I know that the Qasm Simulator execute quantum circuits with presence of noise, even if I do not specify the model of noise in options for that backend.

But how can I execute the circuits without noise at all?


This is incorrect. The QASM simulator by default has no noise. The fluctuations in its results are a result of finite sampling of the output statevector. Thus, the QASM simulator is equivalent to running a quantum circuit on an ideal quantum computer. To add noise you can follow the example here: https://www.qiskit.org/aer

  • $\begingroup$ Thank you for your answer. Would it be right to say that real quantum computer has no such issue because It spans the whole Hilbert space of the problem? But it has other sources of fluctuations in its results such as noise. $\endgroup$
    – C-Roux
    Feb 10 '20 at 13:25
  • 1
    $\begingroup$ The answer on a quantum computer is in the form of a probability distribution that you can only sample a finite number of times. So there will always be sampling error, but these can be reduced by taking more shots. And, as you mentioned, there is always noise to deal with on near-term devices. $\endgroup$ Feb 10 '20 at 14:42

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