Entanglement

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Part of Womanium Quantum + AI 2024 program

Definition

Entanglement is a fundamental phenomenon in quantum mechanics where two or more particles become interconnected in such a way that the state of one particle instantly influences the state of the other(s), regardless of the distance between them.

Entanglement is one of the fundamental mechanics that make the basis of quantum computing, in fact, most quantum algorithms exploit this mechanic in some way to achieve a quantum advantage over classical systems.

Entanglement vs Perfect Correlation

Usually entanglement is demonstrated with an example where we have 2 entangled qubits such that they both will have the same state when measure (e.g. Bell state $|{\mathrm{\Phi }}^{+}⟩$), then these qubits will be given to two persons and each will go in their way. The lemma is that if the first person measure their qubit and found it in state 1 then they would immediately know the state of the second qubit, which will be also 1.

However, there is nothing quantum about this example, the same scenario can be explained classically. For example, think of a game where two players get 1 of 2 total sticks, the game has a tall and a short stick, if the first player got the short stick they would immediately conclude that the other player got the longer one, and vice-versa. A similar scenario can be made where both of them would get the same "state".

So this explanation doesn't show the difference between quantum entanglement and classical perfect correlation.

The actual difference is subtle. In classical perfect correlation we know that the values/states are fixed when they were distributed, however, in entanglement the states are fixed on measurement after the distribution.

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