  The Quantum Private Query is a quantum cryptographic protocol to recover
information from a database, preserving both user and data privacy: the user
can test whether someone has retained information on which query was asked, and
the database provider can test the quantity of information released. Here we
introduce a new variant Quantum Private Query algorithm which admits a simple
linear optical implementation: it employs the photon's momentum (or time slot)
as address qubits and its polarization as bus qubit. A proof-of-principle
experimental realization is implemented.
