77 research outputs found
Moving frames for cotangent bundles
Cartan's moving frames method is a standard tool in riemannian geometry. We
set up the machinery for applying moving frames to cotangent bundles and its
sub-bundles defined by non-holonomic constraints.Comment: 13 pages, to appear in Rep. Math. Phy
Disentangling the exchange coupling of entangled donors in the Si quantum computer architecture
We develop a theory for micro-Raman scattering by single and coupled
two-donor states in silicon. We find the Raman spectra to have significant
dependence on the donor exchange splitting and the relative spatial positions
of the two donor sites. In particular, we establish a strong correlation
between the temperature dependence of the Raman peak intensity and the
interdonor exchange coupling. Micro-Raman scattering can therefore potentially
become a powerful tool to measure interqubit coupling in the development of a
Si quantum computer architecture.Comment: Title changed. Other minor change
Silicon-based spin and charge quantum computation
Silicon-based quantum-computer architectures have attracted attention because
of their promise for scalability and their potential for synergetically
utilizing the available resources associated with the existing Si technology
infrastructure. Electronic and nuclear spins of shallow donors (e.g.
phosphorus) in Si are ideal candidates for qubits in such proposals due to the
relatively long spin coherence times. For these spin qubits, donor electron
charge manipulation by external gates is a key ingredient for control and
read-out of single-qubit operations, while shallow donor exchange gates are
frequently invoked to perform two-qubit operations. More recently, charge
qubits based on tunnel coupling in P substitutional molecular ions in Si
have also been proposed. We discuss the feasibility of the building blocks
involved in shallow donor quantum computation in silicon, taking into account
the peculiarities of silicon electronic structure, in particular the six
degenerate states at the conduction band edge. We show that quantum
interference among these states does not significantly affect operations
involving a single donor, but leads to fast oscillations in electron exchange
coupling and on tunnel-coupling strength when the donor pair relative position
is changed on a lattice-parameter scale. These studies illustrate the
considerable potential as well as the tremendous challenges posed by donor spin
and charge as candidates for qubits in silicon.Comment: Review paper (invited) - to appear in Annals of the Brazilian Academy
of Science
Exchange in silicon-based quantum computer architecture
The silicon-based quantum computer proposal has been one of the intensely
pursued ideas during the past three years. Here we calculate the donor electron
exchange in silicon and germanium, and demonstrate an atomic-scale challenge
for quantum computing in Si (and Ge), as the six (four) conduction band minima
in Si (Ge) lead to inter-valley electronic interferences, generating strong
oscillations in the exchange splitting of two-donor two-electron states. Donor
positioning with atomic scale precision within the unit cell thus becomes a
decisive factor in determining the strength of the exchange coupling--a
fundamental ingredient for two-qubit operations in a silicon-based quantum
computer.Comment: 5 pages, 2 figure
Domain size effects in Barkhausen noise
The possible existence of self-organized criticality in Barkhausen noise is
investigated theoretically through a single interface model, and experimentally
from measurements in amorphous magnetostrictive ribbon Metglas 2605TCA under
stress. Contrary to previous interpretations in the literature, both simulation
and experiment indicate that the presence of a cutoff in the avalanche size
distribution may be attributed to finite size effects.Comment: 5 pages, 3 figures, submitted so Physical Review
Electron Exchange Coupling for Single Donor Solid-State Qubits
Inter-valley interference between degenerate conduction band minima has been
shown to lead to oscillations in the exchange energy between neighbouring
phosphorus donor electron states in silicon \cite{Koiller02,Koiller02A}. These
same effects lead to an extreme sensitivity of the exchange energy on the
relative orientation of the donor atoms, an issue of crucial importance in the
construction silicon-based spin quantum computers. In this article we calculate
the donor electron exchange coupling as a function of donor position
incorporating the full Bloch structure of the Kohn-Luttinger electron
wavefunctions. It is found that due to the rapidly oscillating nature of the
terms they produce, the periodic part of the Bloch functions can be safely
ignored in the Heitler-London integrals as was done by Koiller et. al. [Phys.
