Performing private database queries in a real-world environment using a quantum protocol

We present a practical quantum protocol that allows a user to retrieve information from a database while protecting the privacy of the user (i.e. limiting the database's knowledge of what information is retrieved) as well as the privacy of the database (i.e. limiting the amount of information the user can retrieve). This functionality is similar to the cryptographic primitive 1-out-of-N oblivious transfer, which has been well studied in the context of classical information theory. While it has been shown that quantum protocols cannot provide perfect privacy against an arbitrarily powerful quantum computer, they are not vulnerable to improvements in classical computing technology or algorithms. Here we show an experimental demonstration of our new protocol over a deployed fiber channel, and present an analysis showing that our protocol is secure against simple quantum attacks. This makes private queries the second application of quantum communication (after quantum key distribution) that has been demonstrated in a real-world environment, meeting both the requirements of loss- and fault-tolerance.