Many-body localization for decoherence protected quantum memory

Varvelis, Evangelos; DiVincenzo, David (Thesis advisor); Hassler, Fabian (Thesis advisor)

Aachen : RWTH Aachen University (2023)
Dissertation / PhD Thesis

Dissertation, RWTH Aachen University, 2023

Abstract

In recent years, the field of quantum information has witnessed remarkable progress through the utilization of superconducting qubits. Nonetheless, despite these advancements, significant hurdles persist when it comes to scaling up these systems. One critical challenge that this thesis aims to tackle is the phenomenon of decoherence, whereby a quantum system behaves akin to a classical system in thermal equilibrium. However systems can avoid thermalization if they are in the many-body localized phase. The main objective of this thesis is to investigate the potential of MBL in protecting quantum memories from decoherence. We pursue a two-fold approach: firstly, we establish the existence of a thermal to MBL phase transition in disordered transmon arrays. To achieve this, we employ well-established diagnostics such as level spacing distribution and inverse participation ratio (IPR). Additionally, we introduce a new diagnostic tool called the Walsh-Hadamard coefficients, which reinforce the findings of IPR in a basis-independent manner. We apply these diagnostics to both 1D and 2D transmon arrays with Gaussian disorder, as well as chains with designed frequency patterns using the LASIQ technique. Furthermore, we demonstrate that disorder-free systems can also exhibit MBL by compensating for the absence of disorder through the utilization of quasi-periodic frequency patterns. Surprisingly, we find that these systems not only achieve localization, but also surpass the localization observed in comparable systems with Gaussian disorder. Finally, we develop a perturbation theory scheme that enables the determination of the Walsh-Hadamard coefficients for large transmon lattices, which are comparable to experimental devices.

Institutions

• JARA-Institute for Quantum Information [080043]
• Department of Physics [130000]
• Theoretical Physics (condensed matter) Teaching and Research Area [135220]