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It is claimed that the fake backends of IBM will have noise similar to the real backends. However, to what extent can we expect them to produce the same results?

I have run my quantum circuit on the real backend, ibm_kyoto and the result differs vastly from the fake backends (FakeCairoV2, FakeTorino) which I test my algorithm with.

Do the real backends have much more noise than the fake ones?

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    $\begingroup$ To do a proper comparison you should compare the real backend ibm_kyoto to the fake backend fake_kyoto. $\endgroup$
    – Mathias
    Commented Apr 9 at 14:34
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    $\begingroup$ Related: quantumcomputing.stackexchange.com/q/37673/10454 $\endgroup$
    – Tristan Nemoz
    Commented Apr 9 at 14:57
  • $\begingroup$ @mathias There seems to be another issue with that: the transpiler turns my 20 qubit circuit to a circuit with same num of qubits as the real backend. So it becomes 127 qubit circuit whose simulation takes too much memory. I am guessing this is the case because I have error message 'not enough memory' when I try it on FakeTorino(). $\endgroup$
    – Deren Liu
    Commented Apr 9 at 17:01
  • $\begingroup$ @DerenLiu One thing you could do is to construct a custom fake backend using the calibration data of the subset of qubits that you are using on ibm_kyoto. That way you can simulate locally. Do you think this is what you want? I can write a detailed answer on that. $\endgroup$ Commented Apr 19 at 8:56

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Fake backends from qiskit-IBM-runtime package contains calibration results from a snapshot that is hardcoded in the source code. To get a more realistic simulation results, you can use qiskit-aer with the latest calibration results:

from qiskit_aer import AerSimulator
from qiskit_ibm_runtime import QiskitRuntimeService

service = QiskitRuntimeService()
real_backend = service.backend('ibm_kyoto')
aer_simulator = AerSimulator.from_backend(real_backend)

You can read more in the documentation about different local testing modes: https://docs.quantum.ibm.com/verify/local-testing-mode

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