Magnon confinement and trapping at the nanoscale

Quantum metrology has recently emerged as a powerful approach for dark matter (DM) searches, particularly using nonclassical bosonic states in microwave cavities that are sensitive to weak signals. Nonclassical cat states-macroscopic superpositions of coherent states featuring sub-Planck interference structures-offer promising advantages for high-precision measurements. However, their practical utility in DM search remains unexplored. Here, we report the first experimental application of four-component cat states within a high-quality superconducting microwave cavity to search for dark photons, a potential DM candidate. We demonstrate an 8.1-fold enhancement in the signal photon rate and constrain the dark photon kinetic mixing angle to an unprecedented ε<7.32×10^{-16} near 6.44GHz (26.6 μeV). By employing a parametric sideband drive to actively tune the cavity frequency, we achieve dark photon searches and background subtraction across multiple frequency bins, yielding a sensitivity at the 10^{-16} level within a 100kHz bandwidth. Our cat-assisted DM search and frequency-scanning techniques demonstrate substantial improvements over previous results, promising potential implications in quantum-enhanced searches for new physics.

Frequency combs have revolutionized communication, metrology, and spectroscopy. Considerable efforts have been devoted to developing integrated combs, primarily leveraging Pockels or Kerr nonlinearities. Here, we demonstrate an alternative frequency comb generated via cavity Floquet engineering. By periodically modulating the cavity resonance frequency through a driven mechanical oscillator, a Floquet cavity with multiple equally spaced frequency components is created. These sidebands exhibit nearest-neighbor coupling and are phase-locked to the external modulation drive. A pump tone interacts with the pre-modulated cavity to generate the output frequency comb, which we implement in an on-chip microwave cavity optomechanical system. This approach operates independently of a pumping threshold and is insensitive to pump detuning. Consequently, it enables comb generation under far-sideband pumping with nanowatt-scale total power consumption, providing an ultra-low-power platform for integrated frequency comb synthesis.

Microwave joining of electrical steel sheets based on multi-physics simulation

Cavity magnomechanics has emerged as a new platform for the study of macroscopic quantum phenomena. A hybrid magnon squeezing magnomechanical system is proposed to realize the ground-state cooling of the magnomechanical resonator in the unresolved sideband regime, which consists of a magnon mode, two microwave cavity modes, and a mechanical vibration mode. We demonstrate that magnomechanical Stokes scattering can be completely suppressed by magnon squeezing under appropriate conditions, which greatly enhances the cooling performance of the magnomechanical resonator. Additionally, the cooling process can be further promoted by coupling with two microwave cavities. This scheme provides what we believe to be a novel idea for the subsequent ground-state cooling research and makes it possible to realize quantum manipulation of macroscopic mechanical systems.

The hybridization of quantum states hosted in materials with very different natures is a key resource for quantum technologies. A main example is light-matter interactions, a cornerstone of many quantum computing architectures. In some instances, coherently interfacing more than two quantum systems is crucial, as required, for example, in frequency conversion. Such multipartite quantum blocks have long been envisioned in the field of magnonics1,2, but the observation of coherent interactions between more than two systems has remained elusive so far. Here, by combining an antiferromagnetic crystal, a magnetic-field-resilient superconducting circuit and a microwave cavity, we demonstrate strongly hybridized photon-superconducting circuit-magnon states in a microwave cavity. The anharmonicity of the superconducting circuit enables efficient nonlinear interactions between the three modes, which have very different frequencies. As antiferromagnets are naturally suited for coupling to terahertz signals, our work provides a path towards realizing quantum interfaces between microwave and terahertz radiation3,4.

