Group IV color centers in diamond are among the most promising optically active electron-spin systems with strong optical transitions and long spin coherences. The orbital ground-state splitting of the center is particularly important to suppress the interaction with coherence-limiting phonons. A higher orbital ground-state splitting improves the spin-coherence properties and sets the upper limit for the operating temperature. Negatively charged silicon-vacancy centers have an ordinary ground-state splitting from spin-orbit coupling of only $48\mathrm{G}\mathrm{Hz}$, resulting in required temperatures below 1 K, which can only be achieved by dilution refrigerators. Here, we increase the orbital ground-state splitting by up to an order of magnitude by induced strain in a single-crystal diamond membrane. The characterized orbital ground-state splitting in highly strained regions could allow for electron-spin-coherence times of milliseconds at $1.5\mathrm{K}$. We demonstrate cavity-assisted spectroscopy enabled by coupling the emitter ensemble with a selectable strain to the mode of a Fabry-Perot microcavity. Calculation of the absorption cross section yields ${\sigma }_{\mathrm{abs}}^{\mathrm{ens}}\approx 4.9\cdot {10}^{-11}{\mathrm{cm}}^2$. Furthermore, temperature-dependent lifetime measurements reveal a Purcell enhanced twofold reduction in emitter lifetime below $1\mathrm{ns}$ at $4\mathrm{K}$.

We present a comprehensive investigation of quantum transport in silicon nanowire field-effect transistors (SiNWFETs) at the scaling limit. The $\mathrm{Si}$ bulk $\mathbf{k}\cdot \mathbf{p}$ Hamiltonian parameters are rendered invalid at smaller scales due to pronounced quantum confinement effects. Consequently, nanowire $\mathbf{k}\cdot \mathbf{p}$ Hamiltonian parameters are meticulously calibrated with the use of first-principles HSE06 band structures through the fast least-squares method. On the basis of the nonequilibrium Green’s function formalism, we then systematically analyze the performance limits of gate-all-around SiNWFETs under various gate lengths. With diminishing gate lengths, quantum tunneling from source to drain intensifies, leading to a degradation in subthreshold swing. Our findings reveal that the gate-length scaling limit for $n$-type devices is numerically larger than for $p$-type devices, and distinct gate scaling limits are elucidated for various cross-section sizes; for instance, the ultimate scaling limit of $\mathrm{Si}$ nanowires of $4\times 4{\mathrm{nm}}^2$ cross section is approximately 10 nm.

The design and optimization of quantum codes is generally hard because of the prohibitive computational cost. The recently introduced quantum lego framework provides a powerful method for generating complex quantum error-correcting codes (QECCs) out of simple ones. We gamify this process and unlock a new avenue for code design and discovery using reinforcement learning (RL). Already for simple code measures and modest qubit numbers, we produce codes that are both optimal and novel. Moreover, we argue that this framework is both scalable, when combined with efficient tensor contraction methods, and flexible, since we can specify arbitrary properties of the code to be optimized. We train on two such properties, maximizing the code distance, and minimizing the probability of logical error under biased Pauli noise. For the first, we show that the trained agent identifies ways to increase code distance beyond naive concatenation, saturating the linear programming bound for CSS (Calderbank, Shor, Steane) codes on 13 qubits. With a learning objective to minimize the logical error probability under biased Pauli noise, we find the best-known CSS code at this task for $\lesssim 20$ qubits. Compared to other (locally deformed) CSS codes, including Surface, XZZX, and two-dimensional Color codes, our $[[17,1,3]]$ code construction actually has lower adversarial distance, yet better protects the logical information, highlighting the importance of QECC desiderata. Lastly, we comment on how this RL framework can be used in conjunction with physical quantum devices to tailor a code without explicit characterization of the noise model.

In the search for axionic dark matter, the high-frequency part of the quantum chromodynamics (QCD) axion parameter space is favored, as indicated by both cosmological and astrophysical arguments and recent indications from lattice QCD calculations. To extend the probing range of cavity haloscopes, solutions addressing the unfavorable scaling of cavity volume with frequency must be developed. Here, we present a novel type of high-volume thin-shell resonator for high-frequency haloscope dark-matter searches. The cavity is formed by two nested and coaxial right-angled polygonal prisms enclosed within two flat endcaps. For the axion-sensitive (pseudo-)${\mathrm{TM}}_{010}$ mode at about 10 GHz, finite-element simulations yield a form factor on the order of 0.8 and a $Q$ factor on the order of 60 000 for a copper cavity at 4 K. Reasonable tunability of up to $\sim 5\mathrm{\%}$ is achieved by reciprocal rotation of the two prisms, without relevant changes in haloscope sensitivity. A prototype aluminum hexagonal cavity was built and tested, confirming the main characteristics of the design.

