Quantum Private Information Retrieval has Linear Communication Complexity

In private information retrieval (PIR), a client queries an \(n\)-bit database in order to retrieve an entry of her choice, while maintaining privacy of her query value. Chor et al. [J ACM 45(6):965–981, 1998] showed that, in the information-theoretical setting, a linear amount of communication is required for classical PIR protocols (thus the trivial protocol is optimal). This linear lower bound was shown by Nayak [FOCS 1999, pp. 369–376, 1999] to hold also in the quantum setting. Here, we extend Nayak’s result by considering approximate privacy, and requiring security only against specious adversaries, which are, in analogy to classical honest-but-curious adversaries, the weakest reasonable quantum adversaries. We show that, even in this weakened scenario, quantum private information retrieval (QPIR) requires \(n\) qubits of communication. From this follows that Le Gall’s recent QPIR protocol with sublinear communication complexity [Theory Comput. 8(1):369–374, 2012] is not information-theoretically private, against the weakest reasonable cryptographic adversary.