Silicon dioxide (${\text{SiO}}_2$) is arguably one of the most technologically relevant materials due to its use as a native gate dielectric in silicon-based metal-oxide-semiconductor transistors. In such devices, hydrogen is intentionally introduced during a forming gas anneal to passivate electrically active defects, which would otherwise degrade device performance. While the $\mathrm{Si}─\mathrm{H}$ bonds resulting from this passivation process are highly stable and mostly chemically inert, it has been deduced from experiments and first-principles calculations that radiation-induced holes in the ${\text{SiO}}_2$ valence band mediate the release of ${\mathrm{H}}^{+}$ from such bonds in the oxide. Here, we show through first-principles calculations combined with the theory of multiphonon emission that isolated $\mathrm{Si}─\mathrm{H}$ bonds in ${\text{SiO}}_2$ can be easily broken upon capturing a hole. Our calculations show that capturing holes from the ${\text{SiO}}_2$ valence band leads to an efficient $\mathrm{Si}─\mathrm{H}$ bond dissociation mechanism, with a large effective capture cross section of $\sigma =6\times {10}^{-15}{\text{cm}}^2$. The resulting ${\mathrm{H}}^{+}$ can easily diffuse away, leaving behind a neutral paramagnetic $E^{\prime }$ center, i.e., an ${\mathrm{O}}_3=\text{Si}·$ dangling bond, confirming earlier results reporting ${\mathrm{H}}^{+}$ release and $E^{\prime }$ center creation upon hole injection into the ${\text{SiO}}_2$ valence band.

Nonlinear frequency conversion in the terahertz regime is significant for emerging applications in high-speed communications, imaging, and sensing, yet enhancing light-matter interactions in two-dimensional materials often relies on multilayer stacking or complex resonant architectures. Here, we introduce a transmission-type nonlinear metamaterial based on a single-layer graphene architecture that enables geometry-controlled dual-frequency terahertz third-harmonic generation. By employing a rail-like graphene meta-atom, strong and spatially redistributed local electric-field hot spots are generated through geometry-induced edge confinement and narrow-gap resonances. This enables effective coupling between the enhanced electromagnetic fields and graphene’s intrinsic third-order nonlinearity, allowing dual-frequency nonlinear conversion within a single planar structure without multilayer stacking. Within the adopted fixed-temperature perturbative nonlinear surface-current framework and at an incident pump intensity of $10\mathrm{MW}/{\mathrm{cm}}^2$, the proposed structure predicts third-harmonic generation conversion efficiencies of 4.0% and 4.4% at 1.781 and 2.206 THz, respectively, together with local-field enhancement factors that are one to two orders of magnitude higher than those of bare graphene. This work demonstrates a geometry-driven strategy for controlling terahertz nonlinear responses, showing that nonlinear frequency selectivity can be engineered through geometry-induced local-field redistribution rather than structural complexity. The proposed approach provides a compact strategy for designing single-layer graphene metamaterials with controllable dual-frequency terahertz nonlinear functionality within the adopted theoretical framework.

Continuous-variable quantum secure direct communication (CV-QSDC) utilizes quadrature components of the quantized electromagnetic mode to accomplish direct transmission of secret messages over the channel, offering the advantage of its compatibility with telecommunication facilities. However, phase noises during the running of systems destroy the correlation between the data of two parties and severely limit the effective communication rate. In this paper, we design a phase reference scheme to achieve phase synchronization between the signal light and the local oscillator in the coherent-state CV-QSDC system. To deal with the inaccurate modeling of phase drift in reference pulses, we further propose a phase compensation scheme using model-based deep learning, in which KalmanNet learns the Kalman gain from the data to track the true values of phase drift. In the simulation experiment, the frequency noise acts as the source of inaccuracy, and the results show that the phase compensation scheme based on KalmanNet accomplishes phase estimation effectively and thus improves the communication performance of CV-QSDC.

Defects in solid-state systems constitute a promising platform for the realization of deterministic quantum emitters. Among many candidate materials and emitters, point defects in hexagonal boron nitride (hBN) have recently emerged as particularly promising. In this work, we probe the coherence of an individual B center with a zero-phonon line at 436 nm under pulsed resonant excitation. We observe power-dependent Rabi oscillations up to $5\pi$, demonstrating optical coherent control of the transition. We achieve an excellent single-photon purity of 93% at $\pi$-pulse. Furthermore, we probe the coherence of the two-level system using Ramsey interferometry, revealing an inhomogeneous coherence time of $T_2^{*}=0.64\pm 0.02\mathrm{ns}$. These results establish B centers in hBN as viable candidates for triggered, coherent quantum emitters and represent a significant step toward their integration into quantum photonic platforms.

