Fig. 4: Single-photon quantum regime.

Each data point shows intra-resonator drive photon number N_q necessary for the quantum-limited quadrature measurement of phonons as a function of mechanical eigenmode frequency ω_mech∕2π. Green (red) circles indicate the values found in other opto-(electro-)mechanical systems with radiation pressure interaction. The yellow star shows the value obtained in this work. Color curves show the expectations from numerical simulations. Different colors correspond to the values of E_J∕E_C = {10⁵, 10⁴, 10³, 10², 10} from top to bottom. A list of the references indicated by the numbers next to the circles is presented in the reference list.