The article says this technique might be useful for quantum cryptography:

Quantum computation has moved another step closer with the first demonstration of a quantum NOT gate. Although it is impossible to build perfect logic gates for quantum bits of information, a team led by Francesco De Martini of the University of Rome "La Sapienza" and INFM in Italy has achieved almost the maximum theoretical fidelity with its device (F De Martini et al 2002 Nature 419 815).