1. Optical Materials Engineering Laboratory, Department of Mechanical and Process Engineering, ETH Zurich, 8092 Zurich, Switzerland
2. Quantum Photonics Group, Department of Physics, ETH Zurich, 8092 Zurich, Switzerland
3. Physics and Informatics Laboratories, NTT Research, Inc., Sunnyvale, California 94085, United States
4. IBM Research−Zurich, 8803 Rueschlikon, Switzerland