arXiv subjects

Xiaodan Lyu

Publications and source records attributed to Xiaodan Lyu.

10 recordsLinked to original sources

arXiv2026

Topologically Configurable Nonlinear Vortex Generation at van der Waals Heterostructures

van der Waals (vdW) materials offer a highly tunable and efficient platform at nanoscale for nonlinear and quantum optics. Twist-stacked vdW heterostructures enable elegant control of symmetry and interlayer coupling. Prior studies mainly focus on planar twisted interfaces, while neglecting the naturally formed and mandatory defects in such vdW heterostructures. Here, we demonstrate nonlinear singular optics with topologically configurable nonlinear vortex generation at the corner singularity of vdW heterostructures. By tailoring azimuthally discrete second-harmonic phase gradients at each interface, we obtain programmable nonlinear vortex emitters with dominant target OAM components. Nonlinear OAM beams with topological charge $\ell = 1$ and $\ell = -2$ are experimentally realized, respectively. Our work unlocks the untapped potentials of nonlinear singular optics in twisted vdW materials as a reconfigurable and lithography-free platform for nonlinear structured light generation, important in quantum nonlinear optics and related fields.

physics.optics↗

arXiv2026

Experimental High-Dimensional Quantum Overlapping Tomography

Large-scale quantum systems have advanced rapidly via the exploration of more particles and higher dimensions, offering great potential for developing quantum technologies. However, their characterization becomes prohibitive with increasing local dimensionality and particle number. Here we propose high-dimensional quantum overlapping tomography based on a graph-theoretic formulation, which allows one to efficiently reconstruct few-body marginals of multipartite high-dimensional quantum systems. We experimentally realize it on a photonic four-party entangled state in a $4 \times 4 \times 2 \times 2$ system. Using measurements in mutually unbiased bases, we reconstruct all six two-body marginals with only 25 projective measurement settings, compared with 94 and 225 settings for independent tomography of all two-body reduced states and full state tomography, respectively. The reconstructed marginals reveal a layered entanglement structure vital for high-dimensional quantum networks. We further show that these marginals enable more noise-resilient certification of multipartite high-dimensional entanglement than the fidelity-based criterion. Our work thus offers a scalable route for learning multidimensional quantum systems.

quant-ph↗

arXiv2026

Cavity-enhanced superconductivity in the two-dimensional limit of NbSe2

Vacuum electromagnetic fluctuations have emerged as a means of controlling collective quantum phases without external driving. Cavity-induced modification of superconductivity has been widely predicted. What sets the size of the effect, and which microscopic channel carries it, remain open. Here we couple few-layer NbSe2 to a terahertz complementary split-ring resonator (CSRR) and show that the enhancement grows sharply on approaching the two-dimensional limit. In bilayer NbSe2 the superconducting transition temperature rises by 10%, from 3.02 K to 3.41 K, on a cavity resonant at 0.92 THz - roughly four times the shift measured in a ten-layer device at the same resonance. Within a single device the shift maps onto the simulated cavity field profile, falling from 0.39 K at the field maximum to zero outside the resonator, with the lower critical field following the same spatial ordering; because all regions are measured on one continuous flake in a single cooldown, sample-to-sample variation is excluded by construction. The frequency dependence is non-monotonic, with suppression below resonance and maximal enhancement near 0.96 THz. Quantum electrodynamical density functional theory calculations show that cavity coupling redistributes spectral weight in the Eliashberg function, weakening the total electron-phonon coupling while hardening the logarithmic average phonon frequency; competition between the two reproduces a sign change in Tc. These results identify dimensionality, local field amplitude and detuning as the control parameters of cavity-enhanced superconductivity, and point to electron-phonon reweighting as its microscopic origin.

cond-mat.supr-con↗

arXiv2026

Orbitally resolved single-photon emission from an individual atomic vacancy center in a semiconductor

Atomically confined spins are emerging as active components in quantum optoelectronic devices such as quantum bits and sensors. However, interrogating single spins at atomic length-scales remains a sizeable challenge, limited by diffraction in conventional optics. Here we show that the highly-local excitation provided by injecting energetic charge carriers from the atomically sharp probe of a scanning tunneling microscope can trigger single-photon emission from individual atomic vacancy centers in a layered semiconductor. With an effective spatial resolution of <1 nm, we show that the captured light closely mirrors the orbital symmetry of the bound-state wavefunction of the vacancy center while photon correlation measurements confirm single-photon emission, as reflected in clear photon anti-bunching signatures. Our results constitute an important step toward the realization of an electrically addressable single-atom quantum light source and solid-state spinphoton interface, addressed at the atomic-scale.

cond-mat.mes-hall↗

arXiv2025

Room-Temperature Electrical Readout of Spin Defects in van der Waals Materials

Negatively charged boron vacancy ($\mathrm{V_B^-}$) in hexagonal boron nitride (hBN) is the most extensively studied room-temperature quantum spin system in two-dimensional (2D) materials. Nevertheless, the current effective readout of $\mathrm{V_B^-}$ spin states is carried out by systematically optical methods. This limits their exploitation in compact and miniaturized quantum devices, which would otherwise hold substantial promise to address quantum sensing and quantum information tasks. In this study, we demonstrated a photoelectric spin readout technique for $\mathrm{V_B^-}$ spins in hBN. The observed photocurrent signals stem from the spin-dependent ionization dynamics of boron vacancies, mediated by spin-dependent non-radiative transitions to a metastable state. We further extend this electrical detection technique to enable the readout of dynamical decoupling sequences, including the Carr-Purcell-Meiboom-Gill (CPMG) protocols, and of nuclear spins via electron-nuclear double resonance. These results provide a pathway toward on-chip integration and real-field exploitation of quantum functionalities based on 2D material platforms.

