Linked by Eugenia Loli on Tue 1st Jul 2008 23:03 UTC
Quantum computers could become a reality very soon, opening up some fantastic possibilities -- including tele-portation, says Richard Gray.
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quantum computing will never be practical
by asgard on Wed 2nd Jul 2008 05:44 UTC

Member since:
2008-06-07

..and here is why. The size of quantum memory (number of qubits) doesn't scale so well as the size of normal memory.

In quantum computing, adding just a single quibit into existing memory is an engineering challenge. You have to find a new system with enough states that can be quantum entangled. Compare that with normal computers, where adding memory is not engineering challenge at all until you are going to double it.

So in quantum computers, each added bit of memory poses a problem. In normal computers, each doubling of memory poses a problem. So the memory in normal computers can scale exponentially while the quantum memory can scale only linearly.

This means if we were able to factor 1024-bit integers for cracking RSA on quantum computer, then everybody would start using 2048-bit integers, and they would be only very slightly more expensive. About the same order of more expensive as is factoring of 1025-bit integers on quantum computer. So you can see, this will not scale and is going to nowhere.

Reply Score: 2

Member since:
2008-07-02

but the point is that adding a qbit to a QC doubles its memory. It can represent 2^n states simultaneously, so, increasing n by one, doubles the memory.

The point of QC is that classically there is a minimum power associated with the processing of 1bit. A quantum computation is reversible, meaning that it does not dissipate power.

In fact, you can classify codes by how much energy you would need to crack that code. You might not know how powerful a supercomputer your enemy can build, but you can state how much it would cost in energy, and put a price on that information.

I am completing my PhD in experimental QC, and I tell you, we are far far from building anything useful. We can simulate about 25qbits on a mac pro, and experimentally only do about 7. With the exponential difficulty in adding them, it will take some time before we catch up. But when power really starts to be an issue, QC will be there.

Reply Parent Score: 3