Question: If quantum theory is based on probabilties and uncertainty, how will you ensure that the computer will behave predictably, especially with the cold atoms?

Chris Mansell answered on 20 Jun 2013:

This is a very intelligent question.

Quantum mechanics describes

(1) quantum systems changing with time.
(2) measuring quantum systems.

There is nothing at all probabilistic about (1). However, probabilities seem to be inherent to (2). So the answer to the part of your question about probability is that we do almost all of the actual computing by changing the quantum system without measuring it. At the end of the computation, we do have to do a measurement to find out what the answer is! The way quantum computers work is that they try to make the probability of measuring the correct answer as close to 100% as possible and the probability of measuring a wrong answer as close to 0% as possible.

The term “uncertainty” is used in quantum mechanics in two different cases. One case is measuring two different properties of the same particle. The other case is having lots of identically prepared particles, measuring one property for some of them and measuring a different property for the rest. Neither of these cases is of very much importance when considering how quantum computers work. This is because we only measure the one property of our quantum computer at the end of the computation: the property that tells us the answer.

I hope this has helped. Let me know if you have any follow-up questions.