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Chang-Kui Duan

Publications and source records attributed to Chang-Kui Duan.

At least 19 recordsLinked to original sources
arXiv2026

Stringent Constraints on Spin-Spin-Velocity-Dependent Exotic Interactions with a Levitated Magnet Force Sensor

Exotic spin-spin-velocity-dependent interactions, predicted in extensions of the Standard Model involving new bosonic fields, could resolve fundamental puzzles from dark matter to cosmic asymmetry. However, exploring these weak potential interactions at centimeter scales presents formidable challenges, primarily due to the overwhelming dominance of electromagnetic backgrounds that can easily obscure the weak exotic signals. Here, we utilize a levitated magnet force sensor with ultrahigh electron spin density to probe these interactions. We constrain two interactions individually through a designed spin source and a multi-layer magnetic shielding system that suppresses electromagnetic backgrounds. In this study, we constrain two types of interactions: the V_6 potential at force ranges from $10^{-3}$ m to $6 \times 10^{-2}$ m and the V_{14} potential at ranges greater than $10^{-3}$ m. Our measurements establish 95% confidence-level bounds of $|f_6| \leq 2.12 \times 10^{-13}$ and $|f_{14}| \leq 2.34 \times 10^{-23}$ at $\lambda = 1.6 \times 10^{-2}$ m, improving prior limits by up to 12 and 13 orders of magnitude, respectively. Our result demonstrates the levitated magnet as a highly sensitive probe for detecting new bosonic fields in extensions of the Standard Model.

physics.app-ph↗
arXiv2026

Molecular chiral discrimination through symmetry-breaking spin dynamics

Molecular chirality plays a crucial role in physics, chemistry, life sciences and pharmacology. Nowadays, the chiral discrimination and control at the single-molecule level is urgently needed to reveal the origin of the chirality-relevant phenomena by recovering the information disturbed by the ensemble averaging. The method of magnetic resonance (MR), as one of powerful tools for structure analysis, is blind to the molecular chirality in the absence of a chiral reagent. Here we propose and experimentally demonstrate a direct MR-based method for determining the chirality at the single-molecule level through constructing the symmetry-breaking dynamics of nearby nuclear spins. In principle, the mirror asymmetry of two enantiomers in real space is manifested by breaking the joint symmetry of the mirror reflection and time reversal in spin space under spin dynamics. Experimentally, two enantiomers are indistinguishable from the dynamics of strongly-coupled but unpolarized nuclear spins, but diverge evidently in the dynamical results that break the field-inversion symmetry after spins are polarized. Our method and results will benefit the study of chirality-induced properties in the fields of chemistry and biology.

quant-ph↗
arXiv2026

Lanthanide-Dependent Clustering in Yb$^{3+}$/Ln$^{3+}$ Co-Doped CaF$_2$ Nanocrystals: Correlating Spectroscopic Signatures with DFT Insights

The formation of heterogeneous lanthanide-ion clusters in CaF$_2$ was investigated experimentally and computationally. CaF$_2$ nanoparticles co-doped with 20~mol\% Yb$^{3+}$ and 2~mol\% Ln$^{3+}$ (Ln$^{3+}$ = Ce$^{3+}$, Pr$^{3+}$, Nd$^{3+}$, Sm$^{3+}$, Eu$^{3+}$, Gd$^{3+}$, Ho$^{3+}$, Er$^{3+}$, and Tm$^{3+}$) were synthesized via a hydrothermal method. The structural and morphological properties were characterized using powder X-ray diffraction, dynamic light scattering, and transmission electron microscopy techniques. High-resolution Fourier transform infra-red spectroscopy revealed the presence of Yb$^{3+}$ isolated cubic centers and various cluster sites. The relative concentration of the clusters varied with the choice of the co-doping ion. Calculations based on density functional theory were used to estimate the formation energies and local coordination structures of different clusters. The calculations indicate that the neutral $C_{4v}$ aggregations containing Ln$^{3+}$ tend to decrease across the lanthanide series, while the negatively charged derivatives of hexameric clusters are relatively constant. This variation matches the experimental results. This study advances understanding of the clustering mechanisms in lanthanide-doped CaF$_2$ nanoparticles and has implications for luminescence optimization in advanced nanomaterials.

