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I was just curious; when do the qubits get entangled? Do the qubits get entangled right after initialization or do they get entangled when we apply Hadamard Gates to the qubits?

glS
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Soardr
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    related: https://physics.stackexchange.com/q/65007/58382, https://physics.stackexchange.com/q/17913/58382, https://physics.stackexchange.com/q/54975/58382, https://quantumcomputing.stackexchange.com/q/1631/55 – glS Oct 29 '22 at 17:58
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    Just note that Hadamard gate does not prepare a entanglement. You need at least two-qubit gate. For example, apply Hadamard on first qubit and then CNOT on the first and second one. You will get so-called Bell state, where two quits are entangled. – Martin Vesely Oct 29 '22 at 20:15
  • Thanks a lot for your answer @Martin Vesely – Soardr Oct 30 '22 at 04:32
  • Thank you @glS for the related articles, will surely check them out. – Soardr Oct 30 '22 at 04:33

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As mentioned before, qubits are only entangled by means of 2-qubit gates and not by 1-qubit gates, which only cause a rotation of the qubit state in question. Therefore, 2-qubit gates are needed to provide anything meaningful on a quantum computer other than pure random number generation. Practically, 2-qubit gates (typically a CX - gate) are harder to realize than 1-qubit gates, which also leads to a significantly larger quantum error.

siserman
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