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Coherence limitations of a Fourier-engineered cos(2φ) transmon qubit

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Abstract

A multi-junction circuit Fourier-engineers a cos(2φ)-dominated potential and reproduces the predicted parity-protected spectrum, while exposing flux noise from the residual first harmonic as the dominant lifetime limitation.

Key Results

  • Experimentally suppressed odd harmonics in a multi-junction superconducting circuit.
  • Found good agreement between the measured transition spectrum and the effective cos(2φ) model.
  • Identified flux noise near the symmetry point as the limiting mechanism and showed that an otherwise similar fluxonium is less vulnerable.

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