2025/12/09 by Anonymous, Ksenia Shagalov, David Feldstein-Bofill +30 · 1 citation
Computer Science · Physics and Astronomy · #Cold Atom Physics and Bose-Einstein Condensates #Harmonics #Josephson effect #Phase qubit #Quantum Information and Cryptography #Quantum and electron transport phenomena #Qubit #Strong Light-Matter Interactions #Superconductivity #Transmon
paper · pdf · open access · doi:10.1103/fr7x-555s
published in Physical Review Letters 137(5) (American Physical Society)
openalex publication_date 2026/06/26 · openalex created_date 2026/06/27 · openalex updated_date 2026/06/27
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 <a:math xmlns:a="http://www.w3.org/1998/Math/MathML" display="inline"> <a:mo>∼</a:mo> <a:mn>10</a:mn> <a:mo>%</a:mo> </a:math> 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.