Synchronization of Clocks Through 12 km of Strongly Turbulent Air Over a City.

We demonstrate real-time, femtosecond-level clock synchronization across a low-lying, strongly turbulent, 12-km horizontal air path by optical two-way time transfer. For this long horizontal free-space path, the integrated turbulence extends well into the strong turbulence regime corresponding to multiple scattering with a Rytov variance up to 7 and with the number of signal interruptions exceeding 100 per second. Nevertheless, optical two-way time transfer is used to synchronize a remote clock to a master clock with femtosecond-level agreement and with a relative time deviation dropping as low as a few hundred attoseconds. Synchronization is shown for a remote clock based on either an optical or microwave oscillator and using either tip-tilt or adaptive-optics free-space optical terminals. The performance is unaltered from optical two-way time transfer in weak turbulence across short links. These results confirm that the two-way reciprocity of the free-space time-of-flight is maintained both under strong turbulence and with the use of adaptive optics. The demonstrated robustness of optical two-way time transfer against strong turbulence and its compatibility with adaptive optics is encouraging for future femtosecond clock synchronization over very long distance ground-to-air free-space paths.

[1]  Peter Wolf,et al.  Analysis of Sun/Moon gravitational redshift tests with the STE-QUEST space mission , 2015, 1509.02854.

[2]  L. Andrews,et al.  Laser Beam Propagation Through Random Media , 1998 .

[3]  D. Wineland,et al.  Optical Clocks and Relativity , 2010, Science.

[4]  J. Lodewyck,et al.  Atomic clocks: new prospects in metrology and geodesy , 2013, 1308.6766.

[5]  R. Holzwarth,et al.  Einstein Gravity Explorer–a medium-class fundamental physics mission , 2009 .

[6]  J. Shapiro.  Reciprocity of the Turbulent Atmosphere , 1971 .

[7]  Hugo Bergeron,et al.  Optical system design for femtosecond-level synchronization of clocks , 2016, SPIE OPTO.

[8]  M. Pospelov,et al.  Hunting for topological dark matter with atomic clocks , 2013, Nature Physics.

[9]  Esther Baumann,et al.  Optical two-way time and frequency transfer over free space , 2013 .

[10]  Hugo Bergeron,et al.  Synchronization of Distant Optical Clocks at the Femtosecond Level , 2015, 1509.07888.

[11]  I. Coddington,et al.  Tight real-time synchronization of a microwave clock to an optical clock across a turbulent air path. , 2016, Optica.

[12]  Larry B. Stotts,et al.  Analysis of link performance for the FOENEX laser communications system , 2012, Defense + Commercial Sensing.

[13]  J.-F. Cliche,et al.  Applications of control Precision timing control for radioastronomy maintaining femtosecond synchronization in the atacama large millimeter array , 2006, IEEE Control Systems.

[14]  S. Capozziello,et al.  Quantum tests of the Einstein Equivalence Principle with the STE–QUEST space mission , 2014, 1404.4307.

[15]  L. Bougas,et al.  Search for Ultralight Scalar Dark Matter with Atomic Spectroscopy. , 2015, Physical review letters.

[16]  Larry C. Andrews,et al.  Buffer requirements of an optical communication system in atmospheric turbulence , 2013, Defense, Security, and Sensing.

[17]  Fritz Riehle,et al.  Towards a Re-definition of the Second Based on Optical Atomic Clocks , 2015, 1501.02068.

[18]  R. Noll.  Zernike polynomials and atmospheric turbulence , 1976 .

[19]  D. Maoz,et al.  Using Atomic Clocks to Detect Gravitational Waves , 2015, 1501.00996.

[20]  Jean-Marc Conan,et al.  Impact of turbulence on high-precision ground-satellite frequency transfer with two-way coherent optical links , 2016 .

[21]  D. Rugar,et al.  Optical clocks and relativity , 2013 .