E. P. Menzel, R. Di candia, F. Deppe, P. Eder, L. Zhong, M. Ihmig, M. Haeberlein, A. Baust, E. Hoffmann, D. Ballester, K. Inomata, T. Yamamoto, Y. Nakamura, E. Solano, A. Marx, R. Gross: Path Entanglement of Continuous-Variable Quantum Microwaves. 2012Physical Review Letters: 109. ISBN 0031-9007 http://dx.doi.org/10.1103/PhysRevLett.109.250502