  Quantum entanglement permeates the complex ground states of correlated
electron materials defying single-particle descriptions. Coupled magnetic atoms
have potential as model systems for entanglement in condensed matter giving the
opportunity to create artificial many-body states which can be controlled by
tuning the underlying interactions. They provide an avenue to unravel the
complexities of correlated-electron materials. Here we use low temperature
scanning tunnelling microscopy (STM) and atomic manipulation to tune
entanglement in chains of magnetic atoms. We find that a Kondo singlet state
can emerge from this entanglement. The many electron Kondo state is based on
the screening of the entangled spin ground state of the chain by substrate
electrons and can be engineered to envelop at least ten magnetic atoms. The
concomitant Kondo resonance measured in the differential conductance enables
the electric read-out of entanglement. By tuning composition and coupling
strength within atomic chains it is possible to create model spin chains with
defined entanglement. This lays the foundation for a new class of experiments
to construct exotic correlated-electron materials atom by atom.
