This is the first time I am using the IBM quantum computer. I tried to run a circuit (see below the first picture) but the results I got don't match the theoretical results shown (see below the second picture). I am honestly not sure what to do or why this happens. I also tried to run different circuits but they seem to have to same problem so I am probably misinterpreting something. I also tried to use a different quantum processor but they give the same problem.

Thanks in advance.

enter image description here


1 Answer 1


This happens because the effect of measuring a qubit. Half of the time, the qubit $q_0$ collapses to $1$, half of the time to $0$.

In your first example, $q_0$ collapsed to $1$, so your theoretical chances (from the second example) of $*0$ are distributed among $01$ and $11$.

You can see that if you change the Visualisation seed on the top right corner. Here is an example with seed = 1726:

same example with seed 1726"

In this case, $q_0$ collapsed to $0$ and $*1$ is not happening now. As a consequence, the only possible result is $00$, since $10$ was never a possibility (even without the measurement).

  • $\begingroup$ Thanks for the responds. I had no idea about the visualisation seed. As I understand it now this is only used to determine the 'theoretical' results right? Does this impact the results when I run the the circuit on a quantum processor? $\endgroup$
    – Audrique
    May 12, 2021 at 11:43
  • $\begingroup$ when you run in the real device, the default is to run the circuit 1024 times. So you will have the histogram close to your paper result. $\endgroup$
    – luciano
    May 12, 2021 at 17:08

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