Implementation of a quantum metamaterial using superconducting qubits

• Published in: Nature communications 2014-10, Vol.5 (1), p.5146, Article 5146
• Main Authors: Macha, Pascal, Oelsner, Gregor, Reiner, Jan-Michael, Marthaler, Michael, André, Stephan, Schön, Gerd, Hübner, Uwe, Meyer, Hans-Georg, Il’ichev, Evgeni, Ustinov, Alexey V.
• Format: Article
• Language: English
• Subjects: 639/301/119/1003; 639/301/357/1015; 639/766/483/2802; Humanities and Social Sciences; multidisciplinary; Science; Science (multidisciplinary)
• ISSN: 2041-1723; 2041-1723
• DOI: 10.1038/ncomms6146

The key issue for the implementation of a metamaterial is to demonstrate the existence of collective modes corresponding to coherent oscillations of the meta-atoms. Atoms of natural materials interact with electromagnetic fields as quantum two-level systems. Artificial quantum two-level systems can be made, for example, using superconducting nonlinear resonators cooled down to their ground state. Here we perform an experiment in which 20 of these quantum meta-atoms, so-called flux qubits, are embedded into a microwave resonator. We observe the dispersive shift of the resonator frequency imposed by the qubit metamaterial and the collective resonant coupling of eight qubits. The realized prototype represents a mesoscopic limit of naturally occurring spin ensembles and as such we demonstrate the AC-Zeeman shift of a resonant qubit ensemble. The studied system constitutes the implementation of a basic quantum metamaterial in the sense that many artificial atoms are coupled collectively to the quantized mode of a photon field. Superconducting flux qubits operating as two-level systems can act as artificial atoms, and so represent a potential metamaterial building block. Macha et al. assemble 20 such qubits into a metamaterial in which the ‘atoms’ are collectively coupled to the quantized mode of a microwave photon field.