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by Yajirobe 2650 days ago
What's the difference between an entangled state and a mixed state?
1 comments
onorton 2650 days ago
Knowledge is a bit rusty (took a module in Uni) but I'll try to answer.

A mixed state is that which is a linear combination of pure states e.g. a|0> + b|1>

What determines an entangled state is that qubit values will correlate exactly with each other. |00> + |11> would be an example of an entangled state as measuring one qubit determines the value of the other with certainty. If you measure |0> for the first qubit, the second will definitely be |0> and vice versa.

They are also not mutually exclusive as mixed entangled states exist.

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Yajirobe 2649 days ago
> A mixed state is that which is a linear combination of pure states e.g. a|0> + b|1>

Here https://en.wikipedia.org/wiki/Qubit#Mixed_state it says that 'Mixed states can be represented by points inside the Bloch sphere'. However, points ON the sphere correspond to linear combinations of the pure states. How to reconcile this?

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onorton 2648 days ago
Like I said, my knowledge is rusty.

I've gotten pure states mixed up with |0>, |1>

I believe what it's saying is that with a mixed state |a|^2 + |b|^2 does not need to equal 1.

So pure states are a|0> + b|1> where |a|^2 + |b|^2 = 1

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