1. Department of Physics University of Torino Torino 10125 Italy
2. Istituto Nazionale di Fisica Nucleare (INFN) sezione di Torino Torino 10125 Italy
3. Istituto Nazionale di Ricerca Metrologica (INRiM) Torino 10135 Italy
4. Department of Electronics and Communications Politecnico di Torino Torino 10129 Italy
5. Institute for Quantum Optics Universität Ulm D‐89069 Ulm Germany
6. Applied Quantum Systems, Felix‐Bloch Institute for Solid‐State Physics Universität Leipzig 04103 Leipzig Germany
7. V.S. Sobolev Institute of Geology and Mineralogy Siberian Branch of the Russian Academy of Sciences Academician Koptyug Ave., 3 Novosibirsk 630090 Russian Federation