Fix gpi diff 1

ionizer/ops.py

@@ -17,12 +17,17 @@
 """
 Native gates for IonQ hardware as PennyLane operations.
 """
+import functools
+import inspect
 import numpy as np
 
 import pennylane as qml
 from pennylane.operation import Operation
 
 
+stack_last = functools.partial(qml.math.stack, axis=-1)
+
+
 class GPI(Operation):
     r"""
     The single-qubit GPI rotation
@@ -32,6 +37,8 @@ class GPI(Operation):
                 e^{i\phi} & 0
               \end{bmatrix}.
 
+    This operation does not work with adjoint differentiation, it will result in an incorrect output.
+
     Args:
         phi (float): rotation angle :math:`\phi`
         wires (Sequence[int] or int): the wire the operation acts on
@@ -43,6 +50,30 @@ class GPI(Operation):
     num_wires = 1
     num_params = 1
     ndim_params = (0,)
+    grad_method = "A"
+    parameter_frequencies = [(1,)]
+
+    grad_recipe = ([[1, 1, np.pi / 4], [-1, 1, -np.pi / 4]],)
+
+    def generator(self):
+        r"""The Generator for GPI gate.
+
+        .. math::
+            \hat(G) = \frac{\pi}{2}\begin{bmatrix}
+                1 & e^{i\phi} \\
+                e^{-i\phi} & 1
+              \end{bmatrix} = -\frac{\pi}{2}(I-\cos(\phi)X-\sin(\phi)Y).
+        """
+        phi = self.data[0]
+        c = (np.pi / 2) / phi
+        coeffs = np.array([c, -c * qml.math.cos(phi), -c * qml.math.sin(phi)])
+        observables = [
+            qml.Identity(wires=self.wires),
+            qml.PauliX(wires=self.wires),
+            qml.PauliY(wires=self.wires),
+        ]
+
+        return qml.Hamiltonian(coeffs, observables)
 
     # Note: disable pylint complaint about redefined built-in, since the id
     # value itself is coming from the class definition of Operators in PennyLane proper.
@@ -65,7 +96,14 @@ def compute_matrix(phi):  # pylint: disable=arguments-differ
         array([[0.        +0.j        , 0.95533649-0.29552021j],
                [0.95533649+0.29552021j, 0.        +0.j        ]])
         """
-        return qml.math.stack([[0, qml.math.exp(-1j * phi)], [qml.math.exp(1j * phi), 0]])
+        if qml.math.get_interface(phi) == "tensorflow":
+            phi = qml.math.cast_like(phi, 1j)
+
+        a = 0 + 0j
+        b = qml.math.exp((0 - 1j) * phi)
+        return qml.math.stack(
+            qml.math.stack([stack_last([a, b]), stack_last([qml.math.conj(b), a])])
+        )
 
     def adjoint(self):
         # The GPI gate is its own adjoint.

tests/test_ops.py

@@ -0,0 +1,101 @@
+"""
+Unit tests for GPI2 operation and its utility functions.
+"""
+
+import pytest
+
+import pennylane as qml
+from pennylane import numpy as np
+import pennylane.math as math
+from ionizer.ops import GPI
+
+diff_methods = ["backprop", "parameter-shift", "finite-diff"]
+two_pi = 2 * np.pi
+
+
+def get_GPI_matrix(phi):
+    """Gets the GPI expected GPI matrix"""
+    return np.array([[0, math.exp(-1j * phi)], [math.exp(1j * phi), 0]])
+
+
+class State:
+    @staticmethod
+    def set_state():
+        """Quantum function to set a known starting state to use for testing."""
+        qml.RX(0.2, wires=0)
+        qml.RY(1.1, wires=0)
+        qml.RX(0.3, wires=0)
+
+    @classmethod
+    @qml.qnode(qml.device("default.qubit", wires=1))
+    def get_state(cls):
+        """Gets the predefined starting state"""
+        cls.set_state()
+        return qml.state()
+
+    def __init__(self):
+        """Creates an object for an intial quantum state."""
+        self.state = self.get_state()
+        self.a_conj_b = self.state[0] * np.conj(self.state[1])
+        self.b_conj_a = self.state[1] * np.conj(self.state[0])
+
+
+@pytest.mark.parametrize("phi", [0, 0.37 * two_pi, 1.23 * two_pi, two_pi])
+class TestGPI:
+    @staticmethod
+    def circuit(phi):
+        """Circuit that applies the GPI gate to a known starting state
+        returning the expectation value of Pauli Y."""
+        State.set_state()
+        GPI(phi, wires=0)
+        return qml.expval(qml.PauliY(wires=0))
+
+    @pytest.fixture(autouse=True)
+    def state(self):
+        """Starting state to be used in computation"""
+        self._state = State()
+
+    def test_GPI_compute_matrix(self, phi):
+        """Tests wheter the right matrix is returned depending on phi."""
+        gpi_matrix = GPI.compute_matrix(phi)
+        check_matrix = get_GPI_matrix(phi)
+
+        assert math.allclose(gpi_matrix, check_matrix)
+
+    def test_GPI_circuit(self, phi):
+        """Tests whether the expected value is returned for a circuit using GPI"""
+        phi_GPI = np.array(phi)
+        dev = qml.device("default.qubit", wires=1)
+
+        qnode_GPI = qml.QNode(self.circuit, dev)
+        val_GPI = qnode_GPI(phi_GPI)
+
+        expected_inner_product_1 = 1j * self._state.b_conj_a * np.exp(-2j * phi)
+        expected_inner_product_2 = -1j * self._state.a_conj_b * np.exp(2j * phi)
+        expected_val = expected_inner_product_1 + expected_inner_product_2
+
+        assert np.isclose(
+            val_GPI, expected_val, atol=1e-07
+        ), f"Given val: {val_GPI}; Expected val: {expected_val}"
+
+    def get_circuit_grad(self, phi):
+        """Gets the expected gradient of the basic circuit using GPI's output."""
+        expected_inner_product_1 = 1j * self._state.b_conj_a * np.exp(-2j * phi) * (-2j)
+        expected_inner_product_2 = -1j * self._state.a_conj_b * np.exp(2j * phi) * 2j
+        return np.real(expected_inner_product_1 + expected_inner_product_2)
+
+    @pytest.mark.parametrize("diff_method", diff_methods)
+    def test_GPI_grad(self, diff_method, phi):
+        """Tests whether the correct gradient for a circuit using GPI
+        using different gradient computation methods."""
+
+        phi_GPI = np.array(phi)
+        dev = qml.device("default.qubit", wires=1)
+
+        qnode_GPI = qml.QNode(self.circuit, dev, diff_method=diff_method)
+        grad_GPI = qml.grad(qnode_GPI, argnum=0)(phi_GPI)
+
+        expected_grad = self.get_circuit_grad(phi)
+        assert np.isclose(
+            grad_GPI, expected_grad
+        ), f"Given grad: {grad_GPI}; Expected grad: {expected_grad}"