Fixes Clifford, Pauli, CNOTDihedral to handle Delay (#7850)

* Fixes Clifford,Pauli,CNOTDihedreal to handle Delay

Fixes the Clifford, Pauli and CNOTDihedreal operator classes to handle conversion of circuits containing Delay instructions. These instructions are ignored (treated as identity) when converting to operators.

* black

Co-authored-by: mergify[bot] <37929162+mergify[bot]@users.noreply.github.com>
(cherry picked from commit 9c91b6e)

qiskit/quantum_info/operators/dihedral/dihedral_circuits.py

@@ -17,6 +17,8 @@
 
 from qiskit.exceptions import QiskitError
 from qiskit.circuit import QuantumCircuit
+from qiskit.circuit.barrier import Barrier
+from qiskit.circuit.delay import Delay
 
 
 def _append_circuit(elem, circuit, qargs=None):
@@ -35,6 +37,9 @@ def _append_circuit(elem, circuit, qargs=None):
     if qargs is None:
         qargs = list(range(elem.num_qubits))
 
+    if isinstance(circuit, (Barrier, Delay)):
+        return elem
+
     if isinstance(circuit, QuantumCircuit):
         gate = circuit.to_instruction()
     else:
@@ -81,6 +86,8 @@ def _append_circuit(elem, circuit, qargs=None):
     }
 
     for instr, qregs, _ in gate.definition:
+        if isinstance(instr, (Barrier, Delay)):
+            continue
         # Get the integer position of the flat register
         new_qubits = [qargs[bit_indices[tup]] for tup in qregs]
 

qiskit/quantum_info/operators/symplectic/base_pauli.py

@@ -20,6 +20,7 @@
 
 from qiskit.circuit import QuantumCircuit
 from qiskit.circuit.barrier import Barrier
+from qiskit.circuit.delay import Delay
 from qiskit.exceptions import QiskitError
 from qiskit.quantum_info.operators.base_operator import BaseOperator
 from qiskit.quantum_info.operators.mixins import AdjointMixin, MultiplyMixin
@@ -507,7 +508,7 @@ def _append_circuit(self, circuit, qargs=None):
         Raises:
             QiskitError: if input gate cannot be decomposed into Clifford gates.
         """
-        if isinstance(circuit, Barrier):
+        if isinstance(circuit, (Barrier, Delay)):
             return self
 
         if qargs is None:

qiskit/quantum_info/operators/symplectic/clifford_circuits.py

@@ -16,6 +16,7 @@
 from qiskit.exceptions import QiskitError
 from qiskit.circuit import QuantumCircuit
 from qiskit.circuit.barrier import Barrier
+from qiskit.circuit.delay import Delay
 
 
 def _append_circuit(clifford, circuit, qargs=None):
@@ -32,7 +33,7 @@ def _append_circuit(clifford, circuit, qargs=None):
     Raises:
         QiskitError: if input gate cannot be decomposed into Clifford gates.
     """
-    if isinstance(circuit, Barrier):
+    if isinstance(circuit, (Barrier, Delay)):
         return clifford
 
     if qargs is None:

qiskit/quantum_info/operators/symplectic/pauli.py

@@ -22,6 +22,7 @@
 
 from qiskit.circuit import Instruction, QuantumCircuit
 from qiskit.circuit.barrier import Barrier
+from qiskit.circuit.delay import Delay
 from qiskit.circuit.library.generalized_gates import PauliGate
 from qiskit.circuit.library.standard_gates import IGate, XGate, YGate, ZGate
 from qiskit.exceptions import QiskitError
@@ -681,7 +682,7 @@ def _from_circuit(cls, instr):
                 raise QiskitError(
                     f"Cannot apply instruction with classical registers: {dinstr.name}"
                 )
-            if not isinstance(dinstr, Barrier):
+            if not isinstance(dinstr, (Barrier, Delay)):
                 next_instr = BasePauli(*cls._from_circuit(dinstr))
                 if next_instr is not None:
                     qargs = [tup.index for tup in qregs]

releasenotes/notes/clifford_delay-be1a835413e2531e.yaml

@@ -0,0 +1,7 @@
+---
+fixes:
+  - |
+    Fixes the :class:`.Clifford`, :class:`.Pauli` and :class:`.CNOTDihedral`
+    operators support initializing from compatible circuits that contain
+    :class:`~qiskit.circuit.Delay` instructions. These instructions are
+    treated as identity when converting to operators.

test/python/quantum_info/operators/symplectic/test_clifford.py

@@ -392,6 +392,21 @@ def test_2_qubit_relations(self):
             cliff = _append_circuit(cliff, "sdg", [0])
             self.assertEqual(cliff, cliff1)
 
+    def test_barrier_delay_sim(self):
+        """Test barrier and delay instructions can be simulated"""
+        target_circ = QuantumCircuit(2)
+        target_circ.h(0)
+        target_circ.cx(0, 1)
+        target = Clifford(target_circ)
+
+        circ = QuantumCircuit(2)
+        circ.h(0)
+        circ.delay(100, 0)
+        circ.barrier([0, 1])
+        circ.cx(0, 1)
+        value = Clifford(circ)
+        self.assertEqual(value, target)
+
 
 @ddt
 class TestCliffordSynthesis(QiskitTestCase):

test/python/quantum_info/operators/symplectic/test_pauli.py

@@ -21,6 +21,7 @@
 import numpy as np
 from ddt import ddt, data, unpack
 
+from qiskit import QuantumCircuit
 from qiskit.exceptions import QiskitError
 from qiskit.circuit.library import (
     IGate,
@@ -413,6 +414,21 @@ def test_evolve_clifford_qargs(self):
         self.assertEqual(value, value_h)
         self.assertEqual(value_inv, value_s)
 
+    def test_barrier_delay_sim(self):
+        """Test barrier and delay instructions can be simulated"""
+        target_circ = QuantumCircuit(2)
+        target_circ.x(0)
+        target_circ.y(1)
+        target = Pauli(target_circ)
+
+        circ = QuantumCircuit(2)
+        circ.x(0)
+        circ.delay(100, 0)
+        circ.barrier([0, 1])
+        circ.y(1)
+        value = Pauli(circ)
+        self.assertEqual(value, target)
+
 
 if __name__ == "__main__":
     unittest.main()

test/python/quantum_info/operators/test_dihedral.py

@@ -716,6 +716,21 @@ def test_init_from_pauli(self):
                 target = Operator(elem)
                 self.assertTrue(value.equiv(target), "Error: Pauli operator is not the same.")
 
+    def test_barrier_delay_sim(self):
+        """Test barrier and delay instructions can be simulated"""
+        target_circ = QuantumCircuit(2)
+        target_circ.x(0)
+        target_circ.cx(0, 1)
+        target = CNOTDihedral(target_circ)
+
+        circ = QuantumCircuit(2)
+        circ.x(0)
+        circ.delay(100, 0)
+        circ.barrier([0, 1])
+        circ.cx(0, 1)
+        value = CNOTDihedral(circ)
+        self.assertEqual(value, target)
+
 
 if __name__ == "__main__":
     unittest.main()