I will consider in this talk the regime of strong interaction between a two-level system (for instance a superconducting charge qubit) and a bath of quantum oscillators (realized e.g. by a long chain of Josephson junctions). The structure of the environment will be found to involve quantum superpositions of adiabatic and anti-adiabatic bath modes, due to the energetic competition between oscillator potential energy and quantum tunneling. This picture leads to peculiar entanglement properties, that offer an interesting look into the structure of wavefunctions describing strongly correlated many-body states. Based on Wigner tomography ideas, a systematic variational Ansatz will be proposed and confirmed by Numerical Renormalization Group and Bethe Ansatz results.

Institut Néel, Grenoble