1. I. L. Chuang, Edward L. Ginzton Laboratory, Stanford University, Stanford, CA 94305, USA.
2. R. Laflamme and W. H. Zurek, Theoretical Astrophysics, T-6, MS B288, Los Alamos National Laboratory, Los Alamos, NM 87545, USA.
3. P. W. Shor, AT&T Bell Labs, 600 Mountain Avenue, Murray Hill, NJ 07974, USA.