AVVISO DI SEMINARIO

Prof. Saverio Pascazio
Dipartimento di Fisica - Universita' di Bari
Sezione Infn - Bari

Giovedi' 4 Dicembre 2003
ore 16.00, Aula Seminari INFN

Halting decoherence: quantum Zeno subspaces and alternative strategies

Abstract:
If very frequent measurements are performed on a quantum system, in order to ascertain whether it is still in its initial state, transitions to other states are hindered and the quantum Zeno effect takes place.
This phenomenon stems from general features of the Schroedinger equation that yield quadratic behavior of the survival probability at short times. However, the quantum Zeno effect (QZE) does not necessarily
freeze everything. On the contrary, for frequent projections onto a multi-dimensional subspace, the system can evolve away from its initial state, although it remains in the subspace defined by the measurement. This continuing time evolution within the projected subspace is named "quantum Zeno dynamics" and has interesting features. In particular, it is unitary for a wide class of systems.
We give an *elementary* introduction to these issues and then recast the quantum Zeno effect in terms of the dynamical coupling to another quantum system: when the coupling is strong (or very frequent) the
system is forced to evolve in a set of orthogonal "Zeno" subspaces of the total Hilbert space and a dynamical superselection rule arises. Some significant examples will be proposed and their practical relevance discussed.
We focus on decoherence, irreversibility and decoherence-free subspaces.

 

L. Martina