Electron spin and charge in semiconductor quantum dots

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Abstract

In this thesis, the spin and charge degree of freedom of electrons in semiconductor lateral and vertical quantum dots are experimentally investigated. The lateral quantum dot devices are defined in a two-dimensional electron gas (2DEG) below the surface of a GaAs/AlGaAs heterostructure, by metallic surface gates. The vertical quantum dots are sub-micron pillars fabricated in an In/Al/GaAs double-barrier heterostructure, and surrounded by a metal gate electrode. Both kinds of quantum dots behave in many ways as artificial atoms. In the first part of this thesis, we describe experiments aimed at using a single electron in a lateral quantum dot as a spin qubit, building block of a quantum computer. We first develop the spin qubit hardware: a device consisting of two coupled quantum dots that can be filled with one electron spin each, with a controllable inter-dot tunnel coupling. We then use a nearby quantum point contact (QPC) as an electrometer to characterize the quantum dot in the regime of very weak coupling to the reservoirs. In particular, we measure the Zeeman splitting between the proposed qubit states, i.e. the spin-up and spin-down state of a single electron spin in a large magnetic field. Finally, we develop a fully electrical technique to perform single-shot measurement of the spin orientation of an individual electron in a quantum dot. We find a very long spin relaxation time of 0.85 ms at a magnetic field of 8 T, indicating that the electron spin degree of freedom is only weakly disturbed by the environment. We conclude part one of this thesis with an overview of the progress made towards creating a spin qubit. Part two focusses on quantum dots that are strongly coupled to the reservoirs. We observe a strong Kondo effect in a lateral quantum dot, with the conductance reaching the unitary limit. In a vertical quantum dot containing six electrons, we observe an unexpected Kondo effect at the transition between a spin singlet and a spin triplet ground state. Finally, we conclude with an investigation of elastic and inelastic cotunneling in a vertical quantum dot containing two to six electrons.