Givens gate (#135)

* added givens gate decomposition

Co-authored-by: ValentinS4t1qbit <41597680+ValentinS4t1qbit@users.noreply.github.com>

tangelo/toolboxes/ansatz_generator/ansatz_utils.py

@@ -394,3 +394,26 @@ def derangement_circuit(qubit_list, control=None, n_qubits=None, decomp=None):
                                        control=control)]
 
     return Circuit(gate_list, n_qubits=n_qubits)
+
+
+def givens_gate(target, theta, is_variational=False):
+    """Generates the list of gates corresponding to a givens rotation exp(-theta*(XX+YY))
+
+    Explicitly the two-qubit matrix is
+    [[0,      0,           0,       0],
+     [0,  cos(theta), -sin(theta),  0],
+     [0,  sin(theta),  cos(theta),  0],
+     [0,      0,            0,      0]]
+
+    Args:
+        target (list): list of two integers that indicate which qubits are involved in the givens rotation
+        theta (float): the rotation angle
+        is_variational (bool): Whether the rotation angle is a variational parameter.
+
+    Returns:
+        list of Gate: The list of gates corresponding to the givens rotation"""
+    if len(target) != 2:
+        raise ValueError("target must be a list or array of two integers")
+    return [Gate("CNOT", target=target[0], control=target[1]),
+            Gate("CRY", target=target[1], control=target[0], parameter=-theta, is_variational=is_variational),
+            Gate("CNOT", target=target[0], control=target[1])]

tangelo/toolboxes/ansatz_generator/tests/test_ansatz_util.py

@@ -11,9 +11,8 @@
 from tangelo.molecule_library import mol_H4_sto3g
 from tangelo.linq.tests.test_simulator import assert_freq_dict_almost_equal
 from tangelo.toolboxes.qubit_mappings.statevector_mapping import get_reference_circuit
-from tangelo.toolboxes.ansatz_generator.ansatz_utils import trotterize, get_qft_circuit
-from tangelo.toolboxes.ansatz_generator.ansatz_utils import controlled_swap_to_XX_gates
-from tangelo.toolboxes.ansatz_generator.ansatz_utils import derangement_circuit, controlled_pauliwords
+from tangelo.toolboxes.ansatz_generator.ansatz_utils import (givens_gate, trotterize, get_qft_circuit, controlled_swap_to_XX_gates,
+                                                             derangement_circuit, controlled_pauliwords)
 
 # Initiate simulators, Use cirq as it has the same ordering for statevectors as openfermion does for Hamiltonians
 # This is important when converting the openfermion QubitOperator toarray(), propagating exactly and comparing
@@ -302,6 +301,23 @@ def test_derangement_circuit_by_estimating_pauli_string(self):
         measured = sim.get_expectation_value(exp_op, rho3_pa_circuit, initial_statevector=full_start_vec)
         self.assertAlmostEqual(measured, exact, places=6)
 
+    def test_givens_gate(self):
+        """Test of givens gate decomposition into 2 CNOTs and a CRY gate."""
+        theta = 0.3
+
+        # Explicit definition of givens rotation gate
+        mat_rep = np.eye(4)
+        mat_rep[1, 1] = np.cos(theta/2)
+        mat_rep[1, 2] = -np.sin(theta/2)
+        mat_rep[2, 1] = np.sin(theta/2)
+        mat_rep[2, 2] = np.cos(theta/2)
+
+        # Test that explicit definition and circuit return the same state vector
+        vec = np.array([np.sqrt(2)/3, 2/3, np.sqrt(2)/3, 1/3])
+        gvec = mat_rep@vec
+        _, gvec2 = sim.simulate(Circuit(givens_gate([0, 1], theta)), return_statevector=True, initial_statevector=vec)
+        np.testing.assert_array_almost_equal(gvec, gvec2)
+
 
 if __name__ == "__main__":
     unittest.main()