# Relaxation and decoherence of a 28Si/SiGe spin qubit with large valley splitting

Electron spin qubits in gate defined Si/SiGe quantum dots have become one of the most promising platforms for spin based quantum computation. Single-qubit gate fidelities higher than the error correcting threshold and two-qubit gates have been demonstrated. Applying industrial fabrication processes and integrating conventional silicon electronics opens up the perspective of a highly scalable and dense quantum computing architecture. However, quantum dots in Si/SiGe heterostructures reportedly suffer from a relatively low valley splitting, limiting the operation temperature and the scalability of such qubit devices. In this work we demonstrate a robust and large valley splitting exceeding 200$\,\mu$eV in a gate-defined quantum dot, hosted in molecular-beam epitaxy-grown $^{28}$Si/SiGe with a residual $^{29}$Si contribution of only 60 ppm. We model the spin relaxation mechanisms and observe static spin relaxation times $T_1>1$ s at low magnetic fields in our device containing an integrated nanomagnet. At higher magnetic fields, $T_1$ is limited by the valley hotspot and by phonon noise coupling to intrinsic and artificial spin-orbit coupling, including phonon bottlenecking. The large valley splittings with reproducible stability represent a step forward for the realisation of multi-qubit devices and qubits at elevated temperature. We demonstrate single-shot spin read-out and electric dipole spin resonance control with a single domain nanomagnet. The maximal Rabi-frequency of 1 MHz is limited by unintentional electron exchange with the reservoir. We measure a dephasing time of $T_2^*= (19.14\pm 0.40)\,\mu\text{s}$ for a measurement time of four minutes and discuss possible measurements to discriminate the underlying dephasing mechanisms in this device. The measured record spin echo time of $T_2^\text{echo} =(131\pm4)\,\mu\text{s}$ is not effected by the voltage bias and current of the near-by charge sensor. Quantum error correction requires millions of physical qubits and therefore a scalable quantum computer architecture. To solve signal-line bandwidth and fan-out problems, microwave sources required for qubit manipulation might be embedded close to the qubit chip. In this context, we perform the first low temperature measurements of a 130 nm BiCMOS based SiGe voltage controlled oscillator at cryogenic temperature that maintains its full functionality from 300 K to 4 K. We determined the frequency and power dependence on temperature and magnetic field up to 5 T and measured the temperature influence on its noise performance.