Qiskit Quantum Seminar

Currently Playing: Observation of Josephson Harmonics in Tunnel Junctions | Qiskit Seminar Series

Title: Observation of Josephson Harmonics in Tunnel Junctions Abstract: Superconducting quantum processors have a long road ahead to reach fault-tolerant quantum computing. One of the most daunting challenges is taming the numerous microscopic degrees of freedom ubiquitous in solid-state devices. State-of-the-art technologies, including the world's largest quantum processors, employ aluminum oxide (AlOx) tunnel Josephson junctions (JJs) as sources of nonlinearity, assuming an idealized pure sin(phi) current-phase relation (CPR). However, this celebrated sin(phi) CPR is only expected to occur in the unrealistic limit of vanishingly low-transparency channels in the AlOx barrier. Here we show that the standard CPR fails to describe the energy spectra of transmon artificial atoms across various samples and laboratories. Instead, a mesoscopic model of tunneling through an inhomogeneous AlOx barrier predicts %-level contributions from higher Josephson harmonics. By including these in the transmon Hamiltonian, we obtain orders of magnitude better agreement between the computed and measured energy spectra. The reality of Josephson harmonics transforms qubit design and prompts a reevaluation of models for quantum gates, parametric amplification and mixing, Floquet qubits, protected Josephson Rhombus chains, etc. Indeed, we show that engineered Josephson harmonics can reduce the charge dispersion and the associated errors in transmon qubits by an order of magnitude. Bio Dennis Rieger: Dennis Rieger grew up in a small village in the Black Forest not far from Karlsruhe, Germany. He studied physics at Karlsruhe Institute of Technology and graduated with a master’s thesis on quantum jumps of a transmon qubit under shared supervision by Prof. Wolfgang Wernsdorfer and Prof. Ioan Pop. In his PhD project, he continues the strong collaboration between the groups and focuses on hybrid applications involving granular aluminum circuits in relatively strong magnetic fields, up to 1 Tesla. Bio Dennis Willsch: Dennis Willsch was born in Cologne, Germany. He received his PhD degree in physics from RWTH Aachen University in 2020 for simulating the time evolution of transmon quantum computers on digital supercomputers. He is a postdoctoral researcher in the Quantum Information Processing group at Jülich Supercomputing Centre, Forschungszentrum Jülich. His current research interests include modeling quantum technology experiments, benchmarking existing quantum computers and quantum annealers, and simulating the time evolution of quantum information processors.