arXiv subjects

Wei-Jiang Wu

Publications and source records attributed to Wei-Jiang Wu.

10 recordsLinked to original sources

arXiv2026

Sustained macroscopic quantum coherence in a superradiant solid under ambient conditions

Macroscopic quantum coherence, such as in laser, Bose-Einstein condensates, superfluids, and superconductors, is important to fundamental physics and useful for quantum technologies. Superradiance provides a mechanism to produce coherence among a large number of particles and photons. Its implementation, however, has been limited to gaseous systems, solids at very low temperature, or short pulses. Here we demonstrate a solid-state superradiant maser under ambient conditions, which establishes long-lived coherence among about $10^{14}$ nitrogen-vacancy center spins in diamond and about $10^9$ photons in a microwave cavity. By varying the system parameters to access the above-threshold, well-above-threshold, and deep-above-threshold regimes, we observed continuous-wave masing, periodic amplitude modulation, and sequences of superradiant bursts, which are attributed, correspondingly, to macroscopic spin coherence synchronized at a fixed frequency, a coherent time crystal of large spins, and unsynchronized superradiant transients. This work demonstrates that macroscopic quantum coherence can be spontaneously generated and maintained in solids under ambient conditions and provides a solid-state platform for exploring bright quantum lights with many-body correlations.

quant-ph↗

arXiv2026

Enhancement of signal-to-noise ratio at a high-order exceptional point of coherent perfect absorption

Exceptional points (EPs) in non-Hermitian systems offer a remarkably strong response to weak perturbations, but the nonorthogonal nature of the corresponding eigenvectors causes noise to diverge, hindering EPs practical application. Here, we report a twelve-fold enhancement of signal-to-noise ratio (SNR) in magnetic field sensing enabled by a third-order EP of coherent perfect absorption (CPA EP3) in a passive cavity magnonic system. This non-Hermitian magnonic platform comprises two identical yttrium iron garnet (YIG) spheres coherently coupled to a cavity mode, in which the CPA EP3 is realized by engineering the three-mode loss to form a pseudo-Hermitian absorption Hamiltonian. By independently tailoring the absorption EP apart from the resonance EP, the system circumvents the noise divergence caused by eigenbasis collapse. Notably, we harness the sensitivity of the minimum output intensity near CPA to perturbations, yielding a seventyfold SNR improvement and a 400-fold increase in responsivity compared with non-CPA system. A comprehensive noise analysis over one hundred repeated measurements confirms the suppression of frequency noise near the CPA EP3. This demonstrates that our scheme not only avoids the noise divergence plaguing conventional higher-order EP sensors but also provides a general strategy to exploit both CPA and EP for SNR enhancement in passive non-Hermitian systems.

physics.optics↗

arXiv2025

Polaromechanics: cavity-magnon polaritons strongly coupled to phonons

Building hybrid quantum systems is a crucial step for realizing multifunctional quantum technologies, quantum information processing, and hybrid quantum networks. A functional hybrid quantum system requires strong coupling among its components. However, couplings between distinct physical systems are typically very weak. Experimental realization of strong coupling in a hybrid system remains a long-standing challenge, especially when it has multiple components and the components are of different nature. Here we demonstrate the realization of triple strong coupling in a novel {\it polaromechanical} hybrid system, where polaritons, formed by strongly coupled ferromagnetic magnons and microwave photons, are further strongly coupled to phonons. The corresponding polaromechanical normal-mode splitting is observed. A high polaromechanical cooperativity of $9.4\times10^3$ is achieved by significantly reducing the polariton decay rate via exploiting coherent perfect absorption. The quantum cooperativity much greater than unity is achievable if placing the system at cryogenic temperatures, which would enable various quantum applications. Our results pave the way towards coherent quantum control of photons, magnons and phonons, and are a crucial step for building functional hybrid quantum systems based on magnons.

quant-ph↗

arXiv2024

Microwave-to-optics conversion using magnetostatic modes and a tunable optical cavity

Quantum computing, quantum communication and quantum networks rely on hybrid quantum systems operating in different frequency ranges. For instance, the superconducting qubits work in the gigahertz range, while the optical photons used in communication are in the range of hundreds of terahertz. Due to the large frequency mismatch, achieving the direct coupling and information exchange between different information carriers is generally difficult. Accordingly, a quantum interface is demanded, which serves as a bridge to establish information linkage between different quantum systems operating at distinct frequencies. Recently, the magnon mode in ferromagnetic spin systems has received significant attention. While the inherent weak optomagnonic coupling strength restricts the microwave-to-optical photon conversion efficiency using magnons, the versatility of the magnon modes, together with their readily achievable strong coupling with other quantum systems, endow them with many distinct advantages. Here, we realize the magnon-based microwave-light interface by adopting an optical cavity with adjustable free spectrum range and different kinds of magnetostatic modes in two microwave cavity configurations. By optimizing the parameters, an internal conversion efficiency of $1.28 \times 10^{-7}$ is achieved. We analyze the impact of various parameters on the microwave-to-optics conversion. The study provides useful guidance and insights to further enhancing the microwave-to-optics conversion efficiency using magnons.

quant-ph↗

arXiv2024

Synthetically enhanced sensitivity using higher-order exceptional point and coherent perfect absorption

