Science 357, 484–487 (2017)

Interacting electrons in some materials can be 'fractionized' into independent particles carrying spin and charge, respectively — an unusual behaviour known as spin–charge separation. Doped holes can then move freely in the system, changing the charge distribution while leaving the magnetic order unaffected. Timon Hilker and colleagues have now shown that, in hole-doped ultracold Fermi–Hubbard chains, this phenomenon can be seen directly.

In solid-state systems, experimental determination of this quasi-long-range order is far from easy, as the oft-used two-point spin correlation functions are sensitive to the presence of holes, making the order rather hidden. Hilker and co-workers solved this problem by using a fermionic quantum gas microscope, which allows full access to the spin and density distributions in the system with single-site resolution. A string correlation function was constructed to reveal the hidden antiferromagnetic order in the ground state. These results provide a microscopic picture of spin–charge separation independent of the more frequently discussed spectral properties or excitation dynamics.