Universal quantum computation on decoherence-free subspaces and subsystems
(DFSs) is examined with particular emphasis on using only physically releva
nt interactions. A necessary and sufficient condition for the existence of
decoherence-free (noiseless) subsystems in the Markovian regime is derived
here for the first time. A stabilizer formalism for DFSs is then developed
which allows for the explicit understanding of these in their dual role as
quantum error correcting codes. Conditions for the existence of Hamiltonian
s whose induced evolution always preserves a DFS are derived within this st
abilizer formalism. Two possible collective decoherence mechanisms arising
from permutation symmetries of the system-bath coupling are examined within
this framework. It is shown that in both cases universal quantum computati
on which always preserves the DFS (natural fault-tolerant computation) can
be performed using only two-body interactions. This is in marked contrast t
o standard error correcting codes, where all known constructions using one-
or two-body interactions must leave the code space during the on-time of t
he fault-tolerant gates. A further consequence of our universality construc
tion is that a single exchange Hamiltonian can be used to perform universal
quantum computation on an encoded space whose asymptotic coding efficiency
is unity. The exchange Hamiltonian, which is naturally present in many qua
ntum systems, is thus asymptotically universal.