Quantum technologies with superconducting artificial atoms

Abstract

In the present work, we explore possible routes to the future exploitation of quantum technologies in superconducting artificial atoms. The objectives of the thesis are twofold. On the one hand, we study advanced control of the quantum states of individual artificial atoms, tailored at robust and faithful quantum information processing in complex architectures and at the detection of the Ultra Strong Coupling regime. On the other hand, we study and propose a framework to experimentally establish quantum stochastic thermodynamics in circuit-QED.

In chapter 3, the implementation of a Lambda system in superconducting artificial atoms is discussed. Strategies for optimal design are investigated by means of optimal symmetry breaking and dynamical decoupling. Stimulated Raman Adiabatic Passage (STIRAP), an adiabatic population transfer technique in three-level system, is introduced and its transfer efficiency is shown to be around 70% in the Cooper Pair Box. Optimization strategies are also discussed.

In chapter 4, a novel technique for population transfer in a Lambda system is proposed, nicknamed chirped STIRAP (cSTIRAP), its key asset being the possibility of operating with an always on driving field. Robustness against parametric imperfections is assessed and specific regimes of failure due to energy level fluctuations are thoroughly examined.

A novel way to control superconducting qutrits in the Lambda system is introduced in chapter 5. It is shown that, by employing a two-photon pump, the spectrum of the devices is changed in a non-trivial way by AC Stark shifts, that can be then compensated by suitable modulation of the driving phases. A 2+1 photons STIRAP technique is introduced, with transfer efficiency of 97% in last generation devices despite the presence of both low and high frequency noise.

In chapter 6, dynamical detection of the Ultra Strong Coupling (USC) regime is studied. It is shown that, by implementing a three-level Vee scheme with a flux qutrit, non classical effects and exotic light-matter interaction phenomena can be amplified and detected unambiguously.

A Circuit-QED implementation of a non-equilibrium thermodynamic experiment is finally proposed in chapter 7. A stochastic thermodynamics formalism is defined and work and heat are defined at the single quantum trajectory level. Numerical simulations are shown and the possibility to verify detailed fluctuation theorems is demonstrated. Moreover, the entropy production is defined as a witness of irreversibility and trajectory with a negative entropy production are shown, the mean entropy production being non-negative as required by the second law.