Higher Josephson harmonics in a tunable double-junction transmon qubit

• URL: http://arxiv.org/abs/2512.08470v2
• Date: Thu, 11 Dec 2025 10:24:14 GMT
• Title: Higher Josephson harmonics in a tunable double-junction transmon qubit
• Authors: Ksenia Shagalov, David Feldstein-Bofill, Leo Uhre Jakobsen, Zhenhai Sun, Casper Wied, Amalie T. J. Paulsen, Johann Bock Severin, Malthe A. Marciniak, Clinton A. Potts, Anders Kringhøj, Jacob Hastrup, Karsten Flensberg, Svend Krøjer, Morten Kjaergaard,
• Abstract summary: Tunable Josephson harmonics open new avenues for qubit design.<n>This highly tunable element provides a route toward protected qubits and customizable nonlinear microwave devices.
• Score: 0.0
• License: http://creativecommons.org/licenses/by/4.0/
• Abstract: Tunable Josephson harmonics open new avenues for qubit design. We demonstrate a superconducting circuit element consisting of a tunnel junction in series with a SQUID loop, yielding a Josephson potential whose harmonic content is strongly tunable by magnetic flux. Through spectroscopy of the first four qubit transitions, together with an effective single-mode model renormalized by the internal mode, we resolve a second harmonic with an amplitude up to $\sim10\%$ of the fundamental. We identify a flux sweet spot where the dispersive shift vanishes, achieved by balancing the dispersive couplings to the internal and qubit modes. This highly tunable element provides a route toward protected qubits and customizable nonlinear microwave devices.

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