Quantum Circuit Cosmology: The Expansion of the Universe Since the First
  Qubit
  We consider cosmological evolution from the perspective of quantum
information. We present a quantum circuit model for the expansion of a comoving
region of space, in which initially-unentangled ancilla qubits become entangled
as expansion proceeds. We apply this model to the comoving region that now
coincides with our Hubble volume, taking the number of entangled degrees of
freedom in this region to be proportional to the de Sitter entropy. The quantum
circuit model is applicable for at most 140 $e$-folds of inflationary and
post-inflationary expansion: we argue that no geometric description was
possible before the time $t_1$ when our comoving region was one Planck length
across, and contained one pair of entangled degrees of freedom. This approach
could provide a framework for modeling the initial state of inflationary
perturbations.
