1,699 research outputs found
Laser Technologies for Applications in Quantum Information Science
Scientific progress in experimental physics is inevitably dependent on continuing advances in the underlying technologies. Laser technologies enable controlled coherent and dissipative atom-light interactions and micro-optical technologies allow for the implementation of versatile optical systems not accessible with standard optics.
This thesis reports on important advances in both technologies with targeted applications ranging from Rydberg-state mediated quantum simulation and computation with individual atoms in arrays of optical tweezers to high-resolution spectroscopy of highly-charged ions.
A wide range of advances in laser technologies are reported: The long-term stability and maintainability of external-cavity diode laser systems is improved significantly by introducing a mechanically adjustable lens mount. Tapered-amplifier modules based on a similar lens mount are developed. The diode laser systems are complemented by digital controllers for laser frequency and intensity stabilisation. The controllers offer a bandwidth of up to 1.25 MHz and a noise performance set by the commercial STEMlab platform. In addition, shot-noise limited photodetectors optimised for intensity stabilisation and Pound-Drever-Hall frequency stabilisation as well as a fiber based detector for beat notes in the MHz-regime are developed. The capabilities of the presented techniques are demonstrated by analysing the performance of a laser system used for laser cooling of Rb85 at a wavelength of 780 nm. A reference laser system is stabilised to a spectroscopic reference provided by modulation transfer spectroscopy. This spectroscopy scheme is analysed finding optimal operation at high modulation indices. A suitable signal is generated with a compact and cost-efficient module. A scheme for laser offset-frequency stabilisation based on an optical phase-locked loop is realised. All frequency locks derived from the reference laser system offer a Lorentzian linewidth of 60 kHz (FWHM) in combination with a long-term stability of 130 kHz peak-to-peak within 10 days. Intensity stabilisation based on acousto-optic modulators in combination with the digital controller allows for real-time intensity control on microsecond time scales complemented by a sample and hold feature with a response time of 150 ns.
High demands on the spectral properties of the laser systems are put forward for the coherent excitation of quantum states. In this thesis, the performance of active frequency stabilisation is enhanced by introducing a novel current modulation technique for diode lasers. A flat response from DC to 100 MHz and a phase lag below 90° up to 25 MHz are achieved extending the bandwidth available for laserfrequency stabilisation. Applying this technique in combination with a fast proportional-derivative controller, two laser fields with a relative phase noise of 42 mrad for driving rubidium ground state transitions are realised. A laser system for coherent Rydberg excitation via a two-photon scheme provides light at 780 nm and at 480 nm via frequency-doubling from 960 nm. An output power of 0.6 W at 480 nm from a single-mode optical fiber is obtained . The frequencies of both laser systems are stabilised to a high-finesse reference cavity resulting in a linewidth of 1.02 kHz (FWHM) at 960 nm. Numerical simulations quantify the effect of the finite linewidth on the coherence of Rydberg Rabi-oscillations. A laser system similar to the 480 nm Rydberg system is developed for spectroscopy on highly charged bismuth.
Advanced optical technologies are also at the heart of the micro-optical generation of tweezer arrays that offer unprecedented scalability of the system size. By using an optimised lens system in combination with an automatic evaluation routine, a tweezer array with several thousand sites and trap waists below 1 ÎĽm is demonstrated. A similar performance is achieved with a microlens array produced in an additive manufacturing process. The microlens design is optimised for the manufacturing process. Furthermore, scattering rates in dipole traps due to suppressed resonant light are analysed proving the feasibility of dipole trap generation using tapered amplifier systems
Inferences of Local Genetic Adaptation from Palaeolithic Hunter Gatherers
Ancient human genomes are an extremely powerful tool for exploring human demography and local genetic adaptation. Despite the thousands of genomes now available, there have been fewer amalgamated datasets focused on consistently processing samples created using different methods and filters. Therefore, I collated and processed a sample of 299 mostly publicly available ancient genomes sampling genetic diversity in Europe and recapitulating known clustering within ancient individuals. I made inferences of local genetic adaptation in 30 of the oldest Palaeolithic Hunter Gatherers, which were selected as they may reveal novel adaptations not detectable in younger individuals and or highlight earlier adaptations associated with the colonisation of Eurasia. I found many previously characterised candidates and one novel candidate in TCEA3. I also explored the advantage of incorporating ancient genomes in a test for Local Genetic Adaptation and observed a slight increase in candidates. Finally, I investigated functional annotations for the candidates of local genetic adaptation, highlighting some variants that may have influenced gene expression and disease risk and some which fall within Neanderthal- introgressed regions
Matrix continued fractions associated with lattice paths, resolvents of difference operators, and random polynomials
We begin our analysis with the study of two collections of lattice paths in
the plane, denoted and . These
paths consist of sequences of steps, where each step allows movement in
three directions: upward (with a maximum displacement of units), rightward
(exactly one unit), or downward (with a maximum displacement of units). The
paths start from the point and end at the point . In the
collection , it is a crucial constraint that paths never
go below the -axis, while in the collection , paths
have no such restriction. We assign weights to each path in both collections
and introduce weight polynomials and generating series for them. Our main
results demonstrate that certain matrices of size associated with
these generating series can be expressed as matrix continued fractions. These
results extend the notable contributions previously made by P. Flajolet and G.
