  Almost one century ago, string states - complex bound states (Wellenkomplexe)
of magnetic excitations - have been predicted to exist in one-dimensional
quantum magnets and since then become a subject of intensive theoretical study.
However, experimental realization and identification of string states in
condensed-matter systems remains an unsolved challenge up to date. Here we use
high-resolution terahertz spectroscopy to identify string states in the
antiferromagnetic Heisenberg-Ising chain SrCo2V2O8 in strong longitudinal
magnetic fields. We observe complex bound states (strings) and fractional
magnetic excitations (psinons and antipsinons) in the field-induced critical
regime, which are precisely described by the Bethe ansatz. Our study reveals
that two-string and three-string states govern the quantum spin dynamics close
to the quantum criticality, while the fractional excitations are dominant at
low energies, reflecting the antiferromagnetic quantum fluctuations.
