3,491 research outputs found
Continuous-variable multipartite unlockable bound entangled Gaussian states
Continuous-variable (CV) multipartite unlockable bound-entangled states is
investigated in this paper. Comparing with the qubit multipartite unlockable
bound-entangled states, CV multipartite unlockable bound-entangled states
present the new and different properties. CV multipartite unlockable
bound-entangled states may serve as a useful quantum resource for new
multiparty communication schemes. The experimental protocol for generating CV
unlockable bound-entangled states is proposed with a setup that is at present
accessible.Comment: 6 pages, 4 figure
Characterization of Collective Gaussian Attacks and Security of Coherent-State Quantum Cryptography
We provide a simple description of the most general collective Gaussian
attack in continuous-variable quantum cryptography. In the scenario of such
general attacks, we analyze the asymptotic secret-key rates which are
achievable with coherent states, joint measurements of the quadratures and
one-way classical communication.Comment: 4 pages, 1 figure + 1 Table, REVteX. More descriptive titl
Exponentially Enhanced Quantum Metrology
We show that when a suitable entanglement generating unitary operator
depending on a parameter is applied on N qubits in parallel, and an appropriate
observable is measured, a precision of order 2 raised to the power (-N) in
estimating the parameter may be achieved. This exponentially improves the
precision achievable in classical and in quantum non-entangling parallel
strategies. We propose a quantum-optics model of laser light interacting with
an N-qubit system, say a polyatomic molecule, via a generalized Jaynes-Cummings
interaction which, in principle, could achieve the exponentially enhanced
precision.Comment: 4 pages, 1 postscript figure ; typos correcte
Optical implementation of continuous-variable quantum cloning machines
We propose an optical implementation of the Gaussian continuous-variable
quantum cloning machines. We construct a symmetric N -> M cloner which
optimally clones coherent states and we also provide an explicit design of an
asymmetric 1 -> 2 cloning machine. All proposed cloning devices can be built
from just a single non-degenerate optical parametric amplifier and several beam
splitters.Comment: 4 pages, 3 figures, REVTe
Graphical description of local Gaussian operations for continuous-variable weighted graph states
The form of a local Clifford (LC, also called local Gaussian (LG)) operation
for the continuous-variable (CV) weighted graph states is presented in this
paper, which is the counterpart of the LC operation of local complementation
for qubit graph states. The novel property of the CV weighted graph states is
shown, which can be expressed by the stabilizer formalism. It is distinctively
different from the qubit weighted graph states, which can not be expressed by
the stabilizer formalism. The corresponding graph rule, stated in purely graph
theoretical terms, is described, which completely characterizes the evolution
of CV weighted graph states under this LC operation. This LC operation may be
applied repeatedly on a CV weighted graph state, which can generate the
infinite LC equivalent graph states of this graph state. This work is an
important step to characterize the LC equivalence class of CV weighted graph
states.Comment: 5 pages, 6 figure
Clustering with shallow trees
We propose a new method for hierarchical clustering based on the optimisation
of a cost function over trees of limited depth, and we derive a
message--passing method that allows to solve it efficiently. The method and
algorithm can be interpreted as a natural interpolation between two well-known
approaches, namely single linkage and the recently presented Affinity
Propagation. We analyze with this general scheme three biological/medical
structured datasets (human population based on genetic information, proteins
based on sequences and verbal autopsies) and show that the interpolation
technique provides new insight.Comment: 11 pages, 7 figure
Universal cloning of continuous quantum variables
The cloning of quantum variables with continuous spectra is analyzed. A
universal - or Gaussian - quantum cloning machine is exhibited that copies
equally well the states of two conjugate variables such as position and
momentum. It also duplicates all coherent states with a fidelity of 2/3. More
generally, the copies are shown to obey a no-cloning Heisenberg-like
uncertainty relation.Comment: 4 pages, RevTex. Minor revisions, added explicit cloning
transformation, added reference
Multiple membrane cavity optomechanics
We investigate theoretically the extension of cavity optomechanics to
multiple membrane systems. We describe such a system in terms of the coupling
of the collective normal modes of the membrane array to the light fields. We
show these modes can be optically addressed individually and be cooled, trapped
and characterized, e.g. via quantum nondemolition measurements. Analogies
between this system and a linear chain of trapped ions or dipolar molecules
imply the possibility of related applications in the quantum regime.Comment: 4 pages, 2 figure
A Quantum Teleportation Game
We investigate a game where a sender (Alice) teleports coherent states to two
receivers (Bob and Charlie) through a tripartite Gaussian state. The aim of the
receivers is to optimize their teleportation fidelities by means of local
operations and classical communications. We show that a non-cooperative
strategy, corresponding to the standard telecloning protocol, can be
outperformed by a cooperative strategy, which gives rise to a novel
(cooperative) telecloning protocol.Comment: Typographic corrections 4 pages, 4 figure
Optimal Control Theory for Continuous Variable Quantum Gates
We apply the methodology of optimal control theory to the problem of
implementing quantum gates in continuous variable systems with quadratic
Hamiltonians. We demonstrate that it is possible to define a fidelity measure
for continuous variable (CV) gate optimization that is devoid of traps, such
that the search for optimal control fields using local algorithms will not be
hindered. The optimal control of several quantum computing gates, as well as
that of algorithms composed of these primitives, is investigated using several
typical physical models and compared for discrete and continuous quantum
systems. Numerical simulations indicate that the optimization of generic CV
quantum gates is inherently more expensive than that of generic discrete
variable quantum gates, and that the exact-time controllability of CV systems
plays an important role in determining the maximum achievable gate fidelity.
The resulting optimal control fields typically display more complicated Fourier
spectra that suggest a richer variety of possible control mechanisms. Moreover,
the ability to control interactions between qunits is important for delimiting
the total control fluence. The comparative ability of current experimental
protocols to implement such time-dependent controls may help determine which
physical incarnations of CV quantum information processing will be the easiest
to implement with optimal fidelity.Comment: 39 pages, 11 figure
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