12,954 research outputs found
Spin quantum computation in silicon nanostructures
Proposed 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
because of their long spin coherence times due to their limited interactions
with their environments. For these spin qubits, shallow donor exchange gates
are frequently invoked to perform two-qubit operations. We discuss in this
review a particularly important spin decoherence channel, and bandstructure
effects on the exchange gate control. Specifically, we review our work on donor
electron spin spectral diffusion due to background nuclear spin flip-flops, and
how isotopic purification of silicon can significantly enhance the electron
spin dephasing time. We then review our calculation of donor electron exchange
coupling in the presence of degenerate silicon conduction band valleys. We show
that valley interference leads to orders of magnitude variations in electron
exchange coupling when donor configurations are changed on an atomic scale.
These studies illustrate the substantial potential that donor electron/nuclear
spins in silicon have as candidates for qubits and simultaneously the
considerable challenges they pose. In particular, our work on spin decoherence
through spectral diffusion points to the possible importance of isotopic
purification in the fabrication of scalable solid state quantum computer
architectures. We also provide a critical comparison between the two main
proposed spin-based solid state quantum computer architectures, namely, shallow
donor bound states in Si and localized quantum dot states in GaAs.Comment: 14 pages. Review article submitted to Solid State Communication
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
Hilbert space structure of a solid state quantum computer: two-electron states of a double quantum dot artificial molecule
We study theoretically a double quantum dot hydrogen molecule in the GaAs
conduction band as the basic elementary gate for a quantum computer with the
electron spins in the dots serving as qubits. Such a two-dot system provides
the necessary two-qubit entanglement required for quantum computation. We
determine the excitation spectrum of two horizontally coupled quantum dots with
two confined electrons, and study its dependence on an external magnetic field.
In particular, we focus on the splitting of the lowest singlet and triplet
states, the double occupation probability of the lowest states, and the
relative energy scales of these states. We point out that at zero magnetic
field it is difficult to have both a vanishing double occupation probability
for a small error rate and a sizable exchange coupling for fast gating. On the
other hand, finite magnetic fields may provide finite exchange coupling for
quantum computer operations with small errors. We critically discuss the
applicability of the envelope function approach in the current scheme and also
the merits of various quantum chemical approaches in dealing with few-electron
problems in quantum dots, such as the Hartree-Fock self-consistent field
method, the molecular orbital method, the Heisenberg model, and the Hubbard
model. We also discuss a number of relevant issues in quantum dot quantum
computing in the context of our calculations, such as the required design
tolerance, spin decoherence, adiabatic transitions, magnetic field control, and
error correction.Comment: 22 2-column pages, 11 figures. Published versio
Detection of exomoons in simulated light curves with a regularized convolutional neural network
Many moons have been detected around planets in our Solar System, but none
has been detected unambiguously around any of the confirmed extrasolar planets.
We test the feasibility of a supervised convolutional neural network to
classify photometric transit light curves of planet-host stars and identify
exomoon transits, while avoiding false positives caused by stellar variability
or instrumental noise. Convolutional neural networks are known to have
contributed to improving the accuracy of classification tasks. The network
optimization is typically performed without studying the effect of noise on the
training process. Here we design and optimize a 1D convolutional neural network
to classify photometric transit light curves. We regularize the network by the
total variation loss in order to remove unwanted variations in the data
features. Using numerical experiments, we demonstrate the benefits of our
network, which produces results comparable to or better than the standard
network solutions. Most importantly, our network clearly outperforms a
classical method used in exoplanet science to identify moon-like signals. Thus
the proposed network is a promising approach for analyzing real transit light
curves in the future
- …