Commuting2qGateRouter

class qiskit.transpiler.passes.Commuting2qGateRouter(*args, **kwargs)

Bases: TransformationPass

A class to swap route one or more commuting gates to the coupling map.

This pass routes blocks of commuting two-qubit gates encapsulated as Commuting2qBlock instructions. This pass will not apply to other instructions. The mapping to the coupling map is done using swap strategies, see SwapStrategy. The swap strategy should suit the problem and the coupling map. This transpiler pass should ideally be executed before the quantum circuit is enlarged with any idle ancilla qubits. Otherwise, we may swap qubits outside the portion of the chip we want to use. Therefore, the swap strategy and its associated coupling map do not represent physical qubits. Instead, they represent an intermediate mapping that corresponds to the physical qubits once the initial layout is applied. The example below shows how to map a four qubit PauliEvolutionGate to qubits 0, 1, 3, and 4 of the five qubit device with the coupling map

0 -- 1 -- 2
     |
     3
     |
     4

To do this we use a line swap strategy for qubits 0, 1, 3, and 4 defined it in terms of virtual qubits 0, 1, 2, and 3.

from qiskit import QuantumCircuit
from qiskit.opflow import PauliSumOp
from qiskit.circuit.library import PauliEvolutionGate
from qiskit.transpiler import Layout, CouplingMap, PassManager
from qiskit.transpiler.passes import FullAncillaAllocation
from qiskit.transpiler.passes import EnlargeWithAncilla
from qiskit.transpiler.passes import ApplyLayout
from qiskit.transpiler.passes import SetLayout
 
from qiskit.transpiler.passes.routing.commuting_2q_gate_routing import (
    SwapStrategy,
    FindCommutingPauliEvolutions,
    Commuting2qGateRouter,
)
 
# Define the circuit on virtual qubits
op = PauliSumOp.from_list([("IZZI", 1), ("ZIIZ", 2), ("ZIZI", 3)])
circ = QuantumCircuit(4)
circ.append(PauliEvolutionGate(op, 1), range(4))
 
# Define the swap strategy on qubits before the initial_layout is applied.
swap_strat = SwapStrategy.from_line([0, 1, 2, 3])
 
# Chose qubits 0, 1, 3, and 4 from the backend coupling map shown above.
backend_cmap = CouplingMap(couplinglist=[(0, 1), (1, 2), (1, 3), (3, 4)])
initial_layout = Layout.from_intlist([0, 1, 3, 4], *circ.qregs)
 
pm_pre = PassManager(
    [
        FindCommutingPauliEvolutions(),
        Commuting2qGateRouter(swap_strat),
        SetLayout(initial_layout),
        FullAncillaAllocation(backend_cmap),
        EnlargeWithAncilla(),
        ApplyLayout(),
    ]
)
 
# Insert swap gates, map to initial_layout and finally enlarge with ancilla.
pm_pre.run(circ).draw("mpl")

This pass manager relies on the current_layout which corresponds to the qubit layout as swap gates are applied. The pass will traverse all nodes in the dag. If a node should be routed using a swap strategy then it will be decomposed into sub-instructions with swap layers in between and the current_layout will be modified. Nodes that should not be routed using swap strategies will be added back to the dag taking the current_layout into account.

Parameters

• swap_strategy – An instance of a SwapStrategy that holds the swap layers that are used, and the order in which to apply them, to map the instruction to the hardware. If this field is not given, it should be contained in the property set of the pass. This allows other passes to determine the most appropriate swap strategy at run-time.
• edge_coloring – An optional edge coloring of the coupling map (I.e. no two edges that share a node have the same color). If the edge coloring is given then the commuting gates that can be simultaneously applied given the current qubit permutation are grouped according to the edge coloring and applied according to this edge coloring. Here, a color is an int which is used as the index to define and access the groups of commuting gates that can be applied simultaneously. If the edge coloring is not given then the sets will be built-up using a greedy algorithm. The edge coloring is useful to position gates such as RZZGates next to swap gates to exploit CX cancellations.

Attributes

is_analysis_pass

is_transformation_pass

Methods

execute

name

run

swap_decompose

update_status