Fixes circuit_to_instruction() method for circuits containing single bit conditions

qiskit/converters/circuit_to_instruction.py

@@ -15,7 +15,7 @@
 from qiskit.exceptions import QiskitError
 from qiskit.circuit.instruction import Instruction
 from qiskit.circuit.quantumregister import QuantumRegister
-from qiskit.circuit.classicalregister import ClassicalRegister
+from qiskit.circuit.classicalregister import ClassicalRegister, Clbit
 
 
 def circuit_to_instruction(circuit, parameter_map=None, equivalence_library=None, label=None):
@@ -113,7 +113,16 @@ def circuit_to_instruction(circuit, parameter_map=None, equivalence_library=None
         condition = rule[0].condition
         if condition:
             reg, val = condition
-            if reg.size == c.size:
+            if isinstance(reg, Clbit):
+                idx = 0
+                for creg in circuit.cregs:
+                    if reg not in creg:
+                        idx += creg.size
+                    else:
+                        cond_reg = creg
+                        break
+                rule[0].condition = (c[idx + list(cond_reg).index(reg)], val)
+            elif reg.size == c.size:
                 rule[0].condition = (c, val)
             else:
                 raise QiskitError(

qiskit/visualization/utils.py

@@ -21,6 +21,7 @@
 from qiskit.quantum_info.operators.symplectic import PauliTable, SparsePauliOp
 from qiskit.visualization.exceptions import VisualizationError
 from qiskit.circuit import Measure, ControlledGate, Gate, Instruction, Delay, BooleanExpression
+from qiskit.circuit import Clbit
 from qiskit.circuit.tools import pi_check
 from qiskit.exceptions import MissingOptionalLibraryError
 
@@ -298,6 +299,7 @@ def __init__(self, dag, justification, measure_map, reverse_bits):
         self.qubits = dag.qubits
         self.justification = justification
         self.measure_map = measure_map
+        self.cregs = [self.dag.cregs[reg] for reg in self.dag.cregs]
         self.reverse_bits = reverse_bits
 
         if self.justification == "left":
@@ -347,7 +349,11 @@ def slide_from_left(self, node, index):
             last_insertable_index = -1
             index_stop = -1
             if node.op.condition:
-                index_stop = self.measure_map[node.op.condition[0]]
+                if isinstance(node.op.condition[0], Clbit):
+                    cond_reg = [creg for creg in self.cregs if node.op.condition[0] in creg]
+                    index_stop = self.measure_map[cond_reg[0]]
+                else:
+                    index_stop = self.measure_map[node.op.condition[0]]
             elif node.cargs:
                 for carg in node.cargs:
                     try:

test/python/converters/test_circuit_to_instruction.py

@@ -43,6 +43,30 @@ def test_flatten_circuit_registers(self):
         self.assertEqual(inst.definition[1][1], [q[7]])
         self.assertEqual(inst.definition[1][2], [c[4]])
 
+    def test_flatten_registers_of_circuit_single_bit_cond(self):
+        """Check correct mapping of registers gates conditioned on single classical bits."""
+        qr1 = QuantumRegister(2, "qr1")
+        qr2 = QuantumRegister(3, "qr2")
+        cr1 = ClassicalRegister(3, "cr1")
+        cr2 = ClassicalRegister(3, "cr2")
+        circ = QuantumCircuit(qr1, qr2, cr1, cr2)
+        circ.h(qr1[0]).c_if(cr1[1], True)
+        circ.h(qr2[1]).c_if(cr2[0], False)
+        circ.cx(qr1[1], qr2[2]).c_if(cr2[2], True)
+        circ.measure(qr2[2], cr2[0])
+
+        inst = circuit_to_instruction(circ)
+        q = QuantumRegister(5, "q")
+        c = ClassicalRegister(6, "c")
+
+        self.assertEqual(inst.definition[0][1], [q[0]])
+        self.assertEqual(inst.definition[1][1], [q[3]])
+        self.assertEqual(inst.definition[2][1], [q[1], q[4]])
+
+        self.assertEqual(inst.definition[0][0].condition, (c[1], True))
+        self.assertEqual(inst.definition[1][0].condition, (c[3], False))
+        self.assertEqual(inst.definition[2][0].condition, (c[5], True))
+
     def test_flatten_parameters(self):
         """Verify parameters from circuit are moved to instruction.params"""
         qr = QuantumRegister(3, "qr")