Microscopic observation of magnon bound states and their dynamics

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T. Fukuhara, P. Schauß, M. Endres, S. Hild, M. Cheneau, I. Bloch and C. Gross
Nature 502, 76-79 (2013)

More than eighty years ago Hans Bethe predicted that elementary spin waves, known as magnons, in onedimensional
quantum magnets can form bound states - i.e. that two magnons can pair up and propogate
together. Identifying signatures of magnon bound states has so far remained the subject of intense theoretical
research, and their detection has proved challenging for experiments.

Ultracold atoms offer an ideal setting in which to find such bound states, and this is exactly where Fukuhara
and colleagues have done just that. They were able to directly observe both single and bound magnon states
and document the slower dynamics of the bound state due to its larger effective mass. Their results, as well
as bringing an eighty year hunt to an end, provide a new way of studying fundamental properties of quantum