poly_to_angles fixing #6978

doc/releases/changelog-dev.md

@@ -443,6 +443,10 @@
   passed to the `QNode`, instead of assuming the method is always `deferred`.
   [(#6903)](https://github.com/PennyLaneAI/pennylane/pull/6903)
 
+* The `poly_to_angles` function has been improved to correctly work with different interfaces and
+  no longer manipulate the input angles tensor internally.
+  [(#6979)](https://github.com/PennyLaneAI/pennylane/pull/6979)
+
 * The `QROM` template is upgraded to decompose more efficiently when `work_wires` are not used.
   [#6967)](https://github.com/PennyLaneAI/pennylane/pull/6967)
 

pennylane/templates/subroutines/qsvt.py

@@ -365,7 +365,7 @@ class QSVT(Operation):
 
         .. code-block::
 
-            poly = [0, -1, 0, 0.5, 0, 0.5]
+            poly = np.array([0, -1, 0, 0.5, 0, 0.5])
             angles = qml.poly_to_angles(poly, "QSVT")
             input_matrix = np.array([[0.2, 0.1], [0.1, -0.1]])
 
@@ -825,7 +825,7 @@ def transform_angles(angles, routine1, routine2):
 
         .. code-block::
 
-            poly = [0, 1.0, 0, -1/2, 0, 1/3]
+            poly = np.array([0, 1.0, 0, -1/2, 0, 1/3])
 
             qsp_angles = qml.poly_to_angles(poly, "QSP")
             qsvt_angles = qml.transform_angles(qsp_angles, "QSP", "QSVT")
@@ -911,7 +911,7 @@ def poly_to_angles(poly, routine, angle_solver: Literal["root-finding"] = "root-
 
     .. code-block::
 
-        >>> poly = [0, 1.0, 0, -1/2, 0, 1/3]
+        >>> poly = np.array([0, 1.0, 0, -1/2, 0, 1/3])
         >>> qsvt_angles = qml.poly_to_angles(poly, "QSVT")
         >>> print(qsvt_angles)
         [-5.49778714  1.57079633  1.57079633  0.5833829   1.61095884  0.74753829]
@@ -925,7 +925,7 @@ def poly_to_angles(poly, routine, angle_solver: Literal["root-finding"] = "root-
 
         .. code-block::
 
-            poly = [0, 1.0, 0, -1/2, 0, 1/3]
+            poly = np.array([0, 1.0, 0, -1/2, 0, 1/3])
 
             qsvt_angles = qml.poly_to_angles(poly, "QSVT")
 
@@ -953,11 +953,7 @@ def circuit_qsvt():
 
     """
 
-    # Trailing zeros are removed from the array
-    for _ in range(len(poly)):
-        if not np.isclose(poly[-1], 0.0):
-            break
-        poly.pop()
+    poly = qml.math.trim_zeros(qml.math.array(poly, like="numpy"), trim="b")
 
     if len(poly) == 1:
         raise AssertionError("The polynomial must have at least degree 1.")

tests/templates/test_subroutines/test_qsvt.py

@@ -792,3 +792,66 @@ def test_raise_error(self, poly, routine, angle_solver, msg_match):
 
         with pytest.raises(AssertionError, match=msg_match):
             _ = qml.poly_to_angles(poly, routine, angle_solver)
+
+    def test_immutable_input(self):
+        """Test `poly_to_angles` does not modify the input"""
+
+        poly = [0, 1.0, 0, -1 / 2, 0, 1 / 3, 0]
+        poly_copy = poly.copy()
+        qml.poly_to_angles(poly, "QSVT")
+
+        assert len(poly) == len(poly_copy)
+        assert np.allclose(poly, poly_copy)
+
+    def test_interface_numpy(self):
+        """Test `poly_to_angles` works with numpy"""
+
+        poly = [0, 1.0, 0, -1 / 2, 0, 1 / 3, 0]
+        angles = qml.poly_to_angles(poly, "QSVT")
+
+        poly_numpy = np.array(poly)
+        angles_numpy = qml.poly_to_angles(poly_numpy, "QSVT")
+
+        assert qml.math.allclose(angles, angles_numpy)
+
+    @pytest.mark.jax
+    def test_interface_jax(self):
+        """Test `poly_to_angles` works with jax"""
+
+        import jax
+
+        poly = [0, 1.0, 0, -1 / 2, 0, 1 / 3, 0]
+        angles = qml.poly_to_angles(poly, "QSVT")
+
+        poly_jax = jax.numpy.array(poly)
+        angles_jax = qml.poly_to_angles(poly_jax, "QSVT")
+
+        assert qml.math.allclose(angles, angles_jax)
+
+    @pytest.mark.torch
+    def test_interface_torch(self):
+        """Test `poly_to_angles` works with torch"""
+
+        import torch
+
+        poly = [0, 1.0, 0, -1 / 2, 0, 1 / 3, 0]
+        angles = qml.poly_to_angles(poly, "QSVT")
+
+        poly_torch = torch.tensor(poly)
+        angles_torch = qml.poly_to_angles(poly_torch, "QSVT")
+
+        assert qml.math.allclose(angles, angles_torch)
+
+    @pytest.mark.tf
+    def test_interface_tf(self):
+        """Test `poly_to_angles` works with tensorflow"""
+
+        import tensorflow as tf
+
+        poly = [0, 1.0, 0, -1 / 2, 0, 1 / 3, 0]
+        angles = qml.poly_to_angles(poly, "QSVT")
+
+        poly_tf = tf.Variable(poly)
+        angles_tf = qml.poly_to_angles(poly_tf, "QSVT")
+
+        assert qml.math.allclose(angles, angles_tf)