The diamond nitrogen-vacancy (N-$V$) center ensemble exhibits substantial potential for room-temperature quantum sensing. However, its practical implementation has long been limited by spectral line broadening and a reduced signal-to-noise ratio, which arise from multiaxis orientations of the N-$V$ centers. The (111) crystal plane is considered an optimal growth direction, as it enables near-perfect axial alignment of N-$V$ centers ($>99\%$) and enhances photon collection efficiency. Nonetheless, the perfect alignment was maintained at the sacrifices, including low growth rates and limited nitrogen-to-N-$V$ conversion efficiencies. In this study, we demonstrate a microwave plasma chemical vapor process in which a growth rate of $94.0\mathrm{\mu }\mathrm{m}/\mathrm{h}$ (14-fold enhancement to the highest previous record) was achieved with uniform N-$V$ ensemble distribution, an N-$V$ concentration of 0.54 ppm, and a nitrogen-to-N-$V$ conversion efficiency of 4.1%–4.7%. The resulting volume-normalized dc and ac magnetic field sensitivities reached $2.15\mathrm{nT}\mathrm{\mu }{\mathrm{m}}^{3/2}{\mathrm{Hz}}^{-1/2}$ and $0.30\mathrm{nT}\mathrm{\mu }{\mathrm{m}}^{3/2}{\mathrm{Hz}}^{-1/2}$, respectively.

The time evolution of initially localized wavepackets in the discrete Hatano-Nelson lattice displays a rich dynamical structure shaped by the interplay between dispersion and nonreciprocity. Our analysis reveals a characteristic evolution of the wave-packet center of mass, which undergoes an initial constant acceleration, subsequently decaying acceleration, and ultimately enters a regime of uniform motion, accompanied throughout by exponential amplification of the wave-packet amplitude. To capture this behavior, we develop a continuum approximation that incorporates higher-order dispersive and nonreciprocal effects and provides accurate analytical predictions across all relevant time scales. We then demonstrate the existence of a non-Hermiticity-induced jump—an abrupt spatial shift of the wave-packet center even in the absence of disorder—and derive its underlying analytical foundation. The analytical predictions are in excellent agreement with direct numerical simulations of the Hatano-Nelson chain. Our results elucidate the interplay between dispersion and nonreciprocity in generating unconventional transport phenomena and pave the way for controlling wave dynamics in nonreciprocal and non-Hermitian metamaterials.

Nitrogen-vacancy (NV) centers in diamond have precipitated profound advances in microwave detection, manifesting themselves both in spatial resolution and sensitivity. However, typical methods based on Rabi oscillations are subject to detunings due to thermal and magnetic fluctuations and/or gradients, which introduce systematic errors and render the measurements susceptible to environmental perturbations. Here, we propose and demonstrate a novel approach for determining both the magnitude and direction of microwaves, by exploiting the spectral line broadening effect in the optically detected magnetic resonance of NV centers. This method eliminates the requirement of aligning the MW frequency to the spin transitions and is therefore immune to variations and inhomogeneities of the magnetic field and temperature, providing an optimal tool for fast imaging applications. With this method, we achieved near-field microwave imaging of a microscale “$\mathrm{\Omega }$”-pattern antenna with a resolution of 800 nm. Using multiaxis NVs, a reconstruction of the vector microwave fields is obtained. Our results augment the applicability of diamond-based microwave near-field imaging in applications under complex scenarios, especially where large dynamic range, fast imaging, and highspatial resolution are demanded.

We demonstrate a triple-key optical encryption system that integrates orbital angular momentum (OAM) multiplexing with physical scattering and ultra-sparse sampling. At its core is an adaptive ResNeXt-SPD decoder that achieves 99.28% accuracy in decoding 24-bit OAM superposition states with only 64 pixels—a sampling point density of 0.13%. The system implements a cross-domain security architecture comprising three keys: a physical key (an unclonable scattering medium), a digital key (a sparse sampling pattern), and a computational key (trained network weights). We validate the sparse sampling performance in the full architecture, explore the sampling threshold under constrained networks, and investigate the criticality of each security key. Security analysis confirms that the absence or mismatch of any single key renders the system inoperative. Substituting the physical key drops accuracy to $\mathrm{approximately}50\%$. One pixel shift in the digital key reduces accuracy from 99.28% to $\mathrm{approximately}50\%$. Untrained model weights yield performance equivalent to random guessing. End-to-end image transmission under extreme sparsity maintains high reconstruction quality. This work advances OAM-based secure communication by achieving the lowest sampling-density and multilayer cryptographic protection simultaneously.

