中国物理B ›› 2019, Vol. 28 ›› Issue (12): 120301-120301.doi: 10.1088/1674-1056/ab4e88

• GENERAL • 上一篇    下一篇

Quantum search for unknown number of target items by hybridizing fixed-point method with trail-and-error method

Tan Li(李坦), Shuo Zhang(张硕), Xiang-Qun Fu(付向群), Xiang Wang(汪翔), Yang Wang(汪洋), Jie Lin(林杰), Wan-Su Bao(鲍皖苏)   

  1. 1 Henan Key Laboratory of Quantum Information and Cryptography, PLA SSF IEU, Zhengzhou 450001, China;
    2 Synergetic Innovation Center of Quantum Information and Quantum Physics, University of Science and Technology of China, Hefei 230026, China
  • 收稿日期:2019-08-11 修回日期:2019-09-16 出版日期:2019-12-05 发布日期:2019-12-05
  • 通讯作者: Wan-Su Bao E-mail:bws@qiclab.cn
  • 基金资助:
    Project supported by the National Natural Science Foundation of China (Grant Nos. 11504430 and 61502526) and the National Basic Research Program of China (Grant No. 2013CB338002).

Quantum search for unknown number of target items by hybridizing fixed-point method with trail-and-error method

Tan Li(李坦)1,2, Shuo Zhang(张硕)1,2, Xiang-Qun Fu(付向群)1,2, Xiang Wang(汪翔)1,2, Yang Wang(汪洋)1,2, Jie Lin(林杰)1,2, Wan-Su Bao(鲍皖苏)1,2   

  1. 1 Henan Key Laboratory of Quantum Information and Cryptography, PLA SSF IEU, Zhengzhou 450001, China;
    2 Synergetic Innovation Center of Quantum Information and Quantum Physics, University of Science and Technology of China, Hefei 230026, China
  • Received:2019-08-11 Revised:2019-09-16 Online:2019-12-05 Published:2019-12-05
  • Contact: Wan-Su Bao E-mail:bws@qiclab.cn
  • Supported by:
    Project supported by the National Natural Science Foundation of China (Grant Nos. 11504430 and 61502526) and the National Basic Research Program of China (Grant No. 2013CB338002).

摘要: For the unsorted database quantum search with the unknown fraction λ of target items, there are mainly two kinds of methods, i.e., fixed-point and trail-and-error. (i) In terms of the fixed-point method, Yoder et al.[Phys. Rev. Lett. 113 210501 (2014)] claimed that the quadratic speedup over classical algorithms has been achieved. However, in this paper, we point out that this is not the case, because the query complexity of Yoder's algorithm is actually in O(1/√λ0) rather than O(1/√λ), where λ0 is a known lower bound of λ. (ii) In terms of the trail-and-error method, currently the algorithm without randomness has to take more than 1 times queries or iterations than the algorithm with randomly selected parameters. For the above problems, we provide the first hybrid quantum search algorithm based on the fixed-point and trail-and-error methods, where the matched multiphase Grover operations are trialed multiple times and the number of iterations increases exponentially along with the number of trials. The upper bound of expected queries as well as the optimal parameters are derived. Compared with Yoder's algorithm, the query complexity of our algorithm indeed achieves the optimal scaling in λ for quantum search, which reconfirms the practicality of the fixed-point method. In addition, our algorithm also does not contain randomness, and compared with the existing deterministic algorithm, the query complexity can be reduced by about 1/3. Our work provides a new idea for the research on fixed-point and trial-and-error quantum search.

关键词: quantum search, fixed-point, trail-and-error, unknown number of target items

Abstract: For the unsorted database quantum search with the unknown fraction λ of target items, there are mainly two kinds of methods, i.e., fixed-point and trail-and-error. (i) In terms of the fixed-point method, Yoder et al.[Phys. Rev. Lett. 113 210501 (2014)] claimed that the quadratic speedup over classical algorithms has been achieved. However, in this paper, we point out that this is not the case, because the query complexity of Yoder's algorithm is actually in O(1/√λ0) rather than O(1/√λ), where λ0 is a known lower bound of λ. (ii) In terms of the trail-and-error method, currently the algorithm without randomness has to take more than 1 times queries or iterations than the algorithm with randomly selected parameters. For the above problems, we provide the first hybrid quantum search algorithm based on the fixed-point and trail-and-error methods, where the matched multiphase Grover operations are trialed multiple times and the number of iterations increases exponentially along with the number of trials. The upper bound of expected queries as well as the optimal parameters are derived. Compared with Yoder's algorithm, the query complexity of our algorithm indeed achieves the optimal scaling in λ for quantum search, which reconfirms the practicality of the fixed-point method. In addition, our algorithm also does not contain randomness, and compared with the existing deterministic algorithm, the query complexity can be reduced by about 1/3. Our work provides a new idea for the research on fixed-point and trial-and-error quantum search.

Key words: quantum search, fixed-point, trail-and-error, unknown number of target items

中图分类号:  (Quantum algorithms, protocols, and simulations)

  • 03.67.Ac
03.67.-a (Quantum information) 03.65.-w (Quantum mechanics)