One of the main challenges in magnonics is the efficiency of the conversion of microwave signals into spin waves. This efficiency is low due to the significant mismatch between microwave and spin-wave wavelengths in the gigahertz range (${10}^{-2}$ and ${10}^{-8}$ m, respectively), leading to high energy consumption in magnonic circuits. To address this issue, we propose an approach based on a planar inverse split-ring resonator (ISRR) loaded with a thin Permalloy (Py) film and exploiting the photon-magnon coherent coupling effect. Our numerical studies show that the ISRR-based antenna achieves more than a fourfold improvement in conversion efficiency compared to a conventional single-microstrip-transmission-line antenna when working at frequencies and bias magnetic fields around the anticrossing frequency gap. This has been demonstrated in the weak photon-magnon coherent coupling regime for a nanometer-thin Py film with micrometer lateral dimensions. Further optimization of the ISRR can help to achieve the strong coherent coupling regime, making the system potentially useful for quantum technology. Our compact and efficient antenna design offers a significant advantage over standard microstrip-line antennas, paving the way for powerful magnonic circuits for microwave signal processing.

We explore topological properties of one-dimensional (1D) bilayered elastic rods (i.e., phononic crystals) using the phason as a virtual dimension and subsequently analyze topological interface modes. Following adoption of the phason, we use a two-dimensional (2D) topological invariant—the Chern number—and show that its differences for various crystals predict the number of gapless interface states in $(1+1)\mathrm{D}$ systems. The latter constitutes a manifestation of the bulk-boundary correspondence principle. We develop equations of periodic systems that depend explicitly on the phason, hence facilitate understanding and bring new insight into wave propagation and dynamics of topological states in these media. For the design and analysis of interface modes in systems with two virtual parameters, we introduce the phason-phason space. We find that the interface states mark strips in this space, separated by linear functions with slopes related to the Chern numbers. Consequently, this space can be used to directly predict the Chern numbers of both adjoined phononic crystals as well as the number of gapless interface modes for different phason changes.

Compact integrated optical parametric oscillators are of high demand in numerous cutting-edge applications within photonics. In particular, degenerate optical parametric oscillators attract flourishing research interest, owing to their unique features, which are essential for optical computing and quantum optics. In this study, we experimentally examine the optimal conditions for highly efficient degenerate optical parametric oscillations (DOPOs) in silicon nitride (${\mathrm{Si}}_3{\mathrm{N}}_4$) microring resonators (MRRs) excited by a dual-frequency pump. We demonstrate, both numerically and experimentally, that synchronous frequency scanning of pump lasers is an effective strategy to achieve optimal coupling of a pump power into a cavity, as well as to obtain efficient DOPOs. Additionally, we show, both experimentally and numerically, that a large thermal nonlinearity of a MRR can significantly increase a system’s overall tolerance with respect to pump frequency drift. Through our investigation, we obtain DOPO spectral line handling over 16% of the total output power. This study provides a technique for highly efficient and stable DOPO realization.

Anonymity and privacy are two key properties of modern communication networks. In quantum networks, distributed quantum sensing has emerged as a powerful use case, with applications to clock synchronization, detecting gravitational effects, and more. In this work, we develop a new protocol that, for the first time, combines the different cryptographic properties of anonymity and privacy for the task of distributed parameter estimation. That is, we present a protocol that allows a selected subset of network participants to anonymously collaborate in estimating the average of their private parameters. Crucially, this is achieved without disclosing either the individual parameter values or the identities of the participants, neither to each other nor to the broader network.