We demonstrate a robust, high-visibility nonlocal Franson interferometry for fiber-based quantum networks by copropagating a classical radio-over-fiber clock signal with energy-time entangled photon pairs in the same fiber. Utilizing cross-band allocation (O-band for classical, L-band for quantum signals), the spontaneous Raman scattering noise photons are effectively suppressed. At the same time, their environmental delay fluctuations remain highly correlated for common-mode noise cancellation, achieving a passive synchronization with picoseconds precision. Over 50 km of single-mode fiber, this copropagation enables nonlocal quantum interference with a visibility of $(88.35\pm 3.62)\%$, without relying on external dedicated timing infrastructure. This work provides a practical, scalable synchronization solution for metropolitan-scale entanglement-based quantum networks.

We propose and validate a framework for analog simulation of the Heisenberg spin model using a circuit-quantum-electrodynamics (circuit-QED) platform. To this end, we develop an $f$-deformed boson representation of the $SU(2)$ algebra, which includes the Holstein–Primakoff and Dyson–Maleev transformations as special cases. While this representation is generally non-Hermitian, for spin-$1/2$ systems, we introduce a procedure to circumvent this limitation, showing that the mapping yields the extended Bose-Hubbard (EBH) Hamiltonian. For the experimental realization of this EBH model, we design a scalable circuit-QED architecture based on an engineered Josephson junction array. Numerical simulations confirm that the microwave photon dynamics in this simulator accurately reproduces the original spin dynamics. Our work establishes an experimentally accessible method for investigating complex quantum spin dynamics in a highly controllable bosonic setting.

Nondestructive detection of single-electron motion is crucial for quantum information processing with electrons trapped in Paul traps. The standard approach in Penning traps is to detect the image current induced on the trap electrodes by the electron’s oscillatory motion. However, applying this approach in Paul traps for single electrons is currently hindered by motional frequency fluctuations arising from trap anharmonicities and instabilities in the rf trapping field. In this work, we propose a robust detection scheme exploiting the transient dynamics of parametric driving to overcome these limitations. Distinct from traditional steady-state approaches, our method focuses on the transient regime to break the temporal constraints imposed by steady-state assumptions, thereby enabling fast readout. We show that a controlled ramp of the parametric drive effectively locks the frequency of the electron motion in the transient regime, rendering the signal highly resilient to realistic experimental noise and inherent micromotion. This work paves the way for the experimental realization of nondestructive detection of single-electron motion in Paul traps.

Photons are ideal in many aspects for quantum technologies. However, the absence of direct photon-photon interaction and the problem of wave-packet distortion inherent in nonlinear media-based interactions have long posed a grand challenge in constructing two-qubit logic gates, which are indispensable for universal quantum information processing. Here we report on the theory and design to overcome the challenge, leading to a deterministic, passive, and near-unity-fidelity (exceeding 99%) controlled-$\pi$-phase gate for photonic qubits. The proposed gate comprises only a single two-level emitter and a few cavities and operates under the key concept of time-reversal-symmetric photon transport. It ensures a nonlinear $\pi$ phase shift and circumvents the long-standing difficulty of wave- packet distortion inherent in photonic phase gates using nonlinear media. We develop an analytical solution to reveal the dynamics of two-photon wave-packet transport and to obtain the optimal design. The proposed gate architecture is compatible with the platforms of integrated photonics.

Hopfions, associated with higher-dimensional topology through the Hopf fibration, exhibit exotic features like complex knots and improved stability compared to skyrmions, enhancing their appeal for innovative applications. In this paper, we study the nonlinear response of magnetic hopfion to microwave fields. We observe the emergence of higher-order harmonics of the driving microwave field as it interacts with the hopfion. By carefully selecting the driving frequency, the corresponding harmonic can efficiently excite the localized magnon state of a hopfion. Our results demonstrate the promising potential of hopfions in nonlinear magnonics.

Precise, real-time monitoring of magnetic field evolution is significant in applications including magnetic navigation and searches for physics beyond the standard model. One main field-monitoring technique, the spin-precession magnetometer (SPM), observes electron, nucleus, color center, or muon spins as they precess in response to their local magnetic field. Here, we study Bayesian signal-recovery methods for SPMs in the free-induction decay mode. In particular, we study tracking of field changes well within the coherence time of the spin system and thus well beyond the response bandwidth, as in [R. Jiménez-Martínez et al. Phys. Rev. Lett. 120, 040503 (2018)]. We derive the Bayesian Cramér-Rao bound that dictates the ultimate precision in estimating the Larmor frequency, which we show is attained by the computationally expensive prediction error method. Relative to this benchmark, we show that the extended Kalman filter (EKF) and cubature Kalman filter offer near-optimal tracking that is also computationally efficient, with the use of the latter giving better results only for a large spin number. Focusing thus on the EKF, we show that it is sufficient to accurately track fluctuating and unknown transient signals. Our methods can be easily adapted to other types of sensors undergoing nonlinear dissipative dynamics and experiencing intrinsic Gaussian-like stochastic noises.

