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Die vorliegende Arbeit beschäftigt sich mit der Frage, welchen Einfluss Gase auf die elektronischen Eigenschaften von Kohlenstoff-Nanorohr- (SWNT)-Material haben. Dazu wurden systematische Experimente zur Wechselwirkung von 19 verschiedenen Gasen mit SWNT-, Graphit- und C60 -Oberflächen durchgeführt. Im Einzelnen waren dies Thermische Desorptions-Spektroskopie (TDS), Laserinduzierte Thermische Desorption (LITD) und elektrische 4-Punkt- Widerstandsmessungen sowie Messungen der Thermokraft. Alle Messungen wurden in einer im Rahmen dieser Arbeit aufgebauten UHV-Kammer durchgeführt. Die Bindungsenergien EB der untersuchten Gase und deren Adsorptions- bzw. Desorptionskinetik wurden ermittelt. Die Mehrheit der Gase wechselwirkt nur schwach (Physisorption) mit den defektfreien Oberflächen. Diffusion durch das poröse SWNT-Material beeinflusst die Wechselwirkung mit den Gasen stark, was sich in einer Verbreiterung der TD-Spektren und einer Verschiebung zu höheren Temperaturen im Vergleich zu den Graphit-Spektren ausdrückt. An Inhomogenitäten in Form von funktionalisierten Defektstellen im SWNT-Material findet eine stärkere Wechselwirkung statt. Diese äußert sich in Veränderungen der elektrischen Leitfähigkeit, aufgrund von Ladungstransfer zwischen Adsorbat und Substrat (Dotierung) und einer extremen Verbreiterung der TD-Spektren der entsprechenden polaren Gase. Die Art der Majoritäts-Ladungsträger (Elektronen oder Elektronenlöcher) wurde durch die Thermokraftmessungen bestimmt. Inhomogenitäten sind auch die Orte, wo die hier nachgewiesene chemische Reaktion von NO2 stattfindet. Durch eine systematische Auswertung konnte die gemessene Wechselwirkung der Gase in Einflüsse von verschiedenen VdW- Potentialtermen (wie Dispersionsterm, Orientierungsterm, Dipol-induzierter Dipol-Term) aufgeteilt werden. Aussagen über das Benetzungsverhalten der Gase auf den drei Oberflächen sind aufgrund der durchgeführten Experimente ebenfalls möglich. Als herausragendes Beispiel sei Wasser genannt, das keine der untersuchten Oberflächen vollständig benetzt. Die untersuchten Wechselwirkungen sind für mögliche Anwendungen z.B. im Bereich der Sensorik von Interesse., This thesis aims at the understanding of the influence of adsorbed gases on the electronic properties of single-wall carbon nanotubes (SWNTs). A systematic analysis of the interaction of 19 gases with SWNT, graphite and C60 surfaces was carried out to achive this goal. From the kinetics of adsorption and desorption we determine the binding energies between the gases and the substrates. All gases are found to be only weakly physisorbed (van der Waals- interaction) on the pristine surfaces. Gas diffusion takes place through the porous SWNT-samples and broadens the desorption features in comparison to the graphite desorption spectra. The SWNT desorption features are also shifted to higher desorption temperatures for very small gas coverage of the sample, which can be explained by higher coordinated binding sites at the SWNT-bundle surface. For polar molecules we observe a minority species with higher desorption temperatures. Presumably, this is due to a stronger interaction of these gases with SWNT-defects. The corresponding thermal desorption (TD)-spectra show a long falling high temperature edge. This means that the basic gas interaction with SWNT-samples remains the same as with the planar graphitic surface, except at the defect sites on SWNTs. The observed reaction of NO2 with the SWNT-sample can be explained by the oxidation of SWNT-samples, starting from SWNT-defect sites. Our studies reveal that the direct influence of gas adsorption at defect sites on the electric resistance of the SWNTs and the direction of charge transfer between the adsorbed molecules and the SWNTs can in principle be obtained from electric transport measurements. The microscopic wetting properties of the substrates by adsorbates are also obtained from the systematic study of gas interactions with the surfaces. For example, water shows a non-complete wetting of all the carbon surfaces. The presented study contributes to the understanding of gas-SWNT-interaction and to possible application of SWNT based detectors as gas sensors. Experimental techniques used are thermal desorption spectroscopy (TDS), laserinduced thermal desorption (LITD), 4-point electric resistance and thermopower measurements. All experiments are performed under ultra high vacuum (UHV) conditions.

