One of the exceptional features of non-Hermitian systems is the unidirectional wave interactions. Simultaneous modulation of the real and imaginary parts of the interaction potentials (of the refractive index and the gain and/or loss in the case of optical systems) can result in unequal coupling coefficients between the fields of different parts of the system. The unidirectional coupling can be arranged between internal fields and external radiation as well. At a particular (exceptional) point, the situation can be achieved where the external radiation is coupled into the system, but the internal radiation cannot escape backward. As a result, the incident radiation can be trapped inside the non-Hermitian system and, eventually, can be efficiently absorbed there. We realize this idea in non-Hermitically modulated thin films. The modulation consists of the Hermitian part, i.e., the periodic corrugation of the surfaces of a thin film, and a non-Hermitian part, i.e., the modulation of losses along the film. We prove numerically and demonstrate experimentally that the incident radiation, coupled with such a non-Hermitian thin film, is unidirectionally trapped into a planar mode of the film, which does not escape from the film (or escapes weakly due to experimental imperfections), and is efficiently absorbed there.

We report a sequential two-step vapor deposition process for growing mixed-dimensional van der Waals (vdW) materials, specifically $\mathrm{Te}$ nanowires (1D) and ${\mathrm{Mo}\mathrm{S}}_2$ (2D), on a single ${\mathrm{Si}\mathrm{O}}_2$ wafer. Our growth technique offers a unique potential pathway to create large-scale, high-quality, defect-free interfaces. The assembled samples serve a twofold purpose: first, the as-prepared heterostructures ($\mathrm{Te}\mathrm{NW}/{\mathrm{Mo}\mathrm{S}}_2$) provide insights into the atomically thin depletion region of a 1D/2D vdW diode, as revealed by electrical transport measurements and density-functional-theory-based quantum transport calculations. The charge transfer at the heterointerface is confirmed using Raman spectroscopy and Kelvin probe force microscopy. We also observe modulation of the rectification ratio with varying applied gate voltage. Second, the nonhybrid regions on the substrate, consisting of the as-grown individual $\mathrm{Te}$ nanowires and ${\mathrm{Mo}\mathrm{S}}_2$ microstructures, are used to fabricate separate $p$- and $n$-type field-effect transistors (FETs), respectively. Furthermore, the ionic-liquid gating helps to realize a low-power complementary metal-oxide semiconductor (CMOS) inverter and all basic logic-gate operations using a pair of $n$- and $p$-FETs on the $\mathrm{Si}$/${\mathrm{Si}\mathrm{O}}_2$ platform. This approach also demonstrates the potential for unifying diode and CMOS circuits on a single platform, opening opportunities for integrated analog and digital electronics.

We present a theory for optical binding that incorporates second- and third-order nonlinear optical interactions, extending beyond the linear optical binding models typically considered in previous work. By introducing nonlinear optical interactions, we reveal previously unseen stable equilibrium configurations that fundamentally reshape the spatial organization of bound particle arrays. This nonlinear optical binding regime offers greater tunability and external control, enabling dynamically reconfigurable nanomaterials with adaptable structural properties.

In this article, we propose a method for generating single microwave photons in superconducting circuits. We theoretically show that pure single microwave photons can be generated on demand and tuned over a large frequency band by making use of Landau-Zener transitions under a rapid sweep of a control parameter. We devise a protocol that enables fast control of the frequency of the emitted photon over two octaves, without requiring extensive calibration. Additionally, we make theoretical estimates of the generation efficiency, tunability, purity, and linewidth of the photons emitted using this method for both charge- and flux-qubit-based architectures. We also provide estimates of the optimal device parameters required for these architectures to realize the device.

The ability to efficiently infer system parameters is essential in any signal-processing task that requires fast operation. In quantum systems, a serious challenge arises due to substantial growth of the underlying Hilbert space with the system size. As the statistics of the measurement data observed—i.e., the likelihood—can no longer be easily computed, common approaches such as maximum-likelihood estimators or particle filters become impractical. To address this issue, we propose the use of the approximate Bayesian computation (ABC) algorithm, which evades likelihood computation by sampling from a library of measurement data—a priori prepared for a given quantum device. We apply ABC to interpret photodetection click patterns arising from real-time probing of a two-level atom and an optomechanical system. For the latter, we consider both linear and nonlinear regimes to show how to tailor the ABC algorithm by understanding the quantum measurement statistics. Our work demonstrates that fast parameter inference may be possible regardless of the complexity of the quantum device and the measurement scheme involved.

