Microscopic observation of magnon bound states and their dynamics
Sep 25, 2013Citations per year
Abstract: (Springer)
Bound states of elementary spin waves (magnons) have been predicted to occur in one-dimensional quantum magnets; the observation of two-magnon bound states in a system of ultracold bosonic atoms in an optical lattice is now reported. Two papers published in this issue of Nature show that the propagation of energy quanta in very different physical systems can exhibit the same, unusual dynamics, where bound pairs of quanta become dominant. Ofer Firstenberg et al. realize coherent interactions between individual photons — quanta of light — which are massless and do not usually interact. They achieve this using a quantum nonlinear medium inside which individual photons pair up and travel as massive particles with strong mutual attraction. Potential applications of this technique include all-optical switching, deterministic photonic quantum logic and the generation of strongly correlated states of light. The second paper deals with magnons, the quanta that carry energy in magnets. More than eighty years ago, Hans Bethe predicted the existence of bound states of elementary spin waves (magnons) in one-dimensional quantum magnets. Experimental observation of the phenomenon remained elusive, but now Takeshi Fukuhara et al. have observed two-magnon bound states in a system of ultracold bosonic atoms in an optical lattice. The results provide a new way of studying fundamental properties of quantum magnets. In an accompanying News and Views, Sougato Bose puts these two independent findings into a general context of quantum many-body dynamics. The existence of bound states of elementary spin waves (magnons) in one-dimensional quantum magnets was predicted almost 80 years ago1. Identifying signatures of magnon bound states has so far remained the subject of intense theoretical research2,3,4,5, and their detection has proved challenging for experiments. Ultracold atoms offer an ideal setting in which to find such bound states by tracking the spin dynamics with single-spin and single-site resolution6,7 following a local excitation8. Here we use in situ correlation measurements to observe two-magnon bound states directly in a one-dimensional Heisenberg spin chain comprising ultracold bosonic atoms in an optical lattice. We observe the quantum dynamics of free and bound magnon states through time-resolved measurements of two spin impurities. The increased effective mass of the compound magnon state results in slower spin dynamics as compared to single-magnon excitations. We also determine the decay time of bound magnons, which is probably limited by scattering on thermal fluctuations in the system. Our results provide a new way of studying fundamental properties of quantum magnets and, more generally, properties of interacting impurities in quantum many-body systems.- Bose–Einstein condensates
- Quantum information
- Ultracold gases
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