feat(circuits): add QAOAPathCircuit class

quantum_enablement/circuits/__init__.py

@@ -12,8 +12,10 @@
 
 """Quantum circuit library and tools."""
 
+from ._qaoa import QAOAPathCircuit
 from ._utils import compute_uncompute
 
 __all__ = [
+    "QAOAPathCircuit",
     "compute_uncompute",
 ]

quantum_enablement/circuits/_qaoa.py

@@ -0,0 +1,92 @@
+# This code is part of Qiskit.
+#
+# (C) Copyright IBM 2024.
+#
+# This code is licensed under the Apache License, Version 2.0. You may
+# obtain a copy of this license in the LICENSE.txt file in the root directory
+# of this source tree or at http://www.apache.org/licenses/LICENSE-2.0.
+#
+# Any modifications or derivative works of this code must retain this
+# copyright notice, and modified files need to carry a notice indicating
+# that they have been altered from the originals.
+
+"""Quantum Approximate Optimization Algorithm (QAOA) quantum circuits.
+
+[1] Farhi et.al. A Quantum Approximate Optimization Algorithm,
+    https://arxiv.org/abs/1411.4028
+"""
+
+
+from qiskit import QuantumCircuit
+from qiskit.circuit import ParameterVector
+
+
+# TODO: add graph max-cut weights input arg
+class QAOAPathCircuit(QuantumCircuit):
+    """Parameterized QAOA acyclic line graph quantum circuit.
+
+    The cost parameter-vector is labeled γ, and the mixer
+    parameter-vector β. Overall, there will be one parameter per
+    unit of two-qubit depth: half in γ and half in ß [1].
+    Weights in the generating max-cut graph are all equal to one.
+
+    Args:
+        num_qubits: number of qubits (must be even).
+        depth: two-qubit depth (must be even).
+        barriers: if True adds barriers between layers.
+        measurements: if True adds measurements at the end.
+
+    Notes:
+        [1] Farhi et.al. A Quantum Approximate Optimization Algorithm,
+            https://arxiv.org/abs/1411.4028
+    """
+
+    def __init__(
+        self, num_qubits: int, depth: int, *, barriers: bool = False, measurements: bool = False
+    ) -> None:
+        num_qubits = _validate_qaoa_num_qubits(num_qubits)
+        depth = _validate_qaoa_depth(depth)
+        barriers = bool(barriers)
+        measurements = bool(measurements)
+
+        super().__init__(num_qubits, name=f"QAOAPathCircuit<{num_qubits}, {depth}>")
+
+        gammas = ParameterVector("γ", depth // 2)
+        betas = ParameterVector("β", depth // 2)
+
+        self.h(range(num_qubits))
+        for layer in range(depth // 2):
+            if barriers:
+                self.barrier()
+            for qubit in range(0, num_qubits - 1, 2):
+                self.rzz(gammas[layer], qubit, qubit + 1)
+            for qubit in range(1, num_qubits - 1, 2):
+                self.rzz(gammas[layer], qubit, qubit + 1)
+            for qubit in range(num_qubits):
+                self.rx(betas[layer], qubit)
+        if measurements:
+            self.measure_all()
+
+
+def _validate_qaoa_num_qubits(num_qubits: int) -> int:
+    """Validate number of qubits for QAOA circuits."""
+    # pylint: disable=duplicate-code
+    if not isinstance(num_qubits, int):
+        raise TypeError(f"Invalid num. qubits type {type(num_qubits)}, expected <int>.")
+    if num_qubits <= 2:
+        raise ValueError(f"Number of qubits ({num_qubits}) must be greater than two.")
+    if num_qubits % 2:
+        raise ValueError(f"Number of qubits ({num_qubits}) must be even.")
+    return num_qubits
+
+
+def _validate_qaoa_depth(depth: int) -> int:
+    """Validate depth for QAOA circuits."""
+    # pylint: disable=duplicate-code
+    if not isinstance(depth, int):
+        raise TypeError(f"Invalid depth type {type(depth)}, expected <int>.")
+    if depth < 0:
+        raise ValueError(f"Depth ({depth}) must be positive.")
+    if depth % 2:
+        raise ValueError(f"Depth ({depth}) must be even.")
+    return depth

