Bell State
Entanglement is created by first putting one qubit into a superposition, meaning it is in a combination of |0⟩ and |1⟩ at the same time. Then, a controlled operation is applied that links the second qubit to the first, so their states become correlated. After this process, the two qubits act as a single system: measuring one qubit immediately reveals the state of the other, no matter how far apart they are.
Bell State Circuit: Creating Entanglement
Figure A: Bell state circuit uses H gate for superposition on q₀, then CNOT to entangle q₀ and q₁, creating the maximally entangled state (|00⟩ + |11⟩)/√2.
Ready to run - H gate creates superposition, measurement collapses to |0⟩ or |1⟩, then CNOT applies
What is a Bell State?
A Bell state is a term for a maximally entangled state of two qubits. Think of it this way:
- Entanglement is the property - the strange, non-local connection between qubits
- A Bell state is the name given to the specific, pure, two-qubit quantum states that exhibit this property to the absolute maximum degree
- If you are holding a Bell state, you are holding the perfect, strongest form of two-qubit entanglement
If a quantum system is in a Bell state, we say the qubits are entangled.
The Bell State
This equation is the definition of one of the four Bell states, and the reason we use this term is precisely because the state is inseparable and maximally entangled.