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Published on: August 2, 2019
Higher Josephson Harmonics in a Tunable Double-Junction Transmon Qubit
Ksenia Shagalov1,2, David Feldstein-Bofill1,2, Leo Uhre Jakobsen1,2
1University of Copenhagen, Niels Bohr Institute, Center for Quantum Devices, 2100 Copenhagen, Denmark.
None:
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.
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