Thesis presented December 10, 2020

Abstract:
We measure at very low temperature silicon MOS nanowire transistors in which the electronic transport through the channel takes place via one or more quantum dots. We have also built a radio frequency reflectometry device that allows us to probe the charge and spin state of a charge carrier trapped in one of these dots, even in the case of an interdot charge transition that does not give rise to drain-source current. This main task involves measurements at low temperature, under magnetic field, with low signals. The goal is to control the state of charge and spin for two different applications: electron pumps for quantum ampere metrology and spin quantum bits, or qubits.
In the framework of this thesis, we focused on the problems related to spin qubits. Indeed, in 2016, our laboratory realized the first spin qubit of holes in quantum dots realized in silicon MOS technology. More precisely, we focused on two essential points of the functioning of such a qubit and, by extension, of a network of qubits: the control of the coupling between the quantum dots and the readout of the spin.
First, we looked at three different ways of acting on the coupling: via the polarization of the backgate or via the polarization of a metallic line, both capacitively coupled to the nanowire, and finally via a third quantum dot placed in between the two dots. The first two ways are directly related to the design of the devices. The third way is much more subtle. We have shown that when two quantum dots are coupled via a third dot, the effective coupling between the two outer dots strongly depends on the internal state of the central dot.
In a second step, we set up in a cryostat a new way to read the spin of a carrier via energy selectivity, involving fast electrical manipulations on the gates. After setting up and characterizing the necessary lines in the cryostat, we had to configure both the sending and the analysis of pulsed signals. The experiment was a success at zero magnetic field. We are getting exactly what we expect. This allowed us to validate the experimental set-up. At a non-zero magnetic field, on the other hand, the results obtained are non-trivial, indicating that indeed the magnetic field plays a role but it was difficult to pinpoint a spin signal accurately.
This work was carried out in close collaboration with CEA-LETI, for the design and fabrication of the samples on their 300mm SOI (Silicon-On-Insulator) CMOS platform. This compatibility of our devices with industrial microelectronics production lines could be a great asset in the quest for large-scale qubit integration.

Keywords:
Quantum dots, radio frequency reflectometry, cryogenics, silicon, MOSFET, quantum engineering

On-line thesis.