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Nicolas Piot

A Qubit insensitive to noise

Published on 18 April 2023
Thesis presented April 18, 2023

Spin quantum bits (qubits) defined in semiconductor quantum dots have emerged as a promising platform for quantum information processing. In particular, group IV semiconductors such as silicon and germanium show encouraging results. Among their numerous qualities, the biggest strength of silicon is its compatibility with mainstream manufacturing technology and the ability to eliminate spurious nuclear spins through isotopic purification. Furthermore, electron spin qubits can take advantage of electron dipole spin resonance enabled via an artificial spin orbit coupling, allowing for fast and full electrical spin manipulation. Although quantum dots promise to enable high-integration densities due to their small footprint, the necessity to use cumbersome add-ons to leverage the spin orbit coupling worsens the scalability prospect. Conversely, hole states in silicon possess an intrinsic and fully tunable spin orbit coupling, key for practical, fast and potentially scalable qubit control. In this thesis, we report a single hole spin with enhanced coherence time in natural silicon, achieved by the understanding and the optimization of the spin orbit coupling.
It is intrinsically hard to measure the spin state of a single particle. Consequently, we mapped the spin onto the charge state via an energy selective readout. To do so, a built-in sensor is created by bringing into resonance a large hole island with a reservoir, which is then connected to a tank circuit. The signal is finally recorded via radio frequency reflectometry, allowing for fast and efficient charge sensing. A single hole was then isolated close to the sensor, to study its properties. The spin-orbit coupling combined with asymmetric confinement of the hole give rise to an anisotropic Zeeman energy. Consequently, we measured the g-factors and the spin susceptibility as a function of magnetic field orientation. Although, full electrical driving of hole spin state is a strength, it also renders the qubits sensitive to surrounding electrical noise. We demonstrated that for specific magnetic field orientation, we were able to minimize the longitudinal coupling, improving by a factor five the coherence time. The above-mentioned tuning offers a method to hide the qubit from the noise, helping to improve its property. We also studied the origin of noise affecting the spin lifetime. At low frequency, magnetic noise from the remaining silicon 29 isotope emerge as a highly probable candidate, while at high frequency the main noise source is electrical. The absence of phonons, which should be the main mechanism for spin flips, however remains an open question. The presented work offers a new tunable basic unit, made with CMOS technologies for quantum information processing.

CMOS, quantum processor, Hole Spin Qubits