arXiv subjects

J. Q. You

Publications and source records attributed to J. Q. You.

At least 19 recordsLinked to original sources

arXiv2026

From Liouville equation to universal quantum control: A study of generating ultra highly squeezed states

We find that the seemingly disparate control approaches for classical and quantum continuous-variable systems can be unified via differential manifolds of the ancillary representations. For classical systems, the ancillary representation is defined by the time-dependent ancillary canonical variables resulting from symplectic transformation over the original canonical variables. Under the Hamilton-Jacobi theory, the ancillary canonical variables act as dynamical invariants to guide the system nonadiabatically through the entire phase space. The second quantization of the Liouville equation for the dynamical invariants leads to the Heisenberg equation for the relevant ancillary operators, which is found to be a sufficient condition to activate nonadiabatic passages towards arbitrary target states in both Hermitian and non-Hermitian systems and yield constrained exact solutions of the time-dependent Schroedinger equation. Using the non-Hermitian Hamiltonian rigorously derived from the Lindblad master equation, our theory is exemplified by the generation of single-mode squeezed states with a squeezing level about 29.3 dB and double-mode squeezed states with 20.6 dB, respectively.

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↗

arXiv2026

Unidirectional exceptional point of reflectionless states in a magnonic mirror array

Exceptional points (EPs) in non-Hermitian systems are singularities where both eigenvalues and eigenvectors coalesce. In scattering systems, EPs correspond to the merging of scattering states, leading to reflectionless (RL) behavior. A reflectionless exceptional point (RL EP) arises when two RL states further coalesce, yielding an anomalous quartic spectral response. While RL EPs have been explored in bidirectional systems, their unidirectional realization remains elusive. Here, we experimentally demonstrate a unidirectional RL EP by engineering collective states in an anti-Bragg magnonic mirror array. Inversion symmetry is broken using a giant spin ensemble that couples to a waveguide at three spatially separated points, enabling unidirectional reflectionless. At the RL EP, the reflection spectrum flattens and broadens significantly beyond the Lorentzian profile. The observed spectral valleys also expose dark-state behaviors that are typically inaccessible through conventional measurements. Our results provide a route toward controlling collective coherence in open systems and developing broadband unidirectional devices.

physics.optics↗

arXiv2026

Magnon squeezing in the quantum regime

Squeezed states, crucial for quantum metrology and emerging quantum technologies, have been demonstrated in various platforms, but quantum squeezing of magnons in macroscopic spin systems remains elusive. Here we report the experimental observation of quantum-level magnon squeezing in a millimeter-scale yttrium iron garnet (YIG) sphere. By engineering a strong dispersive magnon-superconducting qubit coupling via a microwave cavity, we implement a significant self-Kerr nonlinearity to generate squeezed magnon states with their mean magnon number less than one. Harnessing a magnon-assisted Raman process, we perform Wigner tomography, revealing quadrature variances of $\sim\!0.8$ ($\sim\!1.0$~dB squeezing) relative to the vacuum. These results lay the groundwork for quantum nonlinear magnonics and promise potential applications in quantum metrology.

quant-ph↗

arXiv2026

Macroscopic quantum phenomena and quantum computing

This News & Views article provides a perspective on the 2025 Nobel Prize in Physics, including the groundbreaking discovery of macroscopic quantum tunneling and energy quantization in superconducting circuits, the history and causes giving rise to this breakthrough, and its impact on subsequent progress in quantum computing.

quant-ph↗

arXiv2025

Single-mode magnon-polariton lasing and amplification controlled by dissipative coupling

We demonstrate single-mode lasing of magnon polaritons in a cavity magnonic system enabled by dissipative coupling between two passive modes, microwave cavity mode and magnon mode in a ferrimagnetic spin ensemble. The cavity mode is partially compensated through a feedback circuit, which reduces its linewidth but retains its dissipative nature. By tuning the compensation strength and dissipative coupling strength, we reach a system cooperativity of unity, marking the lasing threshold and the formation of a zero-linewidth polariton mode. This mode also corresponds to a perfect Friedrich-Wintgen bound state in the continuum. Further increase of the cooperativity drives the system into the strong dissipative coupling regime, where magnon polariton amplification arises between two real frequency scattering poles. These results reveal that dissipative coupling cooperativity carries a clear physical meaning and serves as a key parameter for controlling phase transitions. Dissipative coupling offers an alternative paradigm for tailoring light matter interactions, paving the way for advances in both information processing and quantum technologies.

