In this talk I will be discussing my thesis, which is divided into two parts. In the first part I will be discussing an observer independent approach to quantum field theory developed by Sorkin and Johnston. I will also discuss our results on the Sorkin-Johnston vacuum in 2d causal diamond and in conformally ultrastatic slab spacetimes. In the second part I will be discussing a spacetime formulation of entanglement entropy of quantum fields. In particular I will discuss spacetime entanglement entropy calculations in a static patch of de Sitter spacetimes and in a causal diamond embedded in a 2d cylinder slab spacetime.