def _theta_to_cs(theta: np.ndarray) -> np.ndarray:
    """Returns the CS matrix from the cosine sine decomposition.

    Args:
        theta: the 4 angles that result from the CS decomposition.
    Returns:
        the CS matrix
    """
    c = np.diag(np.cos(theta))
    s = np.diag(np.sin(theta))
    return np.block([[c, -s], [s, c]])
Returns the CS matrix from the cosine sine decomposition.

    Args:
        theta: the 4 angles that result from the CS decomposition.
    Returns:
        the CS matrix
    
cirq-core/cirq/transformers/analytical_decompositions/three_qubit_decomposition_test.py