Nature Phys. 9, 405–409 (2013); published online 26 May 2013; corrected online 4 June 2013; corrected after print 23 December 2013

In the version of this Letter originally published, the stated value for the difference in the gyromagnetic ratio for the |↑> and |↓> state, δγ, was missing a factor of 2π. Consequently, the diffusion constants were overestimated by a factor of (2π)2. In particular, the value at ln(kFa2D) = 0 stated in the text should be 6.3(8) × 10-3ħ/m instead of 0.25(3)ħ/m. Other quantities, such as the magnetic field gradient, which was independently calibrated, are not affected. All other statements and observations in the published version are correct and remain unaffected. These errors have now been corrected in the online versions of the Letter.