Path integral derivation of the thermofield double state in causal diamonds

In this article, we adopt the framework developed by Laflamme (1989 Physica A 158 58–63) to analyze the path integral of a massless—conformally invariant—scalar field defined on a causal diamond (CD) of size 2α in 1+1 dimensions. By examining the Euclidean geometry of the CD, we establish that its structure is conformally related to the cylinder , where the Euclidean time coordinate τ has a periodicity of β. This property, along with the conformal symmetry of the fields, allows us to identify the connection between the thermofield double (TFD) state of CDs and the Euclidean path integral defined on the two disconnected manifolds of the cylinder. Furthermore, we demonstrate that the temperature of the TFD state, derived from the conditions in the Euclidean geometry and analytically calculated, coincides with the temperature of the CD known in the literature. This derivation highlights the universality of the connection between the Euclidean path integral formalism and the TFD state of the CD, as well as it further establishes CDs as a model that exhibits all desired properties of a system exhibiting the Unruh effect.