Decoherence free subspace

These computational results demonstrate the way in which decoherence tends eliminate quantum effects in the system. As a consequence, quantum controllf processes must be effectively shielded from decoherence effects in order to sumvd f Below we consider a number of approaches to combating decoherence in solii P tions and other media where collisions are present. An alternative approach, whiclf we do not address, is the method of decoherence free subspaces [169], to tt[i g approach one deals with the explicit design of systems where a particular subspacejif free from decoherence effects. These approaches are of particular interest to development of subspaces in which to carry out quantum computation, an appn in which the computational machinery follows the laws of quantum mechanic [170], By contrast, we deal below with the need to curb decoherence eff traditional preexistent systems. 

Thus the decoherence model of Fig. 8 exactly reproduces the anticipated behavior. All decoherence processes resulting from individual and uncorrelated reservoir interactions of the atoms are either exponentially suppressed by the energy gap (33) or are proportional to l/N. The latter is due to the large effective distance of the collective states in state space. In this way a quasi decoherence free subspace of dimension two is generated which allows to protect a stored photonic qubit from decoherence much more efficiendy than possible in quantum memories based on single particles. 

Quantum interference creates a decoherence-free subspace and thus slows down the relaxation. Therefore, one may expect that nonzero Y and l/T can in this case accelerate decay. Quantum interference will be significant only for sufficiently large k, so that 7r G(u>n) > un+i n, for some n. To observe reduced decay with l/T (the analog of QZE), one should takeT > (ujn—ujx)/2 and l/T > T. 

Shapiro, M. and Brumer, P, S-matrix approach to the construction of decoherence-free subspaces, Phys. Rev. A, 66, 052308, 2002. 

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