Time-domain characterization of the fixed-frequency qubit in the two-qubit device

Determine, via time-domain pulsed measurements, the coherence properties (including the energy relaxation time T1 and dephasing time T2*) of the fixed-frequency single–Josephson-junction graphene transmon in device 2 (the two-qubit architecture with one SQUID-based qubit and one single-junction qubit), for which only continuous-wave two-tone spectroscopy indicated three candidate transitions without time-domain validation.

Background

Device 2 consists of a SQUID-based qubit and a fixed single–Josephson-junction qubit integrated in a 3D cavity. Across multiple cooldowns, power-dependent measurements showed two-stage dispersive shifts consistent with two qubits coupled to a single cavity mode.

Continuous-wave two-tone spectroscopy revealed three transitions that are consistent with the fixed qubit and supported by capacitance and coupling estimates. However, these assignments and, critically, the coherence properties of the fixed qubit were not confirmed by time-domain measurements.

The authors explicitly state that time-domain experiments are required to examine the coherence properties but could not be performed due to equipment limitations, leaving the fixed-qubit coherence (e.g., T1 and T2*) unresolved.

References

Although the above analysis suggests that these three transitions can be associated with the fixed qubit, time-domain measurements are still required to carefully examine their coherence properties. Unfortunately, due to equipment shortage, we were unable to perform further investigations during that time.

— 3D cavity-based graphene superconducting quantum circuits in two-qubit architectures (2512.21213 - Chiu et al., 24 Dec 2025) in Supplementary Materials, Section VI (Device 2 loaded in different cavities in multiple cooldowns), paragraph discussing Fig. S6(b)