Flux-insensitive qubit from a split transmon shunted with a Josephson junction

Split transmons have been widely used in making quantum processors for building scalable and fault-tolerant superconducting quantum computers. With tunable external flux threading the superconducting quantum interference devices (SQUIDs) of qubits, split transmons have a degree of freedom for tuning frequencies but introduce flux noise as an important dephasing factor at the same time. Here, an improved version of a transmon is proposed and is called a ``triple-Josephson-junction transmon'' (tri-JJ transmon). A tri-JJ transmon is obtained by shunting another Josephson junction to the SQUID of a split transmon, which can produce much more different local minima and maxima of qubit frequencies tuned by external flux as ``sweet spots'' than those of split transmons. The first-order effect of the flux noise at the sweet spots can be eliminated and the corresponding pure-dephasing times are estimated as about $200\phantom{\rule{4pt}{0ex}}\textmu{}\mathrm{s}$ by considering only critical current noise. Tri-JJ transmons are compatible with the process and the protocol of control and measurement of split transmons and can be competitive candidates for constructing a high-performance superconducting quantum processor.