Alexey V. Ustinov

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

Fluxon readout of a superconducting qubit

  1. Kirill G. Fedorov,
  2. Anastasia V. Shcherbakova,
  3. Michael J. Wolf,
  4. Detlef Beckmann,
  5. and Alexey V. Ustinov
Quantum computing using superconducting circuits underwent rapid development in the last decade. This field has propelled from quantum manipulation of single two-level systems to complex
designs employing multiple coupled qubits allowing one to execute simple quantum algorithms. On the way to a practical quantum computer, a need for scalable interfaces between classical circuits and the quantum counterparts has arisen. Low-temperature superconducting single-flux quantum (SFQ) logic employs magnetic fluxons in Josephson transmission lines (JTLs) as basic bits for classical computation. Here, we report on an experiment implementing a direct link between SFQ electronics and a superconducting qubit. We demonstrate a readout of the state of a flux qubit through a frequency shift of a single fluxon oscillating in a JTL. The energy spectrum of the flux qubit is measured using this technique. The demonstrated approach may open ways to future full-scale integration of solid-state quantum computers with digital SFQ electronics.
arxiv pdf

Josephson vortex coupled to a flux qubit

  1. Kirill G. Fedorov,
  2. Anastasia V. Shcherbakova,
  3. Roland Schäfer,
  4. and Alexey V. Ustinov
Experiments towards realizing a readout of superconducting qubits by using ballistic Josephson vortices are reported. We measured the microwave radiation induced by a fluxon moving
in an annular Josephson junction. By coupling a flux qubit as a current dipole to the annular junction, we detect periodic variations of the fluxon’s oscillation frequency versus magnetic flux through the qubit. We found that the scattering of a fluxon on a current dipole can lead to the acceleration of a fluxon regardless of a dipole polarity. We use the perturbation theory and numerical simulations of the perturbed sine-Gordon equation to analyze our results.
arxiv pdf