Optimize `ConsolidateBlocks` pass (#10365)

* Initial: Introducing speedup by calculating matrix

* Lint: Reformat using Black

* Fix: Added `basis_count` and `outside_basis`

* Fix: Use `Operator` if `to_matrix` is unavailable.

* Fix: Default to Operator when necessary
- Will default to previous method when blocks have more than 2 qubits.
    - This is a temporary measure.

* Lint: Removed Cyclic import

* Docs: Added release note.

* Docs: Added reference to the issue.

* Fix: Move `block_to_matrix` to ~.passes.utils

* Lint: Fixed cyclical import and order

* CI: Removed type checking

* Add: Exceptions on `_block_to_matrix`

* Docs: Fix release note.

* Fix: Change import path for block_to_matrix

* Update qiskit/transpiler/passes/utils/block_to_matrix.py

---------

Co-authored-by: ewinston <ewinston@us.ibm.com>

qiskit/transpiler/passes/optimization/consolidate_blocks.py

@@ -26,6 +26,7 @@ from qiskit.transpiler.basepasses import TransformationPass
 from qiskit.circuit.controlflow import ControlFlowOp
 from qiskit.transpiler.passmanager import PassManager
 from qiskit.transpiler.passes.synthesis import unitary_synthesis
+from qiskit.transpiler.passes.utils import _block_to_matrix
 from .collect_1q_runs import Collect1qRuns
 from .collect_2q_blocks import Collect2qBlocks
 
@@ -110,19 +111,24 @@ class ConsolidateBlocks(TransformationPass):
                     if isinstance(nd, DAGOpNode) and getattr(nd.op, "condition", None):
                         block_cargs |= set(getattr(nd.op, "condition", None)[0])
                     all_block_gates.add(nd)
-                q = QuantumRegister(len(block_qargs))
-                qc = QuantumCircuit(q)
-                if block_cargs:
-                    c = ClassicalRegister(len(block_cargs))
-                    qc.add_register(c)
                 block_index_map = self._block_qargs_to_indices(block_qargs, global_index_map)
                 for nd in block:
                     if nd.op.name == basis_gate_name:
                         basis_count += 1
                     if self._check_not_in_basis(nd.op.name, nd.qargs, global_index_map):
                         outside_basis = True
+                if len(block_qargs) > 2:
+                    q = QuantumRegister(len(block_qargs))
+                    qc = QuantumCircuit(q)
+                    if block_cargs:
+                        c = ClassicalRegister(len(block_cargs))
+                        qc.add_register(c)
+                    for nd in block:
                         qc.append(nd.op, [q[block_index_map[i]] for i in nd.qargs])
                     unitary = UnitaryGate(Operator(qc))
+                else:
+                    matrix = _block_to_matrix(block, block_index_map)
+                    unitary = UnitaryGate(matrix)
 
                 max_2q_depth = 20  # If depth > 20, there will be 1q gates to consolidate.
                 if (  # pylint: disable=too-many-boolean-expressions

qiskit/transpiler/passes/utils/__init__.py

@@ -32,3 +32,4 @@ from .minimum_point import MinimumPoint
 
 # Utility functions
 from . import control_flow
+from .block_to_matrix import _block_to_matrix

qiskit/transpiler/passes/utils/block_to_matrix.py

@@ -0,0 +1,70 @@
+# This code is part of Qiskit.
+#
+# (C) Copyright IBM 2017, 2018.
+#
+# 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.
+
+"""Converts any block of 2 qubit gates into a matrix."""
+
+from numpy import identity, kron
+from qiskit.circuit.library import SwapGate
+from qiskit.quantum_info import Operator
+from qiskit.exceptions import QiskitError
+
+
+SWAP_GATE = SwapGate()
+SWAP_MATRIX = SWAP_GATE.to_matrix()
+IDENTITY = identity(2, dtype=complex)
+
+
+def _block_to_matrix(block, block_index_map):
+    """
+    The function converts any sequence of operations between two qubits into a matrix
+    that can be utilized to create a gate or a unitary.
+
+    Args:
+        block (List(DAGOpNode)): A block of operations on two qubits.
+        block_index_map (dict(Qubit, int)): The mapping of the qubit indices in the main circuit.
+
+    Returns:
+        NDArray: Matrix representation of the block of operations.
+    """
+    block_index_length = len(block_index_map)
+    if block_index_length != 2:
+        raise QiskitError(
+            "This function can only operate with blocks of 2 qubits."
+            + f"This block had {block_index_length}"
+        )
+    matrix = identity(2**block_index_length, dtype=complex)
+    for node in block:
+        try:
+            current = node.op.to_matrix()
+        except QiskitError:
+            current = Operator(node.op).data
+        q_list = [block_index_map[qubit] for qubit in node.qargs]
+        if len(q_list) > 2:
+            raise QiskitError(
+                f"The operation {node.op.name} in this block has "
+                + f"{len(q_list)} qubits, only 2 max allowed."
+            )
+        basis_change = False
+        if len(q_list) < block_index_length:
+            if q_list[0] == 1:
+                current = kron(current, IDENTITY)
+            else:
+                current = kron(IDENTITY, current)
+        else:
+            if q_list[0] > q_list[1]:
+                if node.op != SWAP_GATE:
+                    basis_change = True
+        if basis_change:
+            matrix = (SWAP_MATRIX @ current) @ (SWAP_MATRIX @ matrix)
+        else:
+            matrix = current @ matrix
+    return matrix

releasenotes/notes/optimize-consolidate-blocks-3ea60c18bc546273.yaml

@@ -0,0 +1,14 @@
+---
+features:
+  - |
+    Added utility function :func:`qiskit.transpiler.passes.utils._block_to_matrix` that can 
+    generate a matrix based on a block of operations between two qubits. This function can
+    be used in transpiler passes that work on some decomposed circuits such as :class:`.ConsolidateBlocks`. 
+fixes:
+  - |
+    Reduced overhead of the :class:`.ConsolidateBlocks` pass by performing matrix operations
+    on all two-qubit blocks using :func:`qiskit.transpiler.passes.utils._block_to_matrix` 
+    instead of creating an instance of :class:`.QuantumCircuit` and passing it to an :class:`.Operator`. 
+    The speedup will only be applicable when consolidating two-qubit blocks. Anything higher 
+    than that will still be handled by the :class:`.Operator` class.
+    Check `#8779 <https://github.com/Qiskit/qiskit-terra/issues/8779>`__ for details.