This paper considers the problem of distinguishing between different dynamical models using continuous weak measurements; that is, whether the evolution is quantum mechanical or given by a classical stochastic differential equation. We examine the conditions that optimize quantum hypothesis testing, maximizing one's ability to discriminate between classical and quantum models. We set upper limits on the temperature and lower limits on the measurement efficiencies required to explore these differences, using experiments in levitated optomechanical systems as an example.

Space-based research can provide a major leap forward in the study of key open questions in the fundamental physics domain. They include the validity of Einstein’s Equivalence principle, the origin and the nature of dark matter and dark energy, decoherence and collapse models in quantum mechanics, and the physics of quantum many-body systems. Cold-atom sensors and quantum technologies have drastically changed the approach to precision measurements. Atomic clocks and atom interferometers as well as classical and quantum links can be used to measure tiny variations of the space-time metric, elusive accelerations, and faint forces to test our knowledge of the physical laws ruling the Universe. In space, such instruments can benefit from unique conditions that allow improving both their precision and the signal to be measured. In this paper, we discuss the scientific priorities of a space-based research program in fundamental physics.

The superposition principle is the cornerstone of quantum mechanics, leading to a variety of genuinely quantum effects. Whether the principle applies also to macroscopic systems or, instead, there is a progressive breakdown when moving to larger scales, is a fundamental and still open question. Spontaneous wavefunction collapse models predict the latter option, thus questioning the universality of quantum mechanics. Technological advances allow to challenge collapse models and the quantum superposition principle more and more with a variety of different experiments. Among them, non-interferometric experiments proved to be the most effective in testing these models. We provide an overview of such experiments, including cold atoms, optomechanical systems, X-rays detection, bulk heating as well as comparisons with cosmological observations. We also discuss avenues for future dedicated experiments, which aim at further testing collapse models and the validity of quantum mechanics.

Advances in quantum technologies are giving rise to a revolution in the way fundamental physics questions are explored at the empirical level. At the same time, they are the seeds for future disruptive technological applications of quantum physics. Remarkably, a space-based environment may open many new avenues for exploring and employing quantum physics and technologies. Recently, space missions employing quantum technologies for fundamental or applied studies have been proposed and implemented with stunning results. The combination of quantum physics and its space application is the focus of this review: we cover both the fundamental scientific questions that can be tackled with quantum technologies in space and the possible implementation of these technologies for a variety of academic and commercial purposes., peer-reviewed

We present a general master equation describing the quantum dynamics of a scalar bosonic field interacting with an external weak and stochastic gravitational field. The dynamics predicts decoherence both in position and in energy momentum. We show how the master equation reproduces, thus generalizing, the previous results in the literature by taking appropriate limits. We estimate the effect of gravitational decoherence in atom interferometers, providing also a straightforward way to assess the magnitude of the effect.

The Copenhagen interpretation of quantum mechanics states that a measurement collapses a wavefunction onto an eigenstate of the corresponding measurement operator. This causes a quantum mechanical wavefunction to break its unitary evolution described by the Schrödinger equation and is the source of the quantum measurement problem. In this thesis we take the first steps to an experiment that might shed light on this century-old problem. We envision a single-microwave-photon interferometer that has a travelling-wave parametric amplifier (TWPA) added to each of its arms. We wonder if the process of amplification of the quantum state causes the wavefunction to collapse, as it might turn into a detector while smoothly increasing the amplifier's gain. To this end we introduce necessary concepts from the field of microwave engineering. Then, we develop a quantum theory to describe TWPAs. This is followed by preliminary calculations on the expected interference visibility of the envisioned interferometer as a function of amplifier gain and losses. Furthermore, we describe how wavefunction collapse might reveal itself in the experimental results. Finally, the thesis describes our efforts to develop a low-loss TWPA based on Josephson junctions.

