Bug: unwanted addition of classical bits via `measure_all`

[Matteo Paltenghi]

Where does the issue happen?

https://qiskit.org/textbook/ch-appendix/qiskit.html

What is the content issue?

Environment

• qiskit.version: 0.22.0
• Python version: 3.8.0
• Operating system: Ubuntu 18.04

What is happening?

In the notebook qiskit.ipynb the measure_all api call creates a register even if the qc already has a classical register of size 5. This is due to the default behavior of measure_all which always insert a new register if not explicitly mentioned otherwise.

Screenshots

How can we reproduce the issue?

Run the cell with the following code:

qc = QuantumCircuit(5,5)
qc.x(0)
for q in range(4):
    qc.cx(0,q+1)
qc.measure_all()
qc.draw()

Output:

        ┌───┐                     ░ ┌─┐
   q_0: ┤ X ├──■────■────■────■───░─┤M├────────────
        └───┘┌─┴─┐  │    │    │   ░ └╥┘┌─┐
   q_1: ─────┤ X ├──┼────┼────┼───░──╫─┤M├─────────
             └───┘┌─┴─┐  │    │   ░  ║ └╥┘┌─┐
   q_2: ──────────┤ X ├──┼────┼───░──╫──╫─┤M├──────
                  └───┘┌─┴─┐  │   ░  ║  ║ └╥┘┌─┐
   q_3: ───────────────┤ X ├──┼───░──╫──╫──╫─┤M├───
                       └───┘┌─┴─┐ ░  ║  ║  ║ └╥┘┌─┐
   q_4: ────────────────────┤ X ├─░──╫──╫──╫──╫─┤M├
                            └───┘ ░  ║  ║  ║  ║ └╥┘
   c: 5/═════════════════════════════╬══╬══╬══╬══╬═
                                     ║  ║  ║  ║  ║
meas: 5/═════════════════════════════╩══╩══╩══╩══╩═
                                     0  1  2  3  4

Where two circuits are present: c and meas. The first one is the one we created, the second one is the one created by measure_all.

What should happen?

I would expect the existing classical register to be used instead to avoid waste of space.

Additional information

Any suggestions?

Following the documentation of the measure_all method, it is possible to measure the qubits directly, without adding an extra register. The code should be changed to:

qc.measure_all(add_bits=False)

[Michael Papadopoulos]

I can handle this, should be a quick fix

[Michael Papadopoulos]

@MattePalte Can you link the corresponding page on the new textbook, or move this issue over to the old textbook repository https://github.com/qiskit-community/qiskit-textbook

[Matteo Paltenghi]

If you mean the link to the actual file, you can find it by clicking the filename qiskit.ipynb in my issue or here.

If I misunderstood, please let me know which specific link you need.
Thanks in advance

[//va]

just for clarification

in regards to the legacy textbook:
the notebook for this page: https://qiskit.org/textbook/ch-appendix/qiskit.html
is located here: https://github.com/qiskit-community/qiskit-textbook/blob/main/content/ch-appendix/qiskit.ipynb

for the (newer) textbook beta:
this notebook: https://github.com/Qiskit/platypus/blob/main/notebooks/ch-appendix/qiskit.ipynb
is for this page: https://learn.qiskit.org/course/ch-appendix/basic-qiskit-syntax

issues for the legacy textbook should be added to the repo: https://github.com/qiskit-community/qiskit-textbook
and issue for the newer textbook should be added in this repo

[//va]

This repository is no longer maintained. It has been superseded by IBM Quantum Learning.