Magnetosensitive spin-correlated radical pairs offer a promising platform for noise-robust quantum metrology; however, unavoidable interradical interactions, such as electron-electron dipolar and exchange couplings, alongside deleterious perturbations resulting from intrinsic radical motion, typically degrade their potential as magnetometers. In contrast, we show how structured molecular motion modulating interradical interactions in a live chemical sensor in cryptochrome can, in fact, increase sensitivity and, moreover, push precision in estimating magnetic field directions closer to the quantum Cramér-Rao bound. The latter is evident both on average over the magnetoreceptor array and, to an even greater extent, for the subset of best-performing magnetoreceptors, thus suggesting near-optimal metrological performance. Remarkably, this approach to optimality is amplified under environmental noise and persists with increasing complexity of the spin system, suggesting that perturbations inherent to such natural systems have enabled them to operate closer to the quantum limit to more effectively extract information from the weak geomagnetic field. This insight opens the possibility of channeling the underlying physical principles of motion-induced modulation of electron spin-spin interactions toward devising efficient handles over emerging molecular quantum information technologies.

It is shown that avalanches of runaway electrons can arise in stellarators, even if there is no net toroidal current in the plasma or the magnetic-field coils, if the current in the latter varies rapidly enough, e.g., due to a superconductor quench. In present-day devices such as W7-X, significant runaway generation is theoretically possible only at very low gas or plasma density in the vacuum vessel, e.g., between discharges, if a seed population of free electrons is present. In reactor-scale stellarators, more dangerous runaway generation may occur both between discharges and during low-density plasma operation. Since a radiation-induced seed population is necessarily present in an activated device, an accidental coil ramp-down could convert substantial magnetic energy into wall-damaging runaway currents. There is, however, much more time to mitigate such events than in tokamak disruptions.

Using an ab initio evolutionary algorithm combined with first-principles calculations, two metastable ${\mathrm{Na}\mathrm{Mn}\mathrm{O}}_2$ polymorphs, $I{4}_1/amd$ and Cmcm, are identified as promising cathode materials for sodium-ion batteries. Both phases exhibit excellent thermodynamic stability, lying within 35 meV/atom of the ground-state Pmmn phase across 0–50 GPa, and are dynamically and thermally stable under ambient conditions following high-pressure synthesis, as confirmed by phonon and ab initio molecular dynamics simulations. During desodiation, a Jahn-Teller-induced magnetic transition enhances $\mathrm{Mn}$-$\mathrm{O}$ hybridization, reduces the band gap, and promotes robust charge compensation and oxygen retention. Remarkably, the Cmcm phase achieves record low ${\mathrm{Na}}^{+}$ migration barriers (0.39 eV at high $\mathrm{Na}$ concentration; 0.28 eV at low concentration), representing 47% and 33% reductions, respectively, compared with conventional $C2/m$, while delivering a higher average voltage (3.19 vs 2.88 V). The $I{4}_1/amd$ phase exhibits concentration-dependent diffusion with a low-energy pathway (0.38 eV) and maintains a competitive voltage (2.94 V). These findings suggest that metastable ${\mathrm{Na}\mathrm{Mn}\mathrm{O}}_2$ polymorphs may offer viable alternatives to conventional cathode materials, particularly where fast ionic conduction is required.

Reliable control of quantum information in matter-based qubits requires precisely applied external fields, and unaccounted-for spatial crosstalk of these fields between adjacent qubits leads to loss of fidelity. We report a CMOS foundry-produced microfabricated silicon nitride (${\mathrm{Si}}_3$${\mathrm{N}}_4$) optical waveguide for addressing a chain of eight unequally spaced trapped barium ions, with crosstalk compatible with scalable quantum information processing. The crosstalk-mitigation techniques incorporated into the chip design result in a reduction of the measured optical field by at least 50.8(1.3) dB between adjacent waveguide outputs near 650 nm and similar behavior for devices designed for 493 nm and 585 nm. The waveguide outputs near 650 nm, along with a global laser near 493 nm, have been used to laser cool a chain of eight ${}^{138}$$\mathrm{Ba}$${}^{+}$ ions, and a camera has imaged the resulting fluorescence at 493 nm.

