Readout of spins in semiconductor quantum dots

Spins in semiconductor quantum dots constitute a promising pathway towards the realization of a large-scale quantum computer. To fulfill their promise as quantum bits (qubits), it is necessary that the spin states are manipulated and read out with a high fidelity. In this thesis, distinct implementations of spin readout are elucidated experimentally in two different spin qubit platforms. Investigating electron spins in the group III-V compound semiconductor gallium arsenide, we employ energy-selective spin readout with an adjacent electron reservoir in order to distinguish the spin-up and spin-down states of a single electron, provided that an external magnetic field is applied. We extract the g-factor, which is proportional to the energy difference of those two spin states, for different in-plane directions of the external magnetic field. This allows us to identify isotropic and anisotropic corrections to the bulk g-factor that arise from the spin-orbit interaction.
Next, we turn to hole spins in the group IV germanium/silicon core/shell nanowires, a platform that is predicted to host the strong and electrically tunable direct Rashba spin-orbit interaction. Making use of the transport phenomenon of Pauli spin blockade (PSB) arising in a pair of tunnel-coupled quantum dots, we demonstrate that a hole spin qubit can be operated at elevated temperatures of up to 1.7 Kelvin. This constitutes the first time that a germanium-based spin qubit has been measured at temperatures above 1 Kelvin, thus allowing for the implementation of on-chip classical control electronics. We also show that this spin qubit allows for compromise-free operation by simultaneously maximizing the qubit speed and coherence, challenging previous notions about a trade-off between these two quantities.
Following this experiment, we turn to the implementation of fast readout via radio-frequency reflectometry. We employ the scalable method of in situ dispersive readout, which uses the readily available gate electrodes in the nanowire devices. We overcome the challenge of impedance matching by developing a varactor – a voltage-tunable capacitor – based on the quantum paraelectric material strontium titanate, which remains highly tunable down to millikelvin temperatures and high magnetic fields of at least 1.5 Tesla. These properties make our varactors ideal circuit elements for the emerging field of cryogenic radio-frequency engineering, and furthermore allow us to boost our readout signal. After characterizing our reflectometry setup, we move on to describe a novel dispersive signature of PSB, which traces the current rectification properties. Altogether, our results show that the repertoire of spin readout techniques can help unveil a wide range of interesting physics.