Protected Josephson Rhombi Chains

Abstract

We have studied the low-energy excitations in a minimalistic protected Josephson circuit which contains two basic elements (rhombi) characterized by the $\pi$ periodicity of the Josephson energy. Novel design of these elements, which reduces their sensitivity to the offset charge fluctuations, has been employed. We have observed that the life time $T_{1}$ of the first excited state of this quantum circuit in the protected regime is increased up to $70\mu s$, a factor of $\sim100$ longer than that in the unprotected state. The quality factor $\omega_{01}T_{1}$ of this qubit exceeds $10^{6}$ . Our results are in agreement with theoretical expectations; they demonstrate the feasibility of symmetry protection in the rhombi-based qubits fabricated with existing technology.