Security Improvements of Several Basic Quantum Private Query Protocols with O(log N) Communication Complexity

TL;DR

This paper introduces enhanced quantum private query protocols that maintain low communication complexity while significantly improving security and user privacy protections against dishonest databases.

Contribution

It presents new quantum private query protocols with O(log N) communication complexity that offer cheat-sensitive properties and better privacy guarantees compared to prior protocols.

Findings

Randomized protocol detects dishonest databases with nonzero probability.

Maximum user privacy leakage is halved in the new protocol compared to previous ones.

User can detect cheating with probability close to 1/2 using O(√N) special queries.

Abstract

New quantum private database (with N elements) query protocols are presented and analyzed. Protocols preserve O(logN) communication complexity of known protocols for the same task, but achieve several significant improvements in security, especially concerning user privacy. For example, the randomized form of our protocol has a cheat-sensitive property - it allows the user to detect a dishonest database with a nonzero probability, while the phase-encoded private query protocols for the same task do not have such a property. Moreover, when the database performs the computational basis measurement, a particular projective measurement which can cause a significant loss of user privacy in the previous private query protocols with O(logN) communication complexity, at most half of the user privacy could leak to such a database in our protocol, while in the QPQ protocol, the entire user privacy could leak out. In addition, it is proved here that for large N, the user could detect a cheating via the computational basis measurement, with a probability close to 1/2 using O(\sqrt{N}) special queries. Finally, it is shown here, for both forms of our protocol, basic and randomized, how a dishonest database has to act in case it could not learn user's queries.