Sensors play a crucial role in advanced apparatuses and it is persistently pursued to improve their sensitivities. Recently, the singularity of a non-Hermitian system, known as the exceptional point (EP), has drawn much attention for this goal. Response of the eigenfrequency shift to a perturbation $ε$ follows the $ε^{1/n}$-dependence at an $n$th-order EP, leading to significantly enhanced sensitivity via a high-order EP. However, due to the requirement of increasingly complicated systems, great difficulties will occur along the path of increasing the EP order to enhance the sensitivity. Here we report that by utilizing the spectral anomaly of the coherent perfect absorption (CPA), the sensitivity at a third-order EP can be further enhanced owing to the cooperative effects of both CPA and EP. We realize this synthetically enhanced sensor using a pseudo-Hermitian cavity magnonic system composed of two yttrium iron garnet spheres and a microwave cavity. The detectable minimum change of the magnetic field reaches $4.2\times10^{-21}$T. It opens a new avenue to design novel sensors using hybrid non-Hermitian quantum systems.

quant-ph↗

arXiv2022

Giant spin ensembles in waveguide magnonics

The dipole approximation is usually employed to describe light-matter interactions under ordinary conditions. With the development of artificial atomic systems, `giant atom' physics is possible, where the scale of atoms is comparable to or even greater than the wavelength of the light they interact with, and the dipole approximation is no longer valid. It reveals interesting physics impossible in small atoms and may offer useful applications. Here, we experimentally demonstrate the giant spin ensemble (GSE), where a ferromagnetic spin ensemble interacts twice with the meandering waveguide, and the coupling strength between them can be continuously tuned from finite (coupled) to zero (decoupled) by varying the frequency. In the nested configuration, we investigate the collective behavior of two GSEs and find extraordinary phenomena that cannot be observed in conventional systems. Our experiment offers a new platform for `giant atom' physics.

quant-ph↗

arXiv2022

Steady entangled-state generation via cross-Kerr effect in a ferrimagnetic crystal

For solid-state spin systems, the collective spin motion in a single crystal embodies multiple magnetostatic modes. Recently, it was found that the cross-Kerr interaction between the higher-order magnetostatic mode and the Kittel mode introduces a new operable degree of freedom. In this work, we propose a scheme to entangle two magnon modes via the cross-Kerr nonlinearity when the bias field is inhomogeneous and the system is driven. Quantum entanglement persists at the steady state, as demonstrated by numerical results using experimentally feasible parameters. Furthermore, we also demonstrate that entangled states can survive better in the system where self-Kerr and cross-Kerr nonlinearities coexist. Our work provides insights and guidance for designing experiments to observe entanglement between different degrees of freedom within a single ferrimagnetic crystal. Additionally, it may stimulate potential applications in quantum information processing using spintronic devices.

quant-ph↗

arXiv2021

Observation of magnon cross-Kerr effect in cavity magnonics

When there is a certain amount of field inhomogeneity, the biased ferrimagnetic crystal will exhibit the higher-order magnetostatic (HMS) mode in addition to the uniform-precession Kittel mode. In cavity magnonics, we show both experimentally and theoretically the cross-Kerr-type interaction between the Kittel mode and HMS mode. When the Kittel mode is driven to generate a certain number of excitations, the HMS mode displays a corresponding frequency shift and vice versa. The cross-Kerr effect is caused by an exchange interaction between these two spin-wave modes. Utilizing the cross-Kerr effect, we realize and integrate a multi-mode cavity magnonic system with only one yttrium iron garnet (YIG) sphere. Our results will bring new methods to magnetization dynamics studies and pave a way for novel cavity magnonic devices by including the magnetostatic mode-mode interaction as an operational degree of freedom.

quant-ph↗

arXiv2021

Quantum network with magnonic and mechanical nodes

A quantum network consisting of magnonic and mechanical nodes connected by light is proposed. Recent years have witnessed a significant development in cavity magnonics based on collective spin excitations in ferrimagnetic crystals, such as yttrium iron garnet (YIG). Magnonic systems are considered to be a promising building block for a future quantum network. However, a major limitation of the system is that the coherence time of the magnon excitations is limited by their intrinsic loss (typically in the order of 1 $μ$s for YIG). Here, we show that by coupling the magnonic system to a mechanical system using optical pulses, an arbitrary magnonic state (either classical or quantum) can be transferred to and stored in a distant long-lived mechanical resonator. The fidelity depends on the pulse parameters and the transmission loss. We further show that the magnonic and mechanical nodes can be prepared in a macroscopic entangled state. These demonstrate the quantum state transfer and entanglement distribution in such a novel quantum network of magnonic and mechanical nodes. Our work shows the possibility to connect two separate fields of optomagnonics and optomechanics, and to build a long-distance quantum network based on magnonic and mechanical systems.

quant-ph↗

arXiv2021

Remote magnon entanglement between two massive ferrimagnetic spheres via cavity optomagnonics

Recent studies show that hybrid quantum systems based on magnonics provide a new and promising platform for generating macroscopic quantum states involving a large number of spins. Here we show how to entangle two magnon modes in two massive yttrium-iron-garnet (YIG) spheres using cavity optomagnonics, where magnons couple to high-quality optical whispering gallery modes supported by the YIG sphere. The spheres can be as large as 1 mm in diameter and each sphere contains more than $10^{18}$ spins. The proposal is based on the asymmetry of the Stokes and anti-Stokes sidebands generated by the magnon-induced Brillouin light scattering in cavity optomagnonics. This allows one to utilize the Stokes and anti-Stokes scattering process, respectively, for generating and verifying the entanglement. Our work indicates that cavity optomagnonics could be a promising system for preparing macroscopic quantum states.

quant-ph↗