Three-dimensional topological insulators possess topologically protected surface states with spin-momentum locking, which enable spin-charge-current interconversion (SCI) by the inverse Edelstein effect (IEE). However, it remains experimentally challenging to separate the surface-related IEE from the bulk-type inverse spin Hall effect (ISHE). Here we search for distinct time-domain signatures of the two SCI phenomena in an $\mathcal{F}$/TI model stack of a ferromagnetic-metal layer $\mathcal{F}$ ($\mathrm{Co}$ and $\mathrm{Fe}$) and a topological-insulator (TI) layer (${\mathrm{Bi}}_2{\mathrm{Te}}_3$, ${\mathrm{Sn}\mathrm{Bi}}_2{\mathrm{Te}}_4$, and ${\mathrm{Bi}}_{1-x}{\mathrm{Sb}}_x$ with x = 0.15 and x = 0.3), where the focus is on ${\mathrm{Bi}}_2{\mathrm{Te}}_3$. A femtosecond laser pulse serves to induce a transient spin voltage ${\mu }_{\mathrm{s}}^{\mathcal{F}}$ in $\mathcal{F}$ and, thus, drive an ultrafast spin current out of $\mathcal{F}$. SCI results in a transverse charge current with a sheet density $I_{\mathrm{c}}$ that is detected by sampling of the emitted terahertz electric field. Analysis of the dynamics of $I_{\mathrm{c}}$(t) vs time t relative to ${\mu }_{\mathrm{s}}^{\mathcal{F}}(t)$ reveals two components with distinct timescales: (1) a quasi-instantaneous response and (2) a longer-lived response with a relaxation time of 270 fs, which is independent of the $\mathcal{F}$ material chosen. Component (1) is consistently ascribed to the ISHE. In contrast, we interpret component (2) as a signature of interfacial spin accumulation and the IEE at the $\mathcal{F}$/${\mathrm{Bi}}_2{\mathrm{Te}}_3$ interface, with a fraction of less than ${10}^{-2}$ of the incident spins participating. This assignment is fully consistent with respect to its dynamics and magnitude. We rate other possible signal contributions, such as spin trapping in intermediate states, as less likely. Our results show that the femtosecond dynamics of photocurrents allow us to differentiate the ISHE and the IEE and, more generally, provide important insights into the mechanisms of spin transport and SCI in $\mathcal{F}$/TI stacks.

The realization of a global quantum network holds the potential to enable groundbreaking applications such as secure quantum communication and blind quantum computing. However, building such a network remains a formidable challenge, primarily due to photon loss in optical fibers. In this work, we propose a quantum repeater architecture for distributing entanglement over intercontinental distances by leveraging low-Earth-orbit satellites equipped with spontaneous parametric down-conversion photon-pair sources and ground stations utilizing single-atom memories in optical cavities and single-photon detectors to implement the cavity-assisted photon scattering gates for high-fidelity entanglement mapping. The efficient entanglement swapping is achieved by performing high-fidelity Rydberg gates and readouts. We evaluate the entanglement distribution rates and fidelities by analyzing several key imperfections, including time-dependent two-photon transmission and time-dependent pair fidelity, for various satellite heights and ground station distances. We also investigate the impact of pair source fidelity, spin decoherence rate, and sky brightness on the repeater performance. Furthermore, we introduce a spatial-frequency multiplexing strategy within this architecture to enhance the design’s performance. Finally, we discuss in detail the practical implementation of this architecture. Our results show that this architecture enables entanglement distribution over intercontinental distances. For example, it can distribute over 10 000 pairs per flyby over 10 000 km with a fidelity above $90\mathrm{\%}$, surpassing the capabilities of terrestrial quantum repeaters.

