Showing 1–11 of 11 results for author: Chai, Z

Entanglement-Enhanced Nanoscale Single-Spin Sensing

Authors: Xu Zhou, Mengqi Wang, Xiangyu Ye, Haoyu Sun, Yuhang Guo, Han Shuo, Zihua Chai, Wentao Ji, Kangwei Xia, Fazhan Shi, Ya Wang, Jiangfeng Du

Abstract: Detecting individual spins--including stable and metastable states--represents a fundamental challenge in quantum sensing with broad applications across condensed matter physics, quantum chemistry, and single-molecule magnetic resonance imaging. While nitrogen-vacancy (NV) centers in diamond have emerged as powerful nanoscale sensors, their performance for single-spin detection remains constrained… ▽ More Detecting individual spins--including stable and metastable states--represents a fundamental challenge in quantum sensing with broad applications across condensed matter physics, quantum chemistry, and single-molecule magnetic resonance imaging. While nitrogen-vacancy (NV) centers in diamond have emerged as powerful nanoscale sensors, their performance for single-spin detection remains constrained by substantial environmental noise and restricted sensing volume. Here, we propose and demonstrate an entanglement-enhanced sensing protocol that overcomes these limitations through the strategic use of entangled NV pairs. Our approach achieves a 3.4-fold enhancement in sensitivity and a 1.6-fold reduction in spatial resolution relative to single NV centers under ambient conditions. The protocol employs carefully engineered entangled states that amplify target spin signals through quantum interference while suppressing environmental noise. Crucially, we extend these capabilities to resolve metastable single-spin dynamics, directly observing stochastic transitions between different spin states by identifying state-dependent coupling strengths. This dual functionality enables simultaneous detection of static and dynamic spin species for studying complex quantum systems. The achieved performance establishes entanglement-enhanced sensing as a viable pathway toward atomic-scale characterization of quantum materials and interface. △ Less

Submitted 30 April, 2025; originally announced April 2025.

Journal ref: Nature 647, 883-888 (2025)

Investigation of Rare-Earth Ion-Photon Interaction and Strong Coupling in Optical Microcavities

Authors: Quanshen Shen, Wentao Ji, Junyu Guan, Li Qian, Zihua Chai, ChangKui Duan, Ya Wang, Kangwei Xia

Abstract: The strong coupling between an emitter and a cavity is significant for advancing quantum networks. Due to their long optical and spin coherence times, rare-earth ions (REIs) represent a compelling platform for quantum networks. However, their inherently weak intra-4f optical transitions typically result in low coupling strength, thus restricting most current achievements to the weak coupling regim… ▽ More The strong coupling between an emitter and a cavity is significant for advancing quantum networks. Due to their long optical and spin coherence times, rare-earth ions (REIs) represent a compelling platform for quantum networks. However, their inherently weak intra-4f optical transitions typically result in low coupling strength, thus restricting most current achievements to the weak coupling regime. This work proposes a scheme to realize an on-chip quantum network by coupling REIs to high-quality whispering gallery mode (WGM) microcavities. Additionally, we provide numerical validation for a parametric amplification technique to enhance the emitter-cavity coupling strength. As an extension of this approach, the coupled system efficiently achieves the quantum entanglement of local and flying qubits. This study deepens the understanding of emitter-cavity interactions and contributes to realizing REIs-based photonic platforms, which are crucial to distributed quantum computing and developing robust quantum networks. △ Less

Submitted 14 April, 2025; originally announced April 2025.

Coherence Properties of Rare-Earth Spins in Micrometer-Thin Films

Authors: Zihua Chai, Zhaocong Wang, Xinghang Chen, Quanshen Shen, Zeyu Gao, Junyu Guan, Hanyu Zhang, Ya Wang, Yang Tan, Feng Chen, Kangwei Xia

