def test_lookahead_swap_finds_minimal_swap_solution(self):
"""Of many valid SWAPs, test that LookaheadSwap finds the cheapest path.
For a two CNOT circuit: cx q[0],q[2]; cx q[0],q[1]
on the initial layout: qN -> qN
(At least) two solutions exist:
- SWAP q[0],[1], cx q[0],q[2], cx q[0],q[1]
- SWAP q[1],[2], cx q[0],q[2], SWAP q[1],q[2], cx q[0],q[1]
Verify that we find the first solution, as it requires fewer SWAPs.
"""
qr = QuantumRegister(3, "q")
circuit = QuantumCircuit(qr)
circuit.cx(qr[0], qr[2])
circuit.cx(qr[0], qr[1])
dag_circuit = circuit_to_dag(circuit)
coupling_map = CouplingMap([[0, 1], [1, 2]])
mapped_dag = LookaheadSwap(coupling_map).run(dag_circuit)
self.assertEqual(
mapped_dag.count_ops().get("swap", 0), dag_circuit.count_ops().get("swap", 0) + 1
)
def test_lookahead_swap_higher_depth_width_is_better(self):
"""Test that lookahead swap finds better circuit with increasing search space.
Increasing the tree width and depth is expected to yield a better (or same) quality
circuit, in the form of fewer SWAPs.
"""
qr = QuantumRegister(8, name='q')
circuit = QuantumCircuit(qr)
circuit.cx(qr[0], qr[1])
circuit.cx(qr[1], qr[2])
circuit.cx(qr[2], qr[3])
circuit.cx(qr[3], qr[4])
circuit.cx(qr[4], qr[5])
circuit.cx(qr[5], qr[6])
circuit.cx(qr[6], qr[7])
circuit.cx(qr[0], qr[3])
circuit.cx(qr[6], qr[4])
circuit.cx(qr[7], qr[1])
circuit.cx(qr[4], qr[2])
circuit.cx(qr[3], qr[7])
circuit.cx(qr[5], qr[3])
circuit.cx(qr[6], qr[2])
circuit.cx(qr[2], qr[7])
circuit.cx(qr[0], qr[6])
circuit.cx(qr[5], qr[7])
original_dag = circuit_to_dag(circuit)
# Create a ring of 8 connected qubits
coupling_map = CouplingMap.from_grid(num_rows=2, num_columns=4)
mapped_dag_1 = LookaheadSwap(coupling_map,
search_depth=3,
search_width=3).run(original_dag)
mapped_dag_2 = LookaheadSwap(coupling_map,
search_depth=5,
search_width=5).run(original_dag)
num_swaps_1 = mapped_dag_1.count_ops().get('swap', 0)
num_swaps_2 = mapped_dag_2.count_ops().get('swap', 0)
self.assertLessEqual(num_swaps_2, num_swaps_1)
def test_global_phase_preservation(self):
"""Test that LookaheadSwap preserves global phase"""
qr = QuantumRegister(3, "q")
circuit = QuantumCircuit(qr)
circuit.global_phase = pi / 3
circuit.cx(qr[0], qr[2])
dag_circuit = circuit_to_dag(circuit)
coupling_map = CouplingMap([[0, 1], [1, 2]])
mapped_dag = LookaheadSwap(coupling_map).run(dag_circuit)
self.assertEqual(mapped_dag.global_phase, circuit.global_phase)
self.assertEqual(mapped_dag.count_ops().get("swap", 0),
dag_circuit.count_ops().get("swap", 0) + 1)
def test_lookahead_swap_should_add_a_single_swap(self):
"""Test that LookaheadSwap will insert a SWAP to match layout.
For a single cx gate which is not available in the current layout, test
that the mapper inserts a single swap to enable the gate.
