A Generalized Schawlow-Townes Limit
Abstract
We study a class of a feedback oscillators realized by a phase-insensitive amplifier in positive feedback, where either the amplifier or the feedback element may determine the oscillator's linewidth. The spectral purity of the output of such a device originates from basic demands of quantum mechanics and causality. The resulting expression generalizes the Schawlow-Townes limit, which is itself one component of a standard quantum limit for feedback oscillators. Recently realized bad-cavity oscillators such as super-radiant lasers and solid-state masers can saturate this generalized Schawlow-Townes limit. This limit can be surpassed through appropriate quantum engineering: for example by atomic spin squeezing in a super-radiant laser.
Cite
@article{arxiv.2501.11861,
title = {A Generalized Schawlow-Townes Limit},
author = {Hudson A. Loughlin and Vivishek Sudhir},
journal= {arXiv preprint arXiv:2501.11861},
year = {2026}
}
Comments
5+2 pages, 2 figures