Qubit measurement and coupling strategies and their applications

A quantum computer can reduce the amount of computational effort for selected applications exponentially by taking advantage of quantum mechanical phenomena of nature. For the realization of a real-world quantum computer, among other things, optimized qubit measurements and qubit coupling schemes are indispensable. This dissertation uses theoretical tools to develop novel measurement and coupling strategies in different superconducting qubit architectures. In a first part of the thesis a protocol for multi-qubit parity measurements of Transmon qubit registers is presented, which takes advantage of the nonlinear energy level structure to strongly increase contrast, while at the same time achieving high fidelities and being quantum non-demolishing. The second part focuses on superconducting flux qubits, which are promising for adiabatic quantum computing. First a novel indirect flux qubit measurement protocol is introduced, which provides the ability to measure in a fixed basis, the persistent current basis, independent of the qubit energy eigenbasis. Second it is shown that the limitation of natural interactions to pairwise interactions can be overcome by using a nonlinear coupler and four flux qubits. The achieved four local interactions between the qubits are proven to be in the strong coupling regime and even exceed the two local ones for the right system parameters.Ein Quantencomputer besitzt das Potenzial den Rechenaufwand für bestimmte Aufgaben gegenüber einem klassischen Computer exponentiell zu reduzieren, indem er sich quantenmechanischer Phänomene der Natur bedient. Zur Realisierung eines echten Quantencomputers sind, neben anderen Bestandteilen, optimierte Mess- und Kopplungsschemata für Qubits unabdingbar. Diese Dissertation befasst sich damit theoretische Mittel zu nutzen, um neue Mess- und Kopplunsstrategien in verschiedenen supraleitenden Qubitarchitekturen zu entwickeln. In dem ersten Teil der Arbeit wird ein neues Protokoll zur Paritätsmessung von Registern aus Transmon Qubits vorgestellt, welches die nichtlineare Energiestruktur ausnutzt, um den Kontrast der Messung stark zu erhöhen und zudem sowohl einen hohe Messgüte aufweist als auch QND ist. Der zweite Teil fokussiert sich auf supraleitende Flussqubits, die vor allem beim adiabatischen Quantencomputer genutzt werden. Zuerst wird ein neues, indirektes Messprotokol vorgestellt, welches die Möglichkeit bietet in einer festen Basis, der Dauerstrombasis, unabhängig von der jeweiligen Energieeigenbasis, zu messen. Danach wird gezeigt, dass die Einschränkung von natürlichen Wechselwirkungen auf paarweise Wechselwirkung überwunden werden kann, indem man vier Flussqubits mittels eines nichtlinearen Kopplers verknüpft. Die erreichten Viererwechselwirkungen zwischen den Qubits sind im Regime starker Kopplung und können für die richtigen Systemparameter die paarweisen Wechselwirkungen überschreiten

Flux Qubit Readout in the Persistent Current Basis at arbitrary Bias Points

Common flux qubit readout schemes are qubit dominated, meaning they measure in the energy eigenbasis of the qubit. For various applications meausrements in a basis different from the energy eigenbasis are required. Here we present an indirect measurement protocol, which is detector dominated instead of qubit dominated, yielding a projective measurements in the persistent current basis for arbitrary bias points. We show that with our setup it is possible to perform a quantum nondemolition measurement (QND) in the persistent current basis at all flux bias points with fidelities reaching almost 100%.Comment: 5 pages, 3 figures + 5 pages Supplementa

Non-stoquastic interactions of superconducting circuits in the low frequency regime

Non-stoquastic interactions are hard to realize in experimental setups using superconducting qubits. On the other hand they are important or even necessary for the construction of adiabatic quantum computers wich show a real quantum speedup. In ArXiv:1903.06139, Ozfidan et al. show that they can realize non-stoquastic qubit-qubit interactions in a superconducting circuit architecture. The non-stoquastic nature only appears when the system is restricted to the low energy qubit subspace, since the full circuit Hamiltonian itself is stoquastic. Here we study the origin of these non-stoquastic interactions arising when projecting stoquastic Hamiltonians to the low energy spectrum. For this we use different theoretical tools, e.g. renormalization group techniques.*This work was funded by IARPA in connection with the quantum enhanced optimization (QEO) program