Coherent Processes in Rare-Earth-Ion-Doped Solids

This thesis describes a number of coherent processes, such as quantum information processing, superradiance and electromagnetically induced transparency, which have been experimentally implemented using rare-earth-ion-doped crystals. The rare-earths are a class of elements that have in common, an atomic structure that allows for very long lifetimes as well as coherence times, both on optical transitions and on spin transitions. These ions can be naturally trapped inside host crystals, and with the phonon vibrations removed by cooling down to cryogenic temperatures (< 4 K), they can be used for a number of different quantum information processes. Most experiments were carried out using PrYSO, but also other crystal types were investigated, such as Nd:YVO4, and La2(WO4)3. The characterization of the ions were done with a variety of methods, including among others, photon echo techniques and electromagnetically induced transparency. Quantum computing is a rapidly growing field and there are still many potential candidates for its implementation. In our work we have utilized the spin states of the Pr rare-earth ion as a qubit, and demonstrated arbitrary single qubit gates, which are important pieces towards a quantum computing realization in these systems. Most of the experimental work done for the thesis was carried out using an ensemble approach, that has the advantage of giving a strong readout signal, but for future scaling to multiple qubits, single instances are more promising, which is discussed. Quantum memories are crucial components in applications such as quantum networks and long distance quantum communication. In this thesis, work has been done to investigate how quantum memories can be implemented in rare-earth crystals. In particular, there are two protocols, CRIB and AFC, which were suggested with rare-earth crystals directly in mind, and both of these require high optical depth, for maximum recall efficiency. In this thesis, implications of being in the high optical depth regime, with phenomena such as superradiance, a collective effect that could cause the stored light to be immediately reemitted, and slow light effects that come from performing the storage inside spectrally structured materials, were investigated. In order to carry out the phase-sensitive experiments, a laser system with a very narrow linewidth of ~1 kHz at 606 nm was constructed, by locking the laser to a semi-persistent spectral hole. In addition, many experiments required advanced pulse shapes, such as complex sechyp pulses or pulses obtained from optimal control theory. In order to be able to accurately create such shapes, an elaborate system using an arbitrary waveform generator and two well calibrated AOMs, controlled from a computer, was also built

Spline Estimation of Principal Curves

Finding low-dimensional approximations to high-dimensional data is one of the most important topics in statistics, which has also multiple applications in economics, engineering and science. One suggestion in the literature ,based on kernel smoothing, is a non-linear generalization of principal components. This kernel-based approach comes with several complications. Therefore the purpose of this thesis is to provide an alternative based on spline smoothing which produces more reliable results

De-standardised pathways to adulthood : european perspectives on informal learning in informal networks

Becoming adult in the context of de-standardised life course transitions means new demands for the young women and men concerned. While those with higher social and cultural capital cope more easily with shaping individualised transitions, those with restricted resources and opportunities are under pressure of adaptation and at risk of exclusion. While de-standardisation can be regarded as a general trend, it takes varying shape across different European transition regimes whereby different regulative, normative and policy contexts of youth transitions are clustered. The article relates to the European YOYO study which has explored new learning demands and new learning strategies of young people in a comparative perspective. An exemplary illustration refers to the way in which young Germans make use of informal learning networks to cope with these demands. This micro perspective is related to the macro perspective by reflecting the scopes of individual agency provided by different transition regimes.Esdevenir adult en el context de les transicions desestandarditzades de la vida comporta noves demandes per part dels joves, els homes i les dones afectats. Mentre que aquells que disposen d'un capital social i cultural més elevat poden modelar més fàcilment les transicions individualitzades, aquells que no disposen de tants recursos ni oportunitats es troben sota la pressió d'adaptar-se i davant el perill de l'exclusió. Si bé la desestandardització es pot veure com una tendència general, adopta diferents formes en els diferents règims de transició europeus a través dels quals s'agrupen diferents contextos regulatius, normatius i polítics de les transicions juvenils. L'article està relacionat amb l'estudi europeu YOYO, que ha explorat noves demandes d'ensenyament i noves estratègies d'ensenyament dels joves des d'un punt de vista comparatiu. Un exemple il·lustratiu fa referència a com els joves alemanys utilitzen xarxes d'aprenentatge informals per poder fer front a aquestes demandes. Aquesta microperspectiva està relacionada amb la macro perspectiva, ja que reflecteix l'abast de l'agència individual que proporcionen els diferents règims de transició

Setup of a Beam Control System for High Power Laser Systems at DLR

Different types of high power or high energy lasers in the multi kW class are currently available or are under development with promising progress reports. A major challenge is to deliver as much as possible of the available power onto a small and fast moving target over a long distance through a disturbing atmosphere. High resolution imaging is a common way to identify the category of targets dedication and to determine the spatial position relative to the observer. By illuminating the target with a laser the imaging system becomes more resilient towards ambient light and the exposure time can be reduced drastically. Fast and deterministic control loops are demanding for the moving parts in order to maintain a high accuracy for the pointing of the turret and aiming of the laser countermeasure system. Here, we report on the progress of such a beam control system developed at the Institute of Technical Physics of DLR. In an overview we present the beam control system and explain different sub-systems. Performance tests were taken at our outdoor test range. We investigated various scenarios for probing the limits of the tracking and pointing accuracy with a target sample mounted on a fast moving linear stage. We present first results of the beam control system performance

Testosterone and Dehydroepiandrosterone Treatment in Ageing Men: Are We All Set?

Although demographic statistics show that populations around the world are rapidly ageing, this rising life expectancy is accompanied by an increase in the number of people living with age-related chronic conditions, such as frailty, cognitive decline, depression, or sexual dysfunction. In men, a progressive decline in androgens occurs with increasing age, and low androgen levels are associated with age-related chronic conditions. However, androgen administration studies are inconclusive, showing differing results according to the androgen used (testosterone [T], dehydroepiandrosterone [DHEA]), the group of men examined (younger vs. older; eugonadal vs. hypogonadal) and the conditions studied (frailty, cognitive decline, depression, sexual dysfunction). In this review, the current state for the use of T and DHEA therapy in men for the age-related conditions is examined. Due to the progressive age-related decline in androgens leading to a higher rate of older men having low androgen levels, the effects of androgen treatment in elderly males will be of particular interest in this review. Dose-response relationships, the role of potential moderators, and the androgen treatment-related risk for adverse events will be discussed. Studies have suggested that T treatment - more so than DHEA treatment - may be an effective therapy against age-related chronic conditions in men with low T levels; especially older men. Such conditions include frailty, depression, or sexual dysfunction. However, T treatment does not emerge as an effective therapy against cognitive decline. Nevertheless, more high-quality, randomised controlled trials using T treatment for age-related chronic conditions are necessary if further conclusions are to be made

Fast all-optical nuclear spin echo technique based on EIT

We demonstrate an all-optical Raman spin echo technique, using Electromagnetically Induced Transparency (EIT) to create the different pulses of the spin echo sequence: initialization, pi-rotation, and readout. The first pulse of the sequence induces coherence directly from a mixed state, and the technique is used to measure the nuclear spin coherence of an inhomogeneously broadened ensemble of rare-earth ions (Pr$^{3+}$). In contrast to previous experiments it does not require any preparatory hole burning pulse sequences, which greatly shortens the total duration of the sequence. The effect of the different pulses is characterized by quantum state tomography and is compared with simulations. We demonstrate two applications of the technique by using the spin echo sequence to accurately compensate a magnetic field across our sample, and to measure the coherence time at high temperatures up to 11 K, where standard preparation techniques are difficult to implement. We explore the potential of the technique and possible applications.Comment: 8 pages, 6 figure