Sub-femtosecond absolute timing precision with a 10 GHz hybrid   photonic-microwave oscillator
  We present an optical-electronic approach to generating microwave signals with high spectral purity. By circumventing shot noise and operating near fundamental thermal limits, we demonstrate 10 GHz signals with an absolute timing jitter for a single hybrid oscillator of 420 attoseconds (1Hz - 5 GHz). 
T.M. Fortier, C.W. Nelson, A. Hati, F. Quinlan, J. Taylor, H. Jiang,   C.W. Chou, T. Rosenband, N. Lemke, A. Ludlow, D. Howe...
physics.optics physics.atom-ph physics.ins-det