This study presents the design, optimization, and experimental validation of a pilot-scale Microwave-Assisted Pyrolysis (MAP) system for converting LDPE waste into value-added products. A 30 kg batch-capacity multimode microwave cavity was developed and powered by an 8 kW, 2.45 GHz RF source. Iron powder was used as both catalyst and microwave susceptor to enhance dielectric absorption, enable uniform volumetric heating, and accelerate cracking reactions. A custom microwave applicator with optimized RF launchers ensured efficient power coupling, validated through FIT- and FEM-based electromagnetic simulations, along with thermal simulations and experiments confirming temperature uniformity. Under optimized conditions (>400°C), the system produced ∼34% liquid oil, 32% char, and 34% gases. Products were characterized using GC, FTIR, FESEM, EDX, Raman, BET, and electrical analyses. Results demonstrate improved conversion efficiency, controlled product selectivity, and formation of functional carbon materials, highlighting a scalable and energy-efficient approach for sustainable plastic-waste valorization.

Abstract We present a theoretical scheme for nonreciprocal entanglement in a quadpartite system via the&amp;#xD;Barnett effect. Our quadpartite system is based on a microwave (MW) cavity, simultaneously&amp;#xD;coupled with phonon, an LC circuit, and magnon via radiation pressure, adjustable capacitance of&amp;#xD;the LC circuit, and Kittel magnetostatics, respectively. By changing the magnetic field’s direction,&amp;#xD;the Barnett effect produces a Barnett frequency shift that may be efficiently controlled to be either&amp;#xD;negative or positive. By controlling system parameters, we show that, by employing a bidirectional&amp;#xD;contrast ratio, multipartite entanglements can be non-reciprocally enhanced via the Barnett shift.&amp;#xD;In addition, Barnett’s shift also affects the robustness of these entanglements against the bath&amp;#xD;temperature. Such a system with a variety of control mechanisms might find a range of applications&amp;#xD;in quantum technologies, computation, and information processing.

Abstract We theoretically investigate the quantum phase transitions of a hybrid quantum system consisting of an ensemble of nitrogen-vacancy (NV) centers coupled to two microwave cavity resonators. By leveraging the Zeeman effect with a static magnetic field, we achieve a tunable switch between different three-level Dicke model configurations. We map out the global ground-state phase diagram and identify three distinct operating regimes: (i) a V type regime characterized by the competition between two superradiant phases, (ii) an effective two-level regime emerging near the level-crossing point due to channel decoupling and (iii) a cascade Ξ type regime induced by level inversion. Particularly in the cascade regime, we analyze a mechanism of ‘sequential phase transition’, where the onset of the second superradiant phase is strictly contingent upon the condensation of the first channel. This mechanism leads to a nested structure in the phase diagram, which is fundamentally distinct from the phase competition observed in V type systems. Furthermore, by calculating the excitation spectra, we observe the emergence of soft modes at continuous critical points, establishing the second-order quantum nature of the relevant transitions. Our proposal provides a feasible solid-state platform for simulating multicritical phenomena and realizing controllable light–matter interactions.

Abstract We present a theoretical study of synchronization between two strongly driven magnon modes indirectly coupled via a single-mode microwave cavity. Each magnon mode, hosted in separate Yttrium Iron Garnet spheres, interacts coherently with the cavity field, leading to cavity-mediated nonlinear coupling. We show, by using input–output formalism, that both classical and quantum synchronization emerge for appropriate choices of coupling, detuning, and driving. We find that thermal noise reduces quantum synchronization, highlighting the importance of low-temperature conditions. This study provides useful insights into tunable magnonic interactions in cavity systems, with possible applications in quantum information processing and hybrid quantum technologies.

Motivated by recent claims, we revisit how coherent perfect absorption (CPA) influences cavity and polaritonic linewidths. Using standard input-output theory and measurements on single-port bare microwave cavities and cavity-magnon hybrids, we find that CPA drives the on-resonance reflection amplitude to zero while the spectral width remains set by the total decay rate. Apparent narrowing observed near CPA is found to be a visual artifact that does not remain upon quantitative analysis. Extending the analysis to cavity magnomechanics, we show that logarithmic plots can exhibit apparent polaromechanical “normal-mode splitting,” whereas linear-scale spectra display no true splitting. These results clarify when CPA modifies amplitudes versus spectral poles, offer practical guidance for data presentation, and indicate that CPA alone is not a route to linewidth suppression or polaromechanical mode splitting in the linear, weak-probe regime.

