1. Department of Physics and Astronomy, University of Florence, I-50019 Sesto Fiorentino, Italy.
2. European Laboratory for Non-Linear Spectroscopy (LENS), I-50019 Sesto Fiorentino, Italy.
3. Institute for Quantum Optics and Quantum Information of the Austrian Academy of Sciences, A-6020 Innsbruck, Austria.
4. Institute for Theoretical Physics, University of Innsbruck, A-6020 Innsbruck, Austria.
5. Istituto Nazionale di Ottica del Consiglio Nazionale delle Ricerche (INO-CNR), Sezione di Sesto Fiorentino, I-50019 Sesto Fiorentino, Italy.
6. Istituto Nazionale di Ricerca Metrologica (INRIM), I-10135 Torino, Italy.