Large, Tunable Valley Splitting and Single-Spin Relaxation Mechanisms in a <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline" overflow="scroll"><mml:mi>Si</mml:mi></mml:math>/<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline" overflow="scroll"><mml:msub><mml:mi>Si</mml:mi><mml:mi>x</mml:mi></mml:msub></mml:math><mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline" overflow="scroll"><mml:msub><mml:mi>Ge</mml:mi><mml:mrow><mml:mn>1</mml:mn><mml:mo>−</mml:mo><mml:mi>x</mml:mi></mml:mrow></mml:msub></mml:math> Quantum Dot
Arne Hollmann, Tom Struck, Veit Langrock, Andreas Schmidbauer, Floyd Schauer, Tim Leonhardt, Kentarou Sawano, Helge Riemann, Nikolay V. Abrosimov, Dominique Bougeard, Lars R. Schreiber
Abstract
Valley splitting is a key feature of silicon-based spin qubits. Quantum dots in $\mathrm{Si}$/${\mathrm{Si}}_{x}{\mathrm{Ge}}_{1\ensuremath{-}x}$ heterostructures reportedly suffer from a relatively low valley splitting, limiting the operation temperature and the scalability of such qubit devices. Here, we demonstrate a robust and large valley splitting exceeding 200 $\ensuremath{\mu}\mathrm{eV}$ in a gate-defined single quantum dot, hosted in molecular-beam-epitaxy-grown $^{28}\mathrm{Si}/{\mathrm{Si}}_{x}{\mathrm{Ge}}_{1\ensuremath{-}x}$. The valley splitting is monotonically and reproducibly tunable up to 15% by gate voltages, originating from a 6-nm lateral displacement of the quantum dot. We 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.