Quantum systems on non-k-hyperfinite complexes: a generalization of classical statistical mechanics on expander graphs

  • Authors:
  • Michael H. Freedman;Matthew B. Hastings

  • Affiliations:
  • Microsoft Research, University of California, Santa Barbara, CA;Microsoft Research, University of California, Santa Barbara, CA

  • Venue:
  • Quantum Information & Computation
  • Year:
  • 2014

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Abstract

We construct families of cell complexes that generalize expander graphs. These families are called non-k-hyperfinite, generalizing the idea of a non-hyperfinite (NH) family of graphs. Roughly speaking, such a complex has the property that one cannot remove a small fraction of points and be left with an object that looks k - 1-dimensional at large scales. We then consider certain quantum systems on these complexes. A future goal is to construct a family of Hamiltonians such that every low energy state has topological order as part of an attempt to prove the quantum PCP conjecture. This goal is approached by constructing a toric code Hamiltonian with the property that every low energy state without vertex defects has topological order, a property that would not hold for any local system in any lattice Zd or indeed on any 1-hyperfinite complex. Further, such NH complexes find application in quantum coding theory. The hypergraph product codes[1] of Tillich and Zémor are generalized using NH complexes.