Superradiant telecom-to-near-infrared biphoton generation in a diamond-type atomic ensemble with broadly tunable bandwidth*

Telecom-to-infrared biphotons bridge the vastly different frequency bands without the aid of quantum interfaces, a capability that is essential for quantum networks and long-distance quantum communications. Herein, we propose a biphoton generation scheme in diamond-type atomic media, which features superradiant characteristics, supporting the generation of highly nonclassical biphotons with telecom-infrared bands. Using the Heisenberg-Langevin approach, our theory allows us to analyze the noise photons from the vacuum fluctuation and thereby calculate the ratio of the pairing (correlated) photons out of the total generation rate, which can only be analyzed using the open quantum model. By increasing the optical depth of the medium, we control the biphoton correlation time from the single-atom spontaneous emission time to the sub-nanosecond scale, resulting in a broadly tunable bandwidth from MHz to GHz. In addition, our theoretical predictions align well with experimental observations, confirming that the biphoton correlation time follows the superradiant scaling behavior observed in a simple two-level system. Furthermore, we extend our model to room-temperature atoms by incorporating the Doppler effect, and show that the biphoton correlation decay time is governed and further reduced by the interference among atomic velocity classes. Our investigations establish a theoretical groundwork for superradiant bichromatic biphoton generation using either cold or warm atomic media, which may facilitate the development of quantum networks across disparate optical frequencies.