We report on an industry-grade complementary metal-oxide semiconductor (CMOS)-compatible qubit fabrication approach using a CMOS pilot line, enabling a yield of functional devices that reaches 92.8%, with a resistance spread evaluated across the full wafer 200 mm in diameter of 12.4% and relaxation times $T_1$ approaching $80\text{μ}\mathrm{s}$. Furthermore, we conducted a comprehensive analysis of wafer-scale room-temperature (RT) characteristics collected from multiple wafers and fabrication runs, focusing on RT measurements and their correlation to low-temperature qubit parameters. From defined test structures, an across-wafer Josephson junction (JJ) area variation of 10.1% and oxide barrier variation of 7.2% was calculated. Additionally, from the room-temperature JJ characterization the qubit frequency can be derived at the wafer level by applying the Ambegaokar-Baratoff model before making low-temperature measurements. This sets the stage for precooldown wafer-level JJ evaluation and sorting. In particular, such early device characterization and validation are crucial for increasing the fabrication yield and qubit frequency targeting, which currently represent major scaling challenges. Furthermore, this enables the fabrication of large multichip quantum systems in the future. Our analysis highlights the great potential of CMOS-compatible industry-style fabrication of superconducting qubits for scalable quantum computing in a foundry pilot line cleanroom.

Semiconductor quantum dots (QDs) play a crucial role in quantum information technologies, particularly in the development of quantum light sources and spin-photon interfaces. Precise control over exciton states is essential to achieve spatial selectivity, ultrafast response, and scalable multinode architectures. Here, we demonstrate selective QD modulation using interdigital transducers with a frequency gradient that combine strain coupling via surface acoustic waves (SAWs) and dynamic quantum-confined Stark effects. By engineering position-dependent SAW eigenfrequencies and localized dynamic electric fields, we achieve selective activation of strain-induced symmetric broadening and electric-field-driven splitting for single QDs. In addition, integration with external magnetic fields further enhances the modulation range. This work establishes a multifunctional QD modulation platform that integrates acoustic, electric, and magnetic control strategies for scalable quantum photonic networks.

Learning about the physical world relies on sensing and postprocessing. When the signals are weak, multidimensional, and correlated, the performance of learning is often bottlenecked by the quality of sensors, calling for the integration of quantum sensing into the learning of such physical-layer data. Examples of such a learning scenario include the stochastic quadrature displacements of electromagnetic fields, modeling optomechanical force sensing, radio-frequency photonic sensing, and microwave cavity weak-signal sensing. We propose a unified protocol that combines machine learning with interferometric photon counting to reduce noise and reveal correlations. By applying variational quantum learning with multimode programmable quantum measurements, we enhance signal extraction. Our results show that multimode interferometric photon counting outperforms conventional homodyne detection proposed in prior works for tasks like principal component analysis and cross-correlation analysis, even below vacuum noise levels. To further enhance performance, we also integrate entanglement-enhanced modules, in the form of squeezed state distribution and antisqueezing at detection, into the protocol. Combining multimode interferometric photon counting and multipartite entanglement, the proposed protocol provides a powerful toolbox for learning weak signals.

Multipartite entangled states are essential for multiuser quantum cryptography. While large-scale continuous-variable (CV) cluster states, particularly the dual-rail cluster state, have been well studied in measurement-based quantum computation, their cryptographic potential remains underexplored. Here, we propose a three-user conference key protocol using a CV dual-rail cluster state. By applying a node-coloring scheme to the infinite dual-rail graph, we create a six-mode pure graph state ideal for cryptographic tasks. Our results demonstrate near-GHZ (Greenberger-Horne-Zeilinger) performance for quantum conference key agreement (QCKA). Crucially, our protocol uniquely enables bipartite keys post-QCKA, which GHZ states cannot provide. It also surpasses two-mode squeezed vacuum states in generating bipartite keys within downstream-access networks. Considering finite-size effects and impure squeezed states, our scheme remains robust despite experimental imperfections. We also introduce an enhanced method to more accurately estimate bipartite key generation capacity in quantum networks, paving the way for practical multiuser quantum cryptography.

