Extended Data Fig. 1: Characterization of the scattering qubit Q₂ acoustic emission spectrum.

a, Qubit Q₂’s emission spectrum, measured by exciting the qubit and monitoring its subsequent excited state population P_e (color scale) as a function of time (vertical scale) as the qubit frequency is tuned across the UDT bandwidth (horizontal scale). The beamsplitter reflection yields features at ~ 550 ns, visible within the transducer unidirectional band from 3.91 GHz to 4.05 GHz. White dashed line indicates the qubit operating frequency of 3.925 GHz. Red dashed lines show the qubit ± χ = ± 189 MHz sideband frequencies when modulating the qubit at that frequency. b, Tunable qubit-UDT excited state population (color scale) as a function of time (vertical scale) and coupler G₂ control pulse amplitude (arbitrary units; horizontal scale), for the initially excited-state qubit set to the operating frequency of 3.925 GHz. Green dots are fit T₁ times as a function of G₂ pulse amplitude. White arrow indicates coupling strength used in panel a, and used for single-phonon release from Q₂. Black arrow indicates maximum coupling strength used in the phonon scattering experiments.