781 research outputs found
Universal resources for approximate and stochastic measurement-based quantum computation
We investigate which quantum states can serve as universal resources for
approximate and stochastic measurement-based quantum computation, in the sense
that any quantum state can be generated from a given resource by means of
single-qubit (local) operations assisted by classical communication. More
precisely, we consider the approximate and stochastic generation of states,
resulting e.g. from a restriction to finite measurement settings or from
possible imperfections in the resources or local operations. We show that
entanglement-based criteria for universality obtained for the exact,
deterministic case can be lifted to the much more general approximate,
stochastic case, moving from the idealized situation considered in previous
works, to the practically relevant context of non-perfect state preparation. We
find that any entanglement measure fulfilling some basic requirements needs to
reach its maximum value on some element of an approximate, stochastic universal
family of resource states, as the resource size grows. This allows us to rule
out various families of states as being approximate, stochastic universal. We
provide examples of resources that are efficient approximate universal, but not
exact deterministic universal. We also study the robustness of universal
resources for measurement-based quantum computation under realistic assumptions
about the (imperfect) generation and manipulation of entangled states, giving
an explicit expression for the impact that errors made in the preparation of
the resource have on the possibility to use it for universal approximate and
stochastic state preparation. Finally, we discuss the relation between our
entanglement-based criteria and recent results regarding the uselessness of
states with a high degree of geometric entanglement as universal resources.Comment: 17 pages; abstract shortened with respect to the published version to
respect the arXiv limit of 1,920 character
Controlling Spin Exchange Interactions of Ultracold Atoms in Optical Lattices
We describe a general technique that allows to induce and control strong
interaction between spin states of neighboring atoms in an optical lattice. We
show that the properties of spin exchange interactions, such as magnitude,
sign, and anisotropy can be designed by adjusting the optical potentials. We
illustrate how this technique can be used to efficiently ``engineer'' quantum
spin systems with desired properties, for specific examples ranging from
scalable quantum computation to probing a model with non-trivial topological
orders that supports exotic non-abelian anyonic excitations.Comment: 5 pages, 2 figures, revte
Multiparticle entanglement purification for two-colorable graph states
We investigate multiparticle entanglement purification schemes which allow
one to purify all two colorable graph states, a class of states which includes
e.g. cluster states, GHZ states and codewords of various error correction
codes. The schemes include both recurrence protocols and hashing protocols. We
analyze these schemes under realistic conditions and observe for a generic
error model that the threshold value for imperfect local operations depends on
the structure of the corresponding interaction graph, but is otherwise
independent of the number of parties. The qualitative behavior can be
understood from an analytically solvable model which deals only with a
restricted class of errors. We compare direct multiparticle entanglement
purification protocols with schemes based on bipartite entanglement
purification and show that the direct multiparticle entanglement purification
is more efficient and the achievable fidelity of the purified states is larger.
We also show that the purification protocol allows one to produce private
entanglement, an important aspect when using the produced entangled states for
secure applications. Finally we discuss an experimental realization of a
multiparty purification protocol in optical lattices which is issued to improve
the fidelity of cluster states created in such systems.Comment: 22 pages, 8 figures; replaced with published versio
Single-Photon Generation from Stored Excitation in an Atomic Ensemble
Single photons are generated from an ensemble of cold Cs atoms via the
protocol of Duan et al. [Nature \textbf{414}, 413 (2001)]. Conditioned upon an
initial detection from field 1 at 852 nm, a photon in field 2 at 894 nm is
produced in a controlled fashion from excitation stored within the atomic
ensemble. The single-quantum character of the field 2 is demonstrated by the
violation of a Cauchy-Schwarz inequality, namely , where describes detection of two events
conditioned upon an initial detection , with
for single photons.Comment: 5 pages, 4 figure
Quantum communication cost of preparing multipartite entanglement
We study the preparation and distribution of high-fidelity multi-party
entangled states via noisy channels and operations. In the particular case of
GHZ and cluster states, we study different strategies using bipartite or
multipartite purification protocols. The most efficient strategy depends on the
target fidelity one wishes to achieve and on the quality of transmission
channel and local operations. We show the existence of a crossing point beyond
which the strategy making use of the purification of the state as a whole is
more efficient than a strategy in which pairs are purified before they are
connected to the final state. We also study the efficiency of intermediate
strategies, including sequences of purification and connection. We show that a
multipartite strategy is to be used if one wishes to achieve high fidelity,
whereas a bipartite strategy gives a better yield for low target fidelity.Comment: 21 pages, 17 figures; accepted for publication in Phys. Rev. A; v2:
corrections in figure
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