Extended Data Fig. 7: Dispersion symmetry and the properties of supermode squeezed states.

a.-c. The real (blue dots) and imaginary (red dots) parts of the eigenvalues of the amplified vacuum supermodes driven by a 2-FSR comb with different values of \({D}_{1}^{{\prime} }\): 0, 3 and 8 MHz respectively. For larger \({D}_{1}^{{\prime} }\) the purely real eigenvalues gradually become complex. Highlighted quadrature supermodes are those close to threshold. d.-g. The frequency(ω)-dependent squeezing spectrum of the supermodes close to threshold (highlighted). Each line represents the supermode spectrum for a linearly swept intra cavity power, and the supermodes can be seen to narrow as they approach threshold. The supermode in f. does not narrow asymptotically because e. reaches threshold first. h.,i. The supermode composition as a function of cavity modes of the multimode squeezed states, for the supermodes in e. and f. respectively (colours correspond to the frequencies).