Quantum computation using decoherence-free states of the physical operator algebra

The states of the physical algebra, namely the algebra generated by the operators involved in encoding and processing qubits, are considered instead of those of the whole-system algebra. If the physical algebra commutes with the interaction Hamiltonian, and the system Hamiltonian is the sum of arbitrary terms either commuting with or belonging to the physical algebra, then its states are decoherence free. One of the examples considered shows that, for a uniform collective coupling to the environment, the smallest number of physical qubits encoding a decoherence-free logical qubit is reduced from four to three.