Experimental realization of a cos(2φ) transmon qubit

Abstract

A soft-transmon cos(2φ) device demonstrates coherent control, single-shot readout, and strong suppression of charge-induced relaxation while exposing residual flux noise as the dominant limitation.

Key Results

• Resolved an opposite-Cooper-pair-parity doublet split by 13.6 MHz.
• Measured a 100-fold suppression of the island charge matrix element relative to the unprotected plasmon transition.
• Reported $T_1=70\mu \mathrm{s}$ and $T_2^{\mathrm{echo}}=2.5\mu \mathrm{s}$, both limited by $1/f$ flux noise.

Links

• arXiv: 2603.13114

Zoo Links

• cos2phi-qubit