Qudits provide a resource-efficient alternative to qubits for quantum information processing. The multilevel nature of the transmon, with its individually resolvable transition frequencies, makes it an attractive platform for superconducting circuit-based qudits. In this work, we systematically analyze the trade-offs associated with encoding high-dimensional quantum information in fixed-frequency transmons. Designing high $E_J/E_C$ ratios of up to 325, we observe up to 12 levels ($d=12$) on a single transmon. Despite the decreased anharmonicity, we demonstrate process infidelities $e_f<3\times {10}^{-3}$ for qubitlike operations in each adjacent-level qubit subspace in the lowest ten levels. Furthermore, we achieve a ten-state readout assignment fidelity of 93.8% with the assistance of deep neural network classification of a multitone dispersive measurement. We find that the Hahn echo time $T_{2E}$ for the higher levels is close to the limit of $T_1$ decay, primarily limited by bosonic enhancement. We verify the recently observed Josephson harmonic model, finding that it yields better predictions for the transition frequencies and charge dispersion. Finally, we show strong $ZZ$-like coupling between the higher energy levels in a two-transmon system. Our high-fidelity control and readout methods, in combination with our comprehensive characterization of the transmon model, suggest that the high-$E_J/E_C$ transmon is a powerful tool for exploring excited states in circuit quantum electrodynamics.

Spin-wave reservoir computing (RC) utilizes spatiotemporal dynamics of spin waves in a magnetic thin film as a physical reservoir to realize effective neuromorphic computation. Because of the complicated nature of spin-wave dynamics, the spin-wave RC systems exhibit intrinsic properties that are potentially different from software-based RC systems, such as echo-state networks, thus making these two systems suitable for different application contexts. Therefore, a deeper understanding of spin-wave dynamics and their role in neuromorphic computing systems is crucial for more effective system design and practical applications. Here, we construct an analytic-signal-based input-output model for spin-wave RC systems. The proposed model allows for the direct construction of system responses without the need for physical experiments or numerical simulations, given knowledge of the system’s standard responses under low input power. Inspired by our model, we refine the spin-wave RC system by proposing an output block based on passband signal learning to exploit the wave-based characteristics of the system responses. We evaluate passband signal learning in the context of multipath interference (MPI) mitigation. Our results demonstrate that spin-wave RC systems with passband signal learning can more effectively solve the MPI mitigation problem, owing to similar inherent properties between the spin-wave RC systems and the communication channels. We expect this work to provide deep insights into spin-wave physics and similar spin-wave-based neuromorphic computing systems.

We provide results of frequency-selective spin-wave channeling in a bilayer magnonic crystal structure. We report the results of an experimental and numerical study of the propagation of spin waves in a multilayer in-plane-magnetized magnonic crystal structure, which consists of two layers of yttrium iron garnet films located one above the other. Each layer has a lateral system of spin-wave waveguide channels separated by an array of grooves in the form of a one-dimensional magnonic crystal on the surface of yttrium iron garnet. The regimes of frequency-selective signal separation inside the evolved channels in either the bottom layer or the upper layer were revealed by the use of Brillouin light scattering and microwave spectroscopy. The simultaneous effect of magnonic band gap formation and dipolar coupling of propagated spin waves reveals itself in the localized spin-wave beam formation and propagation along one of the four channels. This mechanism was investigated by a numerical simulation based on the electrodynamic three-dimensional model and semianalytical eigenmode composition along the micromagnetic simulation. It is demonstrated that the proposed method of spin-wave transport control can find application in the fabrication of demultiplexers and magnonic logic elements based on the principles of spatial-frequency selection of a microwave signal in multilayer topologies of magnonic devices.

Natural frequency tuning is a critical aspect of superconducting gravimeter adjustment. The coupling of the natural frequency tuning with the levitation position of the test mass in our previous design complicates the natural-frequency-adjustment process. We introduce a new magnetic spring oscillator structure, along with novel superconducting circuits. By employing symmetric solenoids in this structure, the positional shift of the test mass is indicated by the excitation response, enabling the optimal levitation position to be identified. The levitation position remains nearly unchanged when the natural frequency is adjusted at this position. This approach decouples natural frequency tuning from position regulation. The experimental results confirm the feasibility and efficiency of this method, with the natural frequency adjusted from 11.9 to 1.7 Hz. Over ten days of solid tide observations demonstrated the reliable performance of the superconducting gravimeter.

