J.C. and T.C. are supported by National Science Foundation grants PHY-1150531 and PHY-1806634. N.A., A.L. and N.M. are supported by National Science Foundation grants PHY-1707840 and PHY-1404245. A portion of this work was performed in the UCSB Nanofabrication Facility. J.C. and T.C. thank D. McClelland, R. Ward and M. J. Yap for comments and discussion. The authors thank C. Wipf and V. Sudhir for comments during the manuscript preparation. This document has been assigned the LIGO document number LIGO-P1800033.

Reviewer information

Nature thanks Warwick Bowen, Yuta Michimura and the other anonymous reviewer(s) for their contribution to the peer review of this work.