A coupled-oscillator system displays enhanced sensitivity of its saturated steady-state (SS) oscillation frequency to small parameter perturbations near an exceptional-point degeneracy (EPD), a property that can be used to realize EPD-based sensors. Linear $\mathcal{P}\mathcal{T}$-symmetric systems, consisting of two coupled resonators, exhibit EPDs around which square-root sensitivity is observed. However, linear models are insufficient for realistic systems that rely on nonlinear, saturable-gain elements, particularly when $\mathcal{P}\mathcal{T}$-symmetry is broken. Thus, we study the SS of a general system of two coupled oscillators featuring EPDs and saturable nonlinear gain, using coupled-mode theory. We do this by synthesizing and extending prior SS analyses of the system’s stability and its square-root and cubic-root oscillation frequency sensitivity at a unique third-order SS-EPD. We add an SS analysis of saturated gain, energy, and oscillation-frequency sensitivity in the vicinity of the third-order SS-EPD, enabling characterization of the various SS regimes. We determine that the stable and bistable regions in parameter space directly depend on the saturated-gain values; that the dynamic range of high sensitivity around degenerate conditions is extended by increasing losses, consequently reducing the system’s stored energy; and that, to exploit the cubic-root-like sensitivity associated with the third-order SS-EPD, the suggested working regime is best confined to operation within the weakly coupled regime and not exactly at the third-order SS-EPD. Finally, we apply the model to two electronic circuits that exhibit cubic-root sensitivity, demonstrating the application and limitations of this analysis.

Scalable quantum computing in frequency-crowded nuclear spin registers promises long coherence times and native many-body logic. However, as system size increases, spectral crowding introduces two fundamentally competing challenges: coherent-control-induced spectral crosstalk and fragility to frequency fluctuations arising from charge and magnetic noise. Conventional pulse shaping optimized for crosstalk suppression is intrinsically incompatible with robustness against frequency fluctuations. In this work, we show that this incompatibility reflects a fundamental breakdown of single-qubit robust control principles in many-body entangling operations, manifested as a robustness inversion under single-tone ESR control: locally robust pulses disrupt intersubspace phase synchronization and degrade the robustness of many-body operations. We further uncover a Hilbert-space dilution effect, whereby $C$ Z gate robustness decays exponentially with system size, scaling as $1/2^M$. Leveraging these insights, we establish a phase-synchronized global multitone control paradigm that simultaneously suppresses frequency fluctuations and spectral crosstalk, in which square pulses—traditionally avoided due to spectral leakage—induce an intersubspace dynamical decoupling mechanism that converts unstructured crosstalk into effectively static, geometrically neutralized frequency noise. Full Hamiltonian simulations demonstrate programmable quantum logic units achieving the fidelities of many-body entangling operations approaching 99.99%, representing an order-of-magnitude improvement over conventional strategies, and establishing a scalable route toward fault-tolerant modular quantum computing.

Hong-Ou-Mandel (HOM) interference is a fundamental tool for assessing photon indistinguishability in quantum information processing. While the effect of chromatic dispersion on HOM interference has been widely studied, the interplay between dispersion and the finite detection window of realistic measurement devices remains underexplored. In this work, we demonstrate that the rectangular coincidence window inherent to modern time-tagging modules, which effectively acts as a temporal filter, breaks the standard dispersion cancellation condition and restores sensitivity to symmetric group-velocity dispersion. We derive an analytical model for type-II SPDC processes that predicts a modification of the HOM dip shape, specifically the emergence of characteristic oscillations and dip broadening. We experimentally validate this theoretical framework using a ppKTP source and transmission through optical fibers of lengths up to 29 km. The experimental data show excellent agreement with the model, confirming the presence of window-induced oscillations and allowing for the precise extraction of the fiber dispersion parameter. These findings underscore the significance of accounting for finite timing resolution in the design and characterization of dispersive quantum communication links.

Nitrogen-vacancy (NV) centers in diamond have been shown to be capable of detecting ac magnetic fields with high sensitivity, spectral resolution, and spatial resolution. However, most studies so far have focused on the regime of time-averaged or time-correlated measurements, while little attention has been paid to the single-shot regime. Here we show that the amplitude and phase of an ac field can be retrieved from a single pair of two consecutive measurements. We demonstrate this concept by measuring a 4 MHz ac field with a per-shot amplitude and phase sensitivity of $78\mathrm{nT}$ and $63\mathrm{mrad}$, respectively, at a temporal resolution of $320\mathrm{\mu }\mathrm{s}$. We also investigate the effects and quantify the errors resulting from probe frequency detunings, as well as operating in the strong field regime. Moreover, we showcase the ability of the measurement protocol to dynamically change the probe frequency in real time. This work advances the use of NV centers for real-time measurements of ac magnetic fields.