152 research outputs found
Is implicit motor imagery a reliable strategy for a brain computer interface?
Explicit motor imagery (eMI) is a widely used brain computer interface (BCI) paradigm, but not everybody can accomplish this task. Here we propose a BCI based on implicit motor imagery (iMI). We compared classification accuracy between eMI and iMI of hands. Fifteen able bodied people were asked to judge the laterality of hand images presented on a computer screen in a lateral or medial orientation. This judgement task is known to require mental rotation of a person’s own hands which in turn is thought to involve iMI. The subjects were also asked to perform eMI of the hands. Their electroencephalography (EEG) was recorded. Linear classifiers were designed based on common spatial patterns. For discrimination between left and right hand the classifier achieved maximum of 81 ± 8% accuracy for eMI and 83 ± 3% for iMI. These results show that iMI can be used to achieve similar classification accuracy as eMI. Additional classification was performed between iMI in medial and lateral orientations of a single hand; the classifier achieved 81 ± 7% for the left and 78 ± 7% for the right hand which indicate distinctive spatial patterns of cortical activity for iMI of a single hand in different directions. These results suggest that a special brain computer interface based on iMI may be constructed, for people who cannot perform explicit imagination, for rehabilitation of movement or for treatment of bodily spatial neglect
Universal decoherence due to gravitational time dilation
The physics of low-energy quantum systems is usually studied without explicit
consideration of the background spacetime. Phenomena inherent to quantum theory
on curved space-time, such as Hawking radiation, are typically assumed to be
only relevant at extreme physical conditions: at high energies and in strong
gravitational fields. Here we consider low-energy quantum mechanics in the
presence of gravitational time dilation and show that the latter leads to
decoherence of quantum superpositions. Time dilation induces a universal
coupling between internal degrees of freedom and the centre-of-mass of a
composite particle. The resulting correlations cause decoherence of the
particle's position, even without any external environment. We also show that
the weak time dilation on Earth is already sufficient to decohere micron scale
objects. Gravity therefore can account for the emergence of classicality and
the effect can in principle be tested in future matter wave experiments.Comment: 6+4 pages, 3 figures. Revised manuscript in Nature Physics (2015
Gravity is not a Pairwise Local Classical Channel
It is currently believed that there is no experimental evidence on possibly
quantum features of gravity or gravity-motivated modifications of quantum
mechanics. Here we show that single-atom interference experi- ments achieving
large spatial superpositions can rule out a framework where the Newtonian
gravitational inter- action is fundamentally classical in the
information-theoretic sense: it cannot convey entanglement. Specifically, in
this framework gravity acts pairwise between massive particles as classical
channels, which effectively induce approximately Newtonian forces between the
masses. The experiments indicate that if gravity does reduce to the pairwise
Newtonian interaction between atoms at the low energies, this interaction
cannot arise from the exchange of just classical information, and in principle
has the capacity to create entanglement. We clarify that, contrary to current
belief, the classical-channel description of gravity differs from the model of
Diosi and Penrose, which is not constrained by the same data.Comment: 13 pages, 5 figures, 2 tables, Late
A generalization of Margolus-Levitin bound
The Margolus-Levitin lower bound on minimal time required for a state to be
transformed into an orthogonal state is generalized. It is shown that for some
initial states new bound is stronger than the Margolus-Levitin one.Comment: 6 pages, no figures; some comments added; final version accepted for
publication in Phys. Rev.
Single electron relativistic clock interferometer
Although time is one of the fundamental notions in physics, it does not have
a unique description. In quantum theory time is a parameter ordering the
succession of the probability amplitudes of a quantum system, while according
to relativity theory each system experiences in general a different proper
time, depending on the system's world line, due to time to time dilation. It is
therefore of fundamental interest to test the notion of time in the regime
where both quantum and relativistic effects play a role, for example, when
different amplitudes of a single quantum clock experience different magnitudes
of time dilation. Here we propose a realization of such an experiment with a
single electron in a Penning trap. The clock can be implemented in the
electronic spin precession and its time dilation then depends on the radial
(cyclotron) state of the electron. We show that coherent manipulation and
detection of the electron can be achieved already with present day technology.
A single electron in a Penning trap is a technologically ready platform where
the notion of time can be probed in a hitherto untested regime, where it
requires a relativistic as well as quantum description.Comment: 9 pages, 4 figure
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