Centre for Quantum Computation and Communication Technology, School of Physics, The University of New South Wales, Sydney, New South Wales 2052, Australia.
Purdue University, West Lafayette, Indiana 47906, USA.
Nat Mater. 2014 Jun;13(6):605-10. doi: 10.1038/nmat3941. Epub 2014 Apr 6.
Electron and nuclear spins of donor ensembles in isotopically pure silicon experience a vacuum-like environment, giving them extraordinary coherence. However, in contrast to a real vacuum, electrons in silicon occupy quantum superpositions of valleys in momentum space. Addressable single-qubit and two-qubit operations in silicon require that qubits are placed near interfaces, modifying the valley degrees of freedom associated with these quantum superpositions and strongly influencing qubit relaxation and exchange processes. Yet to date, spectroscopic measurements have only probed wavefunctions indirectly, preventing direct experimental access to valley population, donor position and environment. Here we directly probe the probability density of single quantum states of individual subsurface donors, in real space and reciprocal space, using scanning tunnelling spectroscopy. We directly observe quantum mechanical valley interference patterns associated with linear superpositions of valleys in the donor ground state. The valley population is found to be within 5% of a bulk donor when 2.85 ± 0.45 nm from the interface, indicating that valley-perturbation-induced enhancement of spin relaxation will be negligible for depths greater than 3 nm. The observed valley interference will render two-qubit exchange gates sensitive to atomic-scale variations in positions of subsurface donors. Moreover, these results will also be of interest for emerging schemes proposing to encode information directly in valley polarization.
在同位素纯硅中，供体集合的电子和核自旋经历类似于真空的环境，赋予它们非凡的相干性。然而，与真正的真空不同，硅中的电子占据了动量空间中谷的量子叠加。在硅中实现可寻址的单量子比特和双量子比特操作需要将量子比特放置在界面附近，这会改变与这些量子叠加相关的谷自由度，并强烈影响量子比特弛豫和交换过程。然而，迄今为止，光谱测量仅间接地探测了波函数，从而无法直接实验访问谷态、供体位形和环境。在这里，我们使用扫描隧道光谱学直接探测单个亚表面供体量子态的实空间和倒空间中的概率密度。我们直接观察到与施主基态中谷的线性叠加相关的量子力学谷干涉模式。当距界面 2.85 ± 0.45 nm 时，谷态的占据数在 5%以内与体施主相当，表明对于大于 3 nm 的深度，谷扰动引起的自旋弛豫增强将可以忽略不计。观察到的谷干涉将使双量子比特交换门对亚表面供体位置的原子级变化敏感。此外，这些结果对于新兴的提议直接在谷极化中编码信息的方案也将具有重要意义。
