Practical private database queries based on a quantum-key-distribution protocol

Private queries allow a user, Alice, to learn an element of a database held by a provider, Bob, without revealing which element she is interested in, while limiting her information about the other elements. We propose to implement private queries based on a quantum-key-distribution protocol, with changes only in the classical postprocessing of the key. This approach makes our scheme both easy to implement and loss tolerant. While unconditionally secure private queries are known to be impossible, we argue that an interesting degree of security can be achieved by relying on fundamental physical principles instead of unverifiable security assumptions in order to protect both the user and the database. We think that the scope exists for such practical private queries to become another remarkable application of quantum information in the footsteps of quantum key distribution.

Figure 1

How to reduce Alice’s information: her information on a sum string is lower than that on the initial strings. Question marks symbolize bits whose value is unknown to Alice.

Figure 2

The upper bound on the success probability of the joint unambiguous state discrimination (USD) measurement on $k$ qubits declines rapidly with $k$.