  The key realisation which lead to the emergence of the new field of quantum
information processing is that quantum mechanics, the theory that describes
microscopic particles, allows the processing of information in fundamentally
new ways. But just as in classical information processing, errors occur in
quantum information processing, and these have to be corrected. A fundamental
breakthrough was the realisation that quantum error correction is in fact
possible. However most work so far has not been concerned with technological
feasibility, but rather with proving that quantum error correction is possible
in principle. Here we describe a method for filtering out errors and
entanglement purification which is particularly suitable for quantum
communication. Our method is conceptually new, and, crucially, it is easy to
implement in a wide variety of physical systems with present day technology and
should therefore be of wide applicability.
