Understanding Quantum Computing: Unanswered Questions

[MikeFromOhio]

I've been reading "Programming the Universe" by Seth Lloyd, a quantum computer scientist. He goes on and on about the power of quantum computers and their ability, for example, to factor large numbers using massive parallelism. He also states that quantum parallelism is different than classical parallelism. In the quantum case, he says, the individual quantum bits (or qbits) are interfering with each other which is helpful.

A quantum computer of 4 qbits, he says, can be carrying out 2**4 or 16 tasks simultaneously. So if your trying to factor the number 20, one task might be trying out 6*3 which does not equal 20 while another task might be trying 4*5 which does.

What he does not explain is:

1) How does the Quantum Computer carry out the multiplication for a given qbit task?

2) If interference is important, then how do the separate trials interfere which each other? And how do failed tasks (6*3) interfere with the tasks which are successful (4*5) ?

3) How does the Quantum Computer know when it is done?

Any feedback on these questions would be appreciated.
Thanks,
Mike

 

[unusualname]

Here's a report of an actual experiment last year which used Shor's algorithm to factor 15:

http://arxiv.org/abs/0911.1242

http://physicsworld.com/cws/article/news/40306

 

[unusualname]

Two earlier demonstrations which provide more detail:

http://arxiv.org/abs/0705.1684

http://arxiv.org/abs/0705.1398

 

[MikeFromOhio]

Unusualname, thanks for the links.
I can see that it's a bit hard to explain.