Effective Two-Level Dynamics in Confined Alkali Vapors at Telecom Wavelengths

We demonstrate the emergence of an effective two-level atomic system at telecom wavelengths in hot rubidium vapor confined to a sub-micron-thick cell. In this extreme confinement regime, frequent atom–surface collisions reshape the atomic dynamics by selecting a narrow velocity class of atoms moving nearly parallel to the cell walls. This confinement-induced filtering suppresses optical pumping into intermediate and uncoupled states. As a result, a closed cycling transition in the telecom band becomes the dominant interaction channel, thereby giving rise to a robust effective two-level response. Through combined absorption and fluorescence measurements, supported by a theoretical model incorporating wall-induced relaxation, we show that optical behavior is governed by effectively isolated two-level dynamics. Our results reveal a new mechanism for engineering simplified quantum systems from complex atomic media without external state preparation. This approach enables telecom-compatible, compact atomic platforms with intrinsically reduced decoherence pathways, opening a route toward integrated quantum photonics, including chip-scale quantum memories, telecom frequency references, and scalable quantum information processing.