In this chapter, we discuss a few hardware issues and proposals. Most certainly you have wondered (perhaps more than once!) whether all we have presented up to now is nothing more than elegant speculation, with no practical impact for the real world.

To bring things down to earth, we must address a very basic question: do we actually know how to build a quantum computer?

It turns out that the implementation of quantum computing machines represents a formidable challenge to the communities of engineers and applied physicists. However, there is some hope in sight: quite recently, some simple quantum devices consisting of a few qubits have been successfully built and tested. Considering the amount of resources that have been poured into this endeavor from different quarters (academia, private sector, and the military), it would not be entirely surprising if noticeable progress were made in the near future.

In Section 11.1 we spell out the hurdles that stand in the way, chiefly related to the quantum phenomenon known as decoherence. We also enumerate the wish list of desirable features for a quantum device. Sections 11.2 and 11.3 are devoted to describing two of the major proposals around: the ion trap and optical quantum computers.