DAG.width needs to be updated to match QuantumCircuit definition

[nonhermitian]

Information

• Qiskit Terra version: master
• Python version:
• Operating system:

What is the current behavior?

qr = QuantumRegister(5,'qr')
cr = ClassicalRegister(5, 'cr')
ghz = QuantumCircuit(qr, cr, name='ghz')

ghz.h(qr[2])
ghz.cx(qr[2], qr[1])
ghz.cx(qr[1], qr[0])
ghz.cx(qr[2], qr[3])
ghz.cx(qr[3], qr[4])
ghz.draw()

ghz_dag = circuit_to_dag(ghz)

print(ghz.width(), ghz_dag.width())

gives: (10, 5).

Steps to reproduce the problem

What is the expected behavior?

Suggested solutions

DAG width needs to count both qubits and cbits for self-consistency.

[jwoehr]

qiskit.dagcircuit.dagcircuit defines width() as

    def width(self):
        """Return the total number of qubits used by the circuit."""
        return len(self.wires) - self.num_cbits()

There may be code that depends on the semantic of width() == number of qubits

[jwoehr]

Fulfilling the literal request of this ticket would be as simple as removing the - self.num_cbits() as shown in my previous comment. But obviously the coder believed that width means number of qubits. How to resolve this? Just change the code? Look for usages?

[jwoehr]

The only test of dagcircuit.width() I have found is in qiskit-terra/test/python/test_dagcircuit.py and it doesn't set any cbits.

class TestCircuitProperties(QiskitTestCase):
    """DAGCircuit properties test."""

    def setUp(self):
        qr1 = QuantumRegister(4)
        qr2 = QuantumRegister(2)
        circ = QuantumCircuit(qr1, qr2)
        circ.h(qr1[0])
        circ.cx(qr1[2], qr1[3])
        circ.h(qr1[2])
        circ.t(qr1[2])
        circ.ch(qr1[2], qr1[1])
        circ.u2(0.1, 0.2, qr1[3])
        circ.ccx(qr2[0], qr2[1], qr1[0])

        self.dag = circuit_to_dag(circ)

...

    def test_circuit_width(self):
        """Test number of qubits in circuit."""
        self.assertEqual(self.dag.width(), 6)

However, in qiskit/transpiler/passes/width.py the dagcircuit.width() member function seems to be used as-is, that is, only counting qubits:

from qiskit.transpiler.basepasses import AnalysisPass


class Width(AnalysisPass):
    """ An analysis pass for calculating the width of a DAG circuit.
    """

    def run(self, dag):
        self.property_set['width'] = dag.width()

So does changing qiskit.dagcircuit.dagcircuit.width() to count cbits as well break the transpiler?

[nonhermitian]

It probably breaks several things which is why it has not been implemented yet.

[jwoehr]

Ah, so maybe the trick is to

1. Define dagcircuit.qubit_width()
2. Sift through all the verticals looking for current invocations of dagcircuit.width() and change them to dagcircuit.qubit_width()

??

[jwoehr]

First step: coded replacement function.

Next, have to look everywhere for usage of DAGCircuit.width() and replace with DAGCircuit.num_qbits().

Then when invocations are all changed, DAGCircuit.width() can be changed to return the total number of bits.

[jwoehr]

Per @ajavadia 's comments after merge of #2708 I'll proceed to

1. Search for invocations or mentions of DAGCircuit.num_cbits() and change to DAGCircuit.num_clbits()

• qiskit-tutorials/qiskit/terra/advanced_circuits.ipynb
• qiskit-aqua/qiskit/aqua/utils/circuit_utils.py
• qiskit/docs/terra/custom_gates.rst

2. Remove DAGCircuit.num_cbits()
3. Search for current usage of DAGCircuit.width() and replace with DAGCircuit.num_qbits()
4. After such invocations are changed, DAGCircuit.width() can be changed to return the total number of bits (qubits + clbits) fulfilling the intent of this issue.

[jwoehr]

The pull requests #2807 and qiskit-aqua number 613 should complete this issue.

[jwoehr]

It's now pull requests #2812 and aqua pr 614

[jwoehr]

I'll make the transpiler pass changes requested in #2812 on 2019-07-22

[jwoehr]

I have pushed the transpiler pass changes requested in #2812