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Large-scale quantum computation requires a fast assessment of the main sources of error in the implemented quantum gates. To this aim, we provide a learning based framework that allows to extract the contribution of each physical noise source to the infidelity of a series of gates with a small number of experimental measurements. To illustrate this method, we consider the case of superconducting transmon architectures, where we focus on the diabatic implementation of the CZ gate with tunable couplers. In this context, we account for all relevant noise sources, including non-Markovian noise, electronics imperfections and the effect of tunable couplers to the error of the computation., 22 + 6 pages, 7 figures

Digital-Analog Quantum Computation (DAQC) has recently been proposed as an alternative to the standard paradigm of digital quantum computation. DAQC creates entanglement through a continuous or analog evolution of the whole device, rather than by applying two-qubit gates. This manuscript describes an in-depth analysis of DAQC by extending its implementation to arbitrary connectivities and by performing the first systematic study of its scaling properties. We specify the analysis for three examples of quantum algorithms, showing that except for a few specific cases, DAQC is in fact disadvantageous with respect to the digital case., Comment: 16+5 pages, 11 figures

A crucial task to obtain optimal and reliable quantum devices is to quantify their overall performance. The average fidelity of quantum gates is a particular figure of merit that can be estimated efficiently by Randomized Benchmarking (RB). However, the concept of gate-fidelity itself relies on the crucial assumption that noise behaves in a predictable, time-local, or so-called Markovian manner, whose breakdown can naturally become the leading source of errors as quantum devices scale in size and depth. We analytically show that error suppression techniques such as Dynamical Decoupling (DD) and Randomized Compiling (RC) can operationally Markovianize RB: i) fast DD reduces non-Markovian RB to an exponential decay plus longer-time corrections, while on the other hand, ii) RC generally does not affect the average, but iii) it always suppresses the variance of such RB outputs. We demonstrate these effects numerically with a qubit noise model. Our results show that simple and efficient error suppression methods can simultaneously tame non-Markovian noise and allow for standard and reliable gate quality estimation, a fundamentally important task in the path toward fully functional quantum devices., Comment: 11+18 pages, 10 figures

Tunable coupling of superconducting qubits has been widely studied due to its importance for isolated gate operations in scalable quantum processor architectures. Here, we demonstrate a tunable qubit-qubit coupler based on a floating transmon device which allows us to place qubits at least 2 mm apart from each other while maintaining over 50 MHz coupling between the coupler and the qubits. In the introduced tunable-coupler design, both the qubit-qubit and the qubit-coupler couplings are mediated by two waveguides instead of relying on direct capacitive couplings between the components, reducing the impact of the qubit-qubit distance on the couplings. This leaves space for each qubit to have an individual readout resonator and a Purcell filter needed for fast high-fidelity readout. In addition, the large qubit-qubit distance reduces unwanted non-nearest neighbor coupling and allows multiple control lines to cross over the structure with minimal crosstalk. Using the proposed flexible and scalable architecture, we demonstrate a controlled-$Z$ gate with $(99.81 \pm 0.02)\%$ fidelity., Comment: 24 pages, 12 figures, Figure 1 updated, Appendix A extended

Spins in a solid-state environment and photons are both promising physical systems regarding the implementation of qubits. Whereas electron and nuclear spins act very well as localized qubits that are able to store and process quantum information, photons are the primary candidates to transmit quantum information between distant locations. Entanglement, on the other hand, is a fundamental resource of quantum information processing enabling, e.g., secure communication over long distances. The generation and manipulation of entanglement are thus fundamental steps in order to leverage the benefits of quantum information processing. The aim of this dissertation is therefore to theoretically investigate entangling mechanisms for various physical systems.A functioning source of entangled photons is a basic hardware component required for quantum communication. To this end, we study a system composed of semiconductor quantum wells inside a microcavity, known as intersubband cavity system, in which the interaction of the electronic system with the cavity field can reach the ultrastrong coupling regime. The ground state contains a finite number of photons, and we find that these photons are entangled. The amount of entanglement is quantified analytically, and maximal entanglement is found to be possible.The technique of entanglement purification allows to restore maximal entanglement that has decreased, e.g. due to decoherence. Purification requires quantum memory, a role for which electron spins in electrically-defined quantum dots are well suited. However, existing purification protocols are rather unpractical in this case. Here, the concept of asymmetric bilateral two-qubit operations is introduced to purify spin entanglement by harnessing the typical interaction between electrons in neighboring quantum dots. As it turns out, this concept can be applied to a variety of qubit systems, e.g. superconducting qubits or spins in nitrogen-vacancy centers in diamond.The latter example offers several possibilities to implement a qubit, of which the intrinsic nitro- gen nuclear spin has proven its viability in many ways. We develop a scheme to deterministically couple two nuclear spin qubits, in which the interaction is mediated by an optical cavity. It is found that an entangling two-qubit gate, also required for universal quantum computation, can be implemented with operation times below 100 nanoseconds, i.e. several orders of magnitude faster compared to the decoherence time of the nuclear spin.The verification of entanglement, which is required e.g. in entanglement-based quantum com- munication to detect an eavesdropping attack, typically involves a measurement of qubit states. Using the input-output formalism, we derive a fully quantum-mechanical model of an optical readout scheme to measure the spin state of an electron in a self-assembled quantum dot.

