Design of The Merged Element Transmon Qubit

Transmon qubits are one of the most promising candidates for building a universal quantum processor. They consist of two discrete elements: a nanoscale Josephson junction (JJ) and a coplanar capacitor. In conventional transmon designs, the plates of the shunt capacitor have to be very large to achieve low participation ratio of the lossy interfaces. To circumvent this issue, we propose to engineer the device so that the electric field distribution is confined to junction dielectric by merging the capacitor and JJ into a single superconductor/dielectric/superconductor trilayer structure such that the frequency, anharmonicity, and E_j/E_c parameters are in the transmon regime. We refer to this architecture a mergemon to distinguish it from conventional designs. We explore different low-loss dielectrics such that the trilayer itself would provide not only the nonlinear inductance but also most of the capacitance of the transmon qubit. By FEM based simulation, we design the mergemon qubit-readout resonator to be in the strong dispersive regime. We also verify that this design can be realized with standard optical lithography process. Finally, we also discuss possibility of leveraging the state-of-the-art molecular-beam epitaxy technology to enhance the coherence of the mergemon.