Innovative 3D Printed Nb47Ti superconducting microwave cavity: Manufacturing and performance

Abstract Vapour–liquid equilibrium measurements are essential for developing equations of state, tuning binary interaction parameters, and improving the thermodynamic description of fluid mixtures. Conventionally, multiple apparatus are needed to quantify equilibrium phase behaviour and fluid properties, including composition, density, and phase fractions (quality). Using several different instruments to acquire the desired data can be laborious and increase measurement uncertainty. Microwave cavities are a promising technology capable of measuring these properties simultaneously in binary mixtures. Here, we apply recent advances in cavity design and signal processing to demonstrate measurements of phase volume fractions, compositions, and densities in mixtures of methane and propane at vapour–liquid equilibrium. Furthermore, we present an uncertainty analysis for phase composition and density measurements made with a single apparatus consisting of several microwave cavities and demonstrate levels of accuracy comparable to those obtained with conventional analytical techniques.

In hybrid microwave structures based on magnonics (so-called magnon cavities), the energy of the electromagnetic field can be transduced into the energy of magnetostatic spin excitations of a nanomagnet (Kittel magnon). This paper proposes a mechanism for transfer of coherent long-range spin excitation between nanomagnets, showing how Kittel magnons can be used for communication between magnon cavities. Analytical expressions for the magnon transport rate are obtained, and key parameters controlling off-resonant and resonant magnon tunneling regimes are indicated. Physical conditions are determined under which peak values of the rate characterizing the resonant transmission of the Kittel magnon arise either at a fixed tunneling energy with an arbitrary number of units in the ferromagnetic chain or at a fixed number of chain units with a varying tunneling energy. The features of resonant tunneling of the Kittel magnon formed in a magnon cavity at frequencies of about 10 GHz are analyzed. It is also shown that due to the strong photon-magnon coupling in the magnon cavity, at a certain magnetic field, the tunneling flow of magnons can reach an additional peak value, which reflects the process of resonant photon-to-magnon conversion.

We propose a new axion dark matter detection strategy that employs optical readout of laser beam ellipticity modulations caused by axion-induced electric fields in a microwave cavity, using electro-optic (EO) crystals, enhanced by externally injected radio-frequency (rf) power. Building upon the variance-based probing method [.], we extend this concept to the optical domain: A weak probe laser interacts with an EO crystal coupled to the resonant microwave cavity field at cryogenic temperatures, and the axion-induced electric field is revealed through induced ellipticity. The injected rf signal coherently interferes with that of the axion field, amplifying the optical response and significantly improving sensitivity. While our EO-based method employs a Fabry-Pérot resonator, we do not require Michelson interferometers. Our method, hence, enables compact, high-frequency axion searches, across the 0.5–50 GHz range. Operating at cryogenic temperatures not only suppresses thermal backgrounds, but, critically, allows the probing method to mitigate the quantum noise. This approach offers a scalable path forward for axion detection over the ∼ ( few − 200 ) μ eV mass range—covering the preferred parameter space for the postinflationary Peccei-Quinn axion dark matter—using compact, tunable systems.

