Another feature that sets qubits apart from classical bits is entanglement, a fundamental quantum mechanical principle. Entanglement can interlink multiple qubits, such that the state of a qubit cannot be described independently of the other qubits. Entangled qubits can therefore only be described by the quantum state of the entire system, which is a defined state as opposed to that of individual qubits. Entanglement, hence, creates a strong link between qubits, such that when you measure the state of one, it instantly affects the state of the other even if they are far apart, e.g., New York and Delft.

There is a special category of entangled states which are called the Bell states (or EPR pairs) – these are so-called maximally entangled states and they are perfectly correlated. Bell states are often used in quantum applications such as quantum teleportation.

(If the video isn't working, click the link below it to watch on YouTube)