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I am new to using the IBM Quantum Emulator, and I encounter this result which I don't understand:

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The input is a single 0 bit that is passed into a Hadamard gate (click the picture to enlarge). I couldn't understand why the measurement outcome after this is answered as 0 with 100% certainty?

Shouldn't this measurement outcome be 50% 0 and 50% 1 after passing through the single H gate?

Thank you.

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1 Answer 1

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The issue is with a measurement gate. For some reason, the composer shows one of all possible results but not all of them in case the measurement gate is included into a circuit. It seems that IBM wants the composer to behave like a real measurement, i.e. from one experiment, you obtain only one result.

Try these:

  1. remove the measurement gate and you should see 50%/50% probability of occurence for both $|0\rangle$ and $|1\rangle$ states.
  2. Add the measurement gate back and run your cirucit on a simulator or real quantum processor with number of shots greater than 1. In case of simulator you should see perfect 50%/50% distribution, in case of the real processor, there will be some noise but the distribution will be close to the uniform one.

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    $\begingroup$ thank you, removing the measurement gate does give the original 50/50 probabiities. $\endgroup$
    – user315366
    Oct 9, 2021 at 16:41

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