We apply the formalism of quantum estimation theory to extract information about potential collapse mechanisms of the continuous spontaneous localisation (CSL) form. In order to estimate the strength with which the field responsible for the CSL mechanism couples to massive systems, we consider the optomechanical interaction between a mechanical resonator and a cavity field. Our estimation strategy passes through the probing of either the state of the oscillator or that of the electromagnetic field that drives its motion. In particular, we concentrate on all-optical measurements, such as homodyne and heterodyne measurements. We also compare the performances of such strategies with those of a spin-assisted optomechanical system, where the estimation of the CSL parameter is performed through time-gated spin-like measurements., 8 pages, 3 figures, RevTeX4-1; accepted for publication in Phys. Rev. A

The test of modifications to quantum mechanics aimed at identifying the fundamental reasons behind the unobservability of quantum mechanical superpositions at the macroscale is a crucial goal of modern quantum mechanics. Within the context of collapse models, current proposals based on interferometric techniques for their falsification are far from the experimental state of the art. Here we discuss an alternative approach to the testing of quantum collapse models that, by bypassing the need for the preparation of quantum superposition states might help us addressing nonlinear stochastic mechanisms such as the one at the basis of the continuous spontaneous localization model.

New technological developments allow to explore the quantum properties of very complex systems, bringing the question of whether also macroscopic systems share such features, within experimental reach. The interest in this question is increased by the fact that, on the theory side, many suggest that the quantum superposition principle is not exact, departures from it being the larger, the more macroscopic the system. Here we propose a novel way to test the possible violation of the superposition principle, by analyzing its effect on the spectral properties of a generic two-level system. We will show that spectral lines shapes are modified, if the superposition principle is violated, and we quantify the magnitude of the violation. We show how this effect can be distinguished from that of standard environmental noises. We argue that accurate enough spectroscopic experiments are within reach, with current technology., comments are welcome!

Despite the unquestionable empirical success of quantum theory, witnessed by the recent uprising of quantum technologies, the debate on how to reconcile the theory with the macroscopic classical world is still open. Spontaneous collapse models are one of the few testable solutions so far proposed. In particular, the continuous spontaneous localization (CSL) model has become subject of an intense experimental research. Experiments looking for the universal force noise predicted by CSL in ultrasensitive mechanical resonators have recently set the strongest unambiguous bounds on CSL; further improving these experiments by direct reduction of mechanical noise is technically challenging. Here, we implement a recently proposed alternative strategy, that aims at enhancing the CSL noise by exploiting a multilayer test mass attached on a high quality factor microcantilever. The test mass is specifically designed to enhance the effect of CSL noise at the characteristic length $r_c=10^{-7}$ m. The measurements are in good agreement with pure thermal motion for temperatures down to 100 mK. From the absence of excess noise we infer a new bound on the collapse rate at the characteristic length $r_c=10^{-7}$ m, which improves over previous mechanical experiments by more than one order of magnitude. Our results are explicitly challenging a well-motivated region of the CSL parameter space proposed by Adler., Comment: 9 pages, 7 figures

What gravitational field is generated by a massive quantum system in a spatial superposition? Despite decades of intensive theoretical and experimental research, we still do not know the answer. On the experimental side, the difficulty lies in the fact that gravity is weak and requires large masses to be detectable. However, it becomes increasingly difficult to generate spatial quantum superpositions for increasingly large masses, in light of the stronger environmental effects on such systems. Clearly, a delicate balance between the need for strong gravitational effects and weak decoherence should be found. We show that such a trade off could be achieved in an optomechanics scenario that allows to determine whether the gravitational field generated by a quantum system in a spatial superposition is in a coherent superposition or not. We estimate the magnitude of the effect and show that it offers perspectives for observability., Comment: This manuscript supersedes arXiv:1507.05733, upon which it builds. However. we decided to leave the latter on the arXiv in light of its own merits