cond-mat.mtrl-sci↗
arXiv2026

Reconfigurable Oxide Nanoelectronics by Tip-induced Electron Delocalization

Reconfigurable oxide nanoelectronics, enabled by conductive atomic force microscope (cAFM) lithography, have established complex oxide interfaces as a promising platform for quantum engineering that harnesses emergent phenomena for advanced functionalities. However, this cAFM nanofabrication process can only occur in the air, with simultaneous device decay described under the "water-cycle" writing mechanism. These restrictions pose ongoing challenges for device optimization in the quantum regime at mK temperatures. Here, we demonstrate a "waterless" cAFM lithography approach that is compatible with vacuum and cryogenic environments. Through oxygen vacancy engineering at the LaAlO$_3$/SrTiO$_3$ interface, we have achieved nonvolatile and reconfigurable cAFM control of nanoscale interfacial polaron-electron liquid transition at mK temperatures with an ultrafine line resolution of 0.85 nm. Supported by first-principles calculations and drift-diffusion modeling, we show that tip-controlled oxygen vacancy electromigration plays a key role. This advancement bridges reconfigurable device fabrication and concurrent characterization in situ at mK temperatures, and establishes a versatile Hubbard toolbox for engineering programmable quantum phases in correlated oxides.

cond-mat.mes-hall↗
arXiv2025

Time-Reversal Symmetry Protected Transport at Correlated Oxide Interfaces

Time-reversal symmetry (TRS) protection is core to topological physics, yet its role in correlated oxides-typically non-topological-remains underexplored. This limit hampers the potential in engineering exotic quantum states by fusing TRS protection and the rich emergent phenomena in the oxide platform. Here, we report evidence of a TRS-protected subband at oxygen vacancy-free LaAlO3/SrTiO3 interfaces. This subband causes a low-field quantum oscillation with anomalous characters: exceptionally light electron mass, aperiodicity, and susceptibility to magnetic fields. All findings align with a Rashba model in which TRS-protected transport occurs along quasi-1D ferroelastic domain walls, which possess a Dirac band topology and a giant Rashba spin-orbit coupling, two orders stronger than the 2D interface. Our results deepen the understanding of SrTiO3-based electron systems, unveiling an appealing new platform for quantum engineering.

cond-mat.mes-hall↗
arXiv2025

Mechanisms of Dual-Band Emission in Sb-Doped Rare-Earth Phosphates Revealed

The Sb$^{3+}$ ion has garnered significant interest due to its effectiveness in boosting the optical properties of host materials. Among the interesting phenomena is the commonly observed dual-band emission, which has often been interpreted by adopting the phenomenological model that explains the dual-band emission (``ultraviolet band'' and ``visible band'') in Sb-doped $L$PO$_{4}$ ($L$ = Sc, Y, Lu). However, the model for Sb-doped $L$PO$_{4}$ series itself has not been well understood theoretically. In this work, we employ first-principles calculations combined with group-theory analysis to clarify the underlying physical mechanism behind dual-band emission in Sb-doped $L$PO$_{4}$ series. We demonstrate that the dual-band arises from two excited-state equilibrium structures, one exhibits a relatively small distortion with respect to the ground-state equilibrium structure, while the other displays a significantly larger distortion, characteristic of an ``off-center'' configuration. The deviations from the ground-state configuration are dominated by two distinct vibrational modes, $b_2$ and $e$ modes, involving the Jahn-Teller effect and the pseudo Jahn-Teller effect, respectively. Furthermore, charge transition levels and energy barriers calculated using the climbing image nudged elastic band (CI-NEB) method have aided in understanding the relaxation between the two excited-state configurations and the property changes across the Sc, Y, and Lu series. These insights provide a basis for understanding the exotic properties of Sb$^{3+}$ in other hosts and may facilitate the design of optical materials in a broader range of systems involving Sb$^{3+}$ ions.

