Bell circuit

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Bell circuit by Ciro Santilli 34 Updated 2024-12-15 +Created 1970-01-01
A quantum circuit which when fed with input produces the Bell state.
Figure 1.
Quantum circuit that generates the Bell state
. Source.
The fundamental intuition for this circuit is as follows.
First the Hadamard gate makes the first qubit be in a 50/50 state.
Then, the CNOT gate gets controlled by that 50/50 value, and the controlled qubit also gets 50/50 chance as a result.
However, both qubits are now entangled: the result of the second qubit depends on the result of the first one. Because:
  • if the first qubit is 0, cnot is not active, and so the second qubit remains 0 as its input
  • if the first qubit is 1, cnot is active, and so the second qubit is flipped to 1
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