IMPLEMENTATIONS OF QUANTUM LOGIC - FUNDAMENTAL AND EXPERIMENTAL LIMITS

Quantum information processing rests on our ability to manipulate quan tum superpositions through coherent unitary transformations. In realit y the quantum information processor (a linear ion trap, or cavity QED implementation for example) exists in a dissipative environment. Depha sing, and other technical sources of noise, as well as more fundamenta l sources of dissipation severely restrict quantum processing capabili ties. The strength of the coherent coupling needed to implement quantu m logic is not always independent of dissipation. The limitations thes e dissipative influences present will be described and the need for ef ficient error correction noted. Even if long and involved quantum comp utations turn out to be hard to realize, one can perform interesting m anipulations of entanglement involving only a few gates and qubits, of which we give examples. Quantum communication also involves manipulat ions of entanglement which are simpler to implement than elaborate com putations. We briefly analyse the notion of the capacity of a quantum communication channel.