Commutative version of the local Hamiltonian problem and common eigenspace problem

  • Authors:
  • Sergey Bravyi;Mikhail Vyalyi

  • Affiliations:
  • Institute for Quantum Information, California Institute of Technology, Pasadena, CA;Independent University of Moscow, Moscow, Russia

  • Venue:
  • Quantum Information & Computation
  • Year:
  • 2005

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Abstract

We study the complexity of a problem "Common Eigenspace" -- verifying consistency of eigenvalue equations for composite quantum systems. The input of the problem is a family of pairwise commuting Hermitian operators H1,..., Hr on a Hilbert space (Cd)⊗n and a string of real numbers λ =(λ1,...,λr). The problem is to determine whether the common eigenspace specified by equalities Ha|Ø〉 = λa|Ø〉, a = 1, ..., r has a positive dimension. We consider two cases: (i) all operators Ha are k-local; (ii) all operators Ha are factorized. It can be easily shown that both problems belong to the class QMA -- quantum analogue of NP, and that some NP-complete problems can be reduced to either (i) or (ii). A non-trivial question is whether the problems (i) or (ii) belong to NP? We show that the answer is positive for some special values of k and d. Also we prove that the problem (ii) can be reduced to its special case, such that all operators Ha are factorized projectors and all λa = 0.