Entanglement and the universe as a computer

[elfstone]

If you take a system of only 400 particles, and it could be, say, 400 electrons, you can put those into a quantum state called an entangled state, which can be described by a certain string of numbers, and it turns out that you need so many numbers to describe that state that it would exhaust the capacity of the entire universe to store it. In other words, there's more information in that state than can be contained in the entire universe. So I suspect that if we built a state with 400 entangled particles we would then begin to expose the limitations of the so-called Platonic nature of physical law.

 

This is a quote by Paul Davies. Can anyone explain what he's talking about? Is the number 400 something that was calculated or just an example?

Btw, I copied it from here.

[5614]

I think 400 is just an example, but it is chosen for a reason.

 

40 would have been too small, and wouldn't have made the point he made in that quote. Whilst 4,000 would fit the quote, it is unnecassarily big.

 

I looked at the link, and would like to quote one other part, it helps answer your question:

Achieving an entangled state of 400 particles is not easy, but it's not obviously hopeless and the quantum computing industry has set its target at several thousand. They're up to about 12 at the moment but they're very optimistic that within our lifetime they will achieve at least this 400

Was it just the number 400 that puzzled you?

[elfstone]

Not really. What I don't see is why does the universe HAVE to store the information of the entangled state. What I understand is that mathematically we will be able to describe that state, I don't see how that will give us any insight to the nature of physical law. Does Davies mean that an entangled state of more than a certain number of particles is physically impossible for some reason?

[5614]

What I don't see is why does the universe HAVE to store the information of the entangled state.

That's just how it works. If the universe didn't store the information of all the particles in it then the particles would not have any information to base their existence on, they would not exist.

 

Although I do not think that was what Davies meant when he said:

"it turns out that you need so many numbers to describe that state that it would exhaust the capacity of the entire universe to store it"

I think he is basically saying is that the mathematical description of the 400 entangled particles is absolutely massive. They contain more information than you can possibly imagine.

 

To try and paint an image of this unimaginably massive amount of data he said that storing this amount of data would overfill the capacity of the entire universe. It's just meant to emphasise a point.

 

Does Davies mean that an entangled state of more than a certain number of particles is physically impossible for some reason?

No. His point is that when you start entangling more than 400 particles in one quantum computer these 400 entangled particles can hold massive amounts of information. This could be used to build a quantum computer capable of doing very large calculations very quickly.

 

If you only have 12 entangled particles (12 is currently the largest quantum computer), then there is only a relatively small amount of information stored between them. They can make a quantum computer, but it is not anything extraordinary as far as it's power is concerned.

 

Davies is saying that when you reach 400 entangled particles the power of a quantum computer made from them starts getting interesting. A quantum computer is supposed to make what you or I would call a 'normal computer' look very slow. So by "getting interesting" I mean that, according to Davies, when you reach a quantum computer containing around 400 particles the power of that computer is, well, getting powerful.

 

I have no idea how a 400-particle QC compares to an electronic computer speed-wise... but obviously Davies feels it is some kind of landmark. Whether that's becoming faster than, the same speed as or slower than an electronic computer I don't know.

[elfstone]

Thanx for your answers!