"Superconducting nanocircuits behave in many aspects similar to natural atoms. Despite the fact that these so-called artificial atoms are huge compared to their natural counterparts, they have a discrete level structure and exhibit properties unique to the world of quantum mechanics. In the simplest case, these artificial atoms form quantum two-level systems, also called quantum bits. We have realized superconducting flux quantum bits where the quantum two-level system is formed by symmetric and anti-symmetric superposition states of persistent currents circulat-ing clock- and anticlockwise in a superconducting loop [1]. Coupling these flux qubits to on-chip superconducting microwave resonators gives rise to the prospering field of superconducting circuit quantum electrodynamics (circuit-QED), which allows us to study the fundamental inter-action between artificial solid-state atoms and single microwave photons as the basis for com-municating quantum information. We discuss the realization of circuit-QED systems operating in the ultra-strong coupling regime, where the atom-cavity coupling rate reaches a considerable fraction of the atom transition frequency [2]. We also address quantum state tomography of propagating microwaves using a novel dual path detection scheme [3]. We have used this scheme to demonstrate for the first time frequency degenerate path entanglement of continu-ous-variable propagating quantum microwave signals. To this end, we entangle two spatially separate modes of the same frequency using a hybrid ring beam splitter and detect the entan-glement by means of cross-correlation techniques. The input fields of the beam splitter are squeezed vacuum and vacuum, respectively, and the correlations are evaluated up to the fourth moments in amplitude."This work is supported by the German Research Foundation via SFB 631 and the German Excellence Initia-tive via the Nanosystems Initiative Munich (NIM).[1] T. Niemczyk et al., Supercond. Sci. Techn. 22, 034009 (2009); F. Deppe et al., PRB 76, 214503 (2007).[2] T. Niemczyk et al., Nat. Phys. 6, 772-776 (2010); F. Deppe et al., Nat. Phys. 4, 686 (2008); T. Niemczyk et al., arXiv:1107.0810v1.[3] E. Menzel et al., Phys. Rev. Lett. 105, 100401 (2010); M. Mariantoni et al., Phys. Rev. Lett. 105, 133601 (2010).
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