  In this paper, we study a quadratic equation on the one-dimensional torus :
$$i \partial_t u = 2J\Pi(|u|^2)+\bar{J}u^2, \quad u(0, \cdot)=u_0,$$ where
$J=\int_\mathbb{T}|u|^2u \in\mathbb{C}$ has constant modulus, and $\Pi$ is the
Szeg\H{o} projector onto functions with nonnegative frequencies. Thanks to a
Lax pair structure, we construct a flow on BMO$(\mathbb{T})\cap \mathrm{Im}\Pi$
which propagates $H^s$ regularity for any $s>0$, whereas the energy level
corresponds to $s=1/2$. Then, for each $s>1/2$, we exhibit solutions whose
$H^s$ norm goes to $+\infty$ exponentially fast, and we show that this growth
is optimal.
