The Bell Circuit

The following circuit—a combination of the Hadamard gate from the Single-Qubit Quantum Gates lesson, acting on a single qubit, and the $\text{CNOT}$ gate acting on two qubits—is named after John Stewart Bell (1928–90). Bell’s theories had (and still have) a major impact on our understanding of the conceptual foundations of quantum mechanics and on quantum computing.

The figure below shows a Bell circuit (BC) composed of a Hadamard gate and a $\text{CNOT}$ gate. We can also represent the Bell circuit as a gate box as shown below. This gate plays a big role in various quantum algorithms.