Prof. Ying Jiang’s group from Peking University substantially enhances the coherence of solid-state qubits by scanning probe microscopy

   Recently, Prof. Ying Jiang from the International Center for Quantum Materials and Research Center for Light-Element Advanced Materials of Peking University, in collaboration with Prof. Sen Yang from the Hong Kong University of Science and Technology and Prof. Jörg Wrachtrup from Stuttgart University, successfully realized the nanoscale manipulation of near-surface electron spin bath and notable coherence enhancement of shallow solid-state qubits on a diamond chip under ambient conditions, by using a home-made scanning probe microscope (SPM). This work titled “Coherence enhancement of solid-state qubits by local manipulation of the electron spin bath”, has been published in Nature Physics on August 25th, with the potential to break the sensitivity bottleneck in the field of quantum sensing.

   Qubit is the basis of modern quantum science and technology such as quantum computation, quantum information, and quantum sensing. The performance of a qubit is determined by its quantum coherence, which is a critical parameter to maintaining its quantum properties. Effectively suppressing the decoherence of qubits is crucial for the applications of quantum science and technology. Nitrogen-vacancy (NV) center is a point defect hosted in the diamond, which is regarded as one of the most promising solid-state qubits. Since the NV has a long coherence time up to milliseconds even under ambient conditions, the theoretically predicted sensitivity of NV is ultrahigh, allowing for detecting a single nuclear spin. To enhance the coupling strength of NVs to external parameters and achieve high signal-to-noise ratios, it is crucial to bring the NVs into proximity to the diamond surface in the experiment. However, due to the ever-present electron paramagnetic noise near the diamond surface after the NV growth, shallow NVs suffer greatly from decoherence and sensitivity degradation. Thus, there’s still a big gap between the practical and theoretical sensitivity, severely limiting the applications of quantum sensing.

Figure 1: a) 3D model of the home-built scanning quantum sensing microscope. All the data in this work were recorded in this system. b) The photo of the scanner head of our home-built microscope.

   Ying Jiang and his group, have been for a long time devoted to the development of advanced SPM systems. Last year, they have successfully built up a new type of SPM, which integrated the quantum sensing technology into the qPlus-based noncontact atomic force microscope (AFM) (Figure 1). With this system, they have, for the first time, realized NV-based nanoscale electric-field imaging and its charge-state control (Bian et al., Nat. Commun. 12, 2457 (2021)).

   In order to directly eliminate the near-surface electron spin noise, Ying Jiang’s group, in collaboration with Sen Yang’s group, developed a so-called “Pull-and-Push” method based on the SPM tip manipulation with nanoscale precision. This method made use of the local electric field from a sharp tip and substantially decreased the electron spin noise near the diamond surface, leading to a notable enhancement of both coherence and sensitivity of shallow NV centers. This technique is particularly effective for extremely shallow NV centers, increasing their spin echo time (T2) by up to 20 fold. This corresponds to an 80-fold enhancement in the sensitivity for detecting individual external proton spins (Figure 2).

Figure 2: a) and b) Schematic showing the manipulation of near-surface electron spin bath by using a local electric field from an SPM tip (“Pull-and-Push” method). c) Notable coherence enhancement of shallow NV centers after charge manipulation (T2 enhancement ~20 fold). d) The sensitivity of the NV with enhanced coherence approaches the limit of single proton nuclear spin. e) Summary of the T2 time enhancement for 25 shallow NVs by using charge manipulation. This method is particularly effective for NVs with depths < 5 nm.

   By performing detailed noise spectra analyses, the researchers found that unpaired electrons both on the surface and at the subsurface were notably depleted after tip manipulation. Furthermore, through the NV-based nuclear magnetic resonance (NMR) measurements, the shallow NVs with enhanced coherence can detect the internal single 13C nuclear spin and external proton spin clusters, which pave the way for nanoscale NMR imaging. This work not only provides a powerful toolkit for suppressing the decoherence of shallow NV centers with the potential to break the sensitivity bottleneck in the field of quantum sensing, but is also suitable for enhancing the coherence of other kinds of solid-state qubits in diamond, silicon carbide, boron nitride, etc.

   Ying Jiang, Sen Yang, and Associate Prof. Ke Bian are the corresponding authors. Dr. Wentian Zheng and Ke Bian are the co-first authors. This work was financially supported by the National Natural Science Foundation of China, the Ministry of science and technology, the Ministry of education, the Beijing municipal government, the Collaborative Innovation Center of Quantum Matter, and the Research Center for Light-Element Advanced Materials of Peking University.

The information and links for this paper:

W. Zheng, K. Bian*, X. Chen, Y. Shen, S. Zhang, R. Stöhr, A. Denisenko, J. Wrachtrup, S. Yang* and Y. Jiang*, Coherence enhancement of solid-state qubits by local manipulation of the electron spin bath, Nat. Phys. DOI: 10.1038/s41567-022-01719-4 (2022). (