Quantum communication complexity of block-composed functions

  • Authors:
  • Yaoyun Shi;Yufan Zhu

  • Affiliations:
  • Department of Electrical and Computer Engineering, The University of Michigan, Ann Arbor, Michigan;Google Inc., Amphitheatre Parkway, Mountain View, California

  • Venue:
  • Quantum Information & Computation
  • Year:
  • 2009

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Abstract

A major open problem in communication complexity is whether or not quantum protocolscan be exponentially more efficient than classical ones for computing a total Booleanfunction in the two-party interactive model. Razborov's result (Izvestiya: Mathematics,67(1):145-159, 2002) implies the conjectured negative answer for functions F of thefollowing form: F(x, y) = fn(x1 ċ y1, x2 ċ y2, ..., xn ċ yn), where fn is a symmetric Booleanfunction on n Boolean inputs, and xi, yi are the i'th bit of x and y, respectively. Hisproof critically depends on the symmetry of fn. We develop a lower-bound method that does not require symmetry and prove theconjecture for a broader class of functions. Each of those functions F is the "block-composition" of a "building block" gk : {0, 1}k × {0, 1}k → {0, 1}, and an fn : {0, 1}n →{0, 1}, such that F(x, y) = fn(gk(x1, y1), gk(x2, y2), ..., gk(xn, yn)), where xi and xi arethe i'th k-bit block of x, y ∈ {0, 1}nk, respectively. We show that as long as gk itselfis "hard" enough, its block-composition with an arbitrary fn has polynomially relatedquantum and classical communication complexities. For example, when gk is the InnerProduct function with k = Ω(log n), the deterministic communication complexity of itsblock-composition with any fn is asymptotically at most the quantum complexity to thepower of 7.