Detailed knowledge of the local environments of spin defects in semiconductors, such as nitrogen-vacancy (NV) centers in diamond or divacancies in silicon carbide, is crucial for optimizing control and entanglement protocols in quantum sensing and information applications. However, at present a direct experimental characterization of individual defect environments is not scalable, as conventional spin-bath measurements are time consuming and difficult to automate. Achieving high-throughput characterization requires short experiments to probe the spin bath. However, with fewer and noisier measurements, the inverse problem of recovering spin-bath properties from measured data becomes ill posed, with multiple spin baths having a high likelihood of yielding the same data. In this work, we present a set of computational tools to resolve the ill-posed inverse problem of recovering the atomic positions and hyperfine couplings of random nuclei surrounding spin defects from sparse, noisy experimental coherence data, which can be obtained in hours. We use a trans-dimensional Bayesian approach that incorporates ab initio data to yield full posterior distributions over nuclear spin environments, enabling robust recovery from limited data. We also provide practical tools and guidelines to determine the limits of detectability for hyperfine couplings under specific dynamical decoupling sequences and sampling conditions. In addition, we demonstrate how the tools developed here, in combination with ab initio simulations of spin baths, can guide the design of efficient experimental protocols for application-specific high-throughput screening. To showcase the utility of our approach, we apply it to design fast dynamical decoupling experiments to characterize the spin baths of ten individual NV centers in diamond. While the primary focus is on accelerating spin-bath characterization of spin defects, this Bayesian approach also lays the foundation for digital-twin studies of spin defects, where a virtual model of the spin-defect system evolves in real time with ongoing experimental measurements. Together, the set of tools we designed and applied paves the way for scalable deployment of spin defects in semiconductors for quantum sensing and information applications.

Sensing periodic fields using quantum sensors has been an active field of research. In many of these scenarios, the quantum state of the probe is flipped regularly by the application of $\pi$ pulses to accumulate information about the target periodic field. The emergence of a discrete time crystalline phase, as a nonequilibrium phase of matter, naturally provides oscillations in a many-body system with an inherent controllable frequency. They benefit from long coherence time and robustness against imperfections, which makes them excellent potential quantum sensors. In this paper, through theoretical and numerical analysis, we show that a disorder-free discrete time crystal probe can reach the ultimate achievable precision for sensing a periodic field. As the amplitude of the periodic field increases, the discrete time crystalline order diminishes, and the performance of the probe decreases remarkably. Nevertheless, the obtained quantum enhancement in the discrete time crystal phase, which is experimentally accessible using standard projective measurements, shows robustness against different imperfections and dephasing noise in the protocol. Finally, we propose the implementation of our protocol in ultracold atoms in optical lattices.

To enhance the sensitivity of high-precision measurements based on ultrastable optical cavities such as gravitational-wave detections, achieving low coating mechanical loss is crucial. Nanolaminates have been proposed as potential replacements for the currently used high-refractive-index material ${\mathrm{Ti}\mathrm{O}}_2:{\mathrm{Ta}}_2{\mathrm{O}}_5$. However, due to material selection constraints, nanolaminates inherently face a trade-off between refractive index and optical absorption. Quantized-layer technology offers a promising approach for modifying the optical properties of nanolaminates by introducing quantum effects through reduced well-layer thickness, thereby increasing the optical band gap and suppressing absorption. In this study, we systematically investigate the effects of sublayer thickness on the optical properties and coating mechanical loss of nanolaminates, utilizing a periodic coating structure composed of ${\mathrm{Ti}\mathrm{O}}_2$ and ${\mathrm{Ta}}_2{\mathrm{O}}_5$. By tuning the sublayer thickness and well-layer thickness ratio, we establish general trends linking refractive index, extinction coefficient, optical band gap, Urbach energy, and coating mechanical loss to nanolaminate structural parameters. Notably, when the barrier-layer (${\mathrm{Ta}}_2{\mathrm{O}}_5$) thickness is 3.51 nm and the well-layer (${\mathrm{Ti}\mathrm{O}}_2$) thickness is 1.77 nm, quantized-layer technology enables the fabrication of nanolaminates with a mechanical loss performance superior to that of ${\mathrm{Ti}\mathrm{O}}_2:{\mathrm{Ta}}_2{\mathrm{O}}_5$. Furthermore, we observe a correlation between coating mechanical loss and the short- and medium-range disorder of the atomic structure. This study demonstrates the improvement in room-temperature coating mechanical loss via quantized-layer technology from the perspective of thickness effects.

