Token swapper permutation synthesis plugin

pyproject.toml

@@ -88,6 +88,7 @@ sk = "qiskit.transpiler.passes.synthesis.solovay_kitaev_synthesis:SolovayKitaevS
 "permutation.kms" = "qiskit.transpiler.passes.synthesis.high_level_synthesis:KMSSynthesisPermutation"
 "permutation.basic" = "qiskit.transpiler.passes.synthesis.high_level_synthesis:BasicSynthesisPermutation"
 "permutation.acg" = "qiskit.transpiler.passes.synthesis.high_level_synthesis:ACGSynthesisPermutation"
+"permutation.token_swapper" = "qiskit.transpiler.passes.synthesis.high_level_synthesis:TokenSwapperSynthesisPermutation"
 
 [project.entry-points."qiskit.transpiler.init"]
 default = "qiskit.transpiler.preset_passmanagers.builtin_plugins:DefaultInitPassManager"

qiskit/transpiler/passes/synthesis/high_level_synthesis.py

@@ -15,6 +15,8 @@
 
 from typing import Optional, Union, List, Tuple
 
+import rustworkx as rx
+
 from qiskit.circuit.operation import Operation
 from qiskit.converters import circuit_to_dag, dag_to_circuit
 from qiskit.transpiler.basepasses import TransformationPass
@@ -25,6 +27,7 @@
 from qiskit.transpiler.coupling import CouplingMap
 from qiskit.dagcircuit.dagcircuit import DAGCircuit
 from qiskit.transpiler.exceptions import TranspilerError
+from qiskit.transpiler.passes.routing.algorithms import ApproximateTokenSwapper
 
 from qiskit.circuit.annotated_operation import (
     AnnotatedOperation,
@@ -297,7 +300,7 @@ def _recursively_handle_op(
 
         # Try to apply plugin mechanism
         decomposition = self._synthesize_op_using_plugins(op, qubits)
-        if decomposition:
+        if decomposition is not None:
             return decomposition, True
 
         # Handle annotated operations
@@ -644,3 +647,80 @@ def run(self, high_level_object, coupling_map=None, target=None, qubits=None, **
         """Run synthesis for the given Permutation."""
         decomposition = synth_permutation_acg(high_level_object.pattern)
         return decomposition
+
+
+class TokenSwapperSynthesisPermutation(HighLevelSynthesisPlugin):
+    """The permutation synthesis plugin based on the token swapper algorithm.
+
+    This plugin name is :``permutation.token_swapper`` which can be used as the key on
+    an :class:`~.HLSConfig` object to use this method with :class:`~.HighLevelSynthesis`.
+
+    In more detail, this plugin is used to synthesize objects of type `PermutationGate`.
+    When synthesis succeeds, the plugin outputs a quantum circuit consisting only of swap
+    gates. When synthesis does not succeed, the plugin outputs `None`.
+
+    If either `coupling_map` or `qubits` is None, then the synthesized circuit
+    is not required to adhere to connectivity constraints, as is the case
+    when the synthesis is done before layout/routing.
+
+    On the other hand, if both `coupling_map` and `qubits` are specified, the synthesized
+    circuit is supposed to adhere to connectivity constraints. At the moment, the
+    plugin only creates swap gates between qubits in `qubits`, i.e. it does not use
+    any other qubits in the coupling map (if such synthesis is not possible, the
+    plugin  outputs `None`).
+
+    The plugin supports the following plugin-specific options:
+
+    * trials: The number of trials for the token swapper to perform the mapping. The
+      circuit with the smallest number of SWAPs is returned.
+    * seed: The argument to the token swapper specifying the seed for random trials.
+    * parallel_threshold: The argument to the token swapper specifying the number of nodes
+      in the graph beyond which the algorithm will use parallel processing.
+
+    For more details on the token swapper algorithm, see to the paper:
+    `arXiv:1902.09102 <https://arxiv.org/abs/1902.09102>`__.
+
+    """
+
+    def run(self, high_level_object, coupling_map=None, target=None, qubits=None, **options):
+        """Run synthesis for the given Permutation."""
+
+        trials = options.get("trials", 5)
+        seed = options.get("seed", 0)
+        parallel_threshold = options.get("parallel_threshold", 50)
+
+        pattern = high_level_object.pattern
+        pattern_as_dict = {j: i for i, j in enumerate(pattern)}
+
+        # When the plugin is called from the HighLevelSynthesis transpiler pass,
+        # the coupling map already takes target into account.
+        if coupling_map is None or qubits is None:
+            # The abstract synthesis uses a fully connected coupling map, allowing
+            # arbitrary connections between qubits.
+            used_coupling_map = CouplingMap.from_full(len(pattern))
+        else:
+            # The concrete synthesis uses the coupling map restricted to the set of
+            # qubits over which the permutation gate is defined. If we allow using other
+            # qubits in the coupling map, replacing the node in the DAGCircuit that
+            # defines this PermutationGate by the DAG corresponding to the constructed
+            # decomposition becomes problematic. Note that we allow the reduced
+            # coupling map to be disconnected.
+            used_coupling_map = coupling_map.reduce(qubits, check_if_connected=False)
+
+        graph = used_coupling_map.graph.to_undirected()
+        swapper = ApproximateTokenSwapper(graph, seed=seed)
+
+        try:
+            swapper_result = swapper.map(
+                pattern_as_dict, trials, parallel_threshold=parallel_threshold
+            )
+        except rx.InvalidMapping:
+            swapper_result = None
+
+        if swapper_result is not None:
+            decomposition = QuantumCircuit(len(graph.node_indices()))
+            for swap in swapper_result:
+                decomposition.swap(*swap)
+            return decomposition
+
+        return None

qiskit/transpiler/passes/synthesis/plugin.py

@@ -284,6 +284,36 @@ def run(self, high_level_object, coupling_map=None, target=None, qubits=None, **
 
 will return a list of all the installed Clifford synthesis plugins.
 
