  We describe an explicit mechanism for the emergence of a dynamical
holographic bulk from the structure of entanglement in a quantum state. We
start with a generic system in complete isolation, assuming it has a classical
limit involving coherent states. Then we entangle it with another system of
that kind, and subject the pair to a decohering process. We make a number of
broadly applicable and physically reasonable assumptions about this setup.
First, we assume that the states selected by the decoherence (called pointer
states) have the same local symmetries as the isolated systems, in a sense
which is made precise. We also assume that the modular Hamiltonians of pointer
states scale inversely with Planck's constant, so that the pointer states are
highly entangled in the classical limit. Finally, we require the timescale of
decoherence to scale in a certain way with Planck's constant, so that
decoherence happens very frequently in the classical limit, but not too
frequently. Given these assumptions, we demonstrate that the semiclassical
evolution of the system is dominated by a certain dynamical generalisation of
Uhlmann holonomy. We construct a coherent state path integral for this
evolution, showing that the semiclassical fields evolve in a spacetime with one
more dimension than the isolated case. The additional dimension is generated by
modular flow.
