Observation of Josephson Harmonics in Tunnel Junctions

• URL: http://arxiv.org/abs/2302.09192v2
• Date: Tue, 22 Aug 2023 15:55:30 GMT
• Title: Observation of Josephson Harmonics in Tunnel Junctions
• Authors: Dennis Willsch, Dennis Rieger, Patrick Winkel, Madita Willsch, Christian Dickel, Jonas Krause, Yoichi Ando, Rapha\"el Lescanne, Zaki Leghtas, Nicholas T. Bronn, Pratiti Deb, Olivia Lanes, Zlatko K. Minev, Benedikt Dennig, Simon Geisert, Simon G\"unzler, S\"oren Ihssen, Patrick Paluch, Thomas Reisinger, Roudy Hanna, Jin Hee Bae, Peter Sch\"uffelgen, Detlev Gr\"utzmacher, Luiza Buimaga-Iarinca, Cristian Morari, Wolfgang Wernsdorfer, David P. DiVincenzo, Kristel Michielsen, Gianluigi Catelani, Ioan M. Pop
• Abstract summary: State-of-the-art technologies, including the world's largest quantum processors, employ aluminum oxide (AlO$_x$) tunnel Josephson junctions (JJs) as sources of nonlinearity. Here we show that the standard C$varphi$R fails to accurately describe the energy spectra of transmon artificial atoms. By including these in the transmon Hamiltonian, we obtain orders of magnitude better agreement between the computed and measured energy spectra.
• Score: 0.9263754304004778
• License: http://arxiv.org/licenses/nonexclusive-distrib/1.0/
• 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 (AlO$_x$) tunnel Josephson junctions (JJs) as sources of nonlinearity, assuming an idealized pure $\sin\varphi$ current-phase relation (C$\varphi$R). However, this celebrated $\sin\varphi$ C$\varphi$R is only expected to occur in the limit of vanishingly low-transparency channels in the AlO$_x$ barrier. Here we show that the standard C$\varphi$R fails to accurately describe the energy spectra of transmon artificial atoms across various samples and laboratories. Instead, a mesoscopic model of tunneling through an inhomogeneous AlO$_x$ 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 and readout, parametric amplification and mixing, Floquet qubits, protected Josephson qubits, etc. As an example, we show that engineered Josephson harmonics can reduce the charge dispersion and the associated errors in transmon qubits by an order of magnitude, while preserving anharmonicity.

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