Project A8

Cavity Quantum Electrodynamics with Superconducting Devices

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

The project aims at the study of the rapidly emerging field of superconducting circuit quantum electrodynamics (c-QED), the circuit equivalent of atom-photon interaction in cavity-QED. Particular goals are

  • the coupling of superconducting qubits to high-quality superconducting resonators,
  • the generation and detection of non-classical microwave Fock states,
  • the development of dispersive readout and quantum non-demolition measurements, and
  • the entanglement of superconducting qubits via multiple resonators.

The field of superconducting circuit quantum electrodynamics (c-QED) became one of the most interesting and rapidly emerging fields of quantum information processing. In quantum optics, the regime of strong interaction between individual atoms and quantized electromagnetic fields in cavities has stimulated a large number of fundamental experiments. Recently, the strong coupling regime, which is difficult to achieve in quantum optics, has been reached with artificial atoms (superconducting charge qubits) embedded into superconducting resonators. This important breakthrough opened the new field of c-QED. Here, on the one hand well known concepts from quantum optics are transferred to solid-state based systems, however, on the other hand also new regimes not accessible so far by quantum optics come within reach. In project A8 we are aiming to reach the strong coupling regime for superconducting flux qubits (artificial atoms) placed in properly designed superconducting resonators. In a first step superconducting resonators in coplanar and microstrip layout are designed, fabricated and experimentally analyzed. Superconducting flux qubits fabricated in collaboration with project A3 are then placed in the optimized resonators. The coupling strength of the flux qubit-resonator systems is studied by microwave spectroscopy. Furthermore, we plan to investigate these systems in the dispersive regime (Rabi, Ramsey, and spin echo type experiments) to obtain information on the relevant dephasing mechanisms. In parallel, we implement new schemes based on a hybrid ring acting as an on-chip microwave beam splitter to deterministically generate stationary and propagating microwave Fock states. The relevant field observables are measured using a microwave quantum homodyne detection scheme.

Collaborations and Interactions:

Since project A8 combines principles and experimental methods well known from quantum optics with the huge potential of superconducting qubits as basis elements for quantum information systems, it is naturally linked to the projects, which work on superconducting qubits and principles of quantum optics. Three junction flux qubits and rf-SQUIDs placed in adequate microwave resonators are fabricated in collaboration with project A3. The theory for generation and detection of non-classical microwave states is developed in collaboration with project A2. Together with this project the ideas for detection of microwave single photons and the design of more complex networks of cavities and qubits will be promoted. The idea of the “cavity grid” setup to scale up coupled superconducting qubit-cavity systems is analyzed to come up with possible realistic implementations in close collaboration with project A2. Project A5 is theoretically investigating the dynamics of non-adiabatic changes of system parameters in coupled qubit-cavity systems and also the effects of finite temperatures and the coupling of the oscillator mode to its electromagnetic environment, which is highly relevant for project A8. Furthermore, the influence of noise in setups with more than one qubit and more than one oscillator is investigated in project A5. There is also a significant interaction with project B3, where the interaction of quantum dot based qubits with resonators is studied. The relevance of optimal control strategies for project A8 is investigated in project A9.

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