Many physical systems that you might like to use for building a quantum computer, such as Optical Lattices, allow particularly precise control over parts of the system. However, it is very rare that you can do everything that the standard theory of quantum computation requires. In Optical Lattices, for example, you use lasers to manipulate individual atoms in order to create the one and two qubit gates that the network model of quantum computation requires. However, there is a minimum size that a laser can be focussed to (or the order of the wavelength of the light that is being used), and in optical alttices the atoms are typically closer together than that, so you end up addressing several atoms at once.
Given that you can only address several atoms at once (and, in the limit, can only address the whole device in one go) is it still possible to implement quantum computation? Yes! This is the subject of global control 1.
- quant-ph/0104034, quant-ph/0104117, quant-ph/0308113, quant-ph/0504197↩