Evading the Schawlow-Townes limit in feedback oscillators

We develop a general quantum theory of feedback oscillators, such as lasers and masers, and use it to study the origin of quantum noise in such systems, the limitations posed by quantum noise, and systematic techniques to evade these limitations. We show that the Schawlow-Townes limit naturally arises in all such oscillators when they are limited by quantum noise with symmetric quantum contributions to their amplitude and phase fluctuations. After studying the quantum origins of the Schawlow-Townes limit, we explore methods of evading it through squeezing, EPR entanglement, and phase-sensitive amplification. We find that squeezing the oscillator's state can reduce its frequency fluctuations below the Schawlow-Townes limit at the expense of increasing its intensity fluctuations. On the other hand, EPR entanglement and phase-sensitive amplification allow us to avoid this tradeoff and reduce an oscillator's frequency and intensity fluctuations simultaneously.