Use automatic partition_labels in new CutWire how-to (#370)

* Add single-qubit wire cutting how-to, and `expand_observables` function * Don't unpack the result of `partition_problem` * Use automatic `partition_labels` in new `CutWire` how-to * peer review https://github.com/Qiskit-Extensions/circuit-knitting-toolbox/pull/368/files#r1297182836 * Apply suggestions from code review Co-authored-by: Caleb Johnson <[email protected]> * Clarify superset of `Qubit`s * Fix lint * arXiv -> link * Run notebook cells * Update prose in how-to * Update text in wire cutting tutorial to mention how-to * Forgot the save before committing * Add link to Lukas Brenner, Christophe Piveteau, David Sutter paper * Rename `cut_wire_to_move` to `wire_cutting_transforms` * Fix double underscore #370 (comment) --------- Co-authored-by: Caleb Johnson <[email protected]>
Qiskit · Aug 18, 2023 · dbd778c · dbd778c
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diff --git a/circuit_knitting/cutting/__init__.py b/circuit_knitting/cutting/__init__.py
@@ -21,7 +21,7 @@
     :toctree: ../stubs/
     :nosignatures:
 
-    transform_cuts_to_moves
+    cut_wires
     expand_observables
     partition_circuit_qubits
     partition_problem
@@ -88,7 +88,7 @@
 )
 from .cutting_evaluation import execute_experiments, CuttingExperimentResults
 from .cutting_reconstruction import reconstruct_expectation_values
-from .cut_wire_to_move import transform_cuts_to_moves, expand_observables
+from .wire_cutting_transforms import cut_wires, expand_observables
 
 __all__ = [
     "partition_circuit_qubits",
@@ -99,6 +99,6 @@
     "reconstruct_expectation_values",
     "PartitionedCuttingProblem",
     "CuttingExperimentResults",
-    "transform_cuts_to_moves",
+    "cut_wires",
     "expand_observables",
 ]
diff --git a/circuit_knitting/cutting/cut_wire_to_move.py → ...itting/cutting/wire_cutting_transforms.py b/circuit_knitting/cutting/cut_wire_to_move.py → ...itting/cutting/wire_cutting_transforms.py
@@ -13,17 +13,40 @@
 """Function to transform a :class:`.CutWire` instruction to a :class:`.Move` instruction."""
 from __future__ import annotations
 
+from typing import Callable
 from itertools import groupby
 
 import numpy as np
-from qiskit.circuit import Qubit, QuantumCircuit
+from qiskit.circuit import Qubit, QuantumCircuit, Operation
 from qiskit.circuit.exceptions import CircuitError
 from qiskit.quantum_info import PauliList
 
-from .instructions.move import Move
+from .instructions import Move
+from .qpd.instructions import TwoQubitQPDGate
 
 
-def transform_cuts_to_moves(circuit: QuantumCircuit, /) -> QuantumCircuit:
+def cut_wires(circuit: QuantumCircuit, /) -> QuantumCircuit:
+    r"""Transform all :class:`.CutWire` instructions in a circuit to :class:`.Move` instructions marked for cutting.
+
+    The returned circuit will have one newly allocated qubit for every :class:`.CutWire` instruction.
+
+    See Sec. 3 and Appendix A of `2302.03366v1
+    <https://arxiv.org/abs/2302.03366v1>`__ for more information about the two
+    different representations of wire cuts: single-qubit (:class:`.CutWire`)
+    vs. two-qubit (:class:`.Move`).
+
+    Args:
+        circuit: Original circuit with :class:`.CutWire` instructions
+
+    Returns:
+        circuit: New circuit with :class:`.CutWire` instructions replaced by :class:`.Move` instructions wrapped in :class:`TwoQubitQPDGate`\ s
+    """
+    return _transform_cut_wires(
+        circuit, lambda: TwoQubitQPDGate.from_instruction(Move())
+    )
+
+
+def _transform_cuts_to_moves(circuit: QuantumCircuit, /) -> QuantumCircuit:
     """Transform all :class:`.CutWire` instructions in a circuit to :class:`.Move` instructions.
 
