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Bulletin of the American Physical Society

Demonstration of narrowband biphotons with sub-MHz bandwidth from low-optical-depth atomic ensemble

Ravikumar Chinnarasu, Yi-Feng Ding, Chi-Yang Liu, Chih‐Sung Chuu

Published May 29, 2019• 0 citations
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quant-pharXiv:2603.09824•Mar 11, 2026•0 citations
Biphotons of narrow bandwidth and long temporal length play a crucial role in long-distance quantum communication (LDQC) and linear optical quantum computing (LOQC). However, generation of these photons usually requires atomic ensembles with high optical depth or spontaneous parametric down-conversion with sophisticated optical cavity. By manipulating the two-component biphoton wave function generated from a low-optical-depth (low-OD) atomic ensemble, we demonstrate biphotons with subnatural linewidth in the sub-MHz regime. The potential to shape and manipulate the quantum wave packets of these temporally long photons is also demonstrated and discussed. Our work has potential applications in realizing quantum repeaters and large cluster states for LDQC and LOQC, respectively. The possibility to generate and manipulate subnatural-linewidth biphotons with low OD also opens up the opportunity to miniaturize the biphoton source for implementing quantum technologies on chip-scale quantum devices.
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.
We demonstrate high-efficiency telecom frequency conversion of heralded atomic biphoton wavepackets using a diamond-type atomic ensemble. By placing a 2.5 MHz heralded-photon spectrum within the high-efficiency region of the converter response, we achieve a conversion efficiency of 79.4(2.6)% while maintaining strong time-resolved correlations and well-defined temporal wavepackets. For a broader 17.4 MHz input bandwidth, the conversion efficiency is reduced to about 55%, whereas the temporal waveform remains largely preserved. This behavior reflects the nearly flat central response of the converter, which mainly causes spectral-edge loss rather than temporal-mode distortion. These results identify spectral matching as an effective route to efficient and low-distortion telecom conversion of narrowband quantum light from atomic systems.