http://prl.aps.org/abstract/PRL/v106/i25/e253001 (Publisher version)
http://arxiv.org/abs/1104.1206 (Preprint)
Ulmer, S., Rodegheri, C. C., Blaum, K., Kracke, H., Mooser, A., Quint, W., et al. (2011). Observation of Spin Flips with a Single Trapped Proton. Pysical Review Letters, 106(25): 253001, pp. 1-4. doi:10.1103/PhysRevLett.106.253001.