     Args:
@@ -32,6 +55,12 @@ def transform_cuts_to_moves(circuit: QuantumCircuit, /) -> QuantumCircuit:
     Returns:
         circuit: New circuit with :class:`.CutWire` instructions replaced by :class`.Move` instructions
     """
+    return _transform_cut_wires(circuit, Move)
+
+
+def _transform_cut_wires(
+    circuit: QuantumCircuit, factory: Callable[[], Operation], /
+) -> QuantumCircuit:
     new_circuit, mapping = _circuit_structure_mapping(circuit)
 
     for instructions in circuit.data:
@@ -40,7 +69,7 @@ def transform_cuts_to_moves(circuit: QuantumCircuit, /) -> QuantumCircuit:
         if instructions in circuit.get_instructions("cut_wire"):
             # Replace cut_wire with move instruction
             new_circuit.compose(
-                other=Move(),
+                other=factory(),
                 qubits=[mapping[gate_index[0]], mapping[gate_index[0]] + 1],
                 inplace=True,
             )

diff --git a/docs/circuit_cutting/how-tos/how_to_specify_cut_wires.ipynb b/docs/circuit_cutting/how-tos/how_to_specify_cut_wires.ipynb
diff --git a/docs/circuit_cutting/tutorials/03_wire_cutting_via_move_instruction.ipynb b/docs/circuit_cutting/tutorials/03_wire_cutting_via_move_instruction.ipynb
@@ -127,9 +127,11 @@
    "source": [
     "### Create a new circuit where `Move` instructions have been placed at the desired cut locations\n",
     "\n",
-    "Given the above circuit, we would like to place two wire cuts on the middle qubit line, so that the circuit can separate into two circuits of four qubits each.  One way to do this (currently, the only way) is to place two-qubit `Move` instructions that move the state from one qubit wire to another.  A `Move` instruction is conceptually equivalent to a reset operation on the second qubit, followed by a SWAP gate.  The effect of this instruction is to transfer the state of the first (source) qubit to the second (detination) qubit, while discarding the incoming state of the second qubit.  For this to work as intended, it is important that the second (destination) qubit share no entanglement with the remainder of the system; otherwise, the reset operation will cause the state of the remainder of the system to be partially collapsed.\n",
+    "Given the above circuit, we would like to place two wire cuts on the middle qubit line, so that the circuit can separate into two circuits of four qubits each.  One way to do this is to manually place two-qubit `Move` instructions that move the state from one qubit wire to another.  A `Move` instruction is conceptually equivalent to a reset operation on the second qubit, followed by a SWAP gate.  The effect of this instruction is to transfer the state of the first (source) qubit to the second (detination) qubit, while discarding the incoming state of the second qubit.  For this to work as intended, it is important that the second (destination) qubit share no entanglement with the remainder of the system; otherwise, the reset operation will cause the state of the remainder of the system to be partially collapsed.\n",
     "\n",
-    "Here, we build a new circuit with one additional qubit and the `Move` operations in place.  In this example, we are able to reuse a qubit: the source qubit of the first `Move` becomes the destination qubit of the second `Move` operation."
+    "Here, we build a new circuit with one additional qubit and the `Move` operations in place.  In this example, we are able to reuse a qubit: the source qubit of the first `Move` becomes the destination qubit of the second `Move` operation.\n",
+    "\n",
+    "Note: As an alternative to working directly with `Move` instructions, one may choose to mark wire cuts using a single-qubit `CutWire` instruction.  The `cut_wires` function exists to transform `CutWire`s to `Move` instructions on newly allocated qubits.  However, in contrast to the manual method, this automatic method does not allow for the re-use of qubit wires.  See the `CutWire` [how-to guide](../how-tos/how_to_specify_cut_wires.ipynb) for details."
    ]
   },
   {

diff --git a/test/cutting/test_cut_wire_to_move.py → test/cutting/test_wire_cutting_transforms.py b/test/cutting/test_cut_wire_to_move.py → test/cutting/test_wire_cutting_transforms.py
@@ -17,7 +17,9 @@
 from qiskit.circuit import QuantumCircuit, QuantumRegister, Qubit, ClassicalRegister
 from qiskit.quantum_info import PauliList
 from circuit_knitting.cutting.instructions import Move, CutWire
-from circuit_knitting.cutting import transform_cuts_to_moves, expand_observables
+from circuit_knitting.cutting.qpd.instructions import TwoQubitQPDGate
+from circuit_knitting.cutting import cut_wires, expand_observables
+from circuit_knitting.cutting.wire_cutting_transforms import _transform_cuts_to_moves
 