Abstract: Rare-earth ions in bulk crystals are excellent solid-state quantum systems in quantum information science, owing to the exceptional optical and spin coherence properties. However, the weak fluorescence of single rare-earth ions present a significant challenge for scalability, necessitating the integration into micro-cavities. Thin films serve as a promising material platform for the integration, y… ▽ More Rare-earth ions in bulk crystals are excellent solid-state quantum systems in quantum information science, owing to the exceptional optical and spin coherence properties. However, the weak fluorescence of single rare-earth ions present a significant challenge for scalability, necessitating the integration into micro-cavities. Thin films serve as a promising material platform for the integration, yet the fabrication without compromising the properties of the materials and rare-earth ions remains challenging. In this work, we fabricate micrometer-thin yttrium aluminum garnet (YAG) films from bulk crystals using ion implantation techniques. The resulting films preserve the single-crystalline structure of the original bulk crystal. Notably, the embedded rare-earth ions are photo-stable and exhibit bulk-like spin coherence properties. Our results demonstrate the compatibility of bulk-like spin properties with the thin-film fabrication technique, facilitating the efficient integration of rare-earth ions into on-chip photonic devices and advancing the applications of rare-earth ions systems in quantum technologies. △ Less

Submitted 12 March, 2025; originally announced March 2025.

Minimum tracking linear response Hubbard and Hund corrected Density Functional Theory in CP2K

Authors: Ziwei Chai, Rutong Si, Mingyang Chen, Gilberto Teobaldi, David D. O'Regan, Li-Min Liu

Abstract: We present the implementation of the Hubbard ($U$) and Hund ($J$) corrected Density Functional Theory (DFT+$U$+$J$) functionality in the Quickstep program, which is part of the CP2K suite. The tensorial and Löwdin subspace representations are implemented and compared. Full analytical DFT+$U$+$J$ forces are implemented and benchmarked for the tensorial and Löwdin representations. We also present th… ▽ More We present the implementation of the Hubbard ($U$) and Hund ($J$) corrected Density Functional Theory (DFT+$U$+$J$) functionality in the Quickstep program, which is part of the CP2K suite. The tensorial and Löwdin subspace representations are implemented and compared. Full analytical DFT+$U$+$J$ forces are implemented and benchmarked for the tensorial and Löwdin representations. We also present the implementation of the recently proposed minimum-tracking linear-response method that enables the $U$ and $J$ parameters to be calculated on first principles basis without reference to the Kohn-Sham eigensystem. These implementations are benchmarked against recent results for different materials properties including DFT+$U$ band gap opening in NiO, the relative stability of various polaron distributions in TiO$_2$, the dependence of the calculated TiO$_2$ band gap on +$J$ corrections, and, finally, the role of the +$U$ and +$J$ corrections for the computed properties of a series of the hexahydrated transition metals. Our implementation provides results consistent with those already reported in the literature from comparable methods. We conclude the contribution with tests on the influence of the Löwdin orthonormalization on the occupancies, calculated parameters, and derived properties. △ Less

Submitted 24 July, 2024; v1 submitted 18 July, 2024; originally announced July 2024.

Journal ref: J. Chem. Theory Comput. 2024, 20, 20, 8984-9002

Experimental violation of the Leggett-Garg inequality with a single-spin system

Authors: Maimaitiyiming Tusun, Wei Cheng, Zihua Chai, Yang Wu, Ya Wang, Xing Rong, Jiangfeng Du

Abstract: Investigation the boundary between quantum mechanical description and classical realistic view is of fundamental importance. The Leggett-Garg inequality provides a criterion to distinguish between quantum systems and classical systems, and can be used to prove the macroscopic superposition state. A larger upper bound of the LG function can be obtained in a multi-level system. Here, we present an e… ▽ More Investigation the boundary between quantum mechanical description and classical realistic view is of fundamental importance. The Leggett-Garg inequality provides a criterion to distinguish between quantum systems and classical systems, and can be used to prove the macroscopic superposition state. A larger upper bound of the LG function can be obtained in a multi-level system. Here, we present an experimental violation of the Leggett-Garg inequality in a three-level system using nitrogen-vacancy center in diamond by ideal negative result measurement. The experimental maximum value of Leggett-Garg function is $K_{3}^{exp}=1.625\pm0.022$ which exceeds the Lüders bound with a $5σ$ level of confidence. △ Less

Submitted 16 April, 2022; originally announced April 2022.

