Direct observation of optical precursors in a cold potassium gas

J. F. Chen, Heejeong Jeong, M. M. T. Loy +1 more

Maryam Hiradfar, Sridhar Prabhu, Yoo Kyung Lee +4 more

J. F. Chen, M. M. T. Loy, George K. Wong +1 more

Shengwang Du, M. M. T. Loy, George K. Wong

Dong-Sheng Ding, Y. Jiang, Wei Zhang +3 more

We observed optical precursors in four-wave mixing based on a cold-atom gas. Optical precursors appear at the edges of pulses of the generated optical field, and propagate through the atomic medium without absorption. Theoretical analysis suggests that these precursors correspond to high-frequency components of the signal pulse, which means the atoms cannot respond quickly to rapid changes in the electromagnetic field. In contrast, the low-frequency signal components are absorbed by the atoms during transmission. We also showed experimentally that the backward precursor can be stored using a Raman transition of the atomic ensemble and retrieved later.

Shanchao Zhang, J. F. Chen, Chang Liu +3 more

We report the direct observation of optical precursors of heralded single photons with step- and square-modulated wave packets passing through cold atoms. Using electromagnetically induced transparency and the slow-light effect, we separate the single-photon precursor, which always travels at the speed of light in vacuum, from its delayed main wave packet. In the two-level superluminal medium, our result suggests that the causality holds for a single photon.