Can isotropic magnetoactive elastomers (MAEs) undergo giant magnetically induced deformations and exhibit huge magnetorheological effects simultaneously? In this experimental and theoretical study, we reveal how the macroscopic deformation of MAEs relates to the process of particle restructuring caused by application of a magnetic field. For this purpose, MAE cylinders with different aspect ratios and particle loadings are studied in uniform magnetic fields. The axial deformations of the cylinders are acquired using an optical camera. A unified mean-field model proposed in previous studies is adapted to describe the transition of initially isotropic cylinders into transversely isotropic ones. This mechanical transition is caused by the rearrangement of particles into dense columnar structures aligned with the field and is believed to result in a huge magnetorheological effect. Our model however predicts less than a threefold increase in elastic moduli when evaluated along the field direction. This prediction is based on a careful examination of the shear moduli of studied MAEs and the columnar structures. A weak magnetorheological effect explains significant axial deformations measured in the field direction. A strong magnetorheological effect would hinder axial deformations due to an increase in the modulus by several orders of magnitude. Not only are the moduli and macroscopic deformations influenced by microstructure evolution, but so is the magnetization of particles, which increases as they rearrange into dense columns. With this study, we show that the unified mean-field model provides quantitative access to hidden material properties such as magnetization and stiffness in MAE samples with different shapes and evolving microstructures.

A Sagnac-loop structure can help overcome the major difficulty in the practical implementation of a twin-field quantum key distribution (TFQKD) network, namely, the need to stabilize the phase of a quantum state over many kilometers of fiber. Unfortunately, Rayleigh backscattering noise limits the signal-to-noise ratio for Sagnac systems containing long fibers and lossy photonic devices. Here, we solve this problem by sending optical pulses in long on-off bursts and using time postselection on measurements taken with free-run single-photon avalanche detectors. We also investigate the impact of the residual phase noise uncompensated by the Sagnac structure and find that the variance of the phase noise scales as loop length to the third power, verifying an existing calculation in the literature. We measure the interference visibility in Sagnac loops of varying length without active phase stabilization and achieve $>97\mathrm{\%}$ visibility in 200-km ultralow-loss fiber, which is, to our knowledge, the longest fiber Sagnac interferometer demonstrated. Our results suggest that a Sagnac system is feasible for long-distance TFQKD networks, a key step towards the practical implementation of metropolitan quantum networks.

An important goal of spintronics research is to discover efficient methods of spin current generation. The symmetry conditions generally restrict spin polarizations to be orthogonal to both charge and spin flows in heavy metals. In contrast, the combination of time-reversal and specific structural symmetries breaking in noncollinear antiferromagnets with chiral spin structures represented by ${\mathrm{Mn}}_3\mathrm{Sn}$ permits the generation of spin currents with different orientations. Furthermore, the interfacial-generated spin current has proven to have strengths similar to those generated by bulk heavy metals. Here, we report efficient spin-orbit torques (SOT) in the ${\mathrm{Mn}}_3\mathrm{Sn}/\mathrm{Pt}/$ferromagnets multilayers, in which the ${\mathrm{Mn}}_3\mathrm{Sn}/\mathrm{Pt}$ bilayer system acts as an efficient spin current source due to giant interfacial spin-orbit scattering. Through SOT-induced magnetization switching, spin-torque ferromagnetic resonance, and second harmonic Hall resistance measurements, a robust spin current with x- and y-polarizations (${\sigma }_x$ and ${\sigma }_y$, respectively) generated in the ${\mathrm{Mn}}_3\mathrm{Sn}/\mathrm{Pt}/$ferromagnets multilayers is confirmed. Furthermore, all the results reveal magnetic octupole-dependent characteristics. As the current is applied along the magnetic octupole direction of the ${\mathrm{Mn}}_3\mathrm{Sn}$ layer, the robust ${\sigma }_x$ spin current results in a zero-field partial magnetization switching in the ${\mathrm{Mn}}_3\mathrm{Sn}/\mathrm{Pt}/\mathrm{Co}$ multilayers. Compared with the single ${\mathrm{Mn}}_3\mathrm{Sn}$ layer, the spin Hall angle (SHA) dramatically changes in the ${\mathrm{Mn}}_3\mathrm{Sn}/\mathrm{Pt}$ bilayers. The temperature-dependent SHA indicates that the large spin current generation efficiency in the ${\mathrm{Mn}}_3\mathrm{Sn}/\mathrm{Pt}$ bilayers is much larger than the single $\mathrm{Pt}$ layer, which is determined by the combination of magnetic order and giant interfacial spin-orbit scattering. Our work illustrates that the noncollinear antiferromagnet/heavy metal bilayer systems present an ideal spin current source characteristic.

