Conditional Frequency-Domain Beamsplitters Using Phase Modulators

We propose and explore the families of fully tunable, conditional 2 × 2 frequency-domain beamsplitters that result from sideband coupling in phase modulators. Full tunability is obtained by tailoring the driving voltage feeding the modulators, in a design based on the symmetries of this voltage waveform. The most favorable case in terms of success probability is that of first-sideband coupling, which can be implemented by the use of single-tone phase modulation. The use of this device is illustrated by means of several examples based on the use of a wavelength-routed source of pairs of single-photon frequency-entangled states, showing immediate applications as a means of translating to the frequency-domain quantum-processing protocols based on linear optics.