In den letzten Jahrzehnten wuchs das Verst��ndnis der Bev��lkerung f��r Umweltprobleme immens und den Menschen wurde bewusst, dass eine Reduktion von Treibhausgasen wesentlich zur Verbesserung des Weltklimas beitragen kann. Zur Verminderung des Treibhausgasausto��es gibt es viele Str��nge, an denen gezogen werden kann; einer davon ist die energetische Optimierung bestehender Kraftwerksanlagen, um so bei gleichbleibender Produktion, Brennstoff einzusparen und so Emissionen zu mindern. Meine Arbeit besch��ftigt sich mit der Bilanzierung von Masse- und Energiestr��men der M��llverbrennungsanlage Arnoldstein. Sie soll Aufschluss geben ��ber die ein- und austretenden Str��me der Teilkomponenten dieser Anlage, eine Lokalisierung von Energieverlusten erleichtern und die Gr����enordnung von Massestr��men darstellen. Au��erdem soll so, eine aktuelle Dokumentation der Anlage vorliegen, welche auch durch Optimierung ge��nderte Prozessf��hrung seit der Einreichung ber��cksichtigt. Durch die Kenntnis ��ber die Thermodynamik der Anlagenapparate, lassen sich Eingriffe erm��glichen, durch welche weniger Energieverluste entstehen und so eine bessere Brennstoffausnutzung erzielt werden kann. Diese Optimierung erh��ht die Wirtschaftlichkeit der Verbrennungsanlage bei gleichbleibender CO2-Emission und tr��gt so direkt zum Schutz der Umwelt vor unn��tigen Emissionen bei. Durch die Berechnungen liegen jetzt aktuelle Zahlenwerte f��r alle wichtigen Anlagenkomponenten vor. Dies gilt auch f��r die Luftzerlegungsanlage, welche energetisch noch nicht optimal ausgenutzt wird. In etwa die H��lfte des Stromeigenbedarfs der Gesamtanlage ben��tigt alleine der Motor f��r Vakuumpumpe und Gebl��se, der auch das stickstoffreiche Restgas auf eine Temperatur von ��ber 100��C aufw��rmt und in die Atmosph��re bl��st. Weiters zeigte die Bilanzierung, dass ungew��hnlich hohe Luftmengen f��r die Sperrluft des Rostes und der Ru��bl����er aufgewendet werden m��ssen, wodurch eine Neumessung der gef��rderten Luftmengen veranlasst wurde. Hierbei stellte sich heraus, dass die Hersteller einen doppelt so hohen Wert angaben, als tats��chlich gef��rdert wurde., During the last decades human thinking changed; increased awareness of environment pollution extended massively and people realized that a reduction of greenhouse gases can contribute to an improvement of global climate. A decrease of greenhouse gas emissions can be achieved by different ways. One of this ways is the energetic optimization of established power plants, that effects fuel saving and emission reduction with equal production. This thesis deals with energy and mass balances of the waste incineration plant Arnoldstein. It should give information about incoming and outgoing flows of subcomponents, should ease identification of energy losses and should describe dimensions of mass flows. With knowledge of the thermodynamics of single components improvements are allowed and energy losses could be minimized, what effects in fuel saving. This optimization increases the efficiency of the incineration plant and decreases CO2-emissions what protects environment of unnecessary emissions. Because of the calculations of this thesis, current numerical values of all important components of the power plant are on hand. This applies also to the air separation plant that is energetically not perfectly used. Almost half of the auxiliary power of the whole power plant needs the engine for the vacuum pump and the air blower, that increases the temperature of the with nitrogen enriched gas to about 100��C and then blows it to the atmosphere. The balance study also shows that sealing air for reverse-acting grate and for soot blowers are extraordinary high. This induced new measurements of the air flow that show that the information of the producer was double as high as the real air flow.