The state_vector method in StateVectorTrialResult includes a copying of the state vector to resolve issue #3101:

This eliminates the issue of mutating the state vector, but can significantly increase runtime for large circuits without a clear indication of the root cause. We should force users to explicitly make a copy of the state if they need it, either by renaming the method to state_vector_copy or by removing it altogether and requiring users to copy the final_state_vector field.

Related: qsim issue 237