Simple local interactions can cause primitive building blocks to self-assemble into complex and functional patterns; however, even for a small number of blocks, there exist a vast number of plausible and stable configurations with varying degrees of order. The ability to dynamically shift between multistable patterns (i.e., reprogram the self-assembly) entails navigating an intractable search space, which remains a challenge. In this paper, we engineer the self-assembly of macroscopic magnetic particles to create metamaterials with dynamically reversible emergent phases. We use a boundary composed of magnetic hinges to confine free-floating magnetic disks into different stable assemblies. We exploit the nondestructive nature of the magnetic boundaries to create reprogrammable two-dimensional metamaterials that morph from crystalline to quasicrystalline to disordered assembly using the same number of disks and the same boundary. Furthermore, we explore their utility in controlling the propagation of sound waves in an effectively undamped medium with rich nonlinearities. Our findings can broaden the horizon of metamaterials, enabling functional and tunable devices.

We introduce topological invariants into displacement metrology and show that robust topological structures in momentum space can be used to retrieve the displacement of a small particle. Owing to its topological nature, the proposed scheme is general. It does not require phase or polarization stability and works even under broadband unpolarized illumination with randomly fluctuating phases. Remarkably, unpolarized illumination can achieve a superior performance to its coherent counterpart, owing to closely packed in-plane polarization singularity structures with very high displacement sensitivities nearby. Our work opens an avenue for developing topologically protected ultrasensitive metrological methods with randomly fluctuating fields.

High-resolution ultrasound and photoacoustic imaging with flexible, lightweight probes is highly desirable for applications ranging from handheld clinical diagnostics to minimally invasive endoscopy and industrial inspection in confined environments. Piezoelectric transducer arrays, while widely used, face limitations in resolution, bandwidth, and scalability. To overcome these limitations, we present an approach that combines optical ultrasound detection using a Fabry-Pérot ultrasound sensor (FPUS) with light delivery through a single multimode optical fiber (MMF), utilizing wavefront shaping for beam control. This approach offers the capability to combine high-resolution imaging, a compact and flexible form factor, and software-defined reconfigurable array read-out. We present a prototype imaging system in which an FPUS is interrogated through a 200-µm-core-diameter MMF supporting ∼2000 spatial light modes, and use this system to map the ultrasound fields produced by known transducers. We demonstrate high-resolution, high-sensitivity ultrasonic field mapping, paving the way to a new class of versatile handheld imaging devices and endoscopes in medicine and industry.

The spin qubits of color centers are extensively investigated for quantum sensing, communication, and information processing, with their states generally controlled using lasers and microwaves. However, it is challenging to effectively irradiate both lasers and microwaves onto color centers using small-footprint microwave waveguides or antennas that are compatible with semiconductor devices. We show experimentally that, by introducing a compact coplanar waveguide with a thin slit in its signal line, effective irradiation of both lasers and microwaves is enabled, allowing spin-state control of color centers created around the slit. Microwave magnetic fields parallel to the surface, intrinsically generated by a standard coplanar waveguide, persist even after loading the slit, which is necessary to control the color centers whose spin quantization axes are oriented perpendicular to the surface, while laser light for the initialization and readout of spin states can access the color centers through the slit. Continuous and pulsed optically detected magnetic resonance measurements are performed for the silicon vacancies ($V_{\mathrm{Si}}$) in silicon carbide $4H$-$\mathrm{Si}\mathrm{C}$(0001). Experimental results indicate that the spin states of $V_{\mathrm{Si}}$ are effectively controlled by the microwave magnetic fields parallel to the surface, which agrees with numerical results from electromagnetic field simulations. Our small-footprint waveguide is suitable for integrating color-center-based quantum sensors into semiconductor electronic devices and other small-scale systems.

