Frequency hopping in quantum interferometry: efficient up-down conversion for qubits and ebits.

A novel Mach-Zehnder interferometer terminated at two different frequencies realizes in a quantum regime the nonlinear frequency conversion of optical quantum superposition states. The information-preserving character of the relevant unitary transformation has been experimentally demonstrated for input qubits and ebits. Besides its own intrinsic fundamental interest, the new scheme is expected to find important applications in modern quantum information technology.