We introduce a multimode qubit, the protomon, encoded in a fluxonium molecule circuit (see image on the right). Such qubits are intrinsically protected against energy relaxation, as the loss operators have exponentially suppressed matrix elements between the logical states. It is also insensitive to the dephasing, as its spectrum exhibits zero derivative with respect to the external flux fluctuation. Our collaborators from Houck Lab in Princeton University realized four protomon devices and characterized their properties. By tuning the qubits to various operating points identified with calibrated two-tone spectroscopy, we measure
depolarization times ranging from 64 to 73 µs and dephasing times between 0.2 to 0.5 µs for one selected qubit. Unfortunately, we don’t fully understand the discrepancy between the relatively short measured coherence times and theoretical predictions. This calls for future studies investigating the limiting noise factors, informing the direction for improving coherence times of the protomon qubit.

The paper can be found here.