28,787 research outputs found
Efficient Schemes for Reducing Imperfect Collective Decoherences
We propose schemes that are efficient when each pair of qubits undergoes some
imperfect collective decoherence with different baths. In the proposed scheme,
each pair of qubits is first encoded in a decoherence-free subspace composed of
two qubits. Leakage out of the encoding space generated by the imperfection is
reduced by the quantum Zeno effect. Phase errors in the encoded bits generated
by the imperfection are reduced by concatenation of the decoherence-free
subspace with either a three-qubit quantum error correcting code that corrects
only phase errors or a two-qubit quantum error detecting code that detects only
phase errors, connected with the quantum Zeno effect again.Comment: no correction, 3 pages, RevTe
First passage time for random walks in heterogeneous networks
The first passage time (FPT) for random walks is a key indicator of how fast
information diffuses in a given system. Despite the role of FPT as a
fundamental feature in transport phenomena, its behavior, particularly in
heterogeneous networks, is not yet fully understood. Here, we study, both
analytically and numerically, the scaling behavior of the FPT distribution to a
given target node, averaged over all starting nodes. We find that random walks
arrive quickly at a local hub, and therefore, the FPT distribution shows a
crossover with respect to time from fast decay behavior (induced from the
attractive effect to the hub) to slow decay behavior (caused by the exploring
of the entire system). Moreover, the mean FPT is independent of the degree of
the target node in the case of compact exploration. These theoretical results
justify the necessity of using a random jump protocol (empirically used in
search engines) and provide guidelines for designing an effective network to
make information quickly accessible.Comment: 5 pages, 3 figure
Conserved cosmological structures in the one-loop superstring effective action
A generic form of low-energy effective action of superstring theories with
one-loop quantum correction is well known. Based on this action we derive the
complete perturbation equations and general analytic solutions in the
cosmological spacetime. Using the solutions we identify conserved quantities
characterizing the perturbations: the amplitude of gravitational wave and the
perturbed three-space curvature in the uniform-field gauge both in the
large-scale limit, and the angular-momentum of rotational perturbation are
conserved independently of changing gravity sector. Implications for
calculating perturbation spectra generated in the inflation era based on the
string action are presented.Comment: 5 pages, no figure, To appear in Phys. Rev.
Compressibility of graphene
We develop a theory for the compressibility and quantum capacitance of
disordered monolayer and bilayer graphene including the full hyperbolic band
structure and band gap in the latter case. We include the effects of disorder
in our theory, which are of particular importance at the carrier densities near
the Dirac point. We account for this disorder statistically using two different
averaging procedures: first via averaging over the density of carriers
directly, and then via averaging in the density of states to produce an
effective density of carriers. We also compare the results of these two models
with experimental data, and to do this we introduce a model for inter-layer
screening which predicts the size of the band gap between the low-energy
conduction and valence bands for arbitary gate potentials applied to both
layers of bilayer graphene. We find that both models for disorder give
qualitatively correct results for gapless systems, but when there is a band gap
at charge neutrality, the density of states averaging is incorrect and
disagrees with the experimental data.Comment: 10 pages, 7 figures, RevTe
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