Spin Quantum Heat Engine Quantified by Quantum Steering

Authors: Wentao Ji, Zihua Chai, Mengqi Wang, Yuhang Guo, Xing Rong, Fazhan Shi, Changliang Ren, Ya Wang, Jiangfeng Du

Abstract: Following the rising interest in quantum information science, the extension of a heat engine to the quantum regime by exploring microscopic quantum systems has seen a boom of interest in the last decade. Although quantum coherence in the quantum system of the working medium has been investigated to play a nontrivial role, a complete understanding of the intrinsic quantum advantage of quantum heat… ▽ More Following the rising interest in quantum information science, the extension of a heat engine to the quantum regime by exploring microscopic quantum systems has seen a boom of interest in the last decade. Although quantum coherence in the quantum system of the working medium has been investigated to play a nontrivial role, a complete understanding of the intrinsic quantum advantage of quantum heat engines remains elusive. We experimentally demonstrate that the quantum correlation between the working medium and the thermal bath is critical for the quantum advantage of a quantum Szilard engine, where quantum coherence in the working medium is naturally excluded. By quantifying the non-classical correlation through quantum steering, we reveal that the heat engine is quantum when the demon can truly steer the working medium. The average work obtained by taking different ways of work extraction on the working medium can be used to verify the real quantum Szilard engine. △ Less

Submitted 4 February, 2022; originally announced February 2022.

Quantum Anomaly Detection with a Spin Processor in Diamond

Authors: Zihua Chai, Ying Liu, Mengqi Wang, Yuhang Guo, Fazhan Shi, Zhaokai Li, Ya Wang, Jiangfeng Du

Abstract: In the processing of quantum computation, analyzing and learning the pattern of the quantum data are essential for many tasks. Quantum machine learning algorithms can not only deal with the quantum states generated in the preceding quantum procedures, but also the quantum registers encoding classical problems. In this work, we experimentally demonstrate the anomaly detection of quantum states enco… ▽ More In the processing of quantum computation, analyzing and learning the pattern of the quantum data are essential for many tasks. Quantum machine learning algorithms can not only deal with the quantum states generated in the preceding quantum procedures, but also the quantum registers encoding classical problems. In this work, we experimentally demonstrate the anomaly detection of quantum states encoding audio samples with a three-qubit quantum processor consisting of solid-state spins in diamond. By training the quantum machine with a few normal samples, the quantum machine can detect the anomaly samples with a minimum error rate of 15.4%. These results show the power of quantum anomaly detection in dealing with machine learning tasks and the potential to detect abnormal output of quantum devices. △ Less

Submitted 2 March, 2024; v1 submitted 25 January, 2022; originally announced January 2022.

Comments: 10 pages, 8 figures

Journal ref: Advanced Quantum Technologies 7, 2300385 (2024)

Resonant Quantum Principal Component Analysis

Authors: Zhaokai Li, Zihua Chai, Yuhang Guo, Wentao Ji, Mengqi Wang, Fazhan Shi, Ya Wang, Seth Lloyd, Jiangfeng Du

Abstract: Principal component analysis has been widely adopted to reduce the dimension of data while preserving the information. The quantum version of PCA (qPCA) can be used to analyze an unknown low-rank density matrix by rapidly revealing the principal components of it, i.e. the eigenvectors of the density matrix with largest eigenvalues. However, due to the substantial resource requirement, its experime… ▽ More Principal component analysis has been widely adopted to reduce the dimension of data while preserving the information. The quantum version of PCA (qPCA) can be used to analyze an unknown low-rank density matrix by rapidly revealing the principal components of it, i.e. the eigenvectors of the density matrix with largest eigenvalues. However, due to the substantial resource requirement, its experimental implementation remains challenging. Here, we develop a resonant analysis algorithm with the minimal resource for ancillary qubits, in which only one frequency scanning probe qubit is required to extract the principal components. In the experiment, we demonstrate the distillation of the first principal component of a 4$\times$4 density matrix, with the efficiency of 86.0% and fidelity of 0.90. This work shows the speed-up ability of quantum algorithm in dimension reduction of data and thus could be used as part of quantum artificial intelligence algorithms in the future. △ Less

Submitted 25 January, 2022; v1 submitted 6 April, 2021; originally announced April 2021.

