Multi User Quantum Private Query Protocol
Recently, Gao et al. suggested a flexible quantum private query (QPQ) protocol (Opt Express, 2012, 20(16):17411) through Jakobi et al.'s quantum key distribution based QPQ protocol (Phys Rev A, 2011, 83:022301) Inspired by Gao et al.'s QPQ protocol, we design a multi user QPQ protocol by using the product states of single photon states. Its database privacy and user privacy should be compromised by choosing an suitable value for th. Due to the honesty check process for the database owner, an smaller th can be priorly chosen to guarantee better database privacy. Moreover, it naturally inherits some good characteristics of Gao et al.'s QPQ protocol so that it is also loss tolerant, practical to huge database condition, robust against quantum memory attack and flexible. It differs from Gao et al.'s QPQ protocol on three aspects: firstly, it adds the process of checking the presence of an illegal eavesdropper; secondly, it adds the process of checking the database owner's honesty; and finally, it is feasible for n users to query the items of database simultaneously.