AQP -seminaari: Controlling spin-orbit interaction in scalable silicon-MOS quantum dot architectures
Aalto Quantum Physics -seminaari (Nanotalo). Puhuja: Dr. Tuomo Tanttu (UNSW, School of Electrical Engineering, Sydney, Australia)
Spin qubits in silicon are excellent candidates for scalable quantum computation  due to their long coherence times and the enormous investment in silicon CMOS technology. The isotopic purification of the nuclear spin free Si-28 and weak spin-orbit interaction (SOI) allow a quiet noise environment for the spins in silicon. Gate-defined quantum dots formed using standard silicon metal-oxide-semiconductor (SiMOS) technology can be conveniently configured to realize multi-qubit devices where control fidelities exceeding 99% using pulsed electron spin resonance (ESR) have been demonstrated . Neighboring SiMOS quantum dot qubits can also be directly exchange coupled, and two qubit gates can be realized via exchange rotations  or purely via ESR pulses and in the latter case the two-qubit gate fidelity has been shown to exceed 90% . Next challenges on the path to large-scale quantum computing are the demonstration of quantum error correction protocols and the realization a logical qubit which has been recently proposed in a linear array of silicon dots with detailed design and protocols . Preliminary studies for the operation in such architectures has been recently reported . Long term plans for silicon qubits include moving towards two-dimensional CMOS arrays .
Despite of experiencing weak SOI, sharp interfaces in heterostructures induce a noticeable SOI that can degrade the performance of the qubits by increasing decoherence and inducing state leakages. The variations in SOI from dot to another and from device to device place a challenge for future scale-up scenarios in silicon qubits. However, when understood and controlled, it can be turned into powerful resource for instance to drive Global ESR, increase coherence times or to drive qubits electrically without the need for micromagnet. I will present recent results , where we probe the SOI with external magnetic field direction. We demonstrate the control of g-factors, g-factor difference, Stark shift, and state mixing between singlet and polarized triplet states in a SiMOS double dot system. Finally, we exploit the tunability of the Stark shift of one of the dots to reduce its sensitivity to electric noise and observe an expected increase in coherence time .
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