Microscopic observation of magnon bound states and their dynamics

TL;DR

This paper reports the direct observation of magnon bound states in a one-dimensional quantum magnet using ultracold atoms, revealing their dynamics, effective mass, and decay properties through in-situ measurements.

Contribution

First experimental detection of two-magnon bound states in a 1D quantum magnet using ultracold atoms with time-resolved in-situ correlation measurements.

Findings

Observation of quantum walk of bound and free magnons

Determination of the increased effective mass of bound magnons

Measurement of the decay time limited by thermal fluctuations

Abstract

More than eighty years ago, H. Bethe pointed out the existence of bound states of elementary spin waves in one-dimensional quantum magnets. To date, identifying signatures of such magnon bound states has remained a subject of intense theoretical research while their detection has proved challenging for experiments. Ultracold atoms offer an ideal setting to reveal such bound states by tracking the spin dynamics after a local quantum quench with single-spin and single-site resolution. Here we report on the direct observation of two-magnon bound states using in-situ correlation measurements in a one-dimensional Heisenberg spin chain realized with ultracold bosonic atoms in an optical lattice. We observe the quantum walk of free and bound magnon states through time-resolved measurements of the two spin impurities. The increased effective mass of the compound magnon state results in slower spin dynamics as compared to single magnon excitations. In our measurements, we also determine the decay time of bound magnons, which is most likely limited by scattering on thermal fluctuations in the system. Our results open a new pathway for studying fundamental properties of quantum magnets and, more generally, properties of interacting impurities in quantum many-body systems.