A Merged-element Transmon*

The invention of the transmon qubit has led to multiple breakthroughs in quantum information science over the past few years. To reduce the participation ratio of lossy materials, a small Josephson junctions (JJ) with a large-scale, coplanar capacitor shunt is typically employed in transmon designs. However, this method also leads to a large qubit footprint that hinders the scaleup of transmon.
Here, we present a new approach to address these issues: we merge the coplanar capacitor and the JJ into a single junction made of superconductor/semiconductor/superconductor trilayer. We call this new device the merged-element transmon (mergemon). This new geometry reduces the qubit footprint for a factor of 100. For demonstration purposes, we built a prototype using a sputtered Nb/amorphous silicon/Nb film. In a 10 mK measurement we detect a clear qubit response using spectroscopy. A TLS loss analysis based on participation ratio model indicates the mergemon can achieve high coherence when implemented with low-loss, epitaxially grown and lattice-matched trilayers.