A thought-provoking proposal by Tsakmakidis et al. [Science 356, 1260 (2017)]suggests that nonreciprocal optics can break a time-bandwidth limit to passiveresonators. Here I quantize their resonator model and show that quantummechanics does impose a limit, or requires extra noise to be added in the samefashion as amplified spontaneous emission in an active resonator. I also usethermodynamics to argue that extra dissipation or noise must be present intheir proposed device.
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