Project A3

Superconducting Quantum Circuits as Basic Elements for Quantum Information Processing

PIs: Prof. R. Gross, Dr. A. Marx

Project A3 aims at the fabrication and characterization of superconducting quantum circuits. Both standard Josephson junctions based on nanometer sized Al/AlOx/Al trilayer structures fabricated by multi-angle shadow evaporation and Nb/AlOx/NiPd/Nb p-junctions with ferromagnetic NiPd interlayers are used. The key goals are

  • the detailed understanding of the quantum coherent dynamics of superconducting flux and phase qubits,
  • the study and control of sources of decoherence,
  • the optimization of the qubit design and the circuit elements required for qubit manipulation and readout, as well as
  • the development of suitable coupling schemes.

Since the first experimental demonstration of quantum coherent dynamics in a superconducting charge qubit by Nakamura and co-workers in 1999, there has been a huge research activity on the fabrication and the underlying physics of superconducting charge, flux and phase qubits. Although several concepts for superconducting qubits have been successfully implemented until today and promising coherence times have been reported, there is still a large variety of problems to be solved and improvements to be made. On the one hand, the open issues are related to fabrication technology and materials issues for superconducting quantum circuits, on the other hand they concern fundamental questions regarding decoherence, optimization of manipulation and readout schemes, and the coupling of qubits as well as the demonstration of two-qubit entanglement. The present project addresses these issues. Regarding fabrication and materials aspects we aim at the development and optimization of the process technology for superconducting flux and phase qubits as well as additional circuit elements required for their manipulation and readout. A particular goal will be the fabrication of p-Josephson junctions and qubits involving ferromagnetic interlayers. The superconducting qubits are characterized by low-temperature measurements in the frequency and time domain to get a detailed understanding of their quantum coherent dynamics. Here, regarding flux qubits particular goals are the understanding and reduction of decoherence, the improvement of the control and measurement processes, the development of coupling schemes, and the study of entanglement and temporal correlations. With respect to p-Josephson junctions and qubits we aim at the study of their quantum behavior and the realization of p-qubits in a quiet configuration.

Project A3 combines the broad experience of the Walther-Meißner-Institute in the fabrication technology of Josephson junctions and superconducting nanocircuits with the unique expertise in ultra-low temperature technology and low-noise measuring techniques.

Collaborations and Interactions:

Due to the large potential of superconducting qubits as basic elements of future quantum information systems and their use for fundamental experiments, the project A3 occupies a central position within SFB 631. Collaborations and interactions exist with project A2 (von Delft, Marquardt, Siewert, Solano: theory of superconducting qubits, superconducting circuit-QED), A5 (Hänggi, Kohler, Grifoni: control and decoherence of solid-state qubits), A8 (Marx, Gross: superconducting circuit-QED, superconducting qubits in microwave resonators), A9 (Glaser, Schulte-Herbrüggen: optimum control and decoherence protection), A1 (Kotthaus, Ludwig, Strunk, Wegscheider: ultra-low temperature techniques, time-domain measurements, e-beam lithography), and B3 (Finley, Amann, Meyer: coupling of solid-state qubits to resonators, circuit-QED).

Moreover, the very fruitful interaction with the theory group of Frank Wilhelm (now at Institute for Quantum Computing and Physics Department, University of Waterloo, Canada) and the NTT Basic Research Laboratories (K. Semba, H. Takayanagi) will be continued and further intensified.

For more information on the research activities at the Walther-Meißner-Institute please visit the WMI web site: