QUROPE Quantum Information Processing and Communication in Europe

Cette thèse porte sur la dynamique quantique dans un dcSQUID inductif. Ce dispositif est une boucle supraconductrice interrompue par deux jonctions Josephson. Sa dynamique est analogue à celle d’une particule massive évoluant dans un potentiel bidimensionnel. Dans la limite quantique, le dcSQUID se comporte comme un atome artificiel à deux degrés de liberté, contrôlé par le courant et le flux de polarisation.

Macroscopic quantum tunneling of the phase is a fundamental phenomenon in the quantum dynamics of superconducting nanocircuits. The tunneling rate can be controlled in such circuits, where the potential landscape for the phase can be tuned with different external bias parameters. Precise theoretical knowledge of the macroscopic quantum tunneling rate is required in order to simulate and understand the experiments.

We study quantum phase-slip processes in a one-dimensional phase-biased chain containing $N$ Josephson junctions. When the Josephson coupling energy $E_J$ of the junctions is larger than the charging energy $E_C=e^2/2C$ where $C$ is the junction capacitance, the quantum amplitude for the phase-slip processes is exponentially small in the ratio $E_J/E_C$; hence they can occur one by one.

Recently, a lasing effect has been observed in a superconducting nanocircuit where a Cooper-pair box, acting as an artificial three-level atom, was coupled to a resonator [ O. Astafiev, K. Inomata, A. O. Niskanen, T. Yamamoto, Yu. A. Pashkin, Y. Nakamura and J. S. Tsai Nature (London) 449 588 (2007)]. Motivated by this experiment, we analyze the quantum dynamics of a three-level atom coupled to a quantum-mechanical resonator in the presence of a driving on the cavity within the framework of the Lindblad master equation.

A novel method to fabricate large-area superconducting hybrid tunnel junctions with a suspended central normal metal part is presented. The samples are fabricated by combining photo-lithography and chemical etch of a superconductor - insulator - normal metal multilayer. The process involves few fabrication steps, is reliable and produces extremely high-quality tunnel junctions. Under an appropriate voltage bias, a significant electronic cooling is demonstrated. 

We present a novel shadow evaporation technique for the realization of junctions and capacitors. The design by e-beam lithography of strongly asymmetric undercuts on a bilayer resist enables in situ fabrication of junctions and capacitors without the use of the well-known suspended bridge (Dolan 1977 Appl. Phys. Lett. 31 337–9). The absence of bridges increases the mechanical robustness of the resist mask as well as the accessible range of the junction size, from 10 − 2 µm2 to more than 104 µm2.

We present a theoretical analysis of a superconducting quantum circuit based on a highly asymmetric Cooper pair transistor (ACPT) in parallel to a dc superconduction quantum intereference device (SQUID). Starting from the full Hamiltonian we show that the circuit can be modeled as a charge qubit (ACPT) coupled to an anharmonic oscillator (dc SQUID). Depending on the anharmonicity of the SQUID, the Hamiltonian can be reduced either to one that describes two coupled qubits or to the Jaynes-Cummings Hamiltonian. Here the dc SQUID can be viewed as a tunable micrometer-sized resonator.

By adding a large inductance in a dc-SQUID phase qubit loop, one decouples the junctions’ dynamics and creates a superconducting artificial atom with two internal degrees of freedom. In addition to the usual symmetric plasma mode (s mode) which gives rise to the phase qubit, an antisymmetric mode (a mode) appears. These two modes can be described by two anharmonic oscillators with eigenstates |ns⟩ and |na⟩ for the s and a mode, respectively. We show that a strong nonlinear coupling between the modes leads to a large energy splitting between states |0s,1a⟩ and |2s,0a⟩.

We make a detailed theoretical description of the two-dimensional nature of a dc SQUID, analyzing the coupling between its two orthogonal phase oscillation modes. While it has been shown that the mode defined as “longitudinal” can be initialized, manipulated, and measured, so as to encode a quantum bit of information, the mode defined as “transverse” is usually repelled at high frequency and does not interfere in the dynamics. We show that, using typical parameters of existing devices, the transverse mode energy can be made of the order of the longitudinal one.