Viennot in the scalar case . The generating series can also be
interpreted as resolvents of one-sided or two-sided difference operators of
finite order. Additionally, we analyze a class of random banded matrices ,
which have diagonals with entries that are independent and bounded
random variables. These random variables have identical distributions along
diagonals. We investigate the asymptotic behavior of the expected values of
eigenvalue moments for the principal truncation of as tends
to infinity.Comment: 48 pages, 3 figure
Geoarchaeological Investigations of Late Pleistocene Physical Environments and Impacts of Prehistoric Foragers on the Ecosystem in Northern Malawi and Austria
A growing body of research shows that not only did environmental changes play an important role in human evolution, but humans in turn have impacted ecosystems and landscape evolution since the Late Pleistocene. This thesis presents collaborative work on Late Pleistocene open-air sites in the Karonga District of northern Malawi, in which new aspects of forager behavior came to light through the reconstruction of physical environments. My work has helped recognize that late Middle Stone Age (MSA) activity and tool production occurred in locally more open riparian environments within evergreen gallery forest, surrounded by a regional vegetation dominated by miombo woodlands and savanna. Additionally, MSA hunter-gatherers exploited the confluence of river and wetland areas along the shores of Lake Malawi, which likely served as important corridors for the dispersal of biota. By comparing data from the archaeological investigations with lake core records, we were able to identify effects of anthropogenic burning on vegetation structures and sedimentation in the region as early as 80 thousand years ago. These findings not only proved it possible to uncover early impacts of human activity on the ecosystem, but also emphasize the importance of fire in the lives of early foragers.
Publications contained within this dissertation:
A. Wright, D.K., Thompson, J.C., Schilt, F.C., Cohen, A., Choi, J-H., Mercader, J., Nightingale, S., Miller, C.E., Mentzer, S.M., Walde, D., Welling, M., and Gomani-Chindebvu, E. “Approaches to Middle Stone Age landscape archaeology in tropical Africa”. Special issue Geoarchaeology of the Tropics of Journal of Archaeological Science 77:64-77. http://dx.doi.org/10.1016/j.jas.2016.01.014
B. Schilt, F.C., Verpoorte, A., Antl, W. “Micromorphology of an Upper Paleolithic cultural layer at Grub-Kranawetberg, Austria”. Journal of Archaeological Science: Reports 14:152-162. http://dx.doi.org/10.1016/j.jasrep.2017.05.041
C. Nightingale, S., Schilt, F.C., Thompson, J.C., Wright, D.K., Forman, S., Mercader, J., Moss, P., Clarke, S. Itambu, M., Gomani-Chindebvu, E., Welling, M. Late Middle Stone Age Behavior and Environments at Chaminade I (Karonga, Malawi). Journal of Paleolithic Archaeology 2-3:258-397. https://doi.org/10.1007/s41982-019-00035-3
D. Thompson, J.C.*, Wright, D.K.*, Ivory, S.J.*, Choi, J-H., Nightingale, S., Mackay, A., Schilt, F.C., Otárola-Castillo, E., Mercader, J., Forman, S.L., Pietsch, T., Cohen, A.S., Arrowsmith, J.R., Welling, M., Davis, J., Schiery, B., Kaliba, P., Malijani, O., Blome, M.W., O’Driscoll, C., Mentzer, S.M., Miller, C., Heo, S., Choi, J., Tembo, J., Mapemba, F., Simengwa, D., and Gomani-Chindebvu, E. “Early human impacts and ecosystem reorganization in southern-central Africa”. Science Advances 7(19): eabf9776. *equal contribution https://doi.org/10.1126/sciadv.abf9776
E. Schilt, F.C., Miller, C.M., Wright, D.K., Mentzer, S.M., Mercader, J., Moss, Choi, J.-H., Siljedal, G., Clarke, S., Mwambwiga, A., Thomas, K., Barbieri, A., Kaliba, P., Gomani-Chindebvu, E., Thompson, J.C. “Hunter-gatherer environments at the Late Pleistocene sites of Bruce and Mwanganda´s Village, northern Malawi”. Quaternary Science Reviews 292: 107638. https://www.sciencedirect.com/science/article/pii/S0277379122002694 [untranslated
Covariance-modulated optimal transport and gradient flows
We study a variant of the dynamical optimal transport problem in which the
energy to be minimised is modulated by the covariance matrix of the
distribution. Such transport metrics arise naturally in mean-field limits of
certain ensemble Kalman methods for solving inverse problems. We show that the
transport problem splits into two coupled minimization problems: one for the
evolution of mean and covariance of the interpolating curve and one for its
shape. The latter consists in minimising the usual Wasserstein length under the
constraint of maintaining fixed mean and covariance along the interpolation. We
analyse the geometry induced by this modulated transport distance on the space
of probabilities as well as the dynamics of the associated gradient flows.
