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?
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1$\begingroup$ related: physics.stackexchange.com/q/65007/58382, physics.stackexchange.com/q/17913/58382, physics.stackexchange.com/q/54975/58382, quantumcomputing.stackexchange.com/q/1631/55 $\endgroup$– glS ♦Oct 29, 2022 at 17:58
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1$\begingroup$ 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. $\endgroup$– Martin VeselyOct 29, 2022 at 20:15
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$\begingroup$ Thanks a lot for your answer @Martin Vesely $\endgroup$– SoardrOct 30, 2022 at 4:32
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$\begingroup$ Thank you @glS for the related articles, will surely check them out. $\endgroup$– SoardrOct 30, 2022 at 4:33
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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.
