Here are two different IBM quantum online devices,
Both of them have the 5 qubits and the same architectures. But why do they have different error rate? What's the different between them?
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They have different error rates because they are two different physical devices! This relates to the manufacturing processes of these chips. Every device is unique and will have its own fingerprint meaning its own error rate. Of course this is not something that manufactures do on purpose, but a side effect of making these qubit devices. It’s very difficult to make two quantum chips completely identical!
I would add that thermic noise, radiocative background (mainly cosmic rays) can play role in different error rate as those noise sources are different for each quantum processor.
Moreover, as a user of IBM Q, you probably know that quantum processors are sometimes under maintenance. Since each processor is maintained in different time, their runtime is also different and this can influence error rate as well.