[BUG] `simplify` unexpectedly removes all wires with the Identity observable

[SimoneGasperini]

Hello PennyLane Core team! Hope you're all doing well 🥲
I've just found this little bug while working with PennyLane. If you could take a look at it, that would be great, thanks!

Expected behavior

I expect the qml.simplify function to always return a LinearCombination with the same wires of the original object.

Actual behavior

If I call the function on an operator created from the Identity, I'm returned a new object with no wires at all:

>>> qml.simplify(qml.Hamiltonian([1.23], [qml.I(0)])).wires
Wires([])

Additional information

This doesn't happen if I create the same operator as a product between the coefficient and the observable:

>>> qml.simplify(1.23 * qml.I(0)).wires
Wires([0])

System information

Name: pennylane
Version: 0.43.1
Summary: PennyLane is a cross-platform Python library for quantum computing, quantum machine learning, and quantum chemistry. Train a quantum computer the same way as a neural network.
Home-page:
Author:
Author-email:
License-Expression: Apache-2.0
Location: /home/simone/miniconda3/lib/python3.12/site-packages
Requires: appdirs, autograd, autoray, cachetools, diastatic-malt, networkx, numpy, packaging, pennylane-lightning, requests, rustworkx, scipy, tomlkit, typing_extensions
Required-by: pennylane_lightning

Platform info:           Linux-5.15.167.4-microsoft-standard-WSL2-x86_64-with-glibc2.35
Python version:          3.12.2
Numpy version:           2.3.5
Scipy version:           1.16.3
JAX version:             None
Installed devices:
- lightning.qubit (pennylane_lightning-0.43.0)
- default.clifford (pennylane-0.43.1)
- default.gaussian (pennylane-0.43.1)
- default.mixed (pennylane-0.43.1)
- default.qubit (pennylane-0.43.1)
- default.qutrit (pennylane-0.43.1)
- default.qutrit.mixed (pennylane-0.43.1)
- default.tensor (pennylane-0.43.1)
- null.qubit (pennylane-0.43.1)
- reference.qubit (pennylane-0.43.1)

Existing GitHub issues

• I have searched existing GitHub issues to make sure the issue does not already exist.

[CatalinaAlbornoz]

Hi @SimoneGasperini , thanks for reporting this! Is this blocking work at the moment?

[SimoneGasperini]

Ciao Catalina! No, this is not a blocker for me at the moment. Here is a simple workaround:

Additional information

This doesn't happen if I create the same operator as a product between the coefficient and the observable:

>>> qml.simplify(1.23 * qml.I(0)).wires
Wires([0])

[CatalinaAlbornoz]

Thanks for confirming @SimoneGasperini!
Our team has taken note of the issue but it may take some time to fix. Please do let us know if it becomes critical at some point. And thanks again for bringing it up!