Quantum computers can theoretically perform certain tasks which classical computers at realistic times could not. Operating a quantum computer requires precise control over the system, for instance achieved by adiabatic evolution, and isolation from the environment to retain coherence. This report combines these two, somewhat contradicting, error preventing techniques. To reduce the run-time a transitionless quantum driving algorithm, or, adiabatic shortcut, is employed. The notion of Noiseless Subsystems (NS), a generalization of decoherence free subspaces, are used for robustness against environmental decoupling, by creating logical qubits which act as a noiseless code. Furthermore, the adiabatic shortcut for the NS code is applied to a refocusing scheme (spin-echo) in order to remove the dynamical phase, sensitive to error propagation, so that only the Berry phase is effectively picked up. The corresponding Hamiltonian is explicitly derived for the only two cases of two-dimensional NS: N=3,4 qubits with total spin of j=1/2,0, respectively. This constitutes geometric quantum computation (GQC) enacting a universal single-qubit gate, which is also explicitly derived. / Kvantdatorer kan teoretiskt utföra vissa uppgifter som klassiska datorer vid realistiska tider inte kan. Att köra en kvantdator kräver exakt kontroll över systemet, till exempel genom adiabatisk utvecking, och isolering från omgiviningen för att behålla koherens. Denna rapport kombinerar dessa två, något motsägelsefulla, tekniker för felhantering. För att minska körtiden används en övergångsfri kvantkörningsalgoritm, också kallad adiabatisk genväg. Konceptet brusfria delsystem, en generalisering av dekoherensfria underrum, används för robusthet mot sammanflätning med omgivningen genom att skapa logiska kvantbitar som fungerar som en brusfri kod. Vidare tillämpas den adiabatiska genvägen för den brusfria koden på ett spinn-eko för att eliminera den dynamiska fasen, som är känslig för felpropagering, så att endast Berrys fas, som är okänslig för felpropagering, effektivt plockas upp. Motsvarande Hamiltonian härleds uttryckligen för de enda två fallen av tvådimensionella brusfria delsystem: 3 eller 4 kvantbitar med respektive totalspinn j = 1/2 och 0. Detta möjliggör beräkning med en geometrisk kvantdator baserad på en universell en-kvantbitsgrind, som också härleds explicit.

The broad range of applications for photochemical reactions is the result of light-matter interaction at the electronic level. The diverse application of photochemistry in various fields, including photovoltaic materials, molecular switches, and biological systems are due to electronic and structural transformations induced by photoexcitation as well as molecular alteration due to electron and charge transfer. An improved understanding of these photochemical events is dependent on the fundamental theoretical evaluation, to model and analyze the ultrafast processes. The studies discussed in this thesis explore such theoretical implementation in two different frontiers. In the first study, dynamic simulations are performed to model the light-induced bond dissociation of phenyl azide. The surface hopping formalism, implemented under the semiclassical molecular dynamics approach helped in tracing the time evolution of the electronic and structural levels, involved in the photodissociation. In the second study, the time-dependent density functional theory has been applied to generate XA spectra of imidazole solutions. The theoretical assessments support experimental measurements and provide more insight into the core excitations and structural influence on the absorption spectra. / Fotokemiska reaktioner styrs av växelverkan mellan ljus of materia på en elektronisk nivå. I olika fält finns det vitt skilda tillämpningarna av fotokemi. Dessa inkluderar ljusinducerade processer i solceller, molekylära strömbrytare, och biologiska system. Reaktionerna beror av elektroniska och strukturella transformationer som induceras av fotoexcitationen, och kan ge upphov till energi- och laddningsöverföring. För att få utökad förståelse av fotokemiska reaktioner behövs grundläggande teoretiska studier där ultrasnabba processer modelleras och analyseras i jämförelse med experiment. Undersökningar som presenteras i denna avhandling använder sig att teoretiska modeller i två olika områden av fotokemi. I den första studien har vi genomfört dynamiska simuleringar för att modellera ljusinducerad dissociation av fenylazid. Vi have använt en semi-klassisk approximation med hopp mellan olika elektroniska tillstånd vilket gör det möjligt att följa utvecklingen i elektroniska och geometriska frihetsgrader under fotodissociationen. I den andra studien har tidsberoende täthetsfunktionalteori använts för att simulera röntgenabsorptionsspektrum för imidazol i lösning. Genom att utvärdera olika geometriska modeller med teoretiska beräkningar kan vi berika tolkningen av experimentella mätningar, och även få detaljerat insikt i innerskalsexcitationer och hur geometrin påverka röntgenspektrum.

