quick fix (Qiskit#1385)

* Adding quick fix

* Cleaning up

* Linting

test/python/quantum_info/test_quaternions.py

@@ -6,10 +6,10 @@
 # the LICENSE.txt file in the root directory of this source tree.
 
 """Tests qiskit/mapper/_quaternion"""
+import math
 import numpy as np
 import scipy.linalg as la
 from qiskit.quantum_info.operators.quaternion import quaternion_from_euler
-from qiskit.quantum_info.operators.unitary import rotation_matrix
 from ..common import QiskitTestCase
 
 
@@ -72,3 +72,39 @@ def test_equiv_quaternions(self):
             euler = quat1.to_zyz()
             quat2 = quaternion_from_euler(euler, 'zyz')
             self.assertTrue(np.allclose(abs(quat1.data.dot(quat2.data)), 1))
+
+
+def rotation_matrix(angle, axis):
+    """Generates a rotation matrix for a given angle and axis.
+
+    Args:
+        angle (float): Rotation angle in radians.
+        axis (str): Axis for rotation: 'x', 'y', 'z'
+
+    Returns:
+        ndarray: Rotation matrix.
+
+    Raises:
+        ValueError: Invalid input axis.
+    """
+    direction = np.zeros(3, dtype=float)
+    if axis == 'x':
+        direction[0] = 1
+    elif axis == 'y':
+        direction[1] = 1
+    elif axis == 'z':
+        direction[2] = 1
+    else:
+        raise ValueError('Invalid axis.')
+    direction = np.asarray(direction, dtype=float)
+    sin_angle = math.sin(angle)
+    cos_angle = math.cos(angle)
+    rot = np.diag([cos_angle, cos_angle, cos_angle])
+    rot += np.outer(direction, direction) * (1.0 - cos_angle)
+    direction *= sin_angle
+    rot += np.array([
+        [0, -direction[2], direction[1]],
+        [direction[2], 0, -direction[0]],
+        [-direction[1], direction[0], 0]
+    ])
+    return rot