714 research outputs found
Engineering many-body quantum dynamics by disorder
Going beyond the currently investigated regimes in experiments on quantum
transport of ultracold atoms in disordered potentials, we predict a crossover
between regular and quantum-chaotic dynamics when varying the strength of
disorder. Our spectral approach is based on the Bose-Hubbard model describing
interacting atoms in deep random potentials. The predicted crossover from
localized to diffusive dynamics depends on the simultaneous presence of
interactions and disorder, and can be verified in the laboratory by monitoring
the evolution of typical experimental initial states.Comment: 4 pages, 4 figures (improved version), to be published in PR
Discriminative training for continuous speech recognition
Discriminative training techniques for Hidden-Markov Models were recently proposed and successfully applied for automatic speech recognition. In this paper a discussion of the Minimum Classification Error and the Maximum Mutual Information objective is presented. An extended reestimation formula is used for the HMM parameter update for both objective functions. The discriminative training methods were utilized in speaker independent phoneme recognition experiments and improved the phoneme recognition rates for both discriminative training techniques
A hybrid RBF-HMM system for continuous speech recognition
A hybrid system for continuous speech recognition, consisting of a neural network with Radial Basis Functions and Hidden Markov Models is described in this paper together with discriminant training techniques. Initially the neural net is trained to approximate a-posteriori probabilities of single HMM states. These probabilities are used by the Viterbi algorithm to calculate the total scores for the individual hybrid phoneme models. The final training of the hybrid system is based on the "Minimum Classification Error\u27; objective function, which approximates the misclassification rate of the hybrid classifier, and the "Generalized Probabilistic Descent\u27; algorithm. The hybrid system was used in continuous speech recognition experiments with phoneme units and shows about 63.8% phoneme recognition rate in a speaker-independent task
Chaos assisted adiabatic passage
We study the exact dynamics underlying stimulated Raman adiabatic passage
(STIRAP) for a particle in a multi-level anharmonic system (the infinite
square-well) driven by two sequential laser pulses, each with constant carrier
frequency. In phase space regions where the laser pulses create chaos, the
particle can be transferred coherently into energy states different from those
predicted by traditional STIRAP. It appears that a transition to chaos can
provide a new tool to control the outcome of STIRAP
Nanosecond-timescale spin transfer using individual electrons in a quadruple-quantum-dot device
The ability to coherently transport electron-spin states between different
sites of gate-defined semiconductor quantum dots is an essential ingredient for
a quantum-dot-based quantum computer. Previous shuttles using electrostatic
gating were too slow to move an electron within the spin dephasing time across
an array. Here we report a nanosecond-timescale spin transfer of individual
electrons across a quadruple-quantum-dot device. Utilizing enhanced relaxation
rates at a so-called `hot spot', we can upper bound the shuttle time to at most
150 ns. While actual shuttle times are likely shorter, 150 ns is already fast
enough to preserve spin coherence in e.g. silicon based quantum dots. This work
therefore realizes an important prerequisite for coherent spin transfer in
quantum dot arrays.Comment: 7 pages including 2 pages of supplementary materia
Coherent shuttle of electron-spin states
We demonstrate a coherent spin shuttle through a GaAs/AlGaAs
quadruple-quantum-dot array. Starting with two electrons in a spin-singlet
state in the first dot, we shuttle one electron over to either the second,
third or fourth dot. We observe that the separated spin-singlet evolves
periodically into the spin-triplet and back before it dephases due to
nuclear spin noise. We attribute the time evolution to differences in the local
Zeeman splitting between the respective dots. With the help of numerical
simulations, we analyse and discuss the visibility of the singlet-triplet
oscillations and connect it to the requirements for coherent spin shuttling in
terms of the inter-dot tunnel coupling strength and rise time of the pulses.
The distribution of entangled spin pairs through tunnel coupled structures may
be of great utility for connecting distant qubit registers on a chip.Comment: 21 pages, 10 figure
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