Protected Josephson Rhombi Chains

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. The novel design of these elements, which reduces their sensitivity to the offset charge fluctuations, has been employed. We have observed that the lifetime $T_{1}$ of the first excited state of this quantum circuit in the protected regime is increased by up to $70\mu$s, a factor of $\sim$100 longer than that in the unprotected state. The decay 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 rhombi-based qubits fabricated with existing technology.