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SparsePauliOp does not yield correct results

See original GitHub issue

Environment

Qiskit Terra version:0.20.0
Python version: 3.10.4
Operating system: MacOS

What is happening?

Two Z gates on the same qubit make the identity, not a single Z gate

How can we reproduce the issue?

>>> from qiskit.quantum_info import SparsePauliOp
>>> sparse_list = [("ZZ", [0,0], 1)]
>>> SparsePauliOp.from_sparse_list(sparse_list, num_qubits = 5)
SparsePauliOp(['IIIIZ'],
              coeffs=[1.+0.j])
>>>

What should happen?

Ideally, it should return

SparsePauliOp(['IIIII'],
              coeffs=[1.+0.j])

Any suggestions?

No response

Issue Analytics

State:
Created a year ago
Comments:10 (7 by maintainers)

Top GitHub Comments

1reaction

Cryoriscommented, May 10, 2022

I agree raising an error would be good. For performance we could also add a do_checks flag or so that is True per default but can be turned off if you know that you’re strings are valid? 😄

0reactions

ikkohamcommented, May 9, 2022

Initialization with the same index should not be allowed and should be explicitly compose (@). The issue is whether to check for errors (/warning) or just document it.

Since this is a difficult behavior for users to find wrong cases, I think it would be better to make it Error. I don’t think it would be that so costly to check if it is already assigned (i.e., not I) here.

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