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This is my first post in this forum so it is likely that I'll make some rookie mistakes here. In the simple Bell state circuit in IBM circuit composer enter image description here

we measure the first qubit and for this seed we get the following statevector enter image description here

Now if we add a second measurement to the second qubit, we should expect the second qubit to also be in the 0 state with certainty, as decided by the first measurement result with state $|00\rangle$. However, instead it seems that the circuit resets somehow. For the same seed the result changes to $|11\rangle$ with the addition of the measurement of the second qubit. enter image description here

Does anyone know why this happens? I'm new to qiskit so any tips and answers will be gladly accepted.

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Before any measurement, the state you're creating is: $$\frac{|00\rangle+|11\rangle}{\sqrt{2}}$$ But if you measure one qubit, the state collapses to either $|00\rangle$ or $|11\rangle$.

In this case, the composer simulates a single shot of your circuit. It just happened that the state collapsed to $|00\rangle$ the first time and to $|11\rangle$ the second time. If you reload the state vector by adding/removing gates, you should be able to see that it can also collapse to the $|00\rangle$ state. Similarly, you can retry the frist experiment to see that you would sometimes get $1$.

What confused you is that the composer redoes a shot of the circuit each time you add or remove a gate. Thus, these two experiments are totally independent.

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  • $\begingroup$ Thanks for the clarification! So I guess then that when one uses the inspect feature, as you click through the circuit, each addition of a new operation will show results from a new shot? Do you know if there is a way to actually follow a specific shot throughout the entire circuit? $\endgroup$
    – VikingPhysics
    Dec 1, 2021 at 10:35
  • $\begingroup$ @VikingPhysics I think that the closest thing to what you want to do would be the following: 1. Simulate an experiment until the first measurement 2. Note down the resulting statevector 3. Create a new circuit where you initialize your state in this statevector 4. Carry out the rest of the computation. This is the only way to know the intermediate statevector in which the system collapses after the first measurement. However, this is obviously only feasible with a simulator. $\endgroup$
    – Tristan Nemoz
    Dec 1, 2021 at 14:05
  • $\begingroup$ Thanks a lot for answer! $\endgroup$
    – VikingPhysics
    Dec 1, 2021 at 14:39
  • $\begingroup$ @VikingPhysics You can accept it using the checkmark if your problem is solved! :) $\endgroup$
    – Tristan Nemoz
    Dec 1, 2021 at 15:40

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