The history of computer
technology has involved a sequence of changes from one type of physical realisation
to another --- from gears to relays to valves to transistors to integrated circuits
and so on. Today's advanced lithographic techniques can squeeze fraction of micron
wide logic gates and wires onto the surface of silicon chips. Soon they will yield
even smaller parts and inevitably reach a point where logic gates are so small
that they are made out of only a handful of atoms; i.e. the size of the logic
gates become comparable to the size of atoms.On
the atomic scale matter obeys the rules of quantum mechanics, which are quite
different from the classical rules that determine the properties of conventional
logic gates. So if computers are to become smaller in the future, new, quantum
technology must replace or supplement what we have now. The point is, however,
that quantum technology can offer much more than cramming more and more bits to
silicon and multiplying the clock-speed of microprocessors. It can support entirely
new kind of computation with qualitatively new algorithms based on quantum principles.

The
story of quantum computation started as early as 1982, when the physicist Richard
Feynman considered simulation of quantum-mechanical objects by other quantum systems.
However, the unusual power of quantum computation was not really anticipated until
the 1985 when David Deutsch of the University of Oxford published a crucial theoretical
paper in which he described a universal quantum computer. After the Deutsch paper,
the hunt was on for something interesting for quantum computers to do. At the
time all that could be found were a few rather contrived mathematical problems
and the whole issue of quantum computation seemed little more than an academic
curiosity.