"""
qr = QuantumRegister(3, 'q')
circuit = QuantumCircuit(qr)
circuit.cx(qr[0], qr[2])
dag_circuit = circuit_to_dag(circuit)
coupling_map = CouplingMap([[0, 1], [1, 2]])
mapped_dag = LookaheadSwap(coupling_map).run(dag_circuit)
self.assertEqual(mapped_dag.count_ops().get('swap', 0),
dag_circuit.count_ops().get('swap', 0) + 1)
def test_lookahead_swap_higher_depth_width_is_better(self):
"""Test that lookahead swap finds better circuit with increasing search space.
Increasing the tree width and depth is expected to yield a better (or same) quality
circuit, in the form of fewer SWAPs.
"""
# q_0: ──■───────────────────■───────────────────────────────────────────────»
# ┌─┴─┐ │ ┌───┐ »
# q_1: ┤ X ├──■──────────────┼─────────────────┤ X ├─────────────────────────»
# └───┘┌─┴─┐ │ └─┬─┘┌───┐ ┌───┐ »
# q_2: ─────┤ X ├──■─────────┼───────────────────┼──┤ X ├──────────┤ X ├──■──»
# └───┘┌─┴─┐ ┌─┴─┐ │ └─┬─┘ ┌───┐└─┬─┘ │ »
# q_3: ──────────┤ X ├──■──┤ X ├─────────────────┼────┼────■──┤ X ├──┼────┼──»
# └───┘┌─┴─┐└───┘ ┌───┐ │ │ │ └─┬─┘ │ │ »
# q_4: ───────────────┤ X ├──■────────────┤ X ├──┼────■────┼────┼────┼────┼──»
# └───┘┌─┴─┐ └─┬─┘ │ │ │ │ │ »
# q_5: ────────────────────┤ X ├──■─────────┼────┼─────────┼────■────┼────┼──»
# └───┘┌─┴─┐ │ │ │ │ │ »
# q_6: ─────────────────────────┤ X ├──■────■────┼─────────┼─────────■────┼──»
# └───┘┌─┴─┐ │ ┌─┴─┐ ┌─┴─┐»
# q_7: ──────────────────────────────┤ X ├───────■───────┤ X ├──────────┤ X ├»
# └───┘ └───┘ └───┘»
# «q_0: ──■───────
# « │
# «q_1: ──┼───────
# « │
# «q_2: ──┼───────
# « │
# «q_3: ──┼───────
# « │
# «q_4: ──┼───────
# « │
# «q_5: ──┼────■──
# « ┌─┴─┐ │
# «q_6: ┤ X ├──┼──
# « └───┘┌─┴─┐
# «q_7: ─────┤ X ├
# « └───┘
qr = QuantumRegister(8, name="q")
circuit = QuantumCircuit(qr)
circuit.cx(qr[0], qr[1])
circuit.cx(qr[1], qr[2])
circuit.cx(qr[2], qr[3])
circuit.cx(qr[3], qr[4])
circuit.cx(qr[4], qr[5])
circuit.cx(qr[5], qr[6])
circuit.cx(qr[6], qr[7])
circuit.cx(qr[0], qr[3])
circuit.cx(qr[6], qr[4])
circuit.cx(qr[7], qr[1])
circuit.cx(qr[4], qr[2])
circuit.cx(qr[3], qr[7])
circuit.cx(qr[5], qr[3])
circuit.cx(qr[6], qr[2])
circuit.cx(qr[2], qr[7])
circuit.cx(qr[0], qr[6])
circuit.cx(qr[5], qr[7])
original_dag = circuit_to_dag(circuit)
# Create a ring of 8 connected qubits
coupling_map = CouplingMap.from_grid(num_rows=2, num_columns=4)
mapped_dag_1 = LookaheadSwap(coupling_map,
search_depth=3,
search_width=3).run(original_dag)
mapped_dag_2 = LookaheadSwap(coupling_map,
search_depth=5,
search_width=5).run(original_dag)
num_swaps_1 = mapped_dag_1.count_ops().get("swap", 0)
num_swaps_2 = mapped_dag_2.count_ops().get("swap", 0)
self.assertLessEqual(num_swaps_2, num_swaps_1)
