import torch


def get_spin_interp(zeta: torch.Tensor) -> torch.Tensor:
    """Compute spin interpolation function from fractional polarization `zeta`."""
    exponent = 4.0 / 3
    scale = 1.0 / (2.0 ** exponent - 2.0)
    return ((1.0 + zeta) ** exponent + (1.0 - zeta) ** exponent - 2.0) * scale