Laser spectroscopy of the ${}^{229}\mathrm{Th}$ isomeric transition has recently emerged as a field with several groundbreaking advancements. However, the employed vacuum-ultraviolet (VUV) laser sources in these studies are all pulsed, resulting in low spectral power densities that are largely inadequate for coherently driving this extraordinarily weak isomeric transition. Here, we propose generating a cw laser at 148.4 nm using four-wave mixing (FWM) in cadmium vapor. The large transition-matrix elements of cadmium, based on state-of-the-art electron-correlation calculations, and the optimized FWM resonance-enhanced scheme with readily accessible incident laser wavelengths, lead to a giant third-order nonlinear susceptibility. Combined with the high coherence of the FWM process, cadmium vapor emerges as a promising nonlinear medium capable of producing a cw VUV laser with over 30-$\text{μ}\mathrm{W}$ power and a narrow linewidth. This approach addresses the lack of an intense and narrow-linewidth laser near the ${}^{229}\mathrm{Th}$ isomeric transition, allowing for the coherent driving of nuclear Rabi oscillations and advancing the development of the nuclear optical clock.

Two-qubit gates constitute fundamental building blocks in the realization of large-scale quantum devices. Using superconducting circuits, two-qubit gates have been implemented in various ways, with each method aiming to maximize gate fidelity. Another important goal of a new gate scheme is to minimize the complexity of gate calibration. In this work, we demonstrate a high-fidelity two-qubit gate between two fluxonium qubits, enabled by an intermediate capacitively coupled transmon. The coupling strengths between the qubits and the coupler are designed to minimize residual crosstalk while still allowing for fast gate operations. The gate is based on frequency selectively exciting the coupler using a microwave drive to complete a $2\pi$ rotation, conditional on the state of the fluxonium qubits. When successful, this drive scheme implements a conditional phase gate. Using analytically derived pulse shapes, we minimize unwanted excitations of the coupler and obtain gate errors of ${10}^{-2}$ for gate times below 60 ns. At longer durations, the gate performance is limited by relaxation of the coupler. Our results show how carefully designed control pulses can speed up frequency-selective entangling gates.

Scaling up superconducting quantum processors necessitates the use of a high routing density for readout and control lines, relying on low-loss interconnects to maintain design flexibility and device performance. We propose and demonstrate a universal subtractive fabrication process for air bridges based on an aluminum hard mask and niobium as the superconducting film. Using this technology, we fabricate superconducting coplanar waveguide resonators incorporating several niobium air bridges in and across their center conductors. Through rigorous cleaning methods, we achieve mean internal quality factors in the single-photon limit exceeding $Q_{\mathrm{int}}=8.2\times {10}^6$. Notably, the loss per air bridge remains below the detection threshold of the resonators. Due to the larger superconducting energy gap of niobium, compared with conventional aluminum air bridges, our approach enables stable performance at elevated temperatures and magnetic fields, which we experimentally confirm in temperatures up to 3.9 K and in a magnetic field of up to 1.60 T. Additionally, we utilize air bridges to realize low-loss vacuum-gap capacitors and demonstrate their successful integration in transmon qubits by achieving median qubit lifetimes of $T_1=51.6\text{μ}\mathrm{s}$.

Efficient and reliable light coupling between optical fibers and photonic integrated circuits is essential for optical interconnects, nonlinear signal conversion, neuromorphic computing, and quantum information processing. A standard approach involves interfacing inverse tapers with lensed fibers, particularly for waveguides with a moderate refractive index, such as silicon nitride ($\mathrm{Si}$${}_3$$\mathrm{N}$${}_4$), silicon oxynitride, and lithium niobate. While this method offers broad bandwidth and high coupling efficiency, prior research has predominantly focused on taper design, largely neglecting the optimization of the lensed fiber itself. Here, we fill this gap and present a comprehensive co-optimization strategy that synergistically refines the geometries of both the taper and the lensed fiber. By incorporating the genuine shape of the lensed fiber into the simulation, we accurately capture the non-Gaussian emission profile, demonstrating the limitations of the widely used paraxial Gaussian approximation. We characterize a wide range of lensed fibers and $\mathrm{Si}$${}_3$$\mathrm{N}$${}_4$ tapers fabricated via different processes, observing remarkable agreement between experiment and simulation with maximum coupling efficiencies exceeding 80% per facet. Finally, we establish design guidelines for lensed fibers and $\mathrm{Si}$${}_3$$\mathrm{N}$${}_4$ tapers compatible with modern CMOS foundries for scalable manufacturing of $\mathrm{Si}$${}_3$$\mathrm{N}$${}_4$ photonic integrated circuits. These findings provide a scalable solution for photonic packaging and optoelectronic assembly, essential for advancing data-center and artificial-intelligence (AI) hardware infrastructure.

