Fast singlet-triplet qubit driven by magnetic field gradient in isotopically purified silicon

The Micromagnet technique has proven effective for manipulating spins in semiconductor quantum dot (QD) nanostructures, especially in silicon. Although the placement of on-chip micromagnets has enabled single-spin qubits in silicon with gate fidelity to reach surface code-based error correction threshold, corresponding results using encoded spin qubits, for example, single-triplet qubits with high-quality quantum oscillations, have not been demonstrated. Instead, the spin-valley coupling has been recently used to enhance the electrical controllability of two-electron spin qubits in silicon at the expense of increased susceptibility to charge noise. Here, we demonstrate fast singlet-triplet qubit oscillation (>100MHz) in isotopically purified 28-Si/SiGe substrate with an on-chip micromagnet in the regime where valley-splitting in each quantum dot exceeds 300 ueV. Combining rf-reflectometry-based single-shot readout and adaptive initialization, we show that the oscillation quality factor of an encoded spin qubit over 300 can be achieved. We further present the measurement of single-triplet qubit oscillation and variation of coherence time near the micro-magnet's magnetization reversal, offering a route to in-situ tune magnetic field gradient and hence the Larmor frequency of the singlet-triplet qubit in silicon.