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Physical quantum computers have noise. Let's take a (simplified) scenario. You've set up your quantum circuit with one qubit, and put it in a position where it will measure as either 0 or 1 with equal chance. In the simulation it'll come out as 0 or 1 with actual equal chance. In the real world, other factors will create a bias one way or the other (this may not be consistent, either) so that it comes out more like 60% 0 to 40% 1, even over 1000s of trials.

If you set up a circuit where you've entangled two qubits so that they should come out as the same value (00 or 11) and the configuration says they should come out with 50% chance of either, the simulation will show that. The outputs of 01 and 10 will never show up in the simulation. But in the real world, there's still a chance that you get those. You'll likely (on IBM's quantum computers) get something like 1-5% 01, 1-5% 10, 45-50% 11, 45-50% 00 (again, over thousands of runs).

If you want to see how this plays out with simulations and real quantum computers, IBM [0] has free access (constrained by credits when you want to run on real quantum computers, they reset each day).

[0] https://quantum-computing.ibm.com/

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