Circuit drawer reverse_bits does not reverse registerless bits

[enavarro51]

Environment

• Qiskit Terra version: Current main
• Python version: 3.8
• Operating system: Ubuntu

What is happening?

When using classical registers only with reverse_bits, the registers and the bits in the registers are reversed before display. However, bits without registers are reversed in terms of usage, but are not reversed when displayed.

bits = [Qubit(), Qubit(), Clbit(), Clbit()]
bits1 = [Clbit()]
cr = ClassicalRegister(2, "cr")
crx = ClassicalRegister(2, "crx")
qc = QuantumCircuit(bits, cr, [Clbit()], crx, bits1)
qc.x(0).c_if(crx[1], 0)
qc.measure(0, bits[3])
qc.draw('mpl', cregbundle=False, reverse_bits=True)

How can we reproduce the issue?

---

What should happen?

Registerless bits should show in descending order.

Any suggestions?

This will require the merge of #7285 before the above circuit will work.

@javabster Can you assign this to me?

[jakelishman]

Assigned, thanks!

[kdk]

I'm not very familiar with the visualization code, but is there benefit to having reverse_bits implemented separately between circuit_drawer(reverse_bits=True) and QuantumCircuit.reverse_bits (I think the former predated the latter)?

[enavarro51]

Oh, yeah. This looks problematic.

qc = QuantumCircuit(3,3)
qc.x(0).c_if(0, 1)
qc = qc.reverse_bits()
qc.draw('mpl', cregbundle=False)

or

        bits = [Qubit(), Qubit(), Clbit(), Clbit()]
        cr = ClassicalRegister(2, "cr")
        crx = ClassicalRegister(3, "cs")
        circuit = QuantumCircuit(bits, cr, [Clbit()], crx)
        circuit.x(0).c_if(crx[1], True)
        circuit.measure(0, cr[1])
        circuit = circuit.reverse_bits()
        circuit.draw('mpl', cregbundle=False)

IndexError Traceback (most recent call last)
in
5 circuit.x(0).c_if(crx[1], True)
6 circuit.measure(0, cr[1])
----> 7 circuit = circuit.reverse_bits()
8 circuit.draw('mpl', cregbundle=False)#, reverse_bits=False)

~/qiskit/qiskit-terra/qiskit/circuit/quantumcircuit.py in reverse_bits(self)
482
483 for inst, qargs, cargs in self.data:
--> 484 new_qargs = [new_qubits[num_qubits - old_qubits.index(q) - 1] for q in qargs]
485 new_cargs = [new_clbits[num_clbits - old_clbits.index(c) - 1] for c in cargs]
486 circ._append(inst, new_qargs, new_cargs)

~/qiskit/qiskit-terra/qiskit/circuit/quantumcircuit.py in (.0)
482
483 for inst, qargs, cargs in self.data:
--> 484 new_qargs = [new_qubits[num_qubits - old_qubits.index(q) - 1] for q in qargs]
485 new_cargs = [new_clbits[num_clbits - old_clbits.index(c) - 1] for c in cargs]
486 circ._append(inst, new_qargs, new_cargs)

IndexError: list index out of range