High-dimensional quantum entanglement is an essential resource in quantum technology since it provides benefits for increasing the information capacity and processing speed. Thus, the controlled harnessing of high-dimensional entanglement has long been hailed as a necessary prerequisite towards practical quantum applications. By using a deterministic quantum state filter implemented through quantum interference, we present a generalized formulation for the complete high-dimensional symmetric and antisymmetric Bell basis and experimentally prepare four-dimensional orbital angular momentum Bell states that provide the well-behaved symmetric or antisymmetric properties. Additionally, we use a concise yet efficient scan of temporal delay to directly observe high-dimensional two-photon interference effects in spatial modes. These results provide an alternative way to harness high-dimensional entanglement and may facilitate the use of quantum interference for more complex quantum information processing tasks beyond qubits.

High-brilliance x-ray sources from synchrotron radiation or free-electron lasers are widely applied across disciplines, including scientific research and industry. To reduce the cost and footprint of synchrotron or free-electron light sources, a microundulator utilizing microscale-ordered electromagnetic fields offers an innovative approach. Here, using 3D particle-in-cell (PIC) simulations, we demonstrate that the amplified quasiperiodic magnetic field arrays generated in the interactions of counterstreaming flows can give rise to a novel undulatorlike light source. The magnetic field arrays, which are amplified to ${10}^4\mathrm{T}$ by the Weibel instability, cause transverse oscillation of the electrons to form a helical undulator or wiggler with periods in the micrometer scale. We show that the growth rate, intensity, and structure of the magnetic field arrays are tunable by varying the density and energy of the injected and background electrons in the plasma. This flexibility of modification enables the production of compact and ultrabroadband radiation sources ranging from ultraviolet to x-ray with photon energy in hundreds of electron volts and a peak brilliance above $5\times {10}^{22}\mathrm{photons}{\mathrm{s}}^{-1}\mathrm{m}{\mathrm{m}}^{-2}\mathrm{mra}{\mathrm{d}}^{-2}$ per $0.1\mathrm{\%}$ bandwidth.

Achieving high-fidelity two-qubit gates is a key requirement for the realization of large-scale quantum computation and simulation. In this paper, we propose a microwave-control protocol for the implementation of a two-qubit gate employing two transmon qubits coupled via a fixed-frequency transmon coupler. This protocol entails applying two microwave pulses exclusively to the coupler, thereby inducing interaction between fixed-frequency transmon qubits. This interaction facilitates the realization of $\sqrt{i\mathrm{SWAP}}$ gates. Additionally, we explore the implementation of the gate scheme in two distinct qubit architectures (ABA and ABC). Without considering decoherence, the ABA architecture $\sqrt{i\mathrm{SWAP}}$ gate has a fidelity of 94.60% in 346 ns, while the ABC architecture achieves 99.63% in 378 ns. When considering the implementation of the $\sqrt{i\mathrm{SWAP}}$-like gate, we show that gate fidelity can exceed 99.9% in both architectures.

The reconstruction of electrical current densities from magnetic field measurements is an important technique with applications in materials science, circuit design, quality control, plasma physics, and biology. Analytic reconstruction methods exist for planar currents, but break down in the presence of high-spatial-frequency noise or large standoff distance, restricting the types of systems that can be studied. Here, we demonstrate the use of a deep convolutional neural network for current density reconstruction from two-dimensional images of vector magnetic fields acquired by a quantum diamond microscope . Trained network performance significantly exceeds analytic reconstruction for data with high noise or large standoff distances. This machine learning technique can perform quality inversions on lower-signal-to-noise-ratio data, significantly reducing the data collection time and permitting reconstructions of weaker and three-dimensional current sources.

We investigate a higher-order topological behavior in elastic lattices characterized by nonsymmorphic symmetries. In the theoretical spring-mass lattice, altering the vertex mass allows for fine tuning of the topological features within the band gap. We analyze the quadrupole topological behavior in square lattices with nonsymmorphic symmetries using nested Wannier bands. Beyond second-order topological metamaterials, a single-phase topological configuration promotes energy localization at the corners due to a nonzero relative quadrupole moment. Our findings are validated through experimental observations of higher-order topological corner states, which show excellent agreement with simulated results and theoretical predictions. Additionally, the elastic lattices in the self-similar system exhibit fractal higher-order topological behaviors, revealing numerous topological edge and corner states. The self-similar lattice also demonstrates enhanced energy localization, with the number of topological states showing a linear correlation to the corner dimension. This study provides insights into elastic higher-order topological insulators and inspires innovative strategies for simulating topological elastic materials.