  Quantum entanglement is crucial for simulating and understanding exotic
physics of strongly correlated many-body systems, such as high--temperature
superconductors, or fractional quantum Hall states. The entanglement of
non-identical particles exhibits richer physics of strong many-body
correlations and offers more opportunities for quantum computation, especially
with neutral atoms where in contrast to ions the interparticle interaction is
widely tunable by Feshbach resonances. Moreover, the inter-species entanglement
forms a basis for the properties of various compound systems, ranging from
Bose-Bose mixtures to photosynthetic light-harvesting complexes. So far, the
inter-species entanglement has only been obtained for trapped ions. Here we
report on the experimental realization of entanglement of two neutral atoms of
different isotopes. A ${}^{87}\mathrm{Rb}$ atom and a ${}^{85}\mathrm{Rb}$ atom
are confined in two single--atom optical traps separated by 3.8 $\mu$m.
Creating a strong Rydberg blockade, we demonstrate a heteronuclear
controlled--NOT (C--NOT) quantum gate and generate a heteronuclear entangled
state, with raw fidelities $0.73 \pm 0.01$ and $0.59 \pm 0.03$, respectively.
Our work, together with the technologies of single--qubit gate and C--NOT gate
developed for identical atoms, can be used for simulating any many--body system
with multi-species interactions. It also has applications in quantum computing
and quantum metrology, since heteronuclear systems exhibit advantages in low
crosstalk and in memory protection.