Fundamental understanding of high strain rate deformation behavior of materials is critical in designing new alloys for wide-ranging applications including military, automobile, spacecraft, and industrial applications. High entropy alloys, consisting of multiple elements in (near) equimolar proportions, represent a new paradigm in structural alloy design providing ample opportunity for achieving excellent performance in high strain rate applications by proper selection of constituent elements and/or thermomechanical processing. This dissertation is focused on fundamental understanding of high strain-rate deformation behavior of several high entropy alloy systems with widely varying microstructures. Ballistic impact testing of face centered cubic Al0.1CoCrFeNi high entropy alloy showed failure by ductile hole growth. The deformed microstructure showed extensive micro-banding and micro-twinning at low velocities while adiabatic shear bands and dynamic recrystallization were seen at higher velocities. The Al0.7CoCrFeNi and AlCoCrFeNi2.1 eutectic high entropy alloys, with BCC and FCC phases in lamellar morphology, showed failure by discing. A network of cracks coupled with small and inhomogeneous plastic deformation led to the brittle mode of failure in these eutectic alloys. Phase-specific mechanical behavior using small-scale techniques revealed higher strength and strain rate sensitivity for the B2 phase compared to the L12 phase. The interphase boundary demonstrated good stability without any cracks at high compressive strain rates. The Al0.3CoCrFeNi high entropy alloy with bimodal microstructure demonstrated an excellent combination of strength and ductility. Ballistic impact testing of Al0.3CoCrFeNi alloy showed failure by ductile hole growth and demonstrated superior performance compared to all the other high entropy alloy systems studied. The failure mechanism was dominated by micro-banding, micro-twining, and adiabatic shear localization. Comparison of all the high entropy alloy systems with currently used state-of-the-art rolled homogenous armor (RHA) steel showed a strong dependence of failure modes on microstructural features.

This dissertation presents a self-contained study of periodically-driven quantum systems. Following a brief introduction to Floquet theory, we introduce the inverse-frequency expansion, variants of which include the Floquet-Magnus, van Vleck, and Brillouin-Wigner expansions. We reveal that the convergence properties of these expansions depend strongly on the rotating frame chosen, and relate the former to the existence of Floquet resonances in the quasienergy spectrum. The theoretical design and experimental realisation (`engineering') of novel Floquet Hamiltonians is discussed introducing three universal high-frequency limits for systems comprising single-particle and many-body linear and nonlinear models. The celebrated Schrieffer-Wolff transformation for strongly-correlated quantum systems is generalised to periodically-driven systems, and a systematic approach to calculate higher-order corrections to the Rotating Wave Approximation is presented. Next, we develop Floquet adiabatic perturbation theory from first principles, and discuss extensively the adiabatic state preparation and the corresponding leading-order non-adiabatic corrections. Special emphasis is thereby put on geometrical and topological objects, such as the Floquet Berry curvature and the Floquet Chern number obtained within linear response in the presence of the drive. Last, pre-thermalisation and thermalisation in closed, clean periodically-driven quantum systems are studied in detail, with the focus put on the crucial role of Floquet many-body resonances for energy absorption.

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The Landau-Zener-Stückelberg-Majorana (LZSM) problem models diabatic transitions between energy levels in quantum two-level systems with an avoided level-crossing. The diabatic transition is a consequence of quantum tunneling in energy space when the system's Hamiltonian is perturbed with a fast-acting bias. The probability of transition between the energy states for a linear bias is known as the LZSM transition probability. The objective of this work is to investigate the LZSM problem through analytical and numerical lenses. The LZSM transition probability is derived in two ways. The first approach is based on Majorana's solution using contour integrals. The second derivation follows Landau's quasi-classical treatment. The derivations demonstrate methods for transitions in the presence of time-dependent perturbations. The ubiquity of the two-level system is discussed and an application on qubits concerning LSZM interferometry is presented, with the latter arising after considering periodic biases. Lastly, a simulation of the two-level system is conducted using Trotter-decomposed time-evolution operators, perturbation theory, and vectorization. The simulated transition probabilities for linear and periodic biases are obtained for varied parameters. The results show that the simulation achieves an accurate and efficient emulation of the LZSM problem.

