Observation of the dynamic Jahn-Teller effect in the excited states of nitrogen-vacancy centers in diamond

The optical transition linewidth and emission polarization of single nitrogen-vacancy (NV) centers are measured from 5 K to room temperature. Inter-excited state population relaxation is shown to broaden the zero-phonon line and both the relaxation and linewidth are found to follow a T^5 dependence for T up to 100 K. This dependence indicates that the dynamic Jahn-Teller effect is the dominant dephasing mechanism for the NV optical transitions at low temperatures

Cold and Ultracold Rydberg Atoms in Strong Magnetic Fields

Cold Rydberg atoms exposed to strong magnetic fields possess unique properties which open the pathway for an intriguing many-body dynamics taking place in Rydberg gases consisting of either matter or anti-matter systems. We review both the foundations and recent developments of the field in the cold and ultracold regime where trapping and cooling of Rydberg atoms have become possible. Exotic states of moving Rydberg atoms such as giant dipole states are discussed in detail, including their formation mechanisms in a strongly magnetized cold plasma. Inhomogeneous field configurations influence the electronic structure of Rydberg atoms, and we describe the utility of corresponding effects for achieving tightly trapped ultracold Rydberg atoms. We review recent work on large, extended cold Rydberg gases in magnetic fields and their formation in strongly magnetized ultracold plasmas through collisional recombination. Implications of these results for current antihydrogen production experiments are pointed out, and techniques for trapping and cooling of such atoms are investigated.Comment: 46 pages, 38 figures, to appear in Physics Report

Doctor of Philosophy

dissertationThe forefront of current nanoscience initiatives includes the investigation and development of semiconducting colloidal nanocrystals for optoelectronic device concepts. Being highly facile in their synthesis, a wide range of sizes, morphologies, materials, interactions, and effects can easily be engineered by current synthetic chemists. Their solution-processability also makes available the use of long established industrial fabrication techniques such as reel-to-reel processing or even simple inkjet printing, offering the prospect of extremely cheap device manufacturing. Aside from anticipated technologies, this material class also makes available a type of "playground" for generating and observing novel quantum effects within reduced dimensions. Since the surface-to-volume ratio is very large in these systems, unsatisfied surface states are able to dominate the energetics of these particles. Although simple methods for satisfying such states are usually employed, they have proven to be only semieffective, often due to a significant change in surface stoichiometry caused by complex atomic reorganization. Serving as charge "trap" states, their effect on observables is readily seen, for instance, in single particle photoluminescence (PL) blinking. Unfortunately, most methods used to observe their influence are inherently blind to the chemical identity of these sites. In absence of such structural information, systematically engineering a robust passivation system becomes problematic. The development of pulsed optically detected magnetic resonance (pODMR) as a method for directly addressing the chemical nature of optically active charges while under trapping conditions is the primary tenet of this thesis. By taking advantage of this technique, a great wealth of knowledge becomes immediately accessible to the researcher. The first chapter of this work imparts the relevant background needed to pursue spin resonance studies in colloidal nanocrystals; the second chapter addresses technical aspects of these studies. In Chapter 3, pODMR is used to explore shallow trap states that dominate the charge transfer process in CdSe/CdS heterostructure nanocrystals. Several trapping channels are observed, while two in particular are correlated, demonstrating for the first time that both electrons and holes are able to be trapped within the same nanoparticle at the same time. The intrinsically long spin coherence lifetime for these states allows for the spin multiplicity and degree of isolation to be explored. Demonstration of novel effects is also performed, such as coherent control of the light-harvesting process and remote readout of spin information. The study presented in Chapter 4 focuses on the spin-dependencies observed in the historically ill-described emissive CdS defect. By monitoring deep-level emission from nanorods of this material, it is shown that the cluster defect can ultimately be fed by the same shallow trap states explored in Chapter 3. The degree of interaction between trap states and the cluster defect is probed. Also, a surprisingly long spin coherence lifetime (T2 « 1.6 /is) for the defect itself is observed, which opens the possibility of highly precise chemical fingerprinting through electron spin echo envelop modulation (ESEEM). This dissertation lays the groundwork for further use of these, and more powerful magnetic resonance probes of the states that fundamentally limit the practical utility of colloidal nanocrystal optoelectronics devices. Furthermore, by gaining access to these optically active electronic states, novel methods of coherent quantum control may be exerted on the energetics of this material system

Consecutive k-within-m-out-of-n:F system with exchangeable components

As a generalization of k-out-of-n:F and consecutive k-out-of-n:F systems, the consecutive k-within-m-out-of-n:F system consists of n linearly ordered components such that the system fails iff there are m consecutive components which include among them at least k failed components. In this article, the reliability properties of consecutive k-within-m-out-of-n:F systems with exchangeable components are studied. The bounds and approximations for the survival function are provided. A Monte Carlo estimator of system signature is obtained and used to approximate survival function. The results are illustrated and numerics are provided for an exchangeable multivariate Pareto distribution

High Fidelity Universal Gates Performed on a Continuously-Decoupled Coherence Enhanced Transmon Qubit

