QFT circuit synthesis for linear nearest neighbor connectivity

qiskit/synthesis/__init__.py

@@ -84,6 +84,11 @@
 
 .. autofunction:: generate_basic_approximations
 
+Basis Change Synthesis
+======================
+
+.. autofunction:: synth_qft_line
+
 """
 
 from .evolution import (
@@ -120,3 +125,4 @@
 )
 from .stabilizer import synth_stabilizer_layers, synth_stabilizer_depth_lnn
 from .discrete_basis import SolovayKitaevDecomposition, generate_basic_approximations
+from .qft import synth_qft_line

qiskit/synthesis/qft/__init__.py

@@ -0,0 +1,15 @@
+# This code is part of Qiskit.
+#
+# (C) Copyright IBM 2023.
+#
+# 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.
+
+"""Module containing stabilizer QFT circuit synthesis."""
+
+from .qft_decompose_lnn import synth_qft_line

qiskit/synthesis/qft/qft_decompose_lnn.py

@@ -0,0 +1,74 @@
+# This code is part of Qiskit.
+#
+# (C) Copyright IBM 2023.
+#
+# 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.
+"""
+Circuit synthesis for a QFT circuit.
+"""
+
+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: int, do_swaps: bool = True, approximation_degree: int = 0
+) -> QuantumCircuit:
+    """Synthesis of a QFT circuit for a linear nearest neighbor connectivity.
+    Based on Fig 2.b in Fowler et al. [1].
+
+    Note that this method *reverts* the order of qubits in the circuit,
+    compared to the original :class:`.QFT` code.
+    Hence, the default value of the ``do_swaps`` parameter is ``True``
+    since it produces a circuit with fewer CX gates.
+
+    Args:
+        num_qubits: The number of qubits on which the QFT acts.
+        approximation_degree: The degree of approximation (0 for no approximation).
+        do_swaps: Whether to include the final swaps in the QFT.
+
+    Return:
+        A circuit implementation of the QFT circuit.
+
+    Reference:
+        1. A. G. Fowler, S. J. Devitt, and L. C. L. Hollenberg,
+           *Implementation of Shor's algorithm on a linear nearest neighbour qubit array*,
+           Quantum Info. Comput. 4, 4 (July 2004), 237–251.
+           `arXiv:quant-ph/0402196 [quant-ph] <https://arxiv.org/abs/quant-ph/0402196>`_
+    """
+
+    qc = QuantumCircuit(num_qubits)
+
+    for i in range(num_qubits):
+        qc.h(num_qubits - 1)
+
+        for j in range(i, num_qubits - 1):
+            if j - i + 2 < num_qubits - approximation_degree + 1:
+                qc.p(np.pi / 2 ** (j - i + 2), num_qubits - j + i - 1)
+                qc.cx(num_qubits - j + i - 1, num_qubits - j + i - 2)
+                qc.p(-np.pi / 2 ** (j - i + 2), num_qubits - j + i - 2)
+                qc.cx(num_qubits - j + i - 2, num_qubits - j + i - 1)
+                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)
+            else:
+                qc.cx(num_qubits - j + i - 1, num_qubits - j + i - 2)
+                qc.cx(num_qubits - j + i - 2, num_qubits - j + i - 1)
+                qc.cx(num_qubits - j + i - 1, num_qubits - j + i - 2)
+
+    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

releasenotes/notes/qft_lnn_synthesis-c917dc00c3a8cabc.yaml

@@ -0,0 +1,6 @@
+---
+features:
+  - |
+    Add a new synthesis method :func:`.synth_qft_line` of a QFT circuit
+    for linear nearest neighbor connectivity, which significantly reduces
+    the number of SWAPs for large numbers of qubits compared to SABRE.

test/python/synthesis/test_qft_synthesis.py

@@ -0,0 +1,62 @@
+# This code is part of Qiskit.
+#
+# (C) Copyright IBM 2023.
+#
+# 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.
+
+"""Tests for QFT synthesis methods."""
+
+
+import unittest
+from test import combine
+from ddt import ddt
+
+from qiskit.test import QiskitTestCase
+from qiskit.circuit.library import QFT
+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=[2, 3, 4, 5, 6, 7, 8], do_swaps=[True, False])
+    def test_qft_lnn(self, num_qubits, do_swaps):
+        """Assert that the original and synthesized QFT circuits are the same."""
+        qft_circ = QFT(num_qubits, do_swaps=do_swaps)
+        qft_lnn = synth_qft_line(num_qubits, do_swaps=do_swaps)
+
+        with self.subTest(msg="original and synthesized QFT circuits are not the same"):
+            self.assertEqual(Operator(qft_circ), Operator(qft_lnn))
+
+        # Check that the output circuit has LNN connectivity
+        with self.subTest(msg="synthesized QFT circuit do not have LNN connectivity"):
+            self.assertTrue(check_lnn_connectivity(qft_lnn))
+
+    @combine(num_qubits=[5, 6, 7, 8], do_swaps=[True, False], approximation_degree=[2, 3])
+    def test_qft_lnn_approximated(self, num_qubits, do_swaps, approximation_degree):
+        """Assert that the original and synthesized QFT circuits are the same with approximation."""
+        qft_circ = QFT(num_qubits, do_swaps=do_swaps, approximation_degree=approximation_degree)
+        qft_lnn = synth_qft_line(
+            num_qubits, do_swaps=do_swaps, approximation_degree=approximation_degree
+        )
+
+        with self.subTest(msg="original and synthesized QFT circuits are not the same"):
+            self.assertEqual(Operator(qft_circ), Operator(qft_lnn))
+
+        # Check that the output circuit has LNN connectivity
+        with self.subTest(msg="synthesized QFT circuit do not have LNN connectivity"):
+            self.assertTrue(check_lnn_connectivity(qft_lnn))
+
+
+if __name__ == "__main__":
+    unittest.main()