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This study demonstrates a tunable double-junction transmon qubit where magnetic flux controls the amplitude of higher-order Josephson harmonics. By resolving transitions up to the second harmonic and identifying a dispersive-shift-free sweet spot, researchers provide a pathway toward protected qubits with customizable nonlinear microwave properties. The work advances fundamental superconducting circuit design by leveraging internal modes rather than external resonators for enhanced control fidelity.
Higher Josephson harmonics in a tunable double-junction transmon qubit
arXiv:2512.08470v3 Announce Type: replace 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.
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: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.
Quantum Physics (quant-ph); Mesoscale and Nanoscale Physics (cond-mat.mes-hall)
arXiv:2512.08470 [quant-ph]
(or arXiv:2512.08470v3 [quant-ph] for this version)
https://doi.org/10.1103/fr7x-555s DOI(s) linking to related resources
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