Higher Josephson Harmonics in a Tunable Double-Junction Transmon Qubit

Physical Review Letters 2026 Open access

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Authors: Ksenia Shagalov, David Feldstein-Bofill, Leo Uhre Jakobsen, Zhenhai Sun, Casper Wied, Amalie T. J. Paulsen, Johann Bock Severin, Marian Marciniak, C. A. Potts, Anders Kringhøj, Jacob Hastrup, Karsten Flensberg
+2 more Svend Krøjer, Morten Kjærgaard
DOI: 10.1103/fr7x-555s
Cited by: 1
References: 46

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 superconducting quantum interference device (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 ∼10% of the fundamental. We identify a flux sweet spot where the ground-state 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.

Fields of Study & Subjects

Quantum Information and Cryptography Quantum and electron transport phenomena Cold Atom Physics and Bose-Einstein Condensates

Publication details

Journal / source
Physical Review Letters
Publisher
American Physical Society (APS)
Volume
137
Issue
5
Pages
057001–057001
Publication date
26 June 2026
Language
EN