physics.optics↗

arXiv2025

Strong coupling between a single-photon and a two-photon Fock state

The realization of strong nonlinear coupling between single photons has been a long-standing goal in quantum optics and quantum information science, promising wide impact applications, such as all-optical deterministic quantum logic and single-photon frequency conversion. Here, we report an experimental observation of the strong coupling between a single-photon and a two-photon Fock state in an ultrastrongly-coupled circuit-QED system. This strong nonlinear interaction is realized by introducing a detuned flux qubit working as an effective coupler between two modes of a superconducting coplanar waveguide resonator. The ultrastrong light--matter interaction breaks the excitation number conservation, and an external flux bias breaks the parity conservation. The combined effect of the two enables the strong one--two-photon coupling. Quantum Rabi-like avoided crossing is resolved when tuning the two-photon resonance frequency of the first mode across the single-photon resonance frequency of the second mode. Within this new photonic regime, we observe the thresholdless second harmonic generation for a mean photon number below one. Our results represent a key step towards a new regime of quantum nonlinear optics, where individual photons can deterministically and coherently interact with each other in the absence of any stimulating fields.

quant-ph↗

arXiv2025

Unidirectional perfect absorption induced by chiral coupling in spin-momentum locked waveguide magnonics

Chiral coupling opens new avenues for controlling and exploiting light-matter interactions. We demonstrate that chiral coupling can be utilized to achieve unidirectional perfect absorption. In our experiments, chiral magnon-photon coupling is realized by coupling the magnon modes in yttrium iron garnet (YIG) spheres with spin-momentum-locked waveguide modes supported by spoof surface plasmon polaritons (SSPPs). These photon modes exhibit transverse spin, with the spin direction determined by the propagation direction. Due to the intrinsic spin properties of the magnon mode, it exclusively couples with microwaves traveling in one direction, effectively suppressing the reflection channel. Under the critical coupling condition, transmission is also eliminated, resulting in unidirectional perfect absorption. By incorporating additional YIG spheres, bidirectional and multi-frequency perfect absorption can be achieved. Our work introduces a novel platform for exploring and harnessing chiral light-matter interactions within spin-momentum locked devices, offering a paradigm for unidirectional signal processing and energy harvesting technologies.

quant-ph↗

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↗

arXiv2025

Frustration Elimination and Excited State Search in Coherent Ising Machines

Frustration, that is, the impossibility of satisfying the energetic preferences between all spin pairs simultaneously, underlies the complexity of many fundamental properties in spin systems, including the computational difficulty in determining their ground states. Coherent Ising machines (CIMs) have been proposed as a promising analog computational approach to efficiently find different degenerate ground states of large and complex Ising models. However, CIMs also face challenges in solving frustrated Ising models: frustration not only reduces the probability of finding good solutions, but it also prohibits the leveraging quantum effects in doing so. To circumvent these detrimental effects of frustration, we show how frustrated Ising models can be mapped to frustration-free CIM configurations by including ancillary modes and modifying the coupling protocol used in current CIM designs. Such frustration elimination may empower current CIMs to benefit from quantum effects in dealing with frustrated Ising models. In addition, these ancillary modes can also enable error detection and searching for excited states.

quant-ph↗

arXiv2025

Exploring Dynamics of Open Quantum Systems in Naturally Inaccessible Regimes

Markovian open quantum systems are governed by the Lindblad master equation where the dissipation contains two parts, i.e., the anti-Hermitian operator and the quantum jumps, which share a common dissipation rate. We generalize the Lindblad master equation via postselection to a generalized Liouvillian formalism in which the effective damping rate of the anti-Hermitian operator can be different from the quantum jump rate. Our formalism provides a parameter space with regimes inaccessible in naturally-occurring systems. We explore these new regimes and find several interesting results including negative damping rates and generalized Liouvillian exceptional points. In a previously unexplored zero-damping Liouvillian regime where the damping rate is negligible, we investigate the effect only due to the quantum jumps and show an unusual polynomial decay of the excited state. This generalized Liouvillian formalism offers opportunities to explore novel phenomena and quantum technologies associated with the peculiar behavior of quantum jumps.

quant-ph↗

arXiv2024

Polarization-entangled photon pairs generation from a single lithium niobate waveguide with single poling period

Polarization-entangled photon pairs are essential sources for photonic quantum information processing. However, generating entangled photon pairs with large detuning via spontaneous parametric down-conversion (SPDC) often requires complex configurations to compensate for phase matching. Here, we propose a simple and efficient scheme to generate polarization-entangled photon pairs based on type-0 SPDC in a thin-film lithium niobate waveguide with a single poling period. By utilizing the strong dispersion engineering capabilities of thin-film waveguides, we can achieve both degenerate and highly detuned entangled photon pairs. Furthermore, we demonstrate on-chip temporal compensation using an integrated waveguide structure. Our approach offers a compact and scalable solution for integrated quantum photonic circuits.

