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Optics The performance of an active system, whether it is optical, electrical, or mechanical, is often described by its quality ( Q ) factor. Typically, one learns the rule that the higher the Q factor, the sharper the resonance—that is, the bandwidth of the device is reduced. Tsakmakidis et al. show that this is indeed the case, but only for symmetric systems. However, for the case of asymmetric (or nonreciprocal) systems, the rule need not be obeyed. They show theoretically that the more asymmetric a system with high Q is, the wider the bandwidth can be. The effect raises the prospect of designing high- Q devices operating over large bandwidths. Science , this issue p. [1260][1] [1]: /lookup/doi/10.1126/science.aam6662