Creating optical components that allow light to propagate in only one direction—that is, that allow non-reciprocal propagation or ‘isolation’ of light—is important for a range of applications. Non-reciprocal propagation of sound can be achieved simply by using mechanical components that spin1,2. Spinning also affects de Broglie waves3, so a similar idea could be applied in optics. However, the extreme rotation rates that would be required, owing to light travelling much faster than sound, lead to unwanted wobbling. This wobbling makes it difficult to maintain the separation between the spinning devices and the couplers to within tolerance ranges of several nanometres, which is essential for critical coupling4,5. Consequently, previous applications of optical6,7,8,9,10,11,12,13,14,15,16,17 and optomechanical10,17,18,19,20 isolation have used alternative methods. In hard-drive technology, the magnetic read heads of a hard-disk drive fly aerodynamically above the rapidly rotating disk with nanometre precision, separated by a thin film of air with near-zero drag that acts as a lubrication layer21. Inspired by this, here we report the fabrication of photonic couplers (tapered fibres that couple light into the resonators) that similarly fly above spherical resonators with a separation of only a few nanometres. The resonators spin fast enough to split their counter-circulating optical modes, making the fibre coupler transparent from one side while simultaneously opaque from the other—that is, generating irreversible transmission. Our setup provides 99.6 per cent isolation of light in standard telecommunication fibres, of the type used for fibre-based quantum interconnects22. Unlike flat geometries, such as between a magnetic head and spinning disk, the saddle-like, convex geometry of the fibre and sphere in our setup makes it relatively easy to bring the two closer together, which could enable surface-science studies at nanometre-scale separations.
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We thank U. Hofi, Z. Katz, Y. Halupovich and B. Khachatryan for their help. This work was funded by the Israeli Centers for Research Excellence (I-CORE), ‘Circle of Light’ Excellence Center, the Israel Science Foundation (2013/15), the Israel Ministry of Science, Technology and Space, the MURI Center for Dynamic Magneto-Optics via the AFOSR Award number FA9550-14-1-0040, the Army Research Office (ARO) under grant number 73315PH, the AOARD under grant number FA2386-18-1-4045, the CREST under grant number JPMJCR1676, the IMPACT programme of JST, the RIKEN-AIST Challenge Research Fund, the JSPS-RFBR under grant number 17-52-50023, and the Sir John Templeton Foundation.
Nature thanks A. Alù and M. Levy for their contribution to the peer review of this work.