  Quantum entanglement of identical particles is essential in quantum
information theory. Yet, its correct determination remains an open issue
hindering the general understanding and exploitation of many-particle systems.
Operator-based methods have been developed that attempt to overcome the issue.
We introduce a state-based method which, as second quantization, does not label
identical particles and presents conceptual and technical advances compared to
the previous ones. It establishes the quantitative role played by arbitrary
wave function overlaps, local measurements and particle nature (bosons or
fermions) in assessing entanglement by notions commonly used in quantum
information theory for distinguishable particles, like partial trace. Our
approach furthermore shows that bringing identical particles into the same
spatial location functions as an entangling gate, providing fundamental
theoretical support to recent experimental observations with ultracold atoms.
These results pave the way to set and interpret experiments for utilizing
quantum correlations in realistic scenarios where overlap of particles can
count, as in Bose-Einstein condensates, quantum dots and biological molecular
aggregates.
