Quantum lower bounds by quantum arguments

  • Authors:
  • Andris Ambainis

  • Affiliations:
  • Department of Computer Sciences, University of California, Berkeley, California

  • Venue:
  • Journal of Computer and System Sciences - Special issue on STOC 2000
  • Year:
  • 2002

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Abstract

We propose a new method for proving lower bounds on quantum query algorithms. Instead of a classical adversary that runs the algorithm with on input and then modifies the input, we use a quantum adversary that runs the algorithm with a superposition of inputs. If the algorithm works correctly, its state becomes entangled with the superposition over inputs. We bound the number of queries needed to achieve a sufficient entanglement and this implies a lower bound on the number of queries for the computation. Using this method, we prove two new Ω(√N) lower bounds on computing AND of ORs and inverting a permutation and also provide more uniform proofs for several known lower bounds which have been previously proven via a variety of different techniques.