I am trying to simulate an experiment on IBM's melbourne quantum computer and here are the results

  1. Qasm simulator :enter image description here

  2. ibmq_16_melbourne: enter image description here

  3. errorenter image description here

why is there such high probability of unwanted states i.e 01 and 10. Also when the circuit is transpiled on the melbourne computer the circuit operates between $q[4]$ and $q[10]$, is these usual.


1 Answer 1


Every rotation or controlled operation you perform on each individual qubit has an error associated to it, you can check it here by selecting a computer and hovering over each Qubit, through the error range bar or through Python as you seem to have done.

What I've found in my experiments is that what usually ruins an experiment is the error associated with the CNOT operation, you can keep track of their count by using something along these lines qc.count_ops().get('cx'). Another thing that may help is setting the layout method to "noise adaptive", by including layout_method='noise_adaptive' into the transpiler parameters. You should check the transpiler documentation and play around with the different parameters in order to best optimize your gate count. Ultimately the transpiler can only do so much and your circuit might just not be able to run on a real device.

I'm currently working on some circuits that require a decent amount of SWAP operations that are decomposed into 3 CNOTs each, at the very minimum. I was thinking that running them on Melbourne computer would yield better results, since it has the best suited connectivity for my specific needs. What i found was that the results were very, very bad when compared to a computer that has less connectivity but also less error associated with the CNOT operation.

  • $\begingroup$ In the error map, the average error for C-NOT is $37\%$, isn't that a bit high, because that could lead extremely inaccurate results $\endgroup$
    – Upstart
    Jul 29, 2020 at 19:45
  • $\begingroup$ Yeah it's pretty high IMO, for instance my circuit for Melbourne has around 42 CNOTs and the results are completely rubbish, while for Johannesburg 70 CNOTs were producing pretty decent results (the error actually went up yesterday so they're back to being messy). $\endgroup$ Jul 29, 2020 at 19:53
  • $\begingroup$ The C-NOT that is counted, is the number that is shown in the transpiled circuit of the Melbourne hardware or the original circuit that we build. Because my original circuit has 7 C-NOT and 2 C-Z,, but in the transpiled version there are 21-CNOT and quite a frew U gates $\endgroup$
    – Upstart
    Jul 29, 2020 at 20:03
  • $\begingroup$ Well it's both. The 7 CNOT and 2CZ version of your circuit assumes all your qubits are connected to one another (i.e. the QASM simulator). In reality, they're usually connected in pairs or trios, meaning if you want to perform a CNOT between two unconnected qubits, you have to perform several SWAPs in order to simulate a connection between them. That's part of the transpiler's job, it maps your circuit to the backend's topology by adding CNOTs (SWAPS). $\endgroup$ Jul 29, 2020 at 20:17
  • $\begingroup$ Let me know if you have any more questions, if not please mark the question as answered. Good luck! $\endgroup$ Jul 30, 2020 at 5:02

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