1. Center for Engineered Quantum Systems, Dept. of Physics & Astronomy, Macquarie University, 2109 NSW, Australia
2. Dipartimento di Fisica and MECENAS, Università di Bari, I-70126 Bari, Italy
3. INFN, Sezione di Bari, I-70126 Bari, Italy
4. Department of Physics, Waseda University, Tokyo 169-8555, Japan