Abstract
Electron spins are amongst the most coherent solid-state systems known. However, to be used in devices for quantum sensing and information processing applications, they must typically be placed near interfaces. Understanding and mitigating the impacts of such interfaces on the coherence and spectral properties of electron spins is critical to realizing such applications, but it is also challenging: Inferring such data from single-spin studies requires many measurements to obtain meaningful results, while ensemble measurements typically give averaged results that hide critical information. Here, we report a comprehensive study of the coherence of near-surface bismuth donor spins in 28-silicon at millikelvin temperatures. In particular, we use strain-induced frequency shifts caused by a metallic electrode to infer spatial maps of spin coherence as a function of position relative to the electrode. By measuring magnetic-field-insensitive clock transitions, we separate magnetic noise caused by surface spins from charge noise. Our results include quantitative models of the strain-split spin resonance spectra and extraction of paramagnetic impurity concentrations at the silicon surface. The interplay of these decoherence mechanisms for such near-surface electron spins is critical for their application in quantum technologies, while the combination of the strain splitting and clock transition extends the coherence lifetimes by up to 2 orders of magnitude, reaching up to 300 ms at a mean depth of only 100 nm. The technique we introduce here to spatially map coherence in near-surface ensembles is directly applicable to other spin systems of active interest, such as defects in diamond, silicon carbide, and rare earth ions in optical crystals.
- Received 11 January 2021
- Accepted 15 June 2021
DOI:https://doi.org/10.1103/PhysRevX.11.031036
Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI.
Published by the American Physical Society
Physics Subject Headings (PhySH)
Popular Summary
Long-lived quantum states are crucial for applications in quantum information processing and sensing. Electron spins make an attractive solid-state system, as their coherence times can exceed seconds when implanted in the bulk of the substrate. However, for efficient manipulation and readout in quantum devices, these spins must be placed near interfaces and metal electrodes, which may potentially degrade their coherence properties. Understanding and mitigating such impacts is challenging: Single-spin studies require many measurements to obtain meaningful results, while ensemble measurements yield averaged results that hide crucial information. To get spatial resolution in one experiment with a spin ensemble, we propose and demonstrate a new method that makes use of strain gradients imparted in the sample containing the spins by a metallic electrode deposited on top.
Strain affects the spin-energy levels, causing their resonance frequency to become correlated with their spatial location relative to the electrode. We apply this principle to bismuth donor spins in silicon and measure a spatial map of their coherence time for various spin transitions and various electrode geometries. We find that on magnetically sensitive transitions, the coherence time ranges from 1 to 10 ms and decreases close to the sample surface, thus proving that decoherence is caused by interaction with surface paramagnetic impurities. Using a magnetically insensitive transition, we extend the coherence time up to 300 ms, a record for spins close to an interface.
Our method is directly applicable to other spin systems of active interest, such as defects in diamond, silicon carbide, and rare-earth ions in optical crystals.
