Private queries allow a user Alice to learn an element of a database held bya provider Bob without revealing which element she was interested in, whilelimiting her information about the other elements. We propose to implementprivate queries based on a quantum key distribution protocol, with changes onlyin the classical post-processing of the key. This approach makes our schemeboth easy to implement and loss-tolerant. While unconditionally secure privatequeries are known to be impossible, we argue that an interesting degree ofsecurity can be achieved, relying on fundamental physical principles instead ofunverifiable security assumptions in order to protect both user and database.We think that there is scope for such practical private queries to becomeanother remarkable application of quantum information in the footsteps ofquantum key distribution.
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