Lucas Verney

PhD Thesis

[Strongly driven quantum Josephson circuits) (2019)

In this thesis, we investigate the behavior of Josephson circuits under the action of strong microwave drives. Josephson circuits in the quantum regime are a building block to emulate a variety of Hamiltonians, useful to process quantum information. We are here considering a transmon device, made of a Josephson junction and a capacitor in parallel. Through numerical simulations and comparison with experimental results, we show that these drives lead to an instability which results in the escape of the circuit state into states which are no longer confined by the Josephson cosine potential. When the transmon occupies such states, the circuit behaves as if the junction had been removed and all non-linearities are lost, which translates into limitations on the emulated Hamiltonian strengths. In a second part, we propose and study an alternative circuit consisting of a transmon device with an extra inductive shunt, providing a harmonic confinement. This circuit is found to be stable for all pump powers. The dynamics of this circuit is also well captured by a time-averaged model, providing a useful tool for analytical investigation and fast numerical simulations. We developed a novel numerical approach that avoids the built-in limitations of perturbative analysis to investigate the dynamical behavior of both of these circuits. This approach, based on the Floquet-Markov theory, resulted in a modular simulation framework which can be used to study other Josephson-based circuits. Last, we study the properties of an asymmetric version of the Josephson Ring Modulator, a circuit currently used for amplification and conversion, as a more robust source of non-linearity to engineer two-photon and four-photon interaction Hamiltonians required for the catstate encoding of quantum information.

Research fields

• Quantum information processing
• Superconducting qubits
• Numerical simulations

Main publications

• Verney, L., Pitaevskii, L., & Stringari, S. (2015). Hybridization of first and second sound in a weakly interacting Bose gas. Europhysics Letters, 111(4), 40005, https://arxiv.org/abs/1506.06690
• Lescanne, R., Verney, L., Ficheux, Q., Devoret, M. H., Huard, B., Mirrahimi, M., & Leghtas, Z. (2019). Escape of a driven quantum Josephson circuit into unconfined states. Physical Review Applied, 11(1), 014030, https://arxiv.org/abs/1805.05198
• Verney, L., Lescanne, R., Devoret, M. H., Leghtas, Z., & Mirrahimi, M. (2019). Structural instability of driven josephson circuits prevented by an inductive shunt. Physical Review Applied, 11(2), 024003, https://arxiv.org/abs/1805.07542