Lossless nonreciprocity using dual-Raman interference

Atomic absorption-induced insertion loss poses a fundamental limitation on signal transmission in optical nonreciprocity (ONR) systems. We propose a lossless and magnetic-field-free ONR scheme based on dual-Raman interference in two far-detuned Λ-type three-level systems. In the forward direction, destructive interference cancels the imaginary part of the susceptibility while retaining a large phase shift, enabling absorption-free transmission. In contrast, Doppler shifts arising from atomic thermal motion disrupt the Raman resonances in the backward direction, suppressing both absorption and phase modulation. This direction-dependent optical response yields nonreciprocal phase shifts which, when implemented in a Mach-Zehnder interferometer, enable an optical isolator with 100% forward transmittance and an isolation of up to 37 dB.