Photon entanglement is indispensable for optical quantum technologies. Measurement-based optical quantum computing and all-optical quantum networks rely on multiphoton cluster states consisting of indistinguishable entangled photons. A promising method for creating such cluster states on demand is spin-photon entanglement using the spin of a resident charge carrier in a quantum dot, precessing in a weak external magnetic field. In this work, we show theoretically and experimentally that spin-photon entanglement is strongly affected by the hidden anisotropy of quantum dots, which can arise from mechanical stress, shape anisotropy and even specific crystal structure. In the measurements of time-resolved photoluminescence and the cross-polarized second-order photon correlation function in a magnetic field, the anisotropy manifests itself in the spin dynamics and, as a consequence, in the spin-photon concurrence. The measured time-filtered spin-photon Bell state fidelity depends strongly on the excitation polarization and reaches an extremely high value of 94% at maximum. We specify the magnetic field and excitation polarization directions that maximize spin-photon entanglement and thereby enhance the fidelity of multiphoton entangled states.

Precise control of spatial spin structures, such as spin helices, is critical for advancing spintronic devices, particularly in nonvolatile low-power information storage and processing. Conventional techniques, including transient spin-grating spectroscopy and spatial- and time-resolved Kerr rotation microscopy, are limited by fixed optical grating periods and uniform light polarization, respectively, which constrain the flexibility of spin helix generation. Here, we introduce an alternative approach utilizing programmable structured light to directly imprint spatial spin structures in a $\mathrm{Ga}\mathrm{As}$/$(\mathrm{Al},\mathrm{Ga})\mathrm{As}$ quantum well. This method allows for precise control over the wave number and configuration of the spin helices, overcoming the limitations of previous techniques. Experiments conducted using pump-probe Kerr rotation microscopy combined with a programmable spatial light modulator revealed the efficient and tunable generation of spin helices. This approach is broadly applicable not only to semiconductors but also to magnetic thin films and two-dimensional materials.

Quantum key distribution (QKD) is a promising technology for secure communication. Nevertheless, QKD is still treated with caution in certain contexts due to potential gaps between theoretical models and actual QKD implementations. A common assumption in security proofs is that the detection probability at the receiver, for a given input state, is independent of the measurement basis, which might not always be verified and could lead to security loopholes. This paper presents a security proof for QKD protocols that does not rely on the aforementioned assumption and is thus applicable in scenarios with detection probability mismatches, even when induced by the adversary. We demonstrate, through simulations, that our proof can extract positive key rates for setups vulnerable to large detection probability mismatches. This is achieved by one monitoring whether an adversary is actively exploiting such vulnerabilities, instead of considering the worst-case scenario as in previous proofs. Our work highlights the importance of accounting for basis-dependent detection probabilities and provides a concrete solution for improving the security of practical QKD systems.

Artificial spin ices are metamaterials composed of interacting nanomagnets exhibiting frustration. Their resonant magnetization dynamics have been broadly investigated from fundamental and applied points of view. In this work, we realize a dynamically driven macroscopic mechano-magnetic artificial spin ice, or macro-ASI, where permanent magnets are allowed to rotate on specially designed hinges and exhibit natural resonance frequencies on the order of several hertz. A nonlinear dynamical regime is achieved experimentally and well reproduced by numerical modeling. The modulation of the magnetic coupling leads to a frequency comb that manifests itself as an amplitude-phase modulation of the magnets’ motion due to a metastable condition, i.e., a Hopf bifurcation. Our results not only demonstrate a striking similarity across different physical systems, but also suggest that the mechanism to enable nonlinear phenomena could be realized in nanoscale systems using microresonators decorated with magnetic materials to dynamically modulate their coupling.

We present a scalable scheme to achieve photon distillation, i.e., the preparation of a single photon with reduced indistinguishability error out of multiple imperfect photons, based on multiphoton interference in Fourier matrices. Our scheme achieves arbitrary error reduction in a single step, removing the need to concatenate multiple rounds of the protocol. Our scheme has resource costs linear in the error reduction, and a constant probability of success for arbitrary error reductions. We find that the scheme both raises the threshold of error correcting codes in photonic quantum computers, as well as reduces the resources required to construct a photonic quantum computer, making it highly promising for implementation in such devices.

We introduce state-of-the-art protocols to distill indistinguishable photons, reducing distinguishability error rates by a factor of $n$, while using a modest amount of resources scaling only linearly in $n$. Our resource requirements are significantly lower and the protocols have fewer hardware requirements than in previous works, making large-scale distillation experimentally feasible for the first time. This efficient reduction of distinguishability error rates has direct applications to fault-tolerant linear optical quantum computation, potentially leading to improved thresholds for photon loss errors and allowing smaller code distances, thus reducing overall resource costs. Our protocols are based on Fourier transforms on finite Abelian groups, special cases of which include the discrete Fourier transform and Hadamard matrices. This general perspective allows us to unify previous results on distillation protocols and introduce a large family of efficient schemes. We utilize the rich mathematical structure of Fourier transforms, including symmetries and related suppression laws, to quantify the performance of these distillation protocols both analytically and numerically. Finally, our work resolves an open question concerning suppression laws for the $n$-photon discrete Fourier transform: the suppression laws are exactly characterized by the well-known zero transmission law if and only if $n$ is a prime power.

