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  • Open Access

Generalized Schawlow-Townes limit

Hudson Loughlin1,* and Vivishek Sudhir1,2

  • *Contact author: hudsonl@mit.edu
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Phys. Rev. A 113, 043717 – Published 8 April, 2026

DOI: https://doi.org/10.1103/6t5c-swxn

Abstract

We study a class of 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 superradiant 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 superradiant laser.

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Phys. Rev. A 113, 043717– Published 8 April, 2026
  • Received 5 February 2025
  • Revised 31 October 2025
  • Accepted 16 March 2026
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Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article's title, journal citation, and DOI.

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