searchers, believe that practical quantum computers will appear in a matter of years rather than decades. This may prove to be a wishful thinking but the fact is the optimism, however naive, makes things happen. After all it used to be a widely accepted ``scientific truth'' that no machine heavier than air will ever fly ! So, many experimentalists do not give up. The current challenge is not to build a full quantum computer right away but rather to move from the experiments in which we merely observe quantum phenomena to experiments in which we can control these phenomena. This is a first step towards quantum logic gates and simple quantum networks.Can we then control nature at the level of single photons and atoms? Yes, to some degree we can! For example in the so called cavity quantum electrodynamics experiments, which were performed by Serge Haroche, Jean-Michel Raimond and colleagues at the Ecole Normale Superieure in Paris, atoms can be controlled by single photons trapped in small superconducting cavities[6]. Another approach, advocated by Christopher Monroe, David Wineland and coworkers from the NIST in Boulder, USA, uses ions sitting in a radio-frequency trap[7]. Ions interact with each other exchanging vibrational excitations and each ion can be separately controlled by a properly focused and polarised laser beam. Experimental and theoretical research in quantum computation is accelerating world-wide. New technologies for realising quantum computers are being proposed, and new types of quantum computation with various advantages over classical computation are continually being discovered and analysed and we believe some of them will bear technological fruit. From a fundamental standpoint, however, it does not matter how useful quantum computation turns out to be, nor does it matter whether we build the first quantum computer tomorrow, next year or centuries from now. The quantum theory of computation must in any case be an inte...
