Epsilon-net method for optimizations over separable states

  • Authors:
  • Yaoyun Shi;Xiaodi Wu

  • Affiliations:
  • Department of EECS, University of Michigan, Ann Arbor;Department of EECS, University of Michigan, Ann Arbor

  • Venue:
  • ICALP'12 Proceedings of the 39th international colloquium conference on Automata, Languages, and Programming - Volume Part I
  • Year:
  • 2012

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Abstract

We give algorithms for the optimization problem: $\max_\rho \left\langle Q , \rho\right\rangle $, where Q is a Hermitian matrix, and the variable ρ is a bipartite separable quantum state. This problem lies at the heart of several problems in quantum computation and information, such as the complexity of QMA(2). While the problem is NP-hard, our algorithms are better than brute force for several instances of interest. In particular, they give PSPACE upper bounds on promise problems admitting a QMA(2) protocol in which the verifier performs only logarithmic number of elementary gate on both proofs, as well as the promise problem of deciding if a bipartite local Hamiltonian has large or small ground energy. For Q≥0, our algorithm runs in time exponential in ||Q||F. While the existence of such an algorithm was first proved recently by Brandão, Christandl and Yard [Proceedings of the 43rd annual ACM Symposium on Theory of Computation , 343---352, 2011], our algorithm is conceptually simpler.