We propose a nanoscale device consisting of a double quantum dot with a full exchange and pair-hopping interaction. In this design, the current can only flow through the upper dot but is sensitive to the spin state of the lower dot. The system is immersed in a highly inhomogeneous magnetic field and only the bottom dot feels a substantial magnetic field, while the top dot experiences only a residual one. We show that our device exhibits very interesting magnetic field–dependent transport properties at low temperatures. The Kondo effect partially survives the presence of the magnetic field and allows conductances that differ by several orders of magnitude to be obtained for the two spin types across the top dot. Interestingly, as a function of the magnetic field, our two-dot device changes from a spin singlet state to a spin triplet state, in which the amplitudes of the spin-dependent conductances are reversed. Our device is able to discriminate between positive and negative magnetic fields with a high sensitivity and is, therefore, particularly interesting for imaging the surface of antiferromagnetic insulating materials with alternated surface magnetic field, as well as for spin-filtering applications.

We present a general approach for analyzing arbitrary parametric processes in Josephson circuits within a single-degree-of-freedom approximation. Introducing a systematic normal-ordered expansion for the Hamiltonian of parametrically driven superconducting circuits, we present a flexible procedure to describe parametric processes and to compare and optimize different circuit designs for particular applications. We obtain formally exact amplitudes [“supercoefficients” (SCs)] of these parametric processes for driven circuits based on superconducting nonlinear asymmetric inductive elements and superconducting quantum interference devices. The corresponding amplitudes contain complete information about the circuit topology, the form of the nonlinearity, and the parametric drive, making them, in particular, well suited for the study of the strong-drive regime. We present a closed-form expression for SCs describing circuits without stray inductors and a tractable formulation for those with it. We demonstrate the versatility of the approach by applying it to the estimation of Kerr-cat qubit Hamiltonian parameters and by examining the criterion for the emergence of chaos in Kerr-cat qubits. Additionally, we extend the approach to multiple-degree-of-freedom circuits comprising multiple linear modes weakly coupled to a single nonlinear mode. We apply this generalized framework to study the activation of a beam-splitter interaction between two cavities coupled via driven nonlinear elements. Finally, utilizing the flexibility of the proposed approach, we separately derive SCs for the higher-harmonics model of Josephson junctions, circuits with multiple drives, and the expansion of the Hamiltonian in the exact eigenstate basis for Josephson circuits with specific symmetries.

Realizing scalable quantum interconnects necessitates the integration of solid-state quantum memories with foundry photonics processes. While prior photonic integration efforts have relied upon specialized, laboratory-scale fabrication techniques, this work demonstrates the monolithic integration of a quantum memory platform with low-loss foundry photonic circuits via back-end-of-line deposition. We deposited thin films of titanium dioxide (${\mathrm{Ti}\mathrm{O}}_2$) doped with erbium ($\mathrm{Er}$) onto silicon nitride nanophotonic waveguides and studied $\mathrm{Er}$ optical coherence at subkelvin temperatures with photon echo techniques. We suppressed optical dephasing through ex-situ oxygen annealing and optimized measurement conditions, which yielded an optical coherence time of $64\text{μ}\mathrm{s}$ (a 5-kHz homogeneous linewidth) and slow spectral diffusion of 27 kHz over 4 ms, results that are comparable to state-of-the-art $\mathrm{Er}$ nanophotonic devices. Combined with second-long electron spin lifetimes and demonstrated electrical control of $\mathrm{Er}$ emission, our findings establish $\mathrm{Er}$:${\mathrm{Ti}\mathrm{O}}_2$ on foundry photonics as a manufacturable platform for ensemble and single-ion quantum memories.