On the Role of Hadamard Gates in Quantum Circuits

  • Authors:
  • D. J. Shepherd

  • Affiliations:
  • Department of Computer Science, University of Bristol, Bristol, UK B58 ITH

  • Venue:
  • Quantum Information Processing
  • Year:
  • 2006

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Abstract

We study a reduced quantum circuit computation paradigm in which the only allowable gates either permute the computational basis states or else apply a "global Hadamard operation", i.e. apply a Hadamard operation to every qubit simultaneously. In this model, we discuss complexity bounds (lower-bounding the number of global Hadamard operations) for common quantum algorithms: we illustrate upper bounds for Shor's Algorithm, and prove lower bounds for Grover's Algorithm. We also use our formalism to display a gate that is neither quantum-universal nor classically simulable, on the assumption that Integer Factoring is not in BPP.