Robust Macroscopic Schr\"odinger's Cat on a Nucleus
We propose an experimentally feasible scheme to create large Schr\"odinger cat states on a high-spin nucleus of a donor atom embedded in a solid-state system. The resulting cat state is robust against decoherence, macroscopic because its size scales linearly with nuclear spin, and tiny -- at the femtometer scale. Our quantum-control scheme utilizes one-axis twisting caused by a non-linear quadrupole interaction and phase-modulated multi-tone radio-frequency pulses for universal high-dimensional rotations. We achieve fast generation and detection for yielding robust cat states and observing rapid collapse-and-revivals -- two orders of magnitude faster than the dephasing timescale.