Human activity recognition with inertial sensors using a deep learning approach

Our focus in this research is on the use of deep learning approaches for human activity recognition (HAR) scenario, in which inputs are multichannel time series signals acquired from a set of body-worn inertial sensors and outputs are predefined human activities. Here, we present a feature learning method that deploys convolutional neural networks (CNN) to automate feature learning from the raw inputs in a systematic way. The influence of various important hyper-parameters such as number of convolutional layers and kernel size on the performance of CNN was monitored. Experimental results indicate that CNNs achieved significant speed-up in computing and deciding the final class and marginal improvement in overall classification accuracy compared to the baseline models such as Support Vector Machines and Multi-layer perceptron networks

Towards a formal description of the collapse approach to the inflationary origin of the seeds of cosmic structure

Inflation plays a central role in our current understanding of the universe. According to the standard viewpoint, the homogeneous and isotropic mode of the inflaton field drove an early phase of nearly exponential expansion of the universe, while the quantum fluctuations (uncertainties) of the other modes gave rise to the seeds of cosmic structure. However, if we accept that the accelerated expansion led the universe into an essentially homogeneous and isotropic space-time, with the state of all the matter fields in their vacuum (except for the zero mode of the inflaton field), we can not escape the conclusion that the state of the universe as a whole would remain always homogeneous and isotropic. It was recently proposed in [A. Perez, H. Sahlmann and D. Sudarsky, "On the quantum origin of the seeds of cosmic structure," Class. Quant. Grav. 23, 2317-2354 (2006)] that a collapse (representing physics beyond the established paradigm, and presumably associated with a quantum-gravity effect a la Penrose) of the state function of the inflaton field might be the missing element, and thus would be responsible for the emergence of the primordial inhomogeneities. Here we will discuss a formalism that relies strongly on quantum field theory on curved space-times, and within which we can implement a detailed description of such a process. The picture that emerges clarifies many aspects of the problem, and is conceptually quite transparent. Nonetheless, we will find that the results lead us to argue that the resulting picture is not fully compatible with a purely geometric description of space-time.Comment: 53 pages, no figures. Revision to match the published versio

Role of electromagnetically induced transparency in resonant four-wave-mixing schemes.

Published versio

Femtosecond laser irradiation of polymethylmethacrylate for refractive index gratings

Polymethylmethacrylate (PMMA) or Perspex is an inexpensive polymer widely used for making the cores of communications grade polymer optical fibres (POFs) and as a substrate for polymer optoelectronic devices and integrated waveguides. Periodic refractive index structures have been written in undoped PMMA using multiple pulses of 40 fs duration from a 1 kHz Ti:sapphire femtosecond laser operating at the fundamental (800 nm). A refractive index change (Δn) of 5 ± 0.5 × 10−4 was observed before the onset of striations. Optimization of writing conditions for refractive index modification of POF fibres or bulk undoped PMMA will enable structures such as Bragg gratings, long-period gratings, mode couplers, microlens arrays, and zone plates to be written