At Swansea University since 2005, Professor Madsen is an experimental physicist who specializes in work on fundamental physics with antimatter. He is co-founder and group leader in the ALPHA collaboration that was the first group to trap antihydrogen and observe the first quantum transitions in it and is pursuing precision comparisons with hydrogen.
He has been actively involved in antihydrogen research since 2001, playing a substantial role in the ATHENA team that first formed low energy antihydrogen in 2002. He has a research group at CERN. His group plays a leading role in the ALPHA experiment, and led the effort to implement several key techniques leading to the first antihydrogen trapping and spectroscopy. For this work he has been awarded a Royal Society Senior Leverhulme Fellowship in 2010, and the 2011 James Dawson award for Excellence in Plasma Physics Research and a fellow of the American Physical Society in 2015.