11,627 research outputs found
Influence of qubit displacements on quantum logic operations in a silicon-based quantum computer with constant interaction
The errors caused by qubit displacements from their prescribed locations in
an ensemble of spin chains are estimated analytically and calculated
numerically for a quantum computer based on phosphorus donors in silicon. We
show that it is possible to polarize (initialize) the nuclear spins even with
displaced qubits by using Controlled NOT gates between the electron and nuclear
spins of the same phosphorus atom. However, a Controlled NOT gate between the
displaced electron spins is implemented with large error because of the
exponential dependence of exchange interaction constant on the distance between
the qubits. If quantum computation is implemented on an ensemble of many spin
chains, the errors can be small if the number of chains with displaced qubits
is small
Graphene-based one-dimensional photonic crystal
A novel type of one-dimensional (1D) photonic crystal formed by the array of
periodically located stacks of alternating graphene and dielectric stripes
embedded into a background dielectric medium is proposed. The wave equation for
the electromagnetic wave propagating in such structure solved in the framework
of the Kronig-Penney model. The frequency band structure of 1D graphene-based
photonic crystal is obtained analytically as a function of the filling factor
and the thickness of the dielectric between graphene stripes. The photonic
frequency corresponding to the electromagnetic wave localized by the defect of
photonic crystal formed by the extra dielectric placed on the place of the
stack of alternating graphene and dielectric stripes is obtained.Comment: 8 pages, 2 figure
Relaxation and Zeno effect in qubit measurements
We consider a qubit interacting with its environment and continuously
monitored by a detector represented by a point contact. Bloch-type equations
describing the entire system of the qubit, the environment and the detector are
derived. Using these equations we evaluate the detector current and its noise
spectrum in terms of the decoherence and relaxation rates of the qubit. Simple
expressions are obtained that show how these quantities can be accurately
measured. We demonstrate that due to interaction with the environment, the
measurement can never localize a qubit even for infinite decoherence rate.Comment: some clarifications added, to appear in Phys. Rev. Let
Magnetoplasmons in layered graphene structures
We calculate the dispersion equations for magnetoplasmons in a single layer,
a pair of parallel layers, a graphite bilayer and a superlattice of graphene
layers in a perpendicular magnetic field. We demonstrate the feasibility of a
drift-induced instability of magnetoplasmons. The magnetoplasmon instability in
a superlattice is enhanced compared to a single graphene layer. The energies of
the unstable magnetoplasmons could be in the terahertz (THz) part of the
electromagnetic spectrum. The enhanced instability makes superlattice graphene
a potential source of THz radiation.Comment: 5 pages, 4 figure
High temperature thermal conductivity of 2-leg spin-1/2 ladders
Based on numerical simulations, a study of the high temperature, finite
frequency, thermal conductivity of spin-1/2 ladders is
presented. The exact diagonalization and a novel Lanczos technique are
employed.The conductivity spectra, analyzed as a function of rung coupling,
point to a non-diverging limit but to an unconventional low frequency
behavior. The results are discussed with perspective recent experiments
indicating a significant magnetic contribution to the energy transport in
quasi-one dimensional compounds.Comment: 4 pages, 4 figure
Double-Slit Interferometry with a Bose-Einstein Condensate
A Bose-Einstein "double-slit" interferometer has been recently realized
experimentally by (Y. Shin et. al., Phys. Rev. Lett. 92 50405 (2004)). We
analyze the interferometric steps by solving numerically the time-dependent
Gross-Pitaevski equation in three-dimensional space. We focus on the
adiabaticity time scales of the problem and on the creation of spurious
collective excitations as a possible source of the strong dephasing observed
experimentally. The role of quantum fluctuations is discussed.Comment: 4 pages, 3 figure
Simplifying and adapting antiretroviral treatment in resource-poor settings: a necessary step to scaling-up.
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