63,448 research outputs found
Analyzing Multiple Nonlinear Time Series with Extended Granger Causality
Identifying causal relations among simultaneously acquired signals is an
important problem in multivariate time series analysis. For linear stochastic
systems Granger proposed a simple procedure called the Granger causality to
detect such relations. In this work we consider nonlinear extensions of
Granger's idea and refer to the result as Extended Granger Causality. A simple
approach implementing the Extended Granger Causality is presented and applied
to multiple chaotic time series and other types of nonlinear signals. In
addition, for situations with three or more time series we propose a
conditional Extended Granger Causality measure that enables us to determine
whether the causal relation between two signals is direct or mediated by
another process.Comment: 16 pages, 6 figure
Optically Probing Spin and Charge Interactions in an Tunable Artificial Molecule
We optically probe and electrically control a single artificial molecule
containing a well defined number of electrons. Charge and spin dependent
inter-dot quantum couplings are probed optically by adding a single
electron-hole pair and detecting the emission from negatively charged exciton
states. Coulomb and Pauli blockade effects are directly observed and
hybridization and electrostatic charging energies are independently measured.
The inter-dot quantum coupling is confirmed to be mediated predominantly by
electron tunneling. Our results are in excellent accord with calculations that
provide a complete picture of negative excitons and few electron states in
quantum dot molecules.Comment: shortened version: 6 pages, 3 figures, 1 table, to appear in Phys.
Rev. Let
Detecting a relic background of scalar waves with LIGO
We discuss the possible detection of a stochastic background of massive,
non-relativistic scalar particles, through the cross correlation of the two
LIGO interferometers in the initial, enhanced and advanced configuration. If
the frequency corresponding to the mass of the scalar field lies in the
detector sensitivity band, and the non-relativistic branch of the spectrum
gives a significant contribution to energy density required to close the
Universe, we find that the scalar background can induce a non-negligible
signal, in competition with a possible signal produced by a stochastic
background of gravitational radiation.Comment: 17 pages, uses revte
Gravitationally-induced entanglement between two massive particles is sufficient evidence of quantum effects in gravity
All existing quantum gravity proposals share the same deep problem. Their
predictions are extremely hard to test in practice. Quantum effects in the
gravitational field are exceptionally small, unlike those in the
electromagnetic field. The fundamental reason is that the gravitational
coupling constant is about 43 orders of magnitude smaller than the fine
structure constant, which governs light-matter interactions. For example, the
detection of gravitons -- the hypothetical quanta of energy of the
gravitational field predicted by certain quantum-gravity proposals -- is deemed
to be practically impossible. In this letter we adopt a radically different,
quantum-information-theoretic approach which circumvents the problem that
quantum gravity is hard to test. We propose an experiment to witness
quantum-like features in the gravitational field, by probing it with two masses
each in a superposition of two locations. First, we prove the fact that any
system (e.g. a field) capable of mediating entanglement between two quantum
systems must itself be quantum. This argument is general and does not rely on
any specific dynamics. Then, we propose an experiment to detect the
entanglement generated between two masses via gravitational interaction. By our
argument, the degree of entanglement between the masses is an indirect witness
of the quantisation of the field mediating the interaction. Remarkably, this
experiment does not require any quantum control over gravity itself. It is also
closer to realisation than other proposals, such as detecting gravitons or
detecting quantum gravitational vacuum fluctuations
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