Abstract: In the field of optical quantum information processing, manipulation of single photon qubits in frequency modes employs a frequency beamsplitter employs an asymmetric two-path interferometer, reversible down to the quantum limit. A first partially transmitting mirror splits photons into first and second paths. A time delay element introduces a differential time delay into the second path. And a second partially transmitting mirror mixes the two paths again to form two outputs. A half-wave plate utilizes two of the beam splitters.