The ability to manipulate the carrier frequency of quantum states of light, through a process called quantum frequency conversion (QFC), has numerous applications for both technology and basic science. For example, one can upconvert a single-photon-level signal in the 1.5-micron telecommunications band (where single-photon detection has been challenging) to a visible wavelength to take advantage of well-developed single-photon detectors based on silicon avalanche photodiodes. On the more fundamental side, the manipulation of a single photon's frequency may enable the construction of networks of dissimilar quantum systems, whereby one can imagine generating many-body entangled quantum states over vast distances.
Quantum frequency conversion will only be useful if it can be done both efficiently and with little added noise. We demonstrated a conversion efficiency exceeding 99.99% using reverse-proton-exchange waveguides in periodically poled lithium niobate with approximately 150 mW of pump power. Noise has been a more serious issue: the generation of noise photons, due to inelastic scattering of light from the strong pump laser used to drive the frequency conversion, has limited the utility of QFC devices in many applications. We present an analysis of the two primary noise processes in QFC devices (spontaneous Raman scattering and spontaneous parametric fluorescence), and offer solutions on how they may be either mitigated or avoided completely.
We then discuss applications of QFC devices for up- and downconversion of single-photon signals. We used a long-wavelength pump to enable high-efficiency and low-noise single-photon detection for 1.5-micron telecom band signals, and demonstrated a cascaded frequency conversion approach that enabled low timing jitter as well. We also demonstrated a downconversion quantum interface, in which the emission from a single semiconductor quantum dot at a wavelength of 910 nm was downconverted to 1560 nm while maintaining the single-photon character of the light. The results presented in this dissertation indicate a promising future for QFC devices as the field of quantum communications matures.