Nature Physics 5, 198–202 (2009); published online: 25 January 2009; corrected after print: 1 December 2009.

The authors wish to point out that in all versions of this Letter originally published, the pure spontaneous emission rate of 356 (±11) MHz quoted on pages 198 and 199 is overestimated by a factor or four owing to an instrumentation-setting error, and erroneously quoted in units of angular frequency. The correct number should read 227 (±7) MHz in linear frequency. The upper bound for fast dephasing mechanisms is therefore limited to 81 MHz, not 18 MHz. The main arguments of the paper are unaffected by this correction. These changes have been made in both PDF and HTML versions of this Letter.