Parametric Excitation and Squeezing in a Many-Body Spin System

We demonstrate a new method to coherently excite and control the quantum spin states of an atomic Bose gas using parametric excitation of the collective spin by time varying the relative strength of the Zeeman and spin-dependent collisional interaction energies at multiples of the natural frequency of the system. Compared to the usual single-particle quantum control techniques used to excite atomic spins (e.g. Rabi oscillations using rf or microwave fields), the method demonstrated here is intrinsically many-body, requiring inter-particle interactions. While parametric excitation of a classical system is ineffective from the ground state, we show that in our quantum system, parametric excitation from the quantum ground state leads to the generation of quantum squeezed states.