add do_swaps option

qiskit/synthesis/qft/qft_decompose_lnn.py

@@ -15,10 +15,12 @@
 
 import numpy as np
 from qiskit.circuit import QuantumCircuit
+from qiskit.synthesis.linear_phase.cz_depth_lnn import _append_cx_stage1, _append_cx_stage2
 
 
-def synth_qft_line(num_qubits):
-    """Synthesis of a QFT circuit for a linear nearest neighbor connectivity."""
+def synth_qft_line(num_qubits, do_swaps=True):
+    """Synthesis of a QFT circuit for a linear nearest neighbor connectivity.
+    Based on Fowler et al. Fig 2.b from https://arxiv.org/abs/quant-ph/0402196"""
 
     qc = QuantumCircuit(num_qubits)
 
@@ -32,4 +34,13 @@ def synth_qft_line(num_qubits):
             qc.cx(num_qubits - j + i - 1, num_qubits - j + i - 2)
             qc.p(np.pi / 2 ** (j - i + 2), num_qubits - j + i - 1)
 
+    if not do_swaps:
+        # Add a reversal network for LNN connectivity in depth 2*n+2,
+        # based on Kutin at al., https://arxiv.org/abs/quant-ph/0701194, Section 5
+        for _ in range((num_qubits + 1) // 2):
+            qc = _append_cx_stage1(qc, num_qubits)
+            qc = _append_cx_stage2(qc, num_qubits)
+        if (num_qubits % 2) == 0:
+            qc = _append_cx_stage1(qc, num_qubits)
+
     return qc

test/python/synthesis/test_qft_synthesis.py

@@ -22,19 +22,35 @@
 from qiskit.synthesis.qft import synth_qft_line
 from qiskit.quantum_info import Operator
 
+from qiskit.synthesis.linear.linear_circuits_utils import check_lnn_connectivity
+
 
 @ddt
 class TestQFTLNN(QiskitTestCase):
     """Tests for QFT synthesis functions."""
 
-    @combine(num_qubits=[4, 5, 6, 7])
+    @combine(num_qubits=[2, 3, 4, 5, 6, 7, 8])
     def test_qft_lnn(self, num_qubits):
         """Assert that the original and synthesized QFT circuits are the same."""
         qft_circ = QFT(num_qubits, do_swaps=True)
         qft_lnn = synth_qft_line(num_qubits)
 
         self.assertTrue(Operator(qft_circ).equiv(Operator(qft_lnn)))
 
+        # Check that the output circuit has LNN connectivity
+        self.assertTrue(check_lnn_connectivity(qft_lnn))
+
+    @combine(num_qubits=[2, 3, 4, 5, 6, 7, 8])
+    def test_qft_lnn_no_swaps(self, num_qubits):
+        """Assert that the original and synthesized QFT circuits are the same w/o swaps."""
+        qft_circ = QFT(num_qubits, do_swaps=False)
+        qft_lnn = synth_qft_line(num_qubits, do_swaps=False)
+
+        self.assertTrue(Operator(qft_circ).equiv(Operator(qft_lnn)))
+
+        # Check that the output circuit has LNN connectivity
+        self.assertTrue(check_lnn_connectivity(qft_lnn))
+
 
 if __name__ == "__main__":
     unittest.main()