Frequency conversion

Phys. Rev. A 85, 013845 (2012)

Quantum communication networks rely on optical devices such as quantum light sources, optical fibres, quantum memories and photodetectors. Unfortunately, each type of device has its own optimal wavelength of operation; 1,550 nm for optical fibre, 800 nm for quantum memories using rubidium vapour, and visible light for a silicon-based photodetector. Quantum frequency converters, which can change the wavelength of a photon while preserving its indistinguishability, single-photon character and entangled state, are therefore extremely important tools. Sven Ramelow and co-workers from the University of Vienna, the Austrian Academy of Sciences and the Austrian Institute of Technology have now demonstrated a quantum frequency-conversion scheme based on sum-frequency generation. First, the researchers generated polarization-entangled photon pairs at a wavelength of 810 nm by spontaneous parametric downconversion in a periodically poled KTiOPO₄ crystal. They were then combined this signal beam with 1,550 nm pump beams in two orthogonally oriented KTiOPO₄ crystals. The horizontally and vertically polarized components of the signal beam were converted to 532 nm beams in the first and second crystals, respectively. To render the output photons indistinguishable, the researchers used a pair of birefringent calcite wedges to compensate for the temporal walk-off of 1.8 ps between the orthogonal polarization components. This resulted in the successful transfer of entanglement between the 810 nm and 532 nm photons.