Quantum search of partially ordered sets

  • Authors:
  • Ashley Montanaro

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

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

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Abstract

We investigate the generalisation of quantum search of unstructured and totally orderedsets to search of partially ordered sets (posets). Two models for poset search are consid-ered. In both models, we show that quantum algorithms can achieve at most a quadraticimprovement in query complexity over classical algorithms, up to logarithmic factors; wealso give quantum algorithms that almost achieve this optimal reduction in complexity.In one model, we give an improved quantum algorithm for searching forest-like posets;in the other, we give an optimal O(√m)-query quantum algorithm for searching posetsderived from m×m arrays sorted by rows and columns. This leads to a quantum algo-rithm that finds the intersection of two sorted lists of n integers in O(√n) time, whichis optimal.