The gatemon: a transmon with a voltage-variable superconductor-semiconductor junction

We have developed a superconducting transmon qubit with a semiconductor-based Josephson junction element.\footnote{T.W. Larsen \textit{et al.}, Phys. Rev. Lett. \textbf{115}, 127001 (2015).}$^{,}$\footnote{G. de Lange \textit{et al.}, Phys. Rev. Lett. \textbf{115}, 127002 (2015),} The junction is made from an InAs nanowire with \textit{in situ} molecular beam epitaxy-grown superconducting Al contacts. This gate-controlled transmon, or gatemon, allows simple tuning of the qubit transition frequency using a gate voltage to vary the density of carriers in the semiconductor region. In the first generations of devices we have measured coherence times up to $\sim$10 $\mu$s. These coherence times, combined with stable qubit operation, permit single qubit rotations with fidelities of $\sim$99.5~\% for all gates including voltage-controlled $Z$~rotations. Towards multi-qubit operation we have also implemented a two qubit voltage-controlled cPhase gate. In contrast to flux-tuned transmons, voltage-tunable gatemons may simplify the task of scaling to multi-qubit circuits and enable new means of control for many qubit architectures.