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The connection between real-time quantum field theory (RTQFT) [see, e.g., A.\Kamenev and A.\ Levchenko, Advances in Physics {58} (2009) 197] and phase-spacetechniques [E.\ Wolf and L.\ Mandel, {\em Optical Coherence and Quantum Optics}(Cambridge, 1995)] is investigated. The Keldysh rotation that forms the basisof RTQFT is shown to be a phase-space mapping of the quantum system based onthe symmetric (Weyl) ordering. Following this observation, we definegeneralised Keldysh rotations based on the class of operator orderingsintroduced by Cahill and Glauber [Phys.\ Rev.\ {177} (1969) 1882]. Eachrotation is a phase-space mapping, generalising the corresponding ordering fromfree to interacting fields. In particular, response transformation [L.P.\ andS.S., Ann.\ Phys. (N.Y.) {323} (2008) 1989] extends the normal ordering offree-field operators to the time-normal ordering of Heisenberg\ operators.Structural properties of the response transformation, such as its associationwith the nonlinear quantum response problem and the related causalityproperties, hold for all generalised Keldysh rotations.  Furthermore, we argue that response transformation is especially suited forRTQFT formulation of spatial, in particular, relativistic, problems, because itextends cancellation of zero-point fluctuations, characteristic of the normalordering, to interacting fields. As an example, we consider quantisedelectromagnetic\ field in the Dirac sea. In the time-normally-orderedrepresentation, dynamics of the field looks essentially classical (fieldsradiated by currents), without any contribution from zero-point fluctuations.For comparison, we calculate zero-point fluctuations of the interactingelectromagnetic\ field under orderings other than time-normal. The resultingexpression is physically inconsistent: it does not obey the Lorentz condition,nor Maxwell's equations.

Causal signal transmission by quantum fields -- V: Generalised Keldysh rotations and electromagnetic response of the Dirac sea