  Quantum entanglement is closely related to the structure of spacetime in
quantum gravity. For quantum field theories or statistical models, we usually
consider base space entanglement. However, target space instead of base space
sometimes directly connects to our spacetime. In these cases, it is natural to
consider a concept of target space entanglement. To define the target space
entanglement, we consider a generalized definition of entanglement entropy
based on an algebraic approach. This approach is reviewed and is applied to the
first quantized particles, in particular, fermions. This article is based on
the paper JHEP 08 (2021) 046 [arXiv:2105.13726].