Spins associated to solid-state color centers are a promising platform for investigating quantum computation and quantum networks. Recent experiments have demonstrated multiqubit quantum processors, optical interconnects, and basic quantum error-correction protocols. One of the key open challenges towards larger-scale systems is to realize high-fidelity universal quantum gates. In this work, we design and demonstrate a complete high-fidelity gate set for the two-qubit system formed by the electron and nuclear spin of a nitrogen-vacancy center in diamond. We use gate set tomography (GST) to systematically optimize the gates and demonstrate single-qubit gate fidelities of up to $99.999(1)\mathrm{\%}$ and a two-qubit gate fidelity of $99.93(5)\mathrm{\%}$. Our gates are designed to decouple unwanted interactions and can be extended to other electron-nuclear spin systems. The high fidelities demonstrated provide opportunities towards larger-scale quantum processing with color-center qubits.

Power limiters are essential devices in modern rf communication systems to protect highly sensitive input channels from large incoming signals. Nowadays-used semiconductor limiters suffer from high electronic noise and switching delays when approaching the GHz range, which is crucial for the modern generation of 5G communication technologies aiming to operate at the EU 5G high band (24.25–27.5 GHz). The proposed solution is to use ferrite-based frequency selective limiters (FSLs), which maintain their efficiency at high GHz frequencies, although they have only been studied at the macroscale so far. In this study, we demonstrate a proof of concept of nanoscale FSLs. The devices are based on spin-wave transmission affected by four-magnon scattering phenomena in a 97-nm-thin yttrium iron garnet (YIG) film. Spin waves were excited and detected using coplanar waveguide (CPW) transducers of the smallest feature size of $250\mathrm{nm}$. The FSLs are tested in the frequency range up to $25\mathrm{G}\mathrm{Hz}$, and the key parameters are extracted (power threshold, power limiting level, insertion losses, bandwidth) for different spin-wave modes and transducer lengths. An analytical theory has been formulated to describe the fundamental physical processes, and a numerical model has been developed to quantitatively describe the insertion losses and power characteristics of the FSLs. Additionally, the perspective of the spin-wave devices is discussed, including the possibility of simultaneously integrating three devices into one: a frequency-selective limiter, an rf filter, and a delay line, allowing for more efficient use of space and energy.

Quantum algorithms implemented on near-term devices require qubit mapping due to noise and limited qubit connectivity. In this paper we propose a strategy called algorithm-oriented qubit mapping (AOQMAP) that aims to bridge the gap between exact and scalable mapping methods by utilizing the inherent structure of algorithms. While exact methods provide optimal solutions, they become intractable for large circuits. Scalable methods, like swap networks, offer fast solutions but lack optimality. AOQMAP bridges this gap by leveraging algorithmic features and their association with specific device substructures to achieve depth-optimal and scalable solutions. The proposed strategy follows a two-stage approach. First, it maps circuits to subtopologies to meet connectivity constraints. Second, it identifies the optimal qubits for execution using a cost function and performs postselection among execution results across subtopologies. Notably, AOQMAP provides both scalable and optimal solutions for variational quantum algorithms with fully connected two-qubit interactions on common subtopologies including linear, T-, and H-shaped, minimizing circuit depth. Benchmarking experiments conducted on IBM quantum devices demonstrate significant reductions in gate count and circuit depth compared to Qiskit, Tket, and swap network. Specifically, AOQMAP achieves up to an 82% reduction in circuit depth and an average 138% increase in success probability. This scalable and algorithm-specific approach holds the potential to optimize a wider range of quantum algorithms.

Achieving precise and adjustable control over laser frequency is an essential requirement in numerous applications such as precision spectroscopy, quantum control, and sensing. In many such applications it is desired to stabilize a laser with a variable detuning from an atomic line. In this study, we employ an offset-stabilization scheme by utilizing phase contrast spectroscopy in microfabricated atomic diffractive element vapor cells. The spectroscopic response of such a device generates oscillating optical fringes, providing multiple optical frequency stabilization points across a bandwidth of tens of gigahertz, centered around the absorption resonances of $\mathrm{Rb}$. Using this device, we demonstrate laser stabilization at various offset frequencies with instabilities reaching submegahertz levels. We further explore the fundamental limitations of our hybrid atomic-photonic device, drawing parallels to birefringent and dichroic spectroscopy apparatuses, which are commonly employed for offset stabilization. Our system showcases a broad offset lock bandwidth, a highly compact footprint, scalability to chip-scale production, and the ability to operate without reliance on magnetic fields. These attributes pave the way for a multitude of applications in quantum technologies.

