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Abstract Feedback oscillators, consisting of an amplifier whose output is partially fed back to its input, provide stable references for standardization and synchronization. Notably, the laser is such an oscillator whose performance can be limited by quantum fluctuations. The resulting frequency instability, quantified by the Schawlow-Townes formula, sets a limit to laser linewidth. Here, we show that the Schawlow-Townes formula applies universally to feedback oscillators beyond lasers. This is because it arises from quantum noise added by the amplifier and out-coupler in the feedback loop. Tracing the precise origin of quantum noise in an oscillator informs techniques to systematically evade it: we show how squeezing and entanglement can enable sub-Schawlow-Townes linewidth feedback oscillators. Our analysis clarifies the quantum limits to the stability of feedback oscillators in general, derives a standard quantum limit (SQL) for all such devices, and quantifies the efficacy of quantum strategies in realizing sub-SQL oscillators. 

Homodyne detection is a common self-referenced technique to extract optical quadratures. Due to ubiquitous fluctuations, experiments measuring optical quadratures require homodyne angle control. Current homodyne angle locking techniques only provide high quality error signals in a span significantly smaller thanπradians, the span required for full state tomography, leading to inevitable discontinuities during full tomography. Here, we present and demonstrate a locking technique using a universally tunable modulator which produces high quality error signals at an arbitrary homodyne angle. Our work enables continuous full-state tomography and paves the way to backaction evasion protocols based on a time-varying homodyne angle.