cond-mat.mtrl-sci↗
arXiv2025

Constraints on Velocity and Spin Dependent Exotic Interaction at the Millimeter Scale with a Diamagnetic-levitated Force Sensor

Light bosons, beyond the standard model and as prominent candidates for dark matter, can mediate velocity and spin dependent exotic interaction between electron spins and nucleons. At short ranges, it remains an open challenge to test this exotic interaction with high precision. Here, we present a method based on diamagnetic-levitated force sensor to detect the exotic interaction at the millimeter scale. Improved constraints for the coupling $g_A^e g_V^N$ are established, within the force range spanning from 0.15 mm to 1.5 mm. And the constraint $|g_A^e g_V^N| \leq 4.39 \times10^{-26}$ at $\lambda$ = 0.5 mm at the $95 \%$ confidence level, significantly surpasses previous results by more than three orders of magnitude. The diamagnetic levitation force measurement system developed here can also be repurposed to probe other exotic spin-dependent interactions, such as the exotic spin-spin interaction. This system provides a platform for the exploration of dark matter.

hep-ph↗
arXiv2025

Non-Hermitian non-Abelian topological transition in the S=1 electron spin system of a nitrogen vacancy centre in diamond

Topological phases and transitions are of fundamental importance in physics, which provide a deep insight into the understanding of materials. Recently, non-Abelian topological transitions have been investigated in Hermitian systems, revealing important topological features. With non-Hermiticity introduced, non-Hermitian non-Abelian topological transitions bring about more intriguing topological features, yet has not been experimentally explored. In this work, we report the observation of the non-Hermitian non-Abelian topological transition at the atomic scale utilizing a nitrogenvacancy center in diamond. While the well-established topological numbers, failed to recognize this transition, we successfully characterized such a transition with the measurement of the complex eigenvalue braids. We obtained the braid invariants from the measured relative phases between eigenvalues. The observed change in braid invariants provides a clear signature of the non-Abelian topological transition. Furthermore, we experimentally revealed an intriguing consequence of this transition, which is the creation of a third-order exceptional point through the collision of two second-order exceptional points with opposite charges. Our experimental findings shed light on the abundant non-Abelian topological phenomena involving non-Hermiticity, and provide insights into manipulating the spectral topology in atomic scale systems to achieve exotic functionalities arising from non-Abelian band braiding.

quant-ph↗
arXiv2025

Towards Identifying the PL6 Center in SiC: From First-Principles Screening to Hyperfine Validation of Competing Defect Candidates

The PL6 color center in 4H-SiC, known for its excellent ambient-temperature spin and optical properties, has an unresolved microscopic origin. In this first-principles study, we systematically investigate potential structures to clarify its nature. We first rigorously examine the DV-antisite hypothesis (a divacancy paired with a carbon antisite, $\mathrm{C_{Si}}$), analyzing the energetic, electronic, and spin properties of various $V_\mathrm{Si}V_\mathrm{C}+\mathrm{C_{Si}}$ configurations. Two $\mathrm{C_{3v}}$-symmetric $\mathrm{kk+C_{Si}}$ complexes emerge as strong candidates within this framework. Subsequently, a critical comparison of hyperfine interaction signatures is performed between these candidates, the alternative OV model [specifically OV(hh) and OV(kk), an oxygen replacing C together with a Si vacancy], and experimental data. This analysis demonstrates that the OV(hh) structure more accurately reproduces PL6's hyperfine features. Furthermore, re-evaluation of the proposed OV(hk) model for the related PL5 center reveals zero-field splitting parameter $E$ inconsistencies with experimental results, suggesting that PL5 and PL6 may have distinct origins. These findings provide crucial theoretical insights and motivate targeted experimental validation for these quantum defects.

