Adiabatic ground state preparation in an expanding lattice

We implement and characterize a numerical algorithm inspired by the s source framework [B. Swingle and J. McGreevy, Phys. Rev. B 93, 045127 (2016)2469-995010.1103/PhysRevB.93.045127] for building a quantum many-body ground state wave function on a lattice of size 2L by applying adiabatic evolution to the corresponding ground state at size L, along with L interleaved ancillae. The procedure can in principle be iterated to repeatedly double the size of the system. We implement the algorithm for several one-dimensional (1D) spin model Hamiltonians, and find that the construction works particularly well when the gap is large and, interestingly, at scale-invariant critical points. We explain this feature as a natural consequence of the lattice expansion procedure. This behavior holds for both the integrable transverse-field Ising model and nonintegrable variations. We also develop an analytic perturbative understanding of the errors deep in either phase of the transverse-field Ising model, and suggest how the circuit could be modified to parametrically reduce errors. In addition to sharpening our perspective on entanglement renormalization in 1D, the algorithm could also potentially be used to build states experimentally, enabling the realization of certain long-range correlated states with low-depth quantum circuits.