Rev. Lett. 88,027903(2002),Phys. Rev. B. 66,115201(2002)], significantly
reducing the complexity of calculations.
We address issues of fabrication and calculate the expected exchange coupling
between neighbouring donors that have been implanted into the silicon substrate
using an 15keV ion beam in the so-called 'top down' fabrication scheme for a
Kane solid-state quantum computer. In addition we calculate the exchange
coupling as a function of the voltage bias on control gates used to manipulate
the electron wavefunctions and implement quantum logic operations in the Kane
proposal, and find that these gate biases can be used to both increase and
decrease the magnitude of the exchange coupling between neighbouring donor
electrons. The zero-bias results reconfirm those previously obtained by
Koiller.Comment: 10 Pages, 8 Figures. To appear in Physical Review
Depinning transition and thermal fluctuations in the random-field Ising model
We analyze the depinning transition of a driven interface in the 3d
random-field Ising model (RFIM) with quenched disorder by means of Monte Carlo
simulations. The interface initially built into the system is perpendicular to
the [111]-direction of a simple cubic lattice. We introduce an algorithm which
is capable of simulating such an interface independent of the considered
dimension and time scale. This algorithm is applied to the 3d-RFIM to study
both the depinning transition and the influence of thermal fluctuations on this
transition. It turns out that in the RFIM characteristics of the depinning
transition depend crucially on the existence of overhangs. Our analysis yields
critical exponents of the interface velocity, the correlation length, and the
thermal rounding of the transition. We find numerical evidence for a scaling
relation for these exponents and the dimension d of the system.Comment: 6 pages, including 9 figures, submitted for publicatio
Swimming using surface acoustic waves
Microactuation of free standing objects in fluids is currently dominated by the rotary propeller, giving rise to a range of potential applications in the military, aeronautic and biomedical fields. Previously, surface acoustic waves (SAWs) have been shown to be of increasing interest in the field of microfluidics, where the refraction of a SAW into a drop of fluid creates a convective flow, a phenomenon generally known as SAW streaming. We now show how SAWs, generated at microelectronic devices, can be used as an efficient method of propulsion actuated by localised fluid streaming. The direction of the force arising from such streaming is optimal when the devices are maintained at the Rayleigh angle. The technique provides propulsion without any moving parts, and, due to the inherent design of the SAW transducer, enables simple control of the direction of travel
Driven interfaces in disordered media: determination of universality classes from experimental data
While there have been important theoretical advances in understanding the
universality classes of interfaces moving in porous media, the developed tools
cannot be directly applied to experiments. Here we introduce a method that can
identify the universality class from snapshots of the interface profile. We
test the method on discrete models whose universality class is well known, and
use it to identify the universality class of interfaces obtained in experiments
on fluid flow in porous media.Comment: 4 pages, 5 figure
Homoclinic chaos in the dynamics of a general Bianchi IX model
The dynamics of a general Bianchi IX model with three scale factors is
examined. The matter content of the model is assumed to be comoving dust plus a
positive cosmological constant. The model presents a critical point of
saddle-center-center type in the finite region of phase space. This critical
point engenders in the phase space dynamics the topology of stable and unstable
four dimensional tubes , where is a saddle direction and
is the manifold of unstable periodic orbits in the center-center sector.
A general characteristic of the dynamical flow is an oscillatory mode about
orbits of an invariant plane of the dynamics which contains the critical point
and a Friedmann-Robertson-Walker (FRW) singularity. We show that a pair of
tubes (one stable, one unstable) emerging from the neighborhood of the critical
point towards the FRW singularity have homoclinic transversal crossings. The
homoclinic intersection manifold has topology and is constituted
of homoclinic orbits which are bi-asymptotic to the center-center
manifold. This is an invariant signature of chaos in the model, and produces
chaotic sets in phase space. The model also presents an asymptotic DeSitter
attractor at infinity and initial conditions sets are shown to have fractal
basin boundaries connected to the escape into the DeSitter configuration
(escape into inflation), characterizing the critical point as a chaotic
scatterer.Comment: 11 pages, 6 ps figures. Accepted for publication in Phys. Rev.
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