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Mercredi 4 février 2015 à 14h00,
Salle des séminaire bât. A

Orateur : Etienne DUMUR
"A V-shape superconducting artificial atom for circuit quantum electrodynamics"


This thesis focuses on the experimental realisation of an artificial atom with a V-shape energy level diagram. Inspired by trapped-ion experiments, we theoretically predict an ultra fast and high fidelity quantum non destructive readout of qubit state by using the V-shape artificial atom in a circuit quantum electrodynamics architecture.

To realise this experiment, we have developed an experimental setup to perform transmission measurements of our superconducting quantum circuits by heterodyne technique at very low temperature (30 mK) low very signal amplitude (fW). We also implemented a hardware and software environment enabling multi-tone spectroscopies and time resolved measurements in order to control the quantum state of the artificial atom and the coherent field in the resonator. In addition in order to optimise the experiment circuit we have characterised quarterwave microwave resonators made from aluminium and epitaxial rhenium thin films.

The original quantum device is fabricated by two inductively coupled transmons. When the coupling inductance is of the order of the Josephson inductance, we observe "in-phase" and "out-of-phase" oscillating modes of the superconducting phase across the junctions. The energy spectrum of the system, measured by two-tone spectroscopy, is magnetic flux dependent. It is precisely described by our theoretical model leading to an accurate determination of the circuit parameters. Because of their anharmonicity, in the low-energy limit, the two modes can be considered as two-level systems called qubits.
At zero magnetic field, it has beaen observed that the two qubits become only coupled by a cross-anharmonicity. This has been revealed, through three-tone spectroscopy, by a conditional frequency shift as large as 115 MHz of one qubit transition depending on the other qubit state.
All these experimental results demonstrate a V-shape energy diagram for our artificial atom which paves the way to original an high performance read-out.

Keywords :
Quantum nano-electronic, Superconductor quantum bit, Dc-SQUID, quantum electrodynamics

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