Nonlinear Amplification of Small Spin Precession Using Long-Range Dipolar Interactions

In measurements of small signals using spin precession the precession angle usually grows linearly in time. We show that a dynamic instability caused by spin interactions can lead to an exponentially growing spin-precession angle, amplifying small signals and raising them above the noise level of a detection system. We demonstrate amplification by a factor of greater than 8 of a spin-precession signal due to a small magnetic field gradient in a spherical cell filled with hyperpolarized liquid ¹²⁹Xe. This technique can improve the sensitivity in many measurements that are limited by the noise of the detection system, rather than the fundamental spin-projection noise.