Figure 3: Spectroscopic signature of the phononic environment of the double quantum dot.

(a–d) Sideband lineshape for weak driving in the one electron configuration, as a function of detuning and for increasing relative powers at f=31.8 GHz (data taken with different tunnel coupling and gate voltages to Fig. 2). In the blue-detuned region (ε<~70 μeV), phonon emission results in asymmetric broadening of the sideband and the appearance of step-like features with a spacing (indicated by double arrows) that is set by the distance between the two quantum dots. Extracting this distance from the spacing of the steps gives 280 nm, consistent with the micrograph shown in Fig. 1c, see Methods. In the red-detuned region (ε>~70 μeV), the sideband also deviates from the ideal-Lorentzian shape, exhibiting additional skirting features, which we attribute to a renormalization of the qubit levels owing to coupling to the phononic environment. Solid lines are Lorentzian fits to the data using only data points in red region and at the top of the peak.