Those show better convergence properties in comparison to the classical
Wasserstein metric in terms of exponential convergence rates independent of the
Gaussian target. On the level of the gradient flows a similar splitting into
the evolution of moments and shapes of the distribution can be observed.Comment: 84 pages, 4 figures. Comments are welcom
A general framework to quantify deviations from structural assumptions in the analysis of nonstationary function-valued processes
We present a general theory to quantify the uncertainty from imposing
structural assumptions on the second-order structure of nonstationary Hilbert
space-valued processes, which can be measured via functionals of time-dependent
spectral density operators. The second-order dynamics are well-known to be
elements of the space of trace-class operators, the latter is a Banach space of
type 1 and of cotype 2, which makes the development of statistical inference
tools more challenging. A part of our contribution is to obtain a weak
invariance principle as well as concentration inequalities for (functionals of)
the sequential time-varying spectral density operator. In addition, we
introduce deviation measures in the nonstationary context, and derive
estimators that are asymptotically pivotal. We then apply this framework and
propose statistical methodology to investigate the validity of structural
assumptions for nonstationary response surface data, such as low-rank
assumptions in the context of time-varying dynamic fPCA and principle separable
component analysis, deviations from stationarity with respect to the square
root distance, and deviations from zero functional canonical coherency
Rate-splitting multiple access for non-terrestrial communication and sensing networks
Rate-splitting multiple access (RSMA) has emerged as a powerful and flexible
non-orthogonal transmission, multiple access (MA) and interference management
scheme for future wireless networks. This thesis is concerned with the application of
RSMA to non-terrestrial communication and sensing networks. Various scenarios
and algorithms are presented and evaluated.
First, we investigate a novel multigroup/multibeam multicast beamforming strategy
based on RSMA in both terrestrial multigroup multicast and multibeam satellite
systems with imperfect channel state information at the transmitter (CSIT). The
max-min fairness (MMF)-degree of freedom (DoF) of RSMA is derived and shown
to provide gains compared with the conventional strategy. The MMF beamforming
optimization problem is formulated and solved using the weighted minimum mean
square error (WMMSE) algorithm. Physical layer design and link-level simulations
are also investigated. RSMA is demonstrated to be very promising for multigroup
multicast and multibeam satellite systems taking into account CSIT uncertainty
and practical challenges in multibeam satellite systems.
Next, we extend the scope of research from multibeam satellite systems to satellite-
terrestrial integrated networks (STINs). Two RSMA-based STIN schemes are
investigated, namely the coordinated scheme relying on CSI sharing and the co-
operative scheme relying on CSI and data sharing. Joint beamforming algorithms
are proposed based on the successive convex approximation (SCA) approach to
optimize the beamforming to achieve MMF amongst all users. The effectiveness and
robustness of the proposed RSMA schemes for STINs are demonstrated.
Finally, we consider RSMA for a multi-antenna integrated sensing and communications (ISAC) system, which simultaneously serves multiple communication users
and estimates the parameters of a moving target. Simulation results demonstrate
that RSMA is beneficial to both terrestrial and multibeam satellite ISAC systems by
evaluating the trade-off between communication MMF rate and sensing Cramer-Rao
bound (CRB).Open Acces
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