Miloš Popović is at the Department of Electrical, Computer and Energy Engineering, University of Colorado, Boulder, Colorado 80302, USA,

Wolfgang Freude is at the Institute of Photonics and Quantum Electronics and Institute of Microstructure Technology, Karlsruhe Institute of Technology, 76131 Karlsruhe, Germany,

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Our best selling floor lamp in a tabletop size- perfect for topping end tables and desks. Hand-crafted in Pennsylvania, the solid oak balls create a fixture that is as sturdy as it is stylish. The grained oak texture shows through the finish revealing a rich history unique to each piece. With a choice of natural, dark wash, or white wash finishes as well as a black or ivory Belgian linen shade, the Hugo can integrate seamlessly into a variety of aesthetics.

Andrea Melloni is at Dipartimento di Elettronica, Informazione e Bioingegneria, Politecnico di Milano, 20133 Milano, Italy,

INTEC Department and Center for Nano- and Biophotonics, Roel Baets is at the Photonics Research Group, Ghent University-IMEC, B-9000 Gent, Belgium,

Hagen Renner is at the Institute of Optical Communication Technology, Hamburg University of Technology, D-21073 Hamburg, Germany,

John D. Joannopoulos is at the Department of Physics, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA,

Mathias Vanwolleghem is at Institut d'Electronique de Micro-electronique et de Nanotechnologie, CNRS, Université Lille 1, 59650 Villeneuve d'Ascq, France,

Dirk Jalas, Alexander Petrov and Manfred Eich are at the Institute of Optical and Electronic Materials, Hamburg University of Technology, D-21073 Hamburg, Germany,

Jalas, D., Petrov, A., Eich, M. et al. What is — and what is not — an optical isolator. Nature Photon 7, 579–582 (2013). https://doi.org/10.1038/nphoton.2013.185

Christopher R. Doerr is at Acacia Communications Inc., Three Clock Tower Place, Suite 210, Maynard, Massachusetts 01754, USA,

Department of Electrical Engineering, Shanhui Fan and Zongfu Yu are at Ginzton Laboratory, Stanford University, Stanford, California 94305, USA,

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The quest for on-chip optical isolators has recently spawned many new isolator structures. However, there has been some confusion about the requirement of nonreciprocity. Here, we review the essential characteristics of an isolator.