+Available Plugins
+-----------------
+
+High-level synthesis plugins that are directly available in Qiskit include plugins
+for synthesizing :class:`.Clifford` objects, :class:`.LinearFunction` objects, and
+:class:`.PermutationGate` objects.
+Some of these plugins implicitly target all-to-all connectivity. This is not a
+practical limitation since
+:class:`~qiskit.transpiler.passes.synthesis.high_level_synthesis.HighLevelSynthesis`
+typically runs before layout and routing, which will ensure that the final circuit
+adheres to the device connectivity by inserting additional SWAP gates. A good example
+is the permutation synthesis plugin ``ACGSynthesisPermutation`` which can synthesize
+any permutation with at most 2 layers of SWAP gates.
+On the other hand, some plugins implicitly target linear connectivity.
+Typically, the synthesizing circuits have larger depth and the number of gates,
+however no additional SWAP gates would be inserted if the following layout pass chose a
+consecutive line of qubits inside the topology of the device. A good example of this is
+the permutation synthesis plugin ``KMSSynthesisPermutation`` which can synthesize any
+permutation of ``n`` qubits in depth ``n``. Typically, it is difficult to know in advance
+which of the two approaches: synthesizing circuits for all-to-all connectivity and
+inserting SWAP gates vs. synthesizing circuits for linear connectivity and inserting less
+or no SWAP gates lead a better final circuit, so it likely makes sense to try both and
+see which gives better results.
+Finally, some plugins can target a given connectivity, and hence should be run after the
+layout is set. In this case the synthesized circuit automatically adheres to
+the topology of the device. A good example of this is the permutation synthesis plugin
+``TokenSwapperSynthesisPermutation`` which is able to synthesize arbitrary permutations
+with respect to arbitrary coupling maps.
+For more detail, please refer to description of each individual plugin.
+
 Plugin API
 ==========
 
@@ -306,7 +336,6 @@ def run(self, high_level_object, coupling_map=None, target=None, qubits=None, **
    HighLevelSynthesisPlugin
    HighLevelSynthesisPluginManager
    high_level_synthesis_plugin_names
-
 """
 
 import abc

releasenotes/notes/add-token-swapper-synthesis-plugin-4ed5009f5f21519d.yaml

@@ -0,0 +1,44 @@
+---
+upgrade:
+  - |
+    Qiskit 1.0 now requires version 0.14.0 of ``rustworkx``.
+features:
+  - |
+    Added a new :class:`.HighLevelSynthesisPlugin` for :class:`.PermutationGate`
+    objects based on Qiskit's token swapper algorithm. To use this plugin,
+    specify ``token_swapper`` when defining high-level-synthesis config.
+
+    This synthesis plugin is able to run before or after the layout is set.
+    When synthesis succeeds, the plugin outputs a quantum circuit consisting only of
+    swap gates. When synthesis does not succeed, the plugin outputs `None`.
+
+    The following code illustrates how the new plugin can be run::
+
+        from qiskit.circuit import QuantumCircuit
+        from qiskit.circuit.library import PermutationGate
+        from qiskit.transpiler import PassManager, CouplingMap
+        from qiskit.transpiler.passes.synthesis.high_level_synthesis import HighLevelSynthesis, HLSConfig
+
+        # This creates a circuit with a permutation gate.
+        qc = QuantumCircuit(8)
+        perm_gate = PermutationGate([0, 1, 4, 3, 2])
+        qc.append(perm_gate, [3, 4, 5, 6, 7])
+
+        # This defines the coupling map.
+        coupling_map = CouplingMap.from_ring(8)
+
+        # This high-level-synthesis config specifies that we want to use
+        # the "token_swapper" plugin for synthesizing permutation gates,
+        # with the option to use 10 trials.
+        synthesis_config = HLSConfig(permutation=[("token_swapper", {"trials": 10})])
+
+        # This creates the pass manager that runs high-level-synthesis on our circuit.
+        # The option use_qubit_indices=True indicates that synthesis run after the layout is set,
+        # and hence should preserve the specified coupling map.
+        pm = PassManager(
+            HighLevelSynthesis(
+                synthesis_config, coupling_map=coupling_map, target=None, use_qubit_indices=True
+            )
+        )
+
+        qc_transpiled = pm.run(qc)

requirements.txt

@@ -1,4 +1,4 @@
-rustworkx>=0.13.0
+rustworkx>=0.14.0
 numpy>=1.17,<2
 scipy>=1.5
 sympy>=1.3