Observation of Subnatural-Linewidth Biphotons In a Two-Level Atomic Ensemble

TL;DR

This paper demonstrates the generation of subnatural-linewidth biphotons and single photons using a two-level atomic ensemble with a novel single-laser pump scheme, advancing quantum communication and computing technologies.

Contribution

It introduces an innovative experimental method to produce narrowband biphotons with record spectral brightness and symmetry, suitable for quantum applications.

Findings

Biphotons with 0.36 MHz bandwidth achieved

Record spectral brightness of 2.28×10^7 s^{-1} mW^{-1} MHz^{-1}

Observation of heralded sub-MHz-linewidth single photons

Abstract

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\times10^7$${\rm s}^{-1}{\rm mW}^{-1}{\rm MHz}^{-1}$, and a temporally symmetric wave packet at moderate optical depth. The strong non-classical 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 ensembles 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.