Extended Data Fig. 1: Measurement of the gap by observation of temporal oscillations of the k = 0 mode.

a, We measure the gap of the quadratic spin mode by a global rotation of the spinor phase. We record the resulting oscillations of the fractional m = 0 population as a function of evolution time after the rotation. We fit a sinusoidal function (solid line) to infer the frequency. b, Extracted oscillation frequency (diamonds) and mean value of the m = 0 population (circles). We compare to theoretical expectations for the easy-plane phase (solid lines; see Methods equation 1). The dashed line extrapolates the expectations to q < 0 under the assumption of equal populations of m = ± 1. For the theory curves we use nc₁ = 1.3Hz.