We propose a scheme to generate four-photon polarization-entangled states, which enables the encoding of an arbitrary qubit into a four-qubit decoherence-free subspace robust against collective decoherence. The scheme is based on linear optical elements and postselection strategy, which is feasible with existing experimental technology.

• Received 22 January 2008

DOI:https://doi.org/10.1103/PhysRevA.77.042317