28 research outputs found
Decoherence-protected quantum gates for a hybrid solid-state spin register
Protecting the dynamics of coupled quantum systems from decoherence by the
environment is a key challenge for solid-state quantum information processing.
An idle qubit can be efficiently insulated from the outside world via dynamical
decoupling, as has recently been demonstrated for individual solid-state
qubits. However, protection of qubit coherence during a multi-qubit gate poses
a non-trivial problem: in general the decoupling disrupts the inter-qubit
dynamics, and hence conflicts with gate operation. This problem is particularly
salient for hybrid systems, wherein different types of qubits evolve and
decohere at vastly different rates. Here we present the integration of
dynamical decoupling into quantum gates for a paradigmatic hybrid system, the
electron-nuclear spin register. Our design harnesses the internal resonance in
the coupled-spin system to resolve the conflict between gate operation and
decoupling. We experimentally demonstrate these gates on a two-qubit register
in diamond operating at room temperature. Quantum tomography reveals that the
qubits involved in the gate operation are protected as accurately as idle
qubits. We further illustrate the power of our design by executing Grover's
quantum search algorithm, achieving fidelities above 90% even though the
execution time exceeds the electron spin dephasing time by two orders of
magnitude. Our results directly enable decoherence-protected interface gates
between different types of promising solid-state qubits. Ultimately, quantum
gates with integrated decoupling may enable reaching the accuracy threshold for
fault-tolerant quantum information processing with solid-state devices.Comment: This is original submitted version of the paper. The revised and
finalized version is in print, and is subjected to the embargo and other
editorial restrictions of the Nature journa
Luminance, colour, viewpoint and border enhanced disparity energy model
The visual cortex is able to extract disparity information through the use of binocular cells. This process is reflected by the Disparity Energy Model, which describes the role and functioning of simple and complex binocular neuron populations, and how they are able to extract disparity. This model uses explicit cell parameters to mathematically determine preferred cell disparities, like spatial frequencies, orientations, binocular phases and receptive field positions. However, the brain cannot access such explicit cell parameters; it must rely on cell responses. In this article, we implemented a trained binocular neuronal population, which encodes disparity information implicitly. This allows the population to learn how to decode disparities, in a similar way to how our visual system could have developed this ability during evolution. At the same time, responses of monocular simple and complex cells can also encode line and edge information, which is useful for refining disparities at object borders. The brain should then be able, starting from a low-level disparity draft, to integrate all information, including colour and viewpoint perspective, in order to propagate better estimates to higher cortical areas.Portuguese Foundation for Science and Technology (FCT); LARSyS FCT [UID/EEA/50009/2013]; EU project NeuroDynamics [FP7-ICT-2009-6, PN: 270247]; FCT project SparseCoding [EXPL/EEI-SII/1982/2013]; FCT PhD grant [SFRH-BD-44941-2008
Impact of inactivity and exercise on the vasculature in humans
The effects of inactivity and exercise training on established and novel cardiovascular risk factors are relatively modest and do not account for the impact of inactivity and exercise on vascular risk. We examine evidence that inactivity and exercise have direct effects on both vasculature function and structure in humans. Physical deconditioning is associated with enhanced vasoconstrictor tone and has profound and rapid effects on arterial remodelling in both large and smaller arteries. Evidence for an effect of deconditioning on vasodilator function is less consistent. Studies of the impact of exercise training suggest that both functional and structural remodelling adaptations occur and that the magnitude and time-course of these changes depends upon training duration and intensity and the vessel beds involved. Inactivity and exercise have direct “vascular deconditioning and conditioning” effects which likely modify cardiovascular risk