Coloring an Orthogonality Graph

  • Authors:
  • C. D. Godsil;M. W. Newman

  • Affiliations:
  • cgodsil@uwaterloo.ca;mikenewman@alumni.uwaterloo.ca

  • Venue:
  • SIAM Journal on Discrete Mathematics
  • Year:
  • 2008

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Abstract

We deal with a graph coloring problem that arises in quantum information theory. Alice and Bob are each given a $\pm1$-vector of length $2^k$ and are to respond with $k$ bits. Their responses must be equal if they are given equal inputs, and distinct if they are given orthogonal inputs; however, they are not allowed to communicate any information about their inputs. They can always succeed using quantum entanglement, but their ability to succeed using only classical physics is equivalent to a graph coloring problem. We resolve the graph coloring problem, thus determining that they can succeed without entanglement exactly when $k\leq3$.