quant-ph↗

arXiv2025

Experimental demonstration of scalable quantum cryptographic conferencing

Quantum network enables a variety of quantum information processing tasks, where multi-user quantum communication is one of the important objectives. Quantum cryptographic conferencing serves as an essential solution to establish secure keys to realize secure multi-user communications. However, existing QCC implementations have been fundamentally limited by the low probability of multi-user coincidence detection to measure or construct the Greenberger-Horne-Zeilinger (GHZ) entangled state. In this work, we report the experimental realization of QCC eliminating the need for coincidence detection, where the GHZ state is constructed by correlating detection events occurring within the coherence time, thereby greatly enhancing the success probability of GHZ-state measurement. Meanwhile, to establish and maintain high-visibility GHZ measurement among three independent users, we developed a three-party phase compensation scheme combined with precise temporal and polarization alignment within a time-bin-phase encoding framework. Furthermore, we designed an efficient pairing strategy to simplify subsequent data processing and enhance processing efficiency. Based on these techniques, we successfully performed QCC experiments over total channel losses of 66.3 dB, corresponding to 331.5 km of commercial fiber (0.2 dB/km), achieving secure key rates of 5.4 bit/s, whereas previous QCC experiments have been limited to 100 km. The results surpass the multi-user repeaterless bound in quantum networks, establishing a new regime of scalable, multi-user quantum communication and paving the way for metropolitan quantum networks.

quant-ph↗

arXiv2025

Boosting classical and quantum nonlinear processes in ultrathin van der Waals materials

Understanding and controlling nonlinear processes is crucial for engineering light-matter interaction and generating non-classical light. A significant challenge in ultra-thin nonlinear materials is the marked diminution of the nonlinear conversion efficiency due to the reduced light-matter interaction length and, in many cases, the centrosymmetric crystalline structures. Here we relax these limitations and report a giant boost of classical and quantum nonlinear processes in ultrathin van der Waals materials. Specifically, with a metal-nonlinear material heterostructure we enhance classical second-harmonic generation in h-BN flakes by two-orders of magnitude. Moreover, we have engineered a metal-SiO2-nonlinear material heterostructure resulting in a remarkable two orders of magnitude augmentation of the quantum spontaneous parametric down-conversion (SPDC) in NbOCl2 flakes. Notably, we demonstrate SPDC in a 16 nm-thick NbOCl2 flake integrated into the proposed structure. These findings simplify on-chip quantum state engineering and accelerate the use of van der Waals materials in nonlinear optoelectronics.

quant-ph↗

arXiv2024

Observation of Ferromagnetic Phase in the Second Moiré Band of Twisted MoTe2

Flat bands and electron correlation in moiré lattices give rise to many exotic phases, including Mott insulators, superconductivity, and topological states. Within the first moiré band, integer and fractional quantum anomalous Hall effects have been observed in twisted bilayer MoTe2 (tMoTe2) at one hole doping and fractional doping per moiré unit cell, respectively. When the second moiré band is fully hole doped, quantum spin Hall insulator has also been reported in tMoTe2 at a certain twist angle. Exotic topological states together with ferromagnetic (FM) states in the high moiré band can potentially exist as well. In this study, we report the observation of a FM phase in the second moiré band in tMoTe2. The FM phase can be tuned by both the doping level and displacement field. At filling around 2.58 holes per moiré unit cell, the FM phase reaches a Curie temperature of 3.5 K. A large displacement field can suppress the FM phase, like the FM phase at the filling of -1. Our results demonstrate the realization of time-reversal symmetry-breaking states in the higher moiré bands in tMoTe2.

cond-mat.mes-hall↗

arXiv2024

Robust Nuclear Spin Polarization via Ground-State Level Anti-Crossing of Boron Vacancy Defects in Hexagonal Boron Nitride

Nuclear spin polarization plays a crucial role in quantum information processing and quantum sensing. In this work, we demonstrate a robust and efficient method for nuclear spin polarization with boron vacancy ($\mathrm{V_B^-}$) defects in hexagonal boron nitride (h-BN) using ground-state level anti-crossing (GSLAC). We show that GSLAC-assisted nuclear polarization can be achieved with significantly lower laser power than excited-state level anti-crossing, making the process experimentally more viable. Furthermore, we have demonstrated direct optical readout of nuclear spins for $\mathrm{V_B^-}$ in h-BN. Our findings suggest that GSLAC is a promising technique for the precise control and manipulation of nuclear spins in $\mathrm{V_B^-}$ defects in h-BN.

quant-ph↗

arXiv2022

Excited-state optically detected magnetic resonance of spin defects in hexagonal boron nitride

Negatively charged boron vacancy (VB-) centers in hexagonal boron nitride (hBN) are promising spin defects in a van der Waals crystal. Understanding the spin properties of the excited state (ES) is critical for realizing dynamic nuclear polarization. Here, we report zero-field splitting in the ES of DES = 2160 MHz and an optically detected magnetic resonance (ODMR) contrast of 12% at cryogenic temperature. The ES has a g-factor similar to the ground state. The ES photodynamics is further elucidated by measuring the level anti-crossing of the VB- defects under varying external magnetic fields. In contrast to nitrogen vacancy (NV-) centers in diamond, the emission change caused by excited-state level anti-crossing (ESLAC) is more prominent at cryo-temperature than at room temperature. Our results provide important information for utilizing the spin defects of hBN in quantum technology.

cond-mat.mes-hall↗