Magnetometry using nitrogen-vacancy (N-$V$) color centers in diamond predominantly relies on microwave spectroscopy. However, microwaves may hinder certain studies involving biological systems or thin conductive samples. This work demonstrates a wide-field, microwave-free imaging magnetometer utilizing N-$V$ centers in nanodiamonds by exploiting the cross-relaxation feature near zero magnetic fields under ambient conditions without applying microwaves. For this purpose, we measure the center shift, contrast, and linewidth of zero-field cross relaxation in 140-nm nanodiamonds dropcast on a current-carrying conductive pattern while scanning a background magnetic field, achieving a sensitivity of $4.5\text{μ}\mathrm{T}/\sqrt{\mathrm{Hz}}$. Our work allows for applying the N-$V$ zero-field feature in nanodiamonds for magnetic-field sensing in the zero- and low-field regimes and highlights the potential for microwave-free all-optical wide-field magnetometry based on nanodiamonds.

Dissipative stabilization of cat qubits autonomously suppresses for bit-flip errors by ensuring that reservoir-engineered two-photon losses dominate over other mechanisms, inducing phase-flip errors. To describe the latter, we derive an effective master equation for an asymmetrically threaded superconducting quantum interference device–based superconducting circuit used to stabilize a dissipative cat qubit. We analyze the dressing of relaxation processes under drives in time-dependent Schrieffer-Wolff perturbation theory for weakly anharmonic bosonic degrees of freedom and in numerically exact Floquet theory. We find that spurious single-photon decay rates can increase under the action of the parametric pump that generates the required interactions for cat-qubit stabilization. Our analysis feeds into mitigation strategies that can inform current experiments, and the methods presented here can be extended to other circuit implementations.

Cavity magnonics is a promising field focusing on the interaction between spin waves (magnons) and other types of signal. In cavity magnonics, isolation of magnons from the cavity to allow signal storage and processing fully in the magnonic domain is highly desired, but its realization is often hindered by the lack of necessary tunability of the interaction. This work shows that by using the collective mode of two yttrium iron garnet spheres and applying Floquet engineering, magnonic signals can be switched on demand to a magnon dark mode that is protected from the environment, enabling a variety of manipulation over the magnon dynamics. Our demonstration can be scaled up to systems with an array of magnonic resonators, paving the way for large-scale programmable hybrid magnonic circuits.

We investigate the existence of interface states induced by broken inversion symmetries in two-dimensional quasicrystal (QC) lattices. We introduce a 10-fold rotationally symmetric QC lattice whose inversion symmetry is broken through a mass dimerization that produces two fivefold symmetric sublattices. By considering resonator scatterers attached to an elastic plate, we illustrate the emergence of bands of interface states that accompany a band inversion of the QC spectrum as a function of the dimerization parameter. These bands are filled by modes which are localized along domain wall interfaces separating regions of opposite inversion symmetry. These features draw parallels to the dynamic behavior of topological interface states in the context of the valley Hall effect, which has so far been limited to periodic lattices. We numerically and experimentally demonstrate waveguiding in a QC lattice featuring a zigzag interface with sharp turns of ${36}^{\circ }$, which goes beyond the limitation of ${60}^{\circ }$ associated with sixfold symmetric (i.e., honeycomb) periodic lattices. Our results provide new opportunities for symmetry-based quasicrystalline topological waveguides that do not require time-reversal symmetry breaking, and that allow for higher freedom in the design of their waveguiding trajectories by leveraging higher-order rotational symmetries.

One method for scaling up quantum systems is to adopt a modular approach. In the ion-trap architecture, an efficient photonic interface between independent linear ion traps would allow for such an expansion. To this end, an optical cavity with a small mode volume can be used to enhance the photon-emission probability from the ion. Miniature fiber-based Fabry-Perot cavities have been integrated into three-dimensional Paul traps that hold a single ion, but an efficient interface between an optical cavity and a linear trap that can keep multiple ions has remained elusive. This presents a barrier for combining the benefits of the motional coupling in a chain of ions with an optical interface between ion traps. In this paper, we show that simple electrically conductive shielding of the fibers could provide a substantial advantage in mitigating the adverse effects of stray charges and motional heating by dielectrics. We also reveal that while conductive shields are not compatible with the conventional radio frequency (rf) drives in ion traps, using two rf signals with opposite phases can solve this issue. Furthermore, the role played by the symmetry of the electrodes when incorporating an element that disrupts the translational symmetry of a linear trap is elucidated analytically. As a result, it is realized that a two-dimensional implementation of a linear ion trap such as a surface trap is inherently unsuitable for integrating a shielded miniature optical cavity due to the lack of geometrical symmetry. Based on the insights obtained through this analysis, we identify essential components and propose a design strategy for successful integration of a miniature optical cavity into a linear ion trap.