NOISELESS QUANTUM CODES

In this paper we study a model quantum register R made of N replicas ( cells) of a given finite-dimensional quantum system S. Assuming that a ll cells are coupled with a common environment with equal strength we show that, for N large enough, in the Hilbert space of R there exists a linear subspace C-N which is dynamically decoupled from the environm ent. The states in C-N evolve unitarily and are therefore decoherence- dissipation free. The space C-N realizes a noiseless quantum code in w hich information can be stored, in principle, for an arbitrarily long time without being affected by errors.