[1] Univ Calif San Diego, Dept Phys, La Jolla, CA 92093 USA
[2] Scuola Normale Super Pisa, NEST, I-56126 Pisa, Italy
[3] CNR, Ist Nanosci, I-56126 Pisa, Italy
[4] Univ Pisa, Dipartimento Fis, I-56127 Pisa, Italy
[5] INFN, I-56127 Pisa, Italy
[6] CNR, Ist Nanosci, NEST, I-56126 Pisa, Italy
[7] Scuola Normale Super Pisa, I-56126 Pisa, Italy
[8] Natl Univ Singapore, Ctr Quantum Technol, Singapore 117548, Singapore