
lattice, such that the quasi-locality of the toric code hamiltonian becomes a
distinctive feature. To this end, a purely geometric procedure to transform a
given toric code setup into an LC-equivalent graph state is derived. In
combination with an algorithmic computation of LC-equivalent graph states, we
find the smallest non-trivial setup on the square lattice to contain 5
plaquettes and 16 qubits; on the triangular lattice the number of plaquettes
and qubits is reduced to 4 and 9, respectively.Comment: 14 pages, 11 figure
Probabilistic Quantum Encoder for Single-Photon Qubits
We describe an experiment in which a physical qubit represented by the
polarization state of a single-photon was probabilistically encoded in the
logical state of two photons. The experiment relied on linear optics,
post-selection, and three-photon interference effects produced by a parametric
down-conversion photon pair and a weak coherent state. An interesting
consequence of the encoding operation was the ability to observe entangled
three-photon Greenberger-Horne-Zeilinger states.Comment: 4 pages, 4 figures; submitted to Phys. Rev.
Wigner-Yanase skew information as tests for quantum entanglement
A Bell-type inequality is proposed in terms of Wigner-Yanase skew
information, which is quadratic and involves only one local spin observable at
each site. This inequality presents a hierarchic classification of all states
of multipartite quantum systems from separable to fully entangled states, which
is more powerful than the one presented by quadratic Bell inequalities from
two-entangled to fully entangled states. In particular, it is proved that the
inequality provides an exact test to distinguish entangled from nonentangled
pure states of two qubits. Our inequality sheds considerable light on
relationships between quantum entanglement and information theory.Comment: 5 page
Highly Entangled Ground States in Tripartite Qubit Systems
We investigate the creation of highly entangled ground states in a system of
three exchange-coupled qubits arranged in a ring geometry. Suitable magnetic
field configurations yielding approximate GHZ and exact W ground states are
identified. The entanglement in the system is studied at finite temperature in
terms of the mixed-state tangle tau. By adapting a steepest-descent
optimization algorithm we demonstrate that tau can be evaluated efficiently and
with high precision. We identify the parameter regime for which the equilibrium
entanglement of the tripartite system reaches its maximum.Comment: 4 pages, 2 figure
Permutation asymmetry inducing entanglement between degrees of freedom in multiphoton states
We describe and examine entanglement between different degrees of freedom in
multiphoton states based on the permutation properties. From the state
description, the entanglement comes from the permutation asymmetry. According
to the different permutation properties, the multiphoton states can be divided
into several parts. It will help to deal with the multiphoton interference,
which can be used as the measurement of the entanglement.Comment: Final versio
Non-local correlations as an information theoretic resource
It is well known that measurements performed on spatially separated entangled
quantum systems can give rise to correlations that are non-local, in the sense
that a Bell inequality is violated. They cannot, however, be used for
super-luminal signalling. It is also known that it is possible to write down
sets of ``super-quantum'' correlations that are more non-local than is allowed
by quantum mechanics, yet are still non-signalling. Viewed as an information
theoretic resource, super-quantum correlations are very powerful at reducing
the amount of communication needed for distributed computational tasks. An
intriguing question is why quantum mechanics does not allow these more powerful
correlations. We aim to shed light on the range of quantum possibilities by
placing them within a wider context. With this in mind, we investigate the set
of correlations that are constrained only by the no-signalling principle. These
correlations form a polytope, which contains the quantum correlations as a
(proper) subset. We determine the vertices of the no-signalling polytope in the
case that two observers each choose from two possible measurements with d
outcomes. We then consider how interconversions between different sorts of
correlations may be achieved. Finally, we consider some multipartite examples.Comment: Revtex. 12 pages, 6 figure
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