Comments: 10 pages, 7 figures

Journal ref: Science Advance 7, 34 abg2589 (2021)

Quantum simulation for three-dimensional chiral topological insulator

Authors: Wentao Ji, Lin Zhang, Mengqi Wang, Long Zhang, Yuhang Guo, Zihua Chai, Xing Rong, Fazhan Shi, Xiong-Jun Liu, Ya Wang, Jiangfeng Du

Abstract: Quantum simulation, as a state-of-art technique, provides the powerful way to explore topological quantum phases beyond natural limits. Nevertheless, a previously-not-realized three-dimensional (3D) chiral topological insulator, and demonstrate by quantum quenches a complete study of both the bulk and surface topological physics. First, a dynamical bulk-surface correspondence in momentum space is… ▽ More Quantum simulation, as a state-of-art technique, provides the powerful way to explore topological quantum phases beyond natural limits. Nevertheless, a previously-not-realized three-dimensional (3D) chiral topological insulator, and demonstrate by quantum quenches a complete study of both the bulk and surface topological physics. First, a dynamical bulk-surface correspondence in momentum space is observed, showing that the bulk topology of the 3D phase uniquely corresponds to the nontrivial quench dynamics emerging on 2D momentum hypersurfaces called band inversion surfaces (BISs), equivalent to the bulk-boundary correspondence in real space. Further, the symmetry protection of the 3D chiral phase is uncovered by measuring dynamical spin textures on BISs, which exhibit perfect (broken) topology when the chiral symmetry is preserved (broken). Finally we measure the topological charges to characterize directly the bulk topology, and identify an emergent dynamical topological transition when varying the quenches from deep to shallow regimes. This work opens a new avenue of quantum simulation towards for the complete study of topological quantum phases. △ Less

Submitted 26 February, 2020; v1 submitted 26 February, 2020; originally announced February 2020.

Comments: 11 pages in total; 6 pages and 4 figures for main manuscript; 5 pages for supplementary materials

Journal ref: Phys. Rev. Lett. 125, 020504 (2020)

Experimental sensing quantum atmosphere of a single spin

Authors: Kehang Zhu, Zhiping Yang, Qing-Dong Jiang, Zihua Chai, Zhijie Li, Zhiyuan Zhao, Ya Wang, Fazhan Shi, Chang-Kui Duan, Xing Rong, Jiangfeng Du

Abstract: Understanding symmetry-breaking states of materials is a major challenge in the modern physical sciences. Quantum atmosphere proposed recently sheds light on the hidden world of these symmetry broken patterns. But the requirements for exquisite sensitivity to the small shift and tremendous spatial resolution to local information pose huge obstacles to its experimental manifestation. In our experim… ▽ More Understanding symmetry-breaking states of materials is a major challenge in the modern physical sciences. Quantum atmosphere proposed recently sheds light on the hidden world of these symmetry broken patterns. But the requirements for exquisite sensitivity to the small shift and tremendous spatial resolution to local information pose huge obstacles to its experimental manifestation. In our experiment, we prepare time-reversal-symmetry conserved and broken quantum atmosphere of a single nuclear spin and successfully observe their symmetry properties. Our work proves in principle that finding symmetry patterns from quantum atmosphere is conceptually viable. It also opens up entirely new possibilities in the potential application of quantum sensing in material diagnosis. △ Less

Submitted 12 January, 2020; originally announced January 2020.

Experimental observation of dynamical bulk-surface correspondence for topological phases

Authors: Ya Wang, Wentao Ji, Zihua Chai, Yuhang Guo, Mengqi Wang, Xiangyu Ye, Pei Yu, Long Zhang, Xi Qin, Pengfei Wang, Fazhan Shi, Xing Rong, Dawei Lu, Xiong-Jun Liu, Jiangfeng Du

Abstract: We experimentally demonstrate a dynamical classification approach for investigation of topological quantum phases using a solid-state spin system through nitrogen-vacancy (NV) center in diamond. Similar to the bulkboundary correspondence in real space at equilibrium, we observe a dynamical bulk-surface correspondence in the momentum space from a dynamical quench process. An emergent dynamical topo… ▽ More We experimentally demonstrate a dynamical classification approach for investigation of topological quantum phases using a solid-state spin system through nitrogen-vacancy (NV) center in diamond. Similar to the bulkboundary correspondence in real space at equilibrium, we observe a dynamical bulk-surface correspondence in the momentum space from a dynamical quench process. An emergent dynamical topological invariant is precisely measured in experiment by imaging the dynamical spin-textures on the recently defined band-inversion surfaces, with high topological numbers being implemented. Importantly, the dynamical classification approach is shown to be independent of quench ways and robust to the decoherence effects, offering a novel and practical strategy for dynamical topology characterization, especially for high dimensional gapped topological phases. △ Less

Submitted 18 April, 2019; originally announced April 2019.

Comments: 6 pages, 4 figures

Journal ref: Phys. Rev. A 100, 052328 (2019)