= Bell circuit
{c}
{tag=Quantum circuit}
= Bell state circuit
{c}
{synonym}
A <quantum circuit> which when fed with input $\ket{00}$ produces the <Bell state>.
In <Qiskit> at: <qiskit/hello.py>{file}.
\Image[https://upload.wikimedia.org/wikipedia/commons/f/fc/The_Hadamard-CNOT_transform_on_the_zero-state.png]
{title=<Quantum circuit> that generates the <Bell state>}
{description=
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
}