Quantum Information Processing and Quantum Communications

Berry phase for a spin 1/2 in a classical fluctuating field

aula 326 - GioVEdì 1° luglio ore 11.00
  MASSIMO PALMA, Università degli Studi di Milano

Actively working in the filed of Quantum Information Theory since the early 90ies he is currently interested in various aspects of quantum information processing, including entanglement manipulation in superconducting nanostructures and quantum optical systems, channel capacities and geometric quantum computation.

Massimo Palma
  Berry phases and related geometrical phases have received renewed interest in recent years due to several proposal for their use in the implementation of quantum computing gates.
Such interest is motivated by the belief that geometric quantum gates should exhibit an intrinsic fault tolerance in the presence of external noise. Such belief is based on the euristic argument that being Berry phases geometrical in their nature, i.e. proportional to the area spanned in parameter space, any fluctuating perturbation of zero average should indeed average out. Although this argument seems convincing to the best of our knowledge it has not been quantitatively probed sofar. In particular although several authors have investigated aspects of Berry phases in the presence of quantum external noise we are not aware of any in which the effect of classical noise in a simple model of qubit, namely a spin 1/2 interacting with an external classical field with a fluctuating component has been analyzed. This is
precisely the aim of this talk. For such system the effects of classical fluctuations in the control parameter on both geometric and dynamic phases is studied and their impact on dephasing analyzed. We will explicitly shown that in the adiabatic limit dephasing is due to fluctuations of the dynamical phase.