Entangled photon pair sources are essential for applications such as quantum communication and metrology. Here we present a compact energy–time entangled photon pair source at telecom wavelengths realized through cascaded second harmonic generation and spontaneous parametric down conversion in a single periodically poled lithium niobate waveguide. We introduce and characterize methods to diminish the effects of Raman scattering, the principal being quasi-CW pumping. The quality of energy–time entanglement produced by the compact source is analyzed using two-photon interference and Franson interference, and visibilities as high as 93.9% ± 0.4% and 90.5% ± 0.6% are achieved, respectively.

We design and implement an atomic frequency comb quantum memory for 793-nm wavelength photons using a monolithic cavity based on a thulium- (Tm-) doped Y3Al5O12 crystal. Approximate impedance matching results in the absorption of 90% of input photons and a memory efficiency of (27.5±2.7)% over a 500-MHz bandwidth. The cavity enhancement leads to a significant improvement over the previous efficiency in Tm-doped crystals using a quantum memory protocol. In turn, this allows us to store and recall quantum states of light in such a memory. Our results demonstrate progress toward efficient and faithful storage of single-photon qubits with a large time-bandwidth product and multimode capacity for quantum networking.