Die vorliegenden Dissertation berichtet von der Entdeckung ungewöhnlicher magnetischer, elektronischer und struktureller Eigenschaften in einer Reihe von neuen intermetallischen Verbindungen auf Selten-Erd-Basis. Die untersuchten Systeme vom Typ RET2X bestehen aus den Selten-Erd-Elementen (RE) Yb oder Lu, den Übergangsmetallen (T) Pt oder Pd sowie den weiteren Liganden (X) Sn oder In. Die Synthese der verwendeten Proben, deren kristallografische Analyse und die Untersuchung ihrer physikalischen Eigenschaften werden im Detail vorgestellt. Diese Arbeit liefert Resultate, die sowohl für die Grundlagenforschung als auch für technische Anwendungen eine große Relevanz besitzen. Die Untersuchungen der neuen Verbindungen YbPt2Sn und YbPt2In zeigen, dass die magnetische Kopplung zwischen benachbarten Yb-Ionen extrem schwach ist. Dies führt zu einem riesigen magnetokalorischen Effekt im Bereich von 0.05 K bis 2 K. Damit besitzen beide metallischen Materialien optimale Eigenschaften, um als Kühlkörper in Entmagnetisierungskryostaten Verwendung zu finden. Zwei zu Testzwecken aufgebaute Kühleinsätze auf YbPt2Sn-Basis bestätigen die Eignung dieser Verbindung als metallisches Kühlmaterial. Die Untersuchungen der Substitutionsreihe Lu(Pt1-xPdx)2In offenbaren einen Ladungsdichtewelle (CDW)-Phasenübergang mit außergewöhnlichen Eigenschaften. Im Gegensatz zu Beobachtungen in den meisten anderen bekannten CDW-Systemen ist der Übergang in LuPt2In kontinuierlich, d. h. zweiter Ordnung. Durch Ersetzen von Pt mit isovalenten Pd kann die Übergangstemperatur T_CDW kontinuierlich zum absoluten Temperaturnullpunkt geführt werden. Die beobachteten Eigenschaften zeigen, dass der Phasenübergang dabei zweiter Ordnung bleibt. Damit wird experimentell bewiesen, dass Lu(Pt1-xPdx)2In eines der seltenen Systeme ist, in denen ein CDW quantenkritischer Punkt in Erscheinung tritt. Noch außergewöhnlicher ist die Beobachtung von Supraleitung mit einem ausgeprägten Maximum in der Sprungtemperatur T_c genau am quantenkritischen Punkt. Das deutet auf eine neuartige Kopplung zwischen quantenkritischer CDW und Supraleitung hin.:Einleitung 1 Grundlagen 2 YbPt2Sn und YbPt2In 3 Adiabatische Entmagnetisierung von YbPt2Sn 4 Struktureller quantenkritischer Punkt in Lu(Pt1-xPdx)2In 5 Zusammenfassung und Ausblick

