Introduction à la mécanique quantique

:Le but de cet article est de présenter une introduction accessible, non technique, au sujet. Pour l'article encyclopédique consulter Mécanique quantique. La mécanique quantique est la science de l'infiniment petit : elle regroupe l'ensemble des travaux scientifiques qui interprètent le comportement des constituants de la matière, et ses interactions avec l'énergie, à l'échelle des atomes et des particules subatomiques. La physique classique décrit la matière et l'énergie à l'échelle humaine, dans leur observation de tous les jours, y compris les corps célestes. Elle reste fondamentale pour tout ce qui concerne les mesures physiques pour la science moderne et la technologie. Mais à la fin du , les scientifiques ont découvert des phénomènes que la physique classique ne pouvait expliquer, tant à l'échelle macroscopique que microscopique. Comme l'explique Thomas Kuhn dans son analyse sur la philosophie des sciences, La Structure des révolutions scientifiques, la compréhension de ces par

Carbon Nanotubes

Jaap Kroes

This thesis studies carbon nanotubes using state-of-the-art computational methods. Using large-scale quantum-mechanical calculations, based on density-functional-theory, we investigate several important aspects of the physics and chemistry of single-walled-nanotubes. The focus is on the effect of defects, namely adparticles and vacancies, on the structural, electronic and dynmical properties of the nanotubes. We also present preliminary results of simulations exploring a possible route for the formation of SWNTs from graphene nanoflakes. Adparticles include atomic hydrogen, oxygen, sulfur at different concentrations as well as nitrogen--oxides. Chemisorption of hydrogen as well as oxygen and isoelectronic species results in the formation of clusters on the sidewall, with characteristic structures corresponding to characteristic signatures in the electronic spectra. Especially in the case of oxygen, we find that relatively high energy barriers separate different structures: this shows that not only thermodynamically favored configurations are relevant for the understanding of oxygen chemisorption but the presence of traps cannot be neglected. The fingerpint of these traps is confirmed by scanning-tunneling spectroscopy. Trends with size and chirality of the nanotubes and oxygen coverage are studied in detail and also explained in terms of simple chemical descriptors. The importance of large-scale atomistic models is also emphasized to obtain convergent results and thus reliable predictions, and comparison is made of results we obtain using different gradient-corrected exchange-correlation functionals and in part with hybrid functionals. The study of nitrogen-oxides faces the difficulty to correctly represent physisorption, also with empirically corrected gradient-corrected exchange-correlation and hybrid functionals. Still our calculations of vibrational frequencies of different molecules on the sidewall, once compared with experiment, are able to distinguish the specific species observed in infrared spectra. Part of our work is centered on the comparison of widely used classical potentials (reactive force fields) with DFT results. Specifically we use them to study hydrogen and oxygen chemisorption, and especially examine the validity of several different force-fields for the description of the structure and energetics of single and double vacancies. In all cases, we find rather limited agreement with DFT results, showing the intrinsic difficulty to represent the subtle and intrinsic quantum effects governing the physico-chemical behavior of carbon nanotubes. Still, the wealth of results we have obtained might be useful for an improvement of these classical schemes for a specific application or other semi-classical models.

EPFL2014

Quantum optomechanics at room temperature

Hendrik Schütz

Why are classical theories often sufficient to describe the physics of our world even though everything around us is entirely composed of microscopic quantum systems? The boundary between these two fundamentally dissimilar theories remains an unsolved problem in modern physics. Position measurements of small objects allow us to probe the area where the classical approximation breaks down. In quantum mechanics, Heisenbergâs uncertainty principle dictates that any measurement of the position must be accompanied by measurement induced back-action---in this case manifested as an uncertainty in the momentum. In recent years, cavity optomechanics has become a powerful tool to perform precise position measurements and investigate their fundamental limitations. The utilization of optical micro-cavities greatly enhances the interaction between light and state-of-the-art nanomechanical oscillators. Therefore, quantum mechanical phenomena have been successfully observed in systems far beyond the microscopic world. In such a cavity optomechanical system, the fluctuations in the position of the oscillator are transduced onto the phase of the light, while fluctuations in the amplitude of the light disturb the momentum of the oscillator during the measurement. As a consequence, correlations are established between the amplitude and phase quadrature of the probe light. However, so far, observation of quantum effects has been limited exclusively to cryogenic experiments, and access to the quantum regime at room temperature has remained an elusive goal because the overwhelming amount of thermal motion masks the weak quantum effects. This thesis describes the engineering of a high-performance cavity optomechanical device and presents experimental results showing, for the first time, the broadband effects of quantum back-action at room temperature. The device strongly couples mechanical and optical modes of exceptionally high quality factors to provide a measurement sensitivity

\sim\!10^4

times below the requirement to resolve the zero-point fluctuations of the mechanical oscillator. The quantum back-action is then observed through the correlations created between the probe light and the motion of the nanomechanical oscillator. A so-called âvariational measurementâ, which detects the transmitted light in a homodyne detector tuned close to the amplitude quadrature, resolves the quantum noise due to these correlations at the level of 10% of the thermal noise over more than an octave of Fourier frequencies around mechanical resonance. Moreover, building on this result, an additional experiment demonstrates the ability to achieve quantum enhanced metrology. In this case, the generated quantum correlations are used to cancel quantum noise in the measurement record, which then leads to an improved relative signal-to-noise ratio in measurements of an external force. In conclusion, the successful observation of broadband quantum behavior on a macroscopic object at room temperature is an important milestone in the field of cavity optomechanics. Specifically, this result heralds the rise of optomechanical systems as a platform for quantum physics at room temperature and shows promise for generation of ponderomotive squeezing in room-temperature interferometers.

EPFL2017