Limitations on the simulation of non-sparse hamiltonians

  • Authors:
  • Andrew M. Childs;Robin Kothari

  • Affiliations:
  • Department of Combinatorics & Optimization and Institute for Quantum Computing, University of Waterloo, Waterloo, Ontario, Canada;David R. Cheriton School of Computer Science and Institute for Quantum Computing, University of Waterloo, Waterloo, Ontario, Canada

  • Venue:
  • Quantum Information & Computation
  • Year:
  • 2010

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Abstract

The problem of simulating sparse Hamiltonians on quantum computers is well studied. The evolution of a sparse N × N Hamiltonian H for time t can be simulated using O(||Ht|| poly(logN)) operations, which is essentially optimal due to a no-fast-forwarding theorem. Here, we consider non-sparse Hamiltonians and show significant limitations ontheir simulation. We generalize the no-fast-forwarding theorem to dense Hamiltonians, ruling out generic simulations taking time o(||Ht||), even though ||H|| is not a uniquemeasure of the size of a dense Hamiltonian H. We also present a stronger limitationruling out the possibility of generic simulations taking time poly(||Ht||; logN), showingthat known simulations based on discrete-time quantum walk cannot be dramatically improved in general. On the positive side, we show that some non-sparse Hamiltonianscan be simulated efficiently, such as those with graphs of small arboricity.