Quantum Frequency Conversion of Single Photons from a Nitrogen-Vacancy Center in Diamond to Telecommunication Wavelengths
Entanglement-based quantum networks are strongly pursued worldwide, because of their potential impact on secure communication, distributed quantum computing, and timekeeping, for example. Among quantum emitters, the N-$V$ center in diamond is a leading candidate for implementing such networks, but high photon loss at the N-$V$ emission wavelength hinders long-distance entanglement (beyond ~1 km). The authors surmount this hurdle by down-converting single N-$V$ photons to a telecom wavelength, via nonlinear optics plus efficient filtering and excellent control of a lone emitter. This technological achievement is a critical step toward large-scale quantum networks.