test/unit/circuits/test_qaoa.py

@@ -0,0 +1,105 @@
+# This code is part of Qiskit.
+#
+# (C) Copyright IBM 2024.
+#
+# This code is licensed under the Apache License, Version 2.0. You may
+# obtain a copy of this license in the LICENSE.txt file in the root directory
+# of this source tree or at http://www.apache.org/licenses/LICENSE-2.0.
+#
+# Any modifications or derivative works of this code must retain this
+# copyright notice, and modified files need to carry a notice indicating
+# that they have been altered from the originals.
+
+"""Test QAOA circuits module."""
+
+
+from pytest import mark, raises  # pylint: disable=import-error
+from qiskit import QuantumCircuit
+
+from quantum_enablement.circuits._qaoa import QAOAPathCircuit
+
+
+def two_qubit_depth_filter(instruction):
+    """Filter instructions which do not contribute to two-qubit depth."""
+    num_qubits = instruction.operation.num_qubits
+    name = instruction.operation.name
+    return num_qubits > 1 and name != "barrier"
+
+
+class TestQAOAPathCircuit:
+    """Test suite for the QAOAPathCircuit class."""
+
+    @mark.parametrize("num_qubits", [4, 6])
+    @mark.parametrize("depth", [0, 2, 4])
+    def test_creation(self, num_qubits, depth):
+        """Test QAOAPathCircuit creation."""
+        circuit = QAOAPathCircuit(num_qubits, depth, barriers=False, measurements=False)
+        op_counts = circuit.count_ops()
+        assert isinstance(circuit, QuantumCircuit), "QAOAPathCircuit must be a QuantumCircuit."
+        assert circuit.num_qubits == num_qubits, "Wrong number of qubits."
+        assert circuit.depth(two_qubit_depth_filter) == depth, "Wrong two-qubit depth."
+        assert circuit.num_parameters == depth, "Wrong number of parameters."
+        assert op_counts.get("h", 0) == num_qubits, "Wrong number of H gates."
+        assert op_counts.get("rzz", 0) == (num_qubits - 1) * (
+            depth // 2
+        ), "Wrong number of RZZ gates."
+        assert op_counts.get("rx", 0) == num_qubits * (depth // 2), "Wrong number of RX gates."
+        assert op_counts.get("barrier", 0) == 0, "Wrong number of barriers."
+        assert op_counts.get("measure", 0) == 0, "Wrong number of measurements."
+
+    def test_defaults(self):
+        """Test QAOAPathCircuit defaults."""
+        circuit = QAOAPathCircuit(4, 4)  # Note: default barriers and measurements are False.
+        op_counts = circuit.count_ops()
+        assert op_counts.get("barrier", 0) == 0, "By default barriers should not be present."
+        assert op_counts.get("measure", 0) == 0, "By default measurements should not be present."
+
+    @mark.parametrize("num_qubits", [4, 6])
+    @mark.parametrize("depth", [0, 2, 4])
+    def test_barriers(self, num_qubits, depth):
+        """Test QAOAPathCircuit barriers."""
+        circuit = QAOAPathCircuit(num_qubits, depth, barriers=True, measurements=False)
+        op_counts = circuit.count_ops()
+        assert op_counts.get("barrier", 0) == depth // 2, "Wrong number of barriers."
+        # circuit.remove_operations("barrier")
+        # assert circuit == QAOAPathCircuit(num_qubits, depth, measurements=False, barriers=False)
+
+    @mark.parametrize("num_qubits", [4, 6])
+    @mark.parametrize("depth", [0, 2, 4])
+    def test_measurements(self, num_qubits, depth):
+        """Test QAOAPathCircuit measurements."""
+        circuit = QAOAPathCircuit(num_qubits, depth, barriers=False, measurements=True)
+        op_counts = circuit.count_ops()
+        assert op_counts.get("measure", 0) == num_qubits, "Wrong number of measurements."
+        assert op_counts.get("barrier", 0) == 1, "Measurements should be preceded by a barrier."
+        # circuit.remove_final_measurements()
+        # assert circuit == QAOAPathCircuit(num_qubits, depth, measurements=False, barriers=False)
+
+    @mark.parametrize("num_qubits", [4, 6])
+    @mark.parametrize("depth", [0, 2, 4])
+    def test_name(self, num_qubits, depth):
+        """Test QAOAPathCircuit name."""
+        circuit = QAOAPathCircuit(num_qubits, depth)
+        assert circuit.name == f"QAOAPathCircuit<{num_qubits}, {depth}>", "Wrong circuit name."
+
+    def test_invalid_num_qubits(self):
+        """Test QAOAPathCircuit with invalid number of qubits."""
+        with raises(ValueError, match=r"Number of qubits \(2\) must be greater than two."):
+            QAOAPathCircuit(2, 2)
+
+        with raises(ValueError, match=r"Number of qubits \(3\) must be even\."):
+            QAOAPathCircuit(3, 2)
+
+        with raises(TypeError, match=r"Invalid num\. qubits type .*, expected <int>\."):
+            QAOAPathCircuit("4", 2)  # type: ignore
+
+    def test_invalid_depth(self):
+        """Test QAOAPathCircuit with invalid depth."""
+        with raises(ValueError, match=r"Depth \(-1\) must be positive\."):
+            QAOAPathCircuit(4, -1)
+
+        with raises(ValueError, match=r"Depth \(1\) must be even\."):
+            QAOAPathCircuit(4, 1)
+
+        with raises(TypeError, match=r"Invalid depth type .*, expected <int>\."):
+            QAOAPathCircuit(4, "2")  # type: ignore