The Fubini-Study angle between states.

    Equal to the Burrs angle for pure states.

def state_angle(ket0: State, ket1: State) -> bk.BKTensor:
    """The Fubini-Study angle between states.

    Equal to the Burrs angle for pure states.
    """
    return fubini_study_angle(ket0.vec, ket1.vec)