Instability-enhanced quantum sensing with tunable multibody interactions
Dynamical instabilities can amplify small perturbations into measurable signals, offering a route to quantum-enhanced sensing. This mechanism was experimentally demonstrated in a collective spin system with quadratic interactions, described by a twist-and-turn Hamiltonian, where quantum evolution near an unstable point leads to exponential growth of spin fluctuations, enabling metrological gain beyond the standard quantum limit. Here, we show that a quartic extension of this Hamiltonian substantially increases the amplification. This additional nonlinear term reshapes the phase-space structure, generating new unstable fixed points with larger local Lyapunov exponents and therefore faster signal amplification at shorter evolution times. Remarkably, even for the same instability rate (equal Lyapunov exponent), collective multibody interactions outperform the quadratic case, demonstrating that metrological gain is not determined by the Lyapunov exponent alone. We show analytically that the finite-time growth of the gain is governed also by the local phase-space curvature around the unstable point. Only for a symmetric unstable point, as in the quadratic case, is the finite-time gain described by the Lyapunov exponent alone. Our results identify local phase-space curvature as an additional resource for optimizing the speed and performance of quantum sensors.