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. 2018 Jun 8;4(6):eaar6782. doi: 10.1126/sciadv.aar6782

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Copyright © 2018 The Authors, some rights reserved; exclusive licensee American Association for the Advancement of Science. No claim to original U.S. Government Works. Distributed under a Creative Commons Attribution NonCommercial License 4.0 (CC BY-NC).

This is an open-access article distributed under the terms of the Creative Commons Attribution-NonCommercial license, which permits use, distribution, and reproduction in any medium, so long as the resultant use is not for commercial advantage and provided the original work is properly cited.

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Fig. 5 — The PT-symmetric OEO is operating with three different loop lengths of 20.31 m, 433.1 m, and 9.166 km. The corresponding phase noise at an offset frequency of 10 kHz is −92.94, −103.4, and −142.5 dBc/Hz. For comparison, the phase noise of a microwave signal generated by a commercial microwave source is also presented, indicating a 26.6-dB phase noise improvement at an offset frequency of 10 kHz for the 9.166-km-long OEO.