Curator's Take
This article reveals a critical limitation in one of the most promising approaches to building better superconducting qubits. The researchers successfully demonstrated a cos(2φ) qubit that should theoretically be protected from charge noise by its intrinsic symmetry, but discovered that their Fourier engineering approach makes the device extremely sensitive to flux noise - the very problem they were trying to solve. The finding exposes a fundamental trade-off in interference-based protection schemes, where suppressing one type of noise can inadvertently amplify sensitivity to another, providing crucial insights for the quantum hardware community working toward more robust qubit designs. This work underscores why achieving truly fault-tolerant quantum computing remains such a formidable engineering challenge, even with theoretically elegant protection mechanisms.
— Mark Eatherly
Summary
Intrinsically protected superconducting qubits are a promising route toward enhancing coherence times and advancing hardware towards applications in quantum computing. The $\cos(2\varphi)$ qubit achieves protection against qubit relaxation by allowing only the coherent tunneling of pairs of Cooper pairs, resulting in Cooper-pair parity symmetry and thereby suppressing charge-induced errors. In this work, we experimentally realize a $\cos(2\varphi)$ qubit by Fourier engineering the energy-phase relation in a multi-junction superconducting circuit. Using an interference-based architecture, we are able to suppress the odd harmonics of an effective qubit potential and we observe good agreement between the measured transition spectrum and the effective theoretical model. We further investigate the energy relaxation time as a function of external flux and find that the qubit lifetime at the flux symmetry point is limited by $1/f$ flux noise. This strong sensitivity arises from residual fluctuations in the first harmonic, which possesses a large prefactor despite being nominally canceled. In contrast, a fluxonium qubit with a similar energy spectrum and noise amplitude is less affected by flux noise, highlighting a key challenge for interference-based protection schemes.