As the third generation of luminescent materials, thermally activated delayed fluorescence (TADF)-type compounds have great potential as emitter molecules in OLEDs allowing for electro-fluorescence with 100 % internal quantum efficiency. For organic electronics, the general wide range of applications from OLEDs, bio-fluorescence imaging to sensor technologies and photonic energy storages roots on the enormous variety of organic materials. Especially in the field of metal- free aromatic designs, the range of possible materials showing diverse triplet harvesting effects is immense, making material development a highly complex task. Firstly, initial efforts in the understanding of the basic concepts behind TADF will be highlighted. A rational design strategy for TADF materials will be illustrated on an innovative material series based on phenylcarbazoles. A reasonable branch of isomers are theoretically constructed and slight stoichiometric modifications are performed to understand how molecular structure and intramolecular steric hindrance affects reverse intersystem crossing (RISC), while simultaneously revealing the strategy for deep blue TADF. The rational design of a bluish green TADF material called 5CzCF3Ph providing CIEy ≤ 0.4 is demonstrated, enabling peak EQE values of 12.1 % with a promising LT50 of 2 hrs at 500 cd∙m-2. Subsequently, the photophysics of five newly designed trimeric donor (D)-acceptor (A)-donor (D) type material compounds, analogue concepts to archetypical TADF designs, highlight the importance of intramolecular electronic couplings between adjacent triplet states for adiabatically-driven TADF, revealing the mechanism of local type triplet state perturbations on 3CT states. The most promising candidate (DMAC-PTO-DMAC) is disclosed and in turn optimized to meet required conditions for deep blue TADF emission. Ultimately, a deep blue luminescent material called isoDMAC-PTO is developed, featuring CIE coordinates of (0.16, 0.14) with an overall quantum yield of (86.4 ± 0.5) %. The focus switches to the fundamental understanding of the underlying mechanism giving rise to TADF in small molecules, leaving the scope of deep blue emission. While investigating the photophysical properties of a synthesized donor (D)-acceptor (A) type thermally activated delayed fluorescence (TADF) emitter named methyl 2-(9,9-dimethylacridin-10-yl)benzoate (DMAC-MB), it is possible to identify the excited state dynamics mediating the spin-flip process and hence the reutilization of non-radiative triplet states allowing for an internal quantum efficiency approaching unity. As experimentally observed by detailed temperature- and time-dependent transient photoluminescence (PL) measurements and consolidated by comprehensive quantum-chemical considerations, excited state configuration interaction by non-adiabatic couplings are anticipated as key property behind triplet up-conversion in the vicinity of conical intersections, contributing to recent research facing the exciton management within the auspicious field of TADF. For the first time, this thesis reports that even a TADF-silent molecule can be converted into efficient TADF systems by increasing the donor π- conjugation length through polymerization of the building block itself. With a total photoluminescence quantum yield up to 71 %, comprehensible research illustrates an efficient thermally activated delayed fluorescence polymer P1, based solely on non-TADF chromophores represented by a model compound 2 (PLQY of 3 % at RT). Finally, as predicted by TDDFT calculations and shown for the first time in the aspiring field of TADF, a thermally activated delayed fluorescence polymer based on a merely radiative, solely phosphorescent repeating unit is demonstrated. Intramolecular π-conjugation is exploited to trigger the charge-transfer excited state energy, revealing a general design tool to provoke TADF, reserved in particular for polymers. While the introduced twisted methyl 2-(9,9-dimethylacridin-10-yl) benzoate (DMAC-MB) reveals efficient thermally activated delayed fluorescence (TADF), a modified analogue 9,9-Dimethyl-5H,9H-quinolino[3,2,1-de]acridin-5-one (DMAC-ACR) shows emerging room temperature phosphorescence (RTP). As for TADF, intramolecular non-adiabatic couplings are unlocked as key feature actuating persistent RTP, linking photophysical analogies between TADF and RTP to structural self-similarities. Last but not least, degradation processes in TADF materials will be addressed. A correlation between theoretically calculated bond-dissociation energies (BDEs) and phenomenological observations reveals that low BDEs, in particular along pronounced charge-transfer bonds, ultimately lead to irreversible TADF material degradation induced by bi-molecular processes comprising TPQ as well as TTA. Finally, this thesis reveals the photophysics of 24 newly designed, synthesized and characterized TADF materials and demonstrates a fundamentally new approach for RTP, based on structural analogues to TADF. Far reaching design principles as conjugation induced TADF in polymers, as well as new design strategies selectively incorporating virbonic couplings yield device performances comprising LT50 of 2 hrs at 500 cd∙m-2 and targeted deep blue emission with CIE (0.16, 0.14). While lighting the way for TADF as future luminescent OLED materials, intrinsic material instabilities due to low bond-dissociation energies are disclosed as key-issues for tomorrows research.

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