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I am experimenting with concepts in Quantum Computing and I have landed on using entangled Qubits to perform certain actions.

My question is this: is it possible to have (let's say) 3 entangled Qubits in a system. If it is, then how much information is retrievable/usable when only 2 of these Qubits are measured? The application I am thinking of is using 2 of the 3 Qubits for a one-time access code and then the 3rd becomes useless (and almost automatically gets locked out).

I realise my mathematics is basic but it is the application I am curious about.

Any help would be appreciated.

glS
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Stephen Coady
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2 Answers2

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Here is a circuit for preparation of state $\frac{1}{\sqrt{2}}(|000\rangle + |111\rangle)$.

GHZ state

And here one for state $\frac{1}{\sqrt{3}}(|001\rangle + |010\rangle+ |100\rangle)$

W state

Martin Vesely
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There is entanglement of formation and entanglement that can be distilled, but the question of how to measure the entanglement of more than 2 qubits is an ongoing area of research. Yes, you can have three-qubit entanglement. For example, the GHZ state (like the Bell states, complete correlation among the qubits) or the W state. In the GHZ state, if you measure one qubit in the z direction, you lose all entanglement. In the W state, if you measure one qubit, the other two remain entangled.

Bennett Brown
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