Transport measurements of hybrid nanowires often rely on the observation of a zero-bias conductance peak as a hallmark of Majorana bound states (MBSs). However, such signatures can also be produced by trivial zero-energy Andreev bound states (ABSs) or by quasi-Majorana bound states (QMBSs), complicating their unambiguous identification. Here we propose microwave absorption visibility, extracted from parity-dependent cavity-nanowire susceptibility measurements, as a complementary probe of MBSs nonlocality. We study a Rashba spin-orbit nanowire consisting of a proximitized superconducting segment and an uncovered quantum-dot region, capacitively coupled to a single-mode microwave cavity. We show that true MBSs yield finite visibility only when both MBSs are simultaneously coupled to the cavity, reflecting their intrinsic nonlocality. In contrast, ABSs and QMBSs exhibit visibility extrema even when the cavity couples only locally to part of the nanowire. We further demonstrate that this distinction persists in the presence of Gaussian disorder, which may otherwise generate trivial subgap states. Motivated by recent experiments, we also analyze ``poor man's" Majoranas in double-quantum-dot setups, where analytical results confirm the same nonlocal visibility criterion. Finally, we discuss a cavity-driven scheme for initializing the electronic system in a given parity state. Our results establish cavity-based visibility as a robust and versatile probe of MBSs, providing a clear route to distinguish them from trivial zero-energy states in hybrid superconducting platforms.

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 ~0.8 (~1.0 dB squeezing) relative to the vacuum. These results lay the groundwork for quantum nonlinear magnonics and promise potential applications in quantum metrology.

Abstract We report the observation of a phase transition in a KTaO$_3$ crystal, corresponding to a paraelectric-to-ferroelectric transition. The crystal was placed inside a copper cavity to form a dielectric-loaded microwave cavity, and the transition was observed to occur near 134 K. As the cavity was cooled, the frequencies of both transverse electric and transverse magnetic resonant modes decreased (corresponding to an increase in permittivity). The mode frequencies converge at the transition temperature (near 134 K) and, below this point, reverse their tuning direction, increasing their frequency with decreasing temperature. This behaviour corresponds to a decrease in dielectric permittivity and is atypical for pure KTaO$_3$. To investigate further, we conducted impurity analysis using Laser Ablation inductively coupled mass spectrometry (LA-ICPMS), revealing a significant concentration ($\sim$ 7\%) of niobium (Nb) in the crystal. This suggests that the observed phase transition is driven by residual Nb impurities, effective as KTN, which induce ferroelectricity in an otherwise paraelectric host. Similar crystals with a lower concentration ($&lt;$ 2\%) did not undergo a phase transition but exhibited a loss peak at this temperature. These findings have practical implications for the design of tunable devices, for example, resonator-based dark matter detectors, where low-loss material phase stability and tunability are crucial.

We propose a scheme to achieve nonreciprocal unconventional magnon blockade (NUMB) based on the Barnett and Sagnac effects in a hybrid nonlinear system, in which a microwave cavity with two modes and a yttrium iron garnet (YIG) sphere are included. Two cavity modes with the fundamental and the second-harmonic frequencies are coupled by virtue of the χ(2) nonlinear materials, and meanwhile, the fundamental mode interacts with the magnon mode via the magnetic dipole interaction. Under weak driving and coupling conditions, the magnon blockade is obtained, where the second-order correlation function has been used to analyze the magnon antibunching based on the numerical simulations and analytical calculations. The underlying mechanism is that quantum destructive interference occurs between different transition paths. In addition, the combination of the Barnett and Sagnac effects, which leads to the simultaneous frequency shifts of the magnon and two microwave modes, could induce the nonreciprocity of magnon antibunching due to the positive and negative detunings via changing the direction of the external fields. The scheme we present is based on the weak near-resonant coupling between quantum modes and the weak driving of magnon and photon modes, which may demonstrate the potential for achieving the single-magnon resource in a hybrid cavity magnonic system and provide valuable guidance for the design of nonreciprocal magnon devices.

We theoretically study the effects of magnonic Kerr nonlinearity on magnon–polaritons (MPs) with a soft-mode in easy-axis ferromagnets coupled to a microwave cavity. Using an effective circuit model capable of describing MPs up to the nonperturbative strong-coupling regime, we show that chaotic and frequency-comb-like behaviors of MPs emerge at the original modes crossing point. Furthermore, we demonstrate that the Kerr nonlinearity induces a finite excitation gap in the soft-mode, particularly in the strong-coupling regime.