physics.optics↗

arXiv2024

Macroscopic Bell state between a millimeter-sized spin system and a superconducting qubit

Entanglement is a fundamental property in quantum mechanics that systems share inseparable quantum correlation regardless of their mutual distances. Owing to the fundamental significance and versatile applications, the generation of quantum entanglement between {\it macroscopic} systems has been a focus of current research. Here we report on the deterministic generation and tomography of the macroscopically entangled Bell state in a hybrid quantum system containing a millimeter-sized spin system ($\sim 1\times10^{19}$ atoms) and a micrometer-sized superconducting qubit. The deterministic generation is realized by coupling the macroscopic spin system and the qubit via a microwave cavity. Also, we develop a joint tomography approach to confirming the deterministic generation of the Bell state, which gives a generation fidelity of $0.90\pm0.01$. Our work makes the macroscopic spin system the {\it largest} system (in the sense of atom number) capable of generating the maximally entangled quantum state.

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↗

arXiv2023

Scattering description of edge states in Aharonov-Bohm triangle chains

Scattering theory has been suggested as a convenient method to identify topological phases of matter, in particular of disordered systems for which the Bloch band-theory approach is inapplicable. Here we examine this idea, employing as a benchmark a one-dimensional triangle chain whose versatility yields a scattering matrix that ``flows" in parameter space among several members of the topology classification scheme. Our results show that the reflection amplitudes (from both ends of a sufficiently long chain) do indicate the appearance of edge states in {\it all} (topological and non-topological) cases. For the topological cases, the transmission has a peak at the topological phase transition, which happens at the Fermi energy. A peak still exists as one moves into the non-topological `trivial' regions, in which another transmission peak may occur at nonzero energy, at which a relevant edge state appears in the isolated chain. For finite chains, the peak in the transmission strongly depends on their coupling of the leads, and {\it not} on the phase transition of the isolated chain. In any case, {\it the appearance of a peak in the transmission is not sufficient to conclude that the system undergoes a topological phase transition.}

cond-mat.mes-hall↗

arXiv2023

Realization of the unidirectional amplification in a cavity magnonic system

We experimentally demonstrate the nonreciprocal microwave amplification using a cavity magnonic system, consisting of a passive cavity (i.e., the split-ring resonator), an active feedback circuit integrated with an amplifier, and a ferromagnetic spin ensemble (i.e., a yttrium-iron-garnet sphere). Combining the amplification provided by the active circuit and the nonreciprocity supported by the cavity magnonics, we implement a nonreciprocal amplifier with the functions of both unidirectional amplification and reverse isolation. The microwave signal is amplified by 11.5 dB in the forward propagating direction and attenuated in the reverse direction by -34.7 dB, giving an isolation ratio of 46.2 dB. Such a unidirectional amplifier can be readily employed in quantum technologies, where the device can simultaneously amplify the weak signal output by the quantum system and isolate the sensitive quantum system from the backscattered external noise. Also, it is promising to explore more functions and applications using a cavity magnonic system with real gain.

physics.app-ph↗

arXiv2023

Coherent-cluster-state generation in networks of degenerate optical parametric oscillators

Cluster states are versatile quantum resources and an essential building block for measurement-based quantum computing. The possibility to generate cluster states in specific systems may thus serve as an indicator regarding if and to what extent these systems can be harnessed for quantum technologies and quantum information processing in particular. Here, we apply this analysis to networks of degenerate optical parametric oscillators (DOPOs), also called coherent Ising machines (CIMs). CIMs are distinguished by their highly flexible coupling capabilities, which makes it possible to use them, e.g., to emulate large spin systems. As CIMs typically operate with coherent states (and superpositions thereof), it is natural to consider cluster states formed by superpositions of coherent states, i.e., coherent cluster states. As we show, such coherent cluster states can, under ideal conditions, be generated in DOPO networks with the help of beam splitters and classical pumps. Our subsequent numerical analysis provides the minimum requirements for the generation of coherent cluster states under realistic conditions. Moreover, we discuss how nonequilibrium pumps can improve the generation of coherent cluster states. In order to assess the quality of the cluster-state generation, we map the generated states to an effective spin space using modular variables, which allows us to apply entanglement criteria tailored for spin-based cluster states.

quant-ph↗