I am trying to find out about the mechanism of measurement in IBM Q devices. To be specific, if I apply two Hadamard gates on the first qubit and identity on the second then is it possible to immediately measure so that the second qubit is not affected by the second Hadamard on the first qubit? Also, are measurements on qubits simultaneous? Does it differ with devices?
I don't know if this is what you are looking for but I will try to write out what I see.
If you are thinking of a circuit like this:
Then the gates on the first qubit, $q_0$, will not be affected the second qubit at all as they not interacting with one another. The entire system is in a separable state. So it doesn't really matter when you make the measurement on the second qubit, the result will be the same.
Now, if you actually run this on the quantum computer then I don't think the measurement is actually being executed until after the second Hadamard gate on the first qubit has been executed. You can check the pulse schedule for the circuit above:
Now, if you looking at a circuit that have entangled gate between the two qubits,
then the measurement on the second qubit $q_1$ won't be executed until the second Hadamard gate is being executed as well. But again, the Hadamard gate on the first qubit won't affect the second qubit as it only acts on the first qubit. Hence, it is fine that the measurement is being executed after the second Hadamard gate have been applied to $q_0$. The result won't change.
Also, if you have a circuit like:
Then the Hadamard gate on the second qubit, $q_1$, won't be executed until when the 4th Hadamard gate is being executed on the first qubit. This is because if we apply the Hadamard gate to the second qubit too early, it will idling for a long time and more decoherence effects will take place.