Decoherence is the primary limiting factor for the utility of modern qubits and qubit networks; most chiefly, pure dephasing which limits the operational time any gate-sequence can produce a high-fidelity result. In this dissertation, I present the results of my experiment, performing fast, high fidelity, universal single-qubit gates, on a qubit which has been decoupled from pure dephasing resulting from environmental noise. This technique can expand operational ranges of qubits–such as allowing the high-coherence operation of a flux-tunable qubit far away from its flux-insensitive sweet-spot; broadening our selection of viable qubits by making otherwise low-coherence qubits operable with high coherence, or improving the coherence of higher order quantum networks which have limited coherence time due to qubit to qubit interactions producing prohibitive amounts of pure dephasing. This technique could be performed on any deterministic qubit of any modality which can receive drives of a physically similar kind as my particular test-platform, the superconducting transmon

A stochastic and dynamical view of pluripotency in mouse embryonic stem cells

Pluripotent embryonic stem cells are of paramount importance for biomedical research thanks to their innate ability for self-renewal and differentiation into all major cell lines. The fateful decision to exit or remain in the pluripotent state is regulated by complex genetic regulatory network. Latest advances in transcriptomics have made it possible to infer basic topologies of pluripotency governing networks. The inferred network topologies, however, only encode boolean information while remaining silent about the roles of dynamics and molecular noise in gene expression. These features are widely considered essential for functional decision making. Herein we developed a framework for extending the boolean level networks into models accounting for individual genetic switches and promoter architecture which allows mechanistic interrogation of the roles of molecular noise, external signaling, and network topology. We demonstrate the pluripotent state of the network to be a broad attractor which is robust to variations of gene expression. Dynamics of exiting the pluripotent state, on the other hand, is significantly influenced by the molecular noise originating from genetic switching events which makes cells more responsive to extracellular signals. Lastly we show that steady state probability landscape can be significantly remodeled by global gene switching rates alone which can be taken as a proxy for how global epigenetic modifications exert control over stability of pluripotent states.Comment: 11 pages, 7 figure

Laser spectroscopy of functional materials

In a world of increasing energy consumption and man-made global warming, the knowledge of electronic and structural properties of organic materials is crucial for their implementation and use in innovative (smart) devices since those determine device performance. Herein, the electronic and structural properties of new promising materials are studied in detail utilising advanced laser spectroscopic techniques. Considering optoelectronic devices such as organic field effect transistors and solar cells, the metal/organic interface plays an important role for device performance. Therefore, the electronic properties of an n-type semiconducting N-substituted pentacene derivative and its change at higher layer thicknesses in contact with a metal electrode is studied via two-photon photoemission spectroscopy as well as the adsorption and desorption properties via temperature-programmed desorption. Several molecular electronic states such as the highest occupied and the lowest unoccupied molecular orbital as well as the optical gap are thereby determined quantitatively. Developing optoelectronic devices further, smart devices covering more complex needs can be achieved by implementing multiresponsive mixtures reacting in differentiated ways to light, which acts as sustainable external stimulus with high spatio-temporal resolution. Hence, the switching behaviour of a mixture of two individually as well as simultaneously addressable photoswitches is researched in detail. Furthermore, the investigated mixture is 3D printable allowing fast and easy implementation in 3D structures. The photoswitches are a push-pull azobenzene derivative and a first generation donor-acceptor Stenhouse adduct absorbing longer wavelengths in the visible region of the spectrum compared to the azobenzene derivative. All steps of the trans-to-cis isomerisation of the former and the linear-to-closed isomerisation of the latter photoresponsive molecule are studied as well as the respective back reactions with special emphasis on the environmental influence and the impact of the photoswitches on each other. Therefore, several techniques, such as femtosecond transient absorption, temperature dependent kinetic visible absorption and kinetic infrared absorption spectroscopy, are used to gain a full picture. Dynamic effects of the environment and the photoswitches on each other are thereby found such that individual switching of both molecules takes place with minor impacts on each other in the mixture. Other 3D printable materials are diblock copolymers exhibiting microphase separation such as a derivative of a standard diblock copolymer for high-resolution nanolithography with additional functional groups allowing 3D printing. The lamellar structure and its regularity is studied by means of a scattering-scanning near-field optical microscope before and after 3D printing upon which a polymeric network is formed. Before printing, lamellae are thereby found whereas the printing process needs to be optimised to maintain this structure after 3D printing

Radiation Effects on an Active Ytterbium-doped Fiber Laser

This is the first published research focused on the impact of gamma and mixed gamma/neutron radiation on an actively lasing ytterbium-doped fiber laser. While the gain medium of the ytterbium-doped fiber laser was irradiated, the power was measured in-situ and the spectrum was recorded intermittently. Two radiation sources were used, a 60Co cell and a reactor. Three irradiation experiments were conducted per radiation source; pristine fibers were used for the first two experiments, and fibers from the second experiment were re-irradiated for the third experiment. The results indicate that as the total dose increased linearly with time, the laser experienced an exponential decay in power with a maximum power loss of 99.84% (at which time it was no longer lasing), and the lasing wavelength blueshifted up to 15 nm. The laser\u27s initial power affects how much the radiation induced attenuation in the fiber. The laser, when exposed to 145 krad(Si), experienced less attenuation with a higher initial power than with a lower initial power. Power recovery experiments were conducted post-irradiation with the fiber laser off and actively lasing. Passively, the power recovered 100 and 550 µW in 18 and 90 hours respectively. Active recovery experienced the same 100 µW recovery in 9.3% of the time (10 min), and total power recovery of 12.6% and 4.4% for YDF1 and YDF2 respectively. The active recovery rate declined as the number of days following irradiation increased. This indicated a saturation of recovery after the less stable color centers were annealed. The active recovery rate for the re-irradiated fiber (gamma only) decreased 31% from the recovery rate just prior to re-irradiation