1,269 research outputs found
Interplay between the electrical transport properties of GeMn thin films and Ge substrates
We present evidence that electrical transport studies of epitaxial p-type
GeMn thin films fabricated on high resistivity Ge substrates are severely
influenced by parallel conduction through the substrate, related to the large
intrinsic conductivity of Ge due to its small bandgap. Anomalous Hall
measurements and large magneto resistance effects are completely understood by
taking a dominating substrate contribution as well as the measurement geometry
into account. It is shown that substrate conduction persists also for well
conducting, degenerate, p-type thin films, giving rise to an effective
two-layer conduction scheme. Using n-type Ge substrates, parallel conduction
through the substrate can be reduced for the p-type epi-layers, as a
consequence of the emerging pn-interface junction. GeMn thin films fabricated
on these substrates exhibit a negligible magneto resistance effect. Our study
underlines the importance of a thorough characterization and understanding of
possible substrate contributions for electrical transport studies of GeMn thin
films.Comment: 9 pages, 9 figure
A scalable architecture for quantum computation with molecular nanomagnets
A proposal for a magnetic quantum processor that consists of individual
molecular spins coupled to superconducting coplanar resonators and transmission
lines is carefully examined. We derive a simple magnetic quantum
electrodynamics Hamiltonian to describe the underlying physics. It is shown
that these hybrid devices can perform arbitrary operations on each spin qubit
and induce tunable interactions between any pair of them. The combination of
these two operations ensures that the processor can perform universal quantum
computations. The feasibility of this proposal is critically discussed using
the results of realistic calculations, based on parameters of existing devices
and molecular qubits. These results show that the proposal is feasible,
provided that molecules with sufficiently long coherence times can be developed
and accurately integrated into specific areas of the device. This architecture
has an enormous potential for scaling up quantum computation thanks to the
microscopic nature of the individual constituents, the molecules, and the
possibility of using their internal spin degrees of freedom.Comment: 27 pages, 6 figure
A new eriophyoid mite species (Acari: Eriophyoidea) damaging Eucalyptus wandoo in Western Australia.
Resumo
Quantum state transfer in arrays of flux qubits
In this work, we describe a possible experimental realization of Bose's idea
to use spin chains for short distance quantum communication [S. Bose, {\it
Phys. Rev. Lett.} {\bf 91} 207901]. Josephson arrays have been proposed and
analyzed as transmission channels for systems of superconducting charge qubits.
Here, we consider a chain of persistent current qubits, that is appropriate for
state transfer with high fidelity in systems containing flux qubits. We
calculate the fidelity of state transfer for this system. In general, the
Hamiltonian of this system is not of XXZ-type, and we analyze the magnitude and
the effect of the terms that don't conserve the z-component of the total spin.Comment: 10 pages, 8 figure
Correlation of internal representations in feed-forward neural networks
Feed-forward multilayer neural networks implementing random input-output
mappings develop characteristic correlations between the activity of their
hidden nodes which are important for the understanding of the storage and
generalization performance of the network. It is shown how these correlations
can be calculated from the joint probability distribution of the aligning
fields at the hidden units for arbitrary decoder function between hidden layer
and output. Explicit results are given for the parity-, and-, and
committee-machines with arbitrary number of hidden nodes near saturation.Comment: 6 pages, latex, 1 figur
Quantum phase estimation algorithm in presence of static imperfections
We study numerically the effects of static imperfections and residual
couplings between qubits for the quantum phase estimation algorithm with two
qubits. We show that the success probability of the algorithm is affected
significantly more by static imperfections than by random noise errors in
quantum gates. An improvement of the algorithm accuracy can be reached by
application of the Pauli-random-error-correction method (PAREC).Comment: 5 pages, 5 figures. Research avilable at
http://www.quantware.ups-tlse.fr
Multi-band quantum ratchets
We investigate directed motion in non-adiabatically rocked ratchet systems
sustaining few bands below the barrier. Upon restricting the dynamics to the
lowest M bands, the total system-plus-bath Hamiltonian is mapped onto a
discrete tight-binding model containing all the information both on the intra-
and inter-well tunneling motion. A closed form for the current in the
incoherent tunneling regime is obtained. In effective single-band ratchets, no
current rectification occurs. We apply our theory to describe rectification
effects in vortex quantum ratchets devices. Current reversals upon variation of
the ac-field amplitude or frequency are predicted.Comment: Accepted for publication in Physical Review Letter
Approaching Unit Visibility for Control of a Superconducting Qubit with Dispersive Readout
In a Rabi oscillation experiment with a superconducting qubit we show that a
visibility in the qubit excited state population of more than 90 % can be
attained. We perform a dispersive measurement of the qubit state by coupling
the qubit non-resonantly to a transmission line resonator and probing the
resonator transmission spectrum. The measurement process is well characterized
and quantitatively understood. The qubit coherence time is determined to be
larger than 500 ns in a measurement of Ramsey fringes.Comment: 4 pages, 5 figures, version with high resolution figures available at
http://www.eng.yale.edu/rslab/Andreas/content/science/PubsPapers.htm
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