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Issue when commuting moments #6659

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cdbf1 opened this issue Jul 8, 2024 · 1 comment · May be fixed by #7082
Open

Issue when commuting moments #6659

cdbf1 opened this issue Jul 8, 2024 · 1 comment · May be fixed by #7082
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good first issue This issue can be resolved by someone who is not familiar with the codebase. A good starting issue. kind/bug-report Something doesn't seem to work. triage/accepted A consensus emerged that this bug report, feature request, or other action should be worked on

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@cdbf1
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cdbf1 commented Jul 8, 2024

Description of the issue
Cirq gives the wrong result when determining whether moments commute if the individual operators within the moments do not commute, but the moment as a whole still commutes. For example, take a pair of single qubit Z gates and a two qubit RXX gate. While [Z, RXX] !=0 (for either single qubit Z), we do in fact have [Z * Z, RXX] = 0. However, when using cirq we have

import cirq
qubits = cirq.LineQubits.range(2)
moment_1 = cirq.Moment([cirq.Z(qubits[0], cirq.Z(qubits[1])])
moment_2 = cirq.XXPowGate(exponent=1 / 2)(*qubits)

print(cirq.commutes(moment_1, moment_2))

Gives False.

Looking through the source code this seems to be because the the commutes() function compares pairs of operators form each moment but doesn't consider the case when the moment as a whole commutes even when the individual operators do not.

How to reproduce the issue
See above

Cirq version
You can get the cirq version by printing cirq.__version__. From the command line:
1.5.0
@cdbf1 cdbf1 added the kind/bug-report Something doesn't seem to work. label Jul 8, 2024
@NoureldinYosri NoureldinYosri added the triage/discuss Needs decision / discussion, bring these up during Cirq Cynque label Jul 8, 2024
@dstrain115 dstrain115 added triage/accepted A consensus emerged that this bug report, feature request, or other action should be worked on and removed triage/discuss Needs decision / discussion, bring these up during Cirq Cynque labels Jul 17, 2024
@dstrain115
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dstrain115 commented Jul 17, 2024 •
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cirq cync: This functionality works for pauli strings, but not for moments.

A naive solution would be to decompose the moments (if commutes is initially determined to be false via gate by gate comparison).

From the documentation:
Either value has a commutes method that returns 'True', 'False', or 'None' (meaning indeterminate).

So, this should either return None (not sure) or should perform an alternative strategy like decomposing the moment into a unitary.

@dstrain115 dstrain115 added the good first issue This issue can be resolved by someone who is not familiar with the codebase. A good starting issue. label Jul 17, 2024
@cdbf1 cdbf1 mentioned this issue Jul 26, 2024
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chamodtharakaperera added a commit to chamodtharakaperera/Cirq that referenced this issue Feb 23, 2025
@chamodtharakaperera
Fix Moment.commutes_ to handle multi-qubit unitaries (quantumlib#6659)
960ff75 
Add pairwise commuting checks and, when inconclusive, fall back to full
                  unitary commutator. Includes new tests showing Z⊗Z commutes with RXX.
                  Fixes quantumlib#6659.
@chamodtharakaperera chamodtharakaperera linked a pull request Feb 23, 2025 that will close this issue
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good first issue This issue can be resolved by someone who is not familiar with the codebase. A good starting issue. kind/bug-report Something doesn't seem to work. triage/accepted A consensus emerged that this bug report, feature request, or other action should be worked on
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@NoureldinYosri @dstrain115 @cdbf1