cond-mat.mtrl-sci↗
arXiv2024

Observation of freezing phenomenon in high-dimensional quantum correlation dynamics

Quantum information processing (QIP) based on high-dimensional quantum systems provides unique advantages and new potentials where high-dimensional quantum correlations (QCs) play vital roles. Exploring the resistance of QCs against noises is crucial as QCs are fragile due to complex and unavoidable system-environment interactions. In this study, we investigate the performance of high-dimensional QCs under local dephasing noise using a single nitrogen-vacancy center in diamond. A freezing phenomenon in the high-dimensional quantum discord dynamics was observed, showing discord is robust against local dephasing noise. Utilizing a robustness metric known as freezing index, we found that the discord of qutrits outperforms their qubits counterpart when confronted with dephasing noise. Furthermore, we developed a geometric picture to explain this intriguing freezing phenomenon phenomenon. Our findings highlight the potential of utilizing discord as a physical resource for advancing QIP in high-dimensional quantum settings.

quant-ph↗
arXiv2024

Challenging theories of dark energy with levitated force sensor

The nature of dark energy is one of the most outstanding problems in physical science, and various theories have been proposed. It is therefore essential to directly verify or rule out these theories experimentally. However, despite substantial efforts in astrophysical observations and laboratory experiments, previous tests have not yet acquired enough accuracy to provide decisive conclusions as to the validity of these theories. Here, using a diamagnetically levitated force sensor, we carry out a test on one of the most compelling explanations for dark energy to date, namely the Chameleon theory, an ultra-light scalar field with screening mechanisms, which couples to normal-matter fields and leaves a detectable fifth force. Our results extend previous results by nearly two orders of magnitude to the entire physical plausible parameter space of cosmologically viable chameleon models. We find no evidence for such a fifth force. Our results decisively rule out the basic chameleon model as a candidate for dark energy. Our work, thus, demonstrates the robustness of laboratory experiments in unveiling the nature of dark energy in the future. The methodology developed here can be further applied to study a broad range of fundamental physics.

gr-qc↗
arXiv2024

New Constraints on Exotic Spin-Spin-Velocity-Dependent Interactions with Solid-State Quantum Sensors

We report new experimental results on exotic spin-spin-velocity-dependent interactions between electron spins. We designed an elaborate setup that is equipped with two nitrogen-vacancy (NV) ensembles in diamonds. One of the NV ensembles serves as the spin source, while the other functions as the spin sensor. By coherently manipulating the quantum states of two NV ensembles and their relative velocity at the micrometer scale, we are able to scrutinize exotic spin-spin-velocity-dependent interactions at short force ranges. For a T-violating interaction, $V_6$, new limits on the corresponding coupling coefficient, $f_6$, have been established for the force range shorter than 1 cm. For a P,T-violating interaction, $V_{14}$, new constraints on the corresponding coupling coefficient, $f_{14}$, have been obtained for the force range shorter than 1 km.

hep-ex↗
arXiv2024

Measurement of the earth tides with a diamagnetic-levitated micro-oscillator at room temperature

The precise measurement of the gravity of the earth plays a pivotal role in various fundamental research and application fields. Although a few gravimeters have been reported to achieve this goal, miniaturization of high-precision gravimetry remains a challenge. In this work, we have proposed and demonstrated a miniaturized gravimetry operating at room temperature based on a diamagnetic levitated micro-oscillator with a proof mass of only 215 mg. Compared with the latest reported miniaturized gravimeters based on Micro-Electro-Mechanical Systems, the performance of our gravimetry has substantial improvements in that an acceleration sensitivity of 15 $\mu Gal/\sqrt{Hz}$ and a drift as low as 61 $\mu Gal$ per day have been reached. Based on this diamagnetic levitation gravimetry, we observed the earth tides, and the correlation coefficient between the experimental data and theoretical data reached 0.97. Some moderate foreseeable improvements can develop this diamagnetic levitation gravimetry into chip size device, making it suitable for mobile platforms such as drones. Our advancement in gravimetry is expected to facilitate a multitude of applications, including underground density surveying and the forecasting of natural hazards.

