A phonon-engineered merged-element transmon qubit: Part II

Improving superconducting qubit coherence often requires reducing dielectric losses associated with large shunt capacitors, either through advanced fabrication or by engineering a lower surface energy participation ratio. In particular, two-level-system (TLS) defects residing in amorphous oxides can relax by emitting phonons, creating a decoherence channel for a qubit when coupled to such defects. In this work, we investigate a phonon-engineered “merged-element transmon” (MET) architecture, where a large-area Al/AlOx/Al Josephson junction (≈ 2 µm²) is embedded in phononic bandgap crystals.

In Part II, we discuss the integration of MET qubits with GHz phononic bandgap structures. In these devices, the small qubit volume reduces the quantity of TLS defects, while the phononic bandgap reduces the bath phononic density of states available for TLS defects to resonantly decay into. We present the characterization of such phonon-protected MET qubits and the resulting modifications to superconducting qubits and two-level system lifetimes in phononic bandgap structures.