Biphoton in a Two-level Atomic Ensemble

Jianming Wen, Shengwang Du, Morton H. Rubin

Dasen Zhang, Zhi‐Ming Zhang

We study the spatiotemporal structure of the biphoton entangled state generated by the four-wave mixing (FWM) process in a cold two-level atomic ensemble. We analyze, for the first time, the X-like shaped structure of the biphoton entangled state and the geometry of the biphoton correlation for different lengths and densities of the cold atomic ensemble. The propagation equations of the photon pairs generated from FWM process are derived in a spatiotemporal framework. By means of the input-output relations of the propagation equations, the biphoton amplitude function is obtained in a spatiotemporal domain. In the given frequency range, the biphoton amplitude displays an X-like shaped geometry, nonfactorizable in the space-time domain. Such an X-like shaped spatiotemporal structure is caused by the phase matching and the FWM gain. The former leads to the X-like shaped envelope of the biphoton correlation, while the latter gives rise to the oscillations around the X-like shaped envelope.

Jianming Wen, Shengwang Du, Morton H. Rubin

The temporal correlation of biphotons, generated from a two-level cooled atomic system, displays a photon anti-bunching effect, which corresponds to the interference between two types of nonlinear four-wave mixing processes occurring in such a medium.

Chien-Tsun Chen

Nikolai B. Delone, В. П. Крайнов

Jyun-Kai Lin, Tzu-Hsiang Chien, Chin-Te Wu +4 more

Biphotons and single photons with narrow bandwidths and long coherence times are essential to the realization of long-distance quantum communication (LDQC) and linear optical quantum computing (LOQC). In this Letter, we manipulate the biphoton wave functions of the spontaneous four-wave mixing in a two-level atomic ensemble with a single-laser pump scheme. Our innovative experimental approach enables the generation of biphotons with a sub-MHz bandwidth of 0.36 MHz, a record spectral brightness of 2.28×10^{7} s^{-1} mW^{-1} MHz^{-1}, and a temporally symmetric wave packet at moderate optical depth. The strong nonclassical cross-correlation of the biphotons also enables the observation of heralded sub-MHz-linewidth single photons with a pronounced single-photon nature. The generation of sub-MHz-linewidth biphotons and single photons with a two-level atomic ensemble not only finds applications in quantum repeaters and large cluster states for LDQC and LOQC but also opens up the opportunity to miniaturize the biphoton or single-photon sources for chip-scale quantum technologies.