The complexity of stoquastic local Hamiltonian problems

  • Authors:
  • Sergey Bravyi;David P. Divincenzo;Roberto Oliveira;Barbara M. Terhal

  • Affiliations:
  • IBM Watson Research Center, Yorktown Heights, NY;IBM Watson Research Center, Yorktown Heights, NY;IBM Watson Research Center, Yorktown Heights, NY;IBM Watson Research Center, Yorktown Heights, NY

  • Venue:
  • Quantum Information & Computation
  • Year:
  • 2008

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Abstract

We study the complexity of the Local Hamiltonian Problem (denoted as LH-MIN) in the special case when a Hamiltonian obeys the condition that all off-diagonal matrix elements in the standard basis are real and non-positive. We will call such Hamiltonians, which are common in the natural world, stoquastic. An equivalent characterization of stoquastic Hamiltonians is that they have an entry-wise non-negative Gibbs density matrix for any temperature. We prove that LH-MIN for stoquastic Hamiltonians belongs to the complexity class AM -- a probabilistic version of NP with two rounds of communication between the prover and the verifier. We also show that 2-local stoquastic LH-MIN is hard for the class MA. With the additional promise of having a polynomial spectral gap, we show that stoquastic LH-MIN belongs to the class PostBPP=BPPpath--a generalization of BPP in which a post-selective readout is allowed. This last result also shows that any problem solved by adiabatic quantum computation using stoquastic Hamiltonians is in PostBPP.