[21] M. Mariantoni, et al., Planck spectroscopy and the quantum noise of microwave beam splitters , In preparation. [22] M. G¨ oppl, et al., J. Appl. Phys. 104 , 113904 (2008). [23] T. Yamamoto, et al., Appl. Phys. Lett. 93 , 042510 (2008). [24] M. A. Castellanos-Beltran, K. D. Irwin, G. C. Hilton, L. R. Vale, and K. W. Lehnert, Nature Physics 4 , 929 (2008). [25] N. Bergeal, et al. (2010), Nature Physics doi:10.1038/nphys1516. [26] A. M. Zagoskin, E. Il’ichev, M. W. McCutcheon, J. F. Young, and F. Nori, Phys. Rev. Lett. 101 , 253602 (2008). 11 Author contributions Acknowledgments We thank C. Probst, K. Neumaier and K. Uhlig for providing their expertise in cryogenic engineering and M. H¨ aberlein and C. Rauh for programming support. Furthermore, we acknowledge financial support by the Deutsche Forschungsgemeinschaft via SFB 631, the German Excellence Initiative via NIM, the European project EuroSQIP, UPV-EHU Grant GIU07/40 and Ministerio de Ciencia e Innovaci´ on FIS2009-12773-C02-01. E.P.M. is responsible for the main contributions regarding the theoretical and experi- mental results presented in this work. F.D. provided important contributions in theory and experiment. E.P.M. and F.D. prepared the manuscript. M.M. provided valuable input to the initial theoretical ideas and helped during the experiments. E.S. supervised the theo- retical aspects of this work. M.A.A.C. and A.B. helped with the experimental setup. T.N. and E.H. contributed to discussions and helped editing the manuscript. A.M. and R.G. supervised the experimental aspects of the project.