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Intermediate12 min read

Entanglement

Two qubits with a definite joint state and no definite individual states. Nothing travels; the correlation was there all along — but it is not the correlation you think.

Before this: Gates & Circuits

In this lesson you’ll learn

  • Test whether a two-qubit state factors, and recognise the Bell states
  • Explain how perfect correlation coexists with locally random marginals
  • State why entanglement cannot be used to send a signal

Predict before you read on

Try to write the two-qubit state (|00⟩ + |11⟩)/√2 in the form (a|0⟩ + b|1⟩) ⊗ (c|0⟩ + d|1⟩) — a definite state for each qubit separately. Can it be done?

Commit to an answer — the next section is the payoff.