1. Department of Electrical Engineering, Technion - Israel Institute of Technology, 32000 Haifa, Israel.
2. Time and Frequency Division, Associate of the National Institute of Standards and Technology, Boulder, CO 80305, USA.
3. Department of Physics, University of Colorado, Boulder, CO 80309, USA.
4. Department of Physics and Research Center OPTIMAS, University of Kaiserslautern, Erwin Schroedinger Strasse 46, 67663 Kaiserslautern, Germany.