  For a practical quantum computer to operate, it will be essential to properly
manage decoherence. One important technique for doing this is the use of
"decoherence-free subspaces" (DFSs), which have recently been demonstrated.
Here we present the first use of DFSs to improve the performance of a quantum
algorithm. An optical implementation of the Deutsch-Jozsa algorithm can be made
insensitive to a particular class of phase noise by encoding information in the
appropriate subspaces; we observe a reduction of the error rate from 35% to
essentially its pre-noise value of 8%.
