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Sébastien LEGER present

Quantum electrodynamics of a Josephson junction

coupled to a strongly dissipative environment

Wednesday 16th june 2021 at 3.00 pm

Link: https://www.twitch.tv/richlet

Salle Erwin Bertaut (F418): capacity is limited to 15 people

The defense will be held in english.


Abstract :

Since the pioneering work of B.D. Josephson, the superconducting tunnel junctions, also called Josephson junctions, have proven their usefulness in testing various predictions of quantum mechanics.

This PhD work focuses on the use of these junctions to study quantum impurity problems. These problems deal with the interaction between a degree of freedom, the quantum impurity, and an environment. They are thus mainly used to describe how dissipation can be induced in quantum mechanics.

To study these problems we designed two kinds of circuits. Both of them relying on the galvanic coupling between a Josephson junction in the nonlinear regime coupled to a chain of Josephson junctions tailored in the linear regime. These chains are composed of thousands of junctions but cannot be made infinite, hence they constitute a quasi-continuum. However, under some circumstances they behave like a thermodynamic bath. Using a quasi-continuum instead of a real bath makes the problem richer since the backaction from the nonlinearity of the environment can be studied.

This problem being complex a new formalism had to be develop to understand these circuit. The main theoretical novelty came from the Self Consistent Harmonic Approximation. It allowed us to obtain two main predictions on our circuit. First, a renormalization of the nonlinear junction Josephson energy coming from the interaction between the latter and the environment induced phase fluctuations. Second, the creation of a dissipative channel coming from the coupling between single-photon states and multi-photon states.

Thanks to these new theoretical tools we were able to interpret the measured data. This allowed us to measure a non perturbative renormalization of the nonlinear junction Josephson Energy, the maximal value we reached was about 50\% of the bare value. In addition the maximal probability of decay per round trip for the photons inserted in the circuit and that we could attribute to the nonlinear element was about 0.1. Both these observations, together with the microscopic modeling of our circuit show that circuit Quantum Electrodynamics can be used to study complex many body problems.

Thesis Directors : Olivier Buisson & Nicolas Roch (Institut Néel)