 
 @fixture
@@ -207,7 +209,7 @@ def resulting_circuit4() -> tuple[QuantumCircuit, list[int]]:
 )
 def test_transform_cuts_to_moves(request, sample_circuit, resulting_circuit):
     """Tests the transformation of CutWire to Move instruction."""
-    assert request.getfixturevalue(resulting_circuit)[0] == transform_cuts_to_moves(
+    assert request.getfixturevalue(resulting_circuit)[0] == _transform_cuts_to_moves(
         request.getfixturevalue(sample_circuit)
     )
 
@@ -226,7 +228,7 @@ def test_circuit_mapping(request, sample_circuit, resulting_circuit):
     sample_circuit = request.getfixturevalue(sample_circuit)
     resulting_mapping = request.getfixturevalue(resulting_circuit)[1]
 
-    final_circuit = transform_cuts_to_moves(sample_circuit)
+    final_circuit = _transform_cuts_to_moves(sample_circuit)
     final_mapping = [
         final_circuit.find_bit(qubit).index for qubit in sample_circuit.qubits
     ]
@@ -249,7 +251,7 @@ def test_circuit_mapping(request, sample_circuit, resulting_circuit):
 def test_qreg_name_num(request, sample_circuit):
     """Tests the number and name of qregs in initial and final circuits."""
     sample_circuit = request.getfixturevalue(sample_circuit)
-    final_circuit = transform_cuts_to_moves(sample_circuit)
+    final_circuit = _transform_cuts_to_moves(sample_circuit)
 
     # Tests number of qregs in initial and final circuits
     assert len(sample_circuit.qregs) == len(final_circuit.qregs)
@@ -273,7 +275,7 @@ def test_qreg_name_num(request, sample_circuit):
 def test_qreg_size(request, sample_circuit):
     """Tests the size of qregs in initial and final circuits."""
     sample_circuit = request.getfixturevalue(sample_circuit)
-    final_circuit = transform_cuts_to_moves(sample_circuit)
+    final_circuit = _transform_cuts_to_moves(sample_circuit)
 
     # Tests size of qregs in initial and final circuits
     for sample_qreg, final_qreg in zip(
@@ -295,7 +297,7 @@ def test_qreg_size(request, sample_circuit):
 def test_circuit_width(request, sample_circuit):
     """Tests the width of the initial and final circuits."""
     sample_circuit = request.getfixturevalue(sample_circuit)
-    final_circuit = transform_cuts_to_moves(sample_circuit)
+    final_circuit = _transform_cuts_to_moves(sample_circuit)
     total_cut_wire = len(sample_circuit.get_instructions("cut_wire"))
 
     # Tests width of initial and final circuit
@@ -314,7 +316,7 @@ def test_circuit_width(request, sample_circuit):
 def test_creg(request, sample_circuit):
     """Tests the number and size of cregs in the initial and final circuits."""
     sample_circuit = request.getfixturevalue(sample_circuit)
-    final_circuit = transform_cuts_to_moves(sample_circuit)
+    final_circuit = _transform_cuts_to_moves(sample_circuit)
 
     # Tests number of cregs in initial and final circuits
     assert len(sample_circuit.cregs) == len(final_circuit.cregs)
@@ -326,6 +328,19 @@ def test_creg(request, sample_circuit):
         assert sample_creg.size == final_creg.size
 
 
+def test_cut_wires():
+    qc = QuantumCircuit(2)
+    qc.h(0)
+    qc.h(1)
+    qc.append(CutWire(), [1])
+    qc.s(0)
+    qc.s(1)
+    qc_out = cut_wires(qc)
+    qpd_gate = qc_out.data[2].operation
+    assert isinstance(qpd_gate, TwoQubitQPDGate)
+    assert qpd_gate.label == "cut_move"
+
+
 class TestExpandObservables:
     def test_expand_observables(self):
         qc0 = QuantumCircuit(3)