Junction-Intrinsic Dissipation in Hybrid Superconductor-Semiconductor Gatemon Qubits

TL;DR

This study compares hybrid superconductor-semiconductor gatemon qubits with traditional transmons, revealing that intrinsic junction dissipation limits coherence in gatemons despite similar fabrication and design.

Contribution

It identifies junction-intrinsic dissipation as the main factor reducing coherence times in gatemon qubits compared to transmons.

Findings

Gatemons have relaxation times in the few-microsecond range.

Transmons reach relaxation times in the tens of microseconds.

Junction-intrinsic dissipation dominates gatemon decoherence.

Abstract

Superconducting transmon qubits based on hybrid superconductor-semiconductor Josephson junctions (gatemons) offer gate tunability, but their relaxation times remain well below those of state-of-the-art transmons, and the origin of this discrepancy is not fully understood. Here, we co-fabricate gatemons and SIS-junction transmons with nominally identical circuit layouts, gate dielectrics, and control lines, so that the Josephson element is the only intentional distinction. Across multiple chips, transmons in this architecture reach relaxation times in the tens of microseconds, whereas gatemons saturate in the few-microsecond range. Using the transmons as on-chip references, we construct a loss budget including Purcell decay, spontaneous emission through the control line, and internal dielectric loss, and find that the corresponding T1 limits exceed all measured gatemon values by more than an order of magnitude. Temperature-dependent T1 measurements follow a common quasiparticle-activation model and yield similar superconducting gaps for S-Sm-S and SIS junctions, indicating that the reduced gatemon coherence is dominated by additional temperature-independent, junction-intrinsic dissipation.