Time-Frequency Entanglement of Narrowband Biphotons

Photonic entanglement has long been recognized as an important type of entanglement which serves as a versatile tool for fundamental entanglement research and a building block of quantum networks. Entangled photon pairs (biphotons) can not only realize faithful entanglement distribution over long distance, but also interconnect with stationary quantum nodes. In this Thesis, we present a systematic study of time-frequency entangled biphotons with narrow bandwidth. We first theoretically analyze photon-atom interaction and discuss some important nonlinear processes that our work is based on, including electromagnetically induced transparency (EIT) and spontaneous four-wave mixing (FWM). After describing our experimental systems in detail, we report generation of narrowband biphotons from laser-cooled 85Rb atoms and a hot 87Rb atomic vapor cell. These biphotons are naturally time-frequency entangled because of energy conservation in the generation process. Working with cold atoms, we can produce high-quality biphotons with bandwidth down to sub-MHz, and working with the hot atom system we can produce biphotons with high spectral brightness and high signal contrast ratio. Although these narrowband biphotons are inherently time-frequency entangled, this continuous entanglement is difficult to be controlled and studied. Therefore, we create discrete frequencyxii bin entanglement and confirm the genuine entanglement through the Bell’s inequality test. Such frequency-bin entangled narrowband biphotons with features of extendable entanglement dimension, propagation-error resilience and efficient photon-atom interaction are expected to find various applications in quantum information processing and quantum metrology. At last, we report a single photon experiment with an atomic beam splitter based on EIT storage. The single photon wave nature and particle nature are well-preserved in this atomic beam splitter.