Quantum Experiments with Superconducting Circuits – QuExSuperC

Superconducting quantum circuits with Josephson junctions have shown in the last decade very rich and successful quantum experiments. They clearly appeared as the most promising solid state scalable quantum system for quantum information processor. These superconducting circuits behave as artificial atoms and were extensively studied to test fundamental concepts of quantum mechanics and Quantum ElectroDynamics (QED), to demonstrate quantum gate operations. This research is always in strong development.

We propose in this basic research project to understand and fight against decoherence processes related to material defects and to investigate new quantum experiments in these superconducting quantum circuits. These two main objectives are strongly linked together and have to be carried out in parallel.

In order to reduce the density of the defects inside the circuits, we will develop epitaxial growth of superconducting layers of aluminum and rhenium and of the tunnel barrier. In parallel to this study, we plan to build superconducting circuits testing a variety of different materials in order to optimize the circuit performances.

To evaluate the quality of the different materials and epitaxial circuits, we will perform preliminary structural and chemical characterization, local probe analysis, and transport measurements. We will develop a novel characterization method based on resonant macroscopic quantum tunneling. Indeed such effect was demonstrated to be very sensitive to noise and defects inside the barrier. This method will enable to evaluate the circuit quality. Finally we will measure the coherence and relaxation times as well as the quantum measurements contrast and fidelity estimate the performances of the optimized circuits.

The second objective of the project is the development of two original quantum experiments. The first one will concern the dynamical coupling, inspired from nuclear magnetic resonance (NMR). It is a generalization of spin-echo technique. This experiment will inform on the decoherence processes and may suppress noise effects on the quantum dynamics. The second experiment concerns the realization of an artificial atom with V-shape three energy levels. Such property is realized in a circuit with two internal strongly coupled degrees of freedom. V-shape quantum systems were extensively used in atomic physics (trap ions, NV-center) but it has never been considered in the superconducting circuits. We propose to realize and study a V-shape artificial atom coupled to a microwave cavity. Such original quantum circuit will open new road for quantum experiments such as QED with three level system, conditional gate operation, lasing effects. As an example the qubit state could be probed by the cavity without direct coupling between cavity and qubit. This original property can lead to quantum non-demolition measurements with high fidelity.

The outcome of the project will present a broad interest for the entire quantum nano-electronics community. The realization of epitaxial quantum circuits will open new research fields in the superconducting device such as hybrid circuits, nanofridge, high sensitivity superconducting detectors, and SQUIDs.