Quantum Computing Hardware
Two qubits are in the entangled state (|00⟩ + |11⟩)/√2. Which statement is correct?
- AEven when the first qubit is measured, the second qubit stays in a state giving 0 and 1 with 50% probability each
- BIf measuring the first qubit gives 0, the second is certain to be 0, and if it gives 1, the second is certain to be 1
- CBy choosing the measurement result of the first qubit, information can be sent faster than light to the second qubit far away
Show answer and explanation
Answer: B. If measuring the first qubit gives 0, the second is certain to be 0, and if it gives 1, the second is certain to be 1
This state is a superposition of |00⟩ and |11⟩ with no |01⟩ or |10⟩ component, so the two measurement results always agree. Looking at either qubit alone, 0 and 1 each appear 50% of the time, but the results are correlated in a way that two independent bits cannot explain; this is quantum entanglement. However, which result appears is random and cannot be chosen by the person measuring, so entanglement cannot be used to send information faster than light. In a quantum computer, the quality of the two-qubit gates that create entanglement strongly affects performance.
- A. This describes two qubits that are not entangled (independent). In this state the two results always agree.
- B. Correct. There is no |01⟩ or |10⟩ component, so the results always match.
- C. The result is random and cannot be chosen by the person measuring; the correlation only becomes visible when the results are compared later. Information cannot be sent faster than light.