Stephan André

Implementation of a Quantum Metamaterial

  1. Pascal Macha,
  2. Gregor Oelsner,
  3. Jan-Michael Reiner,
  4. Michael Marthaler,
  5. Stephan André,
  6. Gerd Schön,
  7. Uwe Huebner,
  8. Hans-Georg Meyer,
  9. Evgeni Il'ichev,
  10. and Alexey V. Ustinov
Manipulating the propagation of electromagnetic waves through sub-wavelength sized artificial structures is the core function of metamaterials. Resonant structures, such as split ring
resonators, play the role of artificial „atoms“ and shape the magnetic response. Superconducting metamaterials moved into the spotlight for their very low ohmic losses and the possibility to tune their resonance frequency by exploiting the Josephson inductance. Moreover, the nonlinear nature of the Josephson inductance enables the fabrication of truly artificial atoms. Arrays of such superconducting quantum two-level systems (qubits) can be used for the implementation of a quantum metamaterial. Here, we perform an experiment in which 20 superconducting flux qubits are embedded into a single microwave resonator. The phase of the signal transmitted through the resonator reveals the collective resonant coupling of up to 8 qubits. Quantum circuits of many artificial atoms based on this proof-of-principle experiment offer a wide range of prospects, from detecting single microwave photons to phase switching, quantum birefringence and superradiant phase transitions.
arxiv pdf

Emission spectrum of the driven nonlinear oscillator

  1. Stephan André,
  2. Lingzhen Guo,
  3. Vittorio Peano,
  4. Michael Marthaler,
  5. and Gerd Schön
Motivated by recent „circuit QED“ experiments we investigate the noise properties of coherently driven nonlinear resonators. By using Josephson junctions in superconducting
circuits, strong nonlinearities can be engineered, which lead to the appearance of pronounced effects already for a low number of photons in the resonator. Based on a master equation approach we determine the emission spectrum and observe for typical circuit QED parameters, in addition to the primary Raman-type peaks, second-order peaks. These peaks describe higher harmonics in the slow noise-induced fluctuations of the oscillation amplitude of the resonator and provide a clear signature of the nonlinear nature of the system.
arxiv pdf