Fix incorrect error estimation in VF2 fallback (backport #14218) #14315
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The VF2 passes include a fall-back error heuristic if the `Target` specifies no error rates: a number proportional to the node degree is used. This was a proxy metric, used historically becasue IBM bowtie devices (amongst others) were unreliable at reporting error rates but had significantly degraded performance on higher-degree nodes. The calculation of the node degree, however, could double count qubits in a directional backend, and since the scaling factor is the total number of qubits, this could cause error rates greater than one, which then cause unpredictable behaviour on the scoring. (cherry picked from commit 45778dd)
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Summary
The VF2 passes include a fall-back error heuristic if the
Targetspecifies no error rates: a number proportional to the node degree is used. This was a proxy metric, used historically becasue IBM bowtie devices (amongst others) were unreliable at reporting error rates but had significantly degraded performance on higher-degree nodes.The calculation of the node degree, however, could double count qubits in a directional backend, and since the scaling factor is the total number of qubits, this could cause error rates greater than one, which then cause unpredictable behaviour on the scoring.
Details and comments
This is an automatic backport of pull request #14218 done by [Mergify](https://mergify.com).