Nature Physics 8, 185–189 (2012); published online 15 January 2012; corrected after print 12 March 2012.

In the version of this Letter originally published, in Fig. 5a a factor of ħ/2 appeared incorrectly against the label 'Heisenberg lower limit' and in the figure caption. This error has been corrected in the HTML and PDF versions of the Letter.