54,203 research outputs found
Toward a more economical cluster state quantum computation
We assess the effects of an intrinsic model for imperfections in cluster
states by introducing {\it noisy cluster states} and characterizing their role
in the one-way model for quantum computation. The action of individual
dephasing channels on cluster qubits is also studied. We show that the effect
of non-idealities is limited by using small clusters, which requires compact
schemes for computation. In light of this, we address an experimentally
realizable four-qubit linear cluster which simulates a controlled-{\sf NOT}
({\sf CNOT}).Comment: 4 pages, 2 figures, RevTeX4; proposal for experimental setup include
A model for - kaon cross section
We calculate the cross section for the dissociation of by kaons
within the framework of a meson exchange model. We find that, depending on the
values of the coupling constants used, the cross section can vary from 5 mb to
30 mb at GeV.Comment: 4 pages, 3 eps figure
Classical simulation of commuting quantum computations implies collapse of the polynomial hierarchy
We consider quantum computations comprising only commuting gates, known as
IQP computations, and provide compelling evidence that the task of sampling
their output probability distributions is unlikely to be achievable by any
efficient classical means. More specifically we introduce the class post-IQP of
languages decided with bounded error by uniform families of IQP circuits with
post-selection, and prove first that post-IQP equals the classical class PP.
Using this result we show that if the output distributions of uniform IQP
circuit families could be classically efficiently sampled, even up to 41%
multiplicative error in the probabilities, then the infinite tower of classical
complexity classes known as the polynomial hierarchy, would collapse to its
third level. We mention some further results on the classical simulation
properties of IQP circuit families, in particular showing that if the output
distribution results from measurements on only O(log n) lines then it may in
fact be classically efficiently sampled.Comment: 13 page
Investigating the tetraquark structure of the new mesons
Using the QCD sum rule approach we investigate the possible four-quark
structure of the recently observed mesons , firstly observed
by BaBaR, X(3872), firstly observed by BELLE and observed by
BELLE. We use diquark-antidiquark currents and work in full QCD, without
relying on expansion. Our results indicate that a four-quark structure
is acceptable for these mesons.Comment: 4 pages 1 eps figure, proceedings of the XVIII Workshop on Hadronic
Interactions (RETINHA-18) Sao Paulo-S
Looking for meson molecules in B decays
We discuss the possibility of observing a loosely bound molecular state in a
B three-body hadronic decay. In particular we use the QCD sum rule approach to
study a molecular current. We consider an isovector-scalar
molecular current and we use the two-point and
three-point functions to study the mass and decay width of such state. We
consider the contributions of condensates up to dimension six and we work at
leading order in . We obtain a mass around 1.1 GeV, consistent with a
loosely bound state, and a decay width
around 10 MeV.Comment: 7 pages, 8 figure
Multiphoton resonances for all-optical quantum logic with multiple cavities
We develop a theory for the interaction of multilevel atoms with multimode cavities yielding cavity-enhanced multiphoton resonances. The locations of the resonances are predicted from the use of effective two- and three-level Hamiltonians. As an application we show that quantum gates can be realized when photonic qubits are encoded on the cavity modes in arrangements where ancilla atoms transit the cavity. The fidelity of operations is increased by conditional measurements on the atom and by the use of a selected, dual-rail, Hilbert space. A universal set of gates is proposed, including the Fredkin gate and iSWAP operation; the system seems promising for scalability
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