1. Single Quantum Dot Project, ERATO, Japan Science and Technology Corporation, 5‐9‐9 Tokodai, Tsukuba 300‐2635, Japan
2. Present address: Hikari Building, 1‐8‐18 Nishi‐Shinsaibashi, Chuo‐ku, Osaka 542‐0086, Japan; Fax: +81‐6‐6245‐9697
3. Institute of Applied Physics, University of Tsukuba, 1‐1‐1 Tennodai, Tsukuba 305‐8573, Japan
4. Japan Advanced Institute of Science and Technology, Tatsunokuchi, Ishikawa 923‐1292, Japan
5. National Institute for Materials Science, 1‐2‐1 Sengen, Tsukuba 305‐0047, Japan