Draft code for remaining circuit ansatz

src/qibochem/ansatz/universal.py

@@ -0,0 +1,83 @@
+"""
+Module documentation
+"""
+
+from qibo import Circuit, gates
+
+# Helper functions
+
+
+def double_excitation_gate(excitation, theta=0.0):
+    """
+    Decomposition of a double excitation gate into single qubit rotations and CNOTs
+
+    Args:
+        n_qubits (int): Number of qubits in the circuit
+        excitation: Iterable of orbitals involved in the excitation; must have an even number of elements
+        theta (float): Rotation angle. Default: 0.0
+
+    Returns:
+        (list): List of gates representing the decomposition of the Givens' double excitation gate
+    """
+    sorted_orbitals = sorted(excitation)
+    # Check size of orbitals input
+    assert len(sorted_orbitals) % 2 == 0, f"{excitation} must have an even number of items"
+
+    if theta is None:
+        theta = 0.0
+
+    result = []
+    result.append(gates.CNOT(sorted_orbitals[2], sorted_orbitals[3]))
+    result.append(gates.CNOT(sorted_orbitals[0], sorted_orbitals[2]))
+    result.append(gates.H(sorted_orbitals[0]))
+    result.append(gates.H(sorted_orbitals[3]))
+    result.append(gates.CNOT(sorted_orbitals[0], sorted_orbitals[1]))
+    result.append(gates.CNOT(sorted_orbitals[2], sorted_orbitals[3]))
+    result.append(gates.RY(sorted_orbitals[0], -0.125 * theta))
+    result.append(gates.RY(sorted_orbitals[1], 0.125 * theta))
+    result.append(gates.CNOT(sorted_orbitals[0], sorted_orbitals[3]))
+    result.append(gates.H(sorted_orbitals[3]))
+    result.append(gates.CNOT(sorted_orbitals[3], sorted_orbitals[1]))
+    result.append(gates.RY(sorted_orbitals[0], -0.125 * theta))
+    result.append(gates.RY(sorted_orbitals[1], 0.125 * theta))
+    result.append(gates.CNOT(sorted_orbitals[2], sorted_orbitals[1]))
+    result.append(gates.CNOT(sorted_orbitals[2], sorted_orbitals[0]))
+    result.append(gates.RY(sorted_orbitals[0], 0.125 * theta))
+    result.append(gates.RY(sorted_orbitals[1], -0.125 * theta))
+    result.append(gates.CNOT(sorted_orbitals[3], sorted_orbitals[1]))
+    result.append(gates.H(sorted_orbitals[3]))
+    result.append(gates.CNOT(sorted_orbitals[0], sorted_orbitals[3]))
+    result.append(gates.RY(sorted_orbitals[0], 0.125 * theta))
+    result.append(gates.RY(sorted_orbitals[1], -0.125 * theta))
+    result.append(gates.CNOT(sorted_orbitals[0], sorted_orbitals[1]))
+    result.append(gates.CNOT(sorted_orbitals[2], sorted_orbitals[0]))
+    result.append(gates.H(sorted_orbitals[0]))
+    result.append(gates.H(sorted_orbitals[3]))
+    result.append(gates.CNOT(sorted_orbitals[0], sorted_orbitals[2]))
+    result.append(gates.CNOT(sorted_orbitals[2], sorted_orbitals[3]))
+    return result
+
+
+# Main function
+def universal_circuit(n_qubits, excitation, theta=0.0):
+    """
+    Universal circuit ansatz from Arrazola et al. Reference: https://doi.org/10.22331/q-2022-06-20-742
+
+    Args:
+        n_qubits: Number of qubits in the quantum circuit
+        n_electrons: Number of electrons in the molecular system
+
+    Returns:
+        Qibo ``Circuit``: Circuit ansatz
+    """
+    circuit = Circuit(n_qubits)
+    if len(excitation) == 2:
+        circuit.add(gates.GIVENS(excitation[0], excitation[1], theta))
+    elif len(excitation) == 4:
+        circuit.add(double_excitation_gate(excitation, theta))
+    return circuit
+
+
+def universal_ansatz(n_qubits, excitation, n_electrons):
+    """TODO: Same implementation as UCC?"""
+    pass