physics.app-ph↗
arXiv2024

Experimental test of the Crooks fluctuation theorem in a single nuclear spin

We experimentally test the Crooks fluctuation theorem in a quantum spin system. Our results show that the Crooks fluctuation theorem is valid for different speeds of the nonequilibrium processes and under various effective temperatures. Work is not an observable in quantum systems, which makes tests of quantum thermodynamic theorems challenging. In this work, we developed high-fidelity single-shot readouts of a single nuclear spin in diamond and implemented the two-point work measurement protocol, enabling a direct experimental test of the Crooks fluctuation theorem. Our results provide a quantum insight into fluctuations and the methods we developed can be utilized to study other quantum thermodynamic theorems.

quant-ph↗
arXiv2024

Third-order exceptional line in a nitrogen-vacancy spin system

The exceptional points (EPs) aroused from the non-Hermiticity bring rich phenomena, such as exceptional nodal topologies, unidirectional invisibility, single-mode lasing, sensitivity enhancement and energy harvesting. Isolated high-order EPs have been observed to exhibit richer topological characteristics and better performance in sensing over 2nd-order EPs. Recently, high-order EP geometries, such as lines or rings formed entirely by high order EPs, are predicted to provide richer phenomena and advantages over stand-alone high-order EPs. However, experimental exploration of high-order EP geometries is hitherto beyond reach due to the demand of more degrees of freedom in the Hamiltonian's parameter space or a higher level of symmetries. Here we report the observation of the third-order exceptional line (EL) at the atomic scale. By introducing multiple symmetries, the emergence of the third-order EL has been successfully realized with a single electron spin of nitrogen-vacancy center in diamond. Furthermore, the behaviors of the EP structure under different symmetries are systematically investigated. The symmetries are shown to play essential roles in the occurrence of high-order EPs and the related EP geometries. Our work opens a new avenue to explore high-order EP-related topological physics at the atomic scale and to the potential applications of high-order EPs in quantum technologies.

quant-ph↗
arXiv2023

Observation of the Knot Topology of Non-Hermitian Systems in a Single Spin

The non-Hermiticity of the system gives rise to distinct knot topology that has no Hermitian counterpart. Here, we report a comprehensive study of the knot topology in gapped non-Hermitian systems based on the universal dilation method with a long coherence time nitrogen-vacancy center in a $^{\text{12}}$C isotope purified diamond. Both the braiding patterns of energy bands and the eigenstate topology are revealed. Furthermore, the global biorthogonal Berry phase related to the eigenstate topology has been successfully observed, which identifies the topological invariance for the non-Hermitian system. Our method paves the way for further exploration of the interplay among band braiding, eigenstate topology and symmetries in non-Hermitian quantum systems.

quant-ph↗
arXiv2023

Experimental test of the Jarzynski equality in a single spin-1 system using high-fidelity single-shot readouts

The Jarzynski equality (JE), which connects the equilibrium free energy with non-equilibrium work statistics, plays a crucial role in quantum thermodynamics. Although practical quantum systems are usually multi-level systems, most tests of the JE were executed in two-level systems. A rigorous test of the JE by directly measuring the work distribution of a physical process in a high-dimensional quantum system remains elusive. Here, we report an experimental test of the JE in a single spin-1 system. We realized nondemolition projective measurement of this three-level system via cascading high-fidelity single-shot readouts and directly measured the work distribution utilizing the two-point measurement protocol. The validity of the JE was verified from the non-adiabatic to adiabatic zone and under different effective temperatures. Our work puts the JE on a solid experimental foundation and makes the NV center system a mature toolbox to perform advanced experiments of stochastic quantum thermodynamics.

quant-ph↗