  Although spin is a core property in fermionic systems, its symmetry can be
easily violated in a variational simulation, especially when strong correlation
plays a vital role therein. In this study, we will demonstrate that the broken
spin-symmetry can be restored exactly in a quantum computer, with little
overhead in circuits, while delivering additional strong correlation energy
with the desired spin quantum number. The proposed scheme permits drastic
reduction of a potentially large number of measurements required to ensure
spin-symmetry by employing a superposition of only a few rotated quantum
states. Our implementation is universal, simple, and, most importantly,
straightforwardly applicable to any ansatz proposed to date.
