Nature 440, 87–90 (2006)

Equation (1) in this paper was incorrect. The correct equation is:

The equation given in Section 1.5 of the Supplementary Information (page 4) was also incorrect (it lacked the minus sign). The Supplementary Information has now been updated. All results reported in the paper were obtained using the correct equations.

Supplementary Information is linked to the online version of the original paper.