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This note is a comment on a recent article [Tsakmakidis, et al., Nat Commun10 (2019)] that presents a thought-provoking proposal to overcome the bandwidthrestrictions of invisibility cloaks based on using media that supportsuperluminal (faster than light in free space) group and phase velocities. Asillustrated in Fig. 1 of the original article, a wave packet propagatingthrough such a fast-light cloak is alleged to be able to reach the side behindthe cloaked object simultaneously with a corresponding wave packet propagatingthrough the shorter, direct route in free space without the object, so that "noshadow or waveform distortion arises." As the authors claim, the "extrapathlength is balanced out by the correspondingly larger group velocity of thepulse in the cloak", which allows to "restore the incident field distributionall around the object in, both, amplitude and phase". This fast-light effectmay be achieved in a broadband fashion using active (gain) materials. Theauthors claim that such a fast-light cloak can hide an object, even fromtime-of-flight detection techniques, and achieve invisibility "over any desiredfrequency band." We disagree with these claims and believe that a thoroughclarification of the ideas put forward in the original article is important andnecessary for the broad wave-physics community. Specifically, in this commentwe clarify that invisibility cloaks based on fast-light media suffer fromfundamental bandwidth restrictions that arise due to causality, the nature ofsuperluminal wave propagation, and the stability issues of active systems.These limitations and issues were not addressed in [Tsakmakidis, et al., NatCommun 10 (2019)]. Most importantly, we show that the material model consideredin the original article is unphysical.

Physical limitations on broadband invisibility based on fast-light media