Predicting the outcomes of quantum measurements is a cornerstone of quantum information theory and a key resource for quantum technologies. Here we introduce a comprehensive framework for quantifying the predictability of measurements on a bipartite quantum system using error measures inherited from statistical learning theory: the Bayes risk and inference variance. We derive analytical expressions for the optimal measurement that minimizes the prediction error for any arbitrary observable and any two-qubit state. We establish a direct, quantitative link between the ability to surpass the fundamental limit of local unpredictability and the presence of Einstein-Podolsky-Rosen steering. Additionally, by optimizing measurement choices according to the minimal Bayes risk, we propose a modified entanglement-based quantum key distribution protocol achieving higher secure key rates than the standard BB84 protocol, demonstrating enhanced resilience to noise. We apply our framework in two scenarios: perfect Bell states affected by local amplitude-damping noises, and top-antitop quark pairs produced in high-energy colliders. Our work offers a perspective on quantum correlations, connecting statistical inference, fundamental quantum phenomena, and cryptographic applications.

The Greenberger-Horne-Zeilinger and W states represent two inequivalent classes of multipartite entangled states that cannot be transformed through local operations and classical communication. The challenge of their interconversion has received considerable attention due to these intrinsic constraints. In this study, we propose a protocol to achieve rapid and deterministic conversion between the W and Greenberger-Horne-Zeilinger states in a spin-chain system based on the inverse-engineering technique. The inherent flexibility of this approach allows for the optimization of energy cost during the conversion process. Furthermore, we present a proof-of-principle experimental demonstration of this protocol on a nuclear magnetic resonance platform. This work highlights the great potential of the inverse-engineering method for rapidly and efficiently manipulating quantum entangled states in many-body systems.

Time-resolved photoconductivity is widely used to characterize nonequilibrium charge-carrier lifetime, impurity content, and solar-cell efficiency in a broad range of semiconductors. Most measurements are limited to the detection of reflection of electromagnetic radiation at a single frequency and a single photoexciting light wavelength. We present a time-resolved photoconductivity instrument that enables broadband frequency detection (essentially from dc to $20\mathrm{GHz}$), temperature-dependent measurements, and multiple excitation photon energy. The measurement is realized with the help of a coplanar waveguide, which acts as an efficient antenna and whose performance was tested over $10\mathrm{MHz}–10\mathrm{GHz}$. The instrument enables the study of surface and bulk charge-recombination specific processes.

$\mathrm{Si}$/$\mathrm{Si}\mathrm{Ge}$ heterostructures on bulk $\mathrm{Si}$ substrates have been shown to host high-fidelity electron spin qubits. Building a scalable quantum processor would, however, benefit from further improvement of critical material properties such as the valley-splitting landscape. Flexible control of the strain field and the out-of-plane electric field ${\mathcal{E}}_z$ may be decisive for valley-splitting enhancement in the presence of alloy disorder. We envision the $\mathrm{Si}$/$\mathrm{Si}\mathrm{Ge}$ membrane as a versatile scientific platform for investigating intervalley scattering mechanisms which have thus far remained elusive in conventional $\mathrm{Si}$/$\mathrm{Si}\mathrm{Ge}$ heterostructures and have the potential to yield favorable valley-splitting distributions. Here, we report the fabrication of locally etched, suspended $\mathrm{Si}\mathrm{Ge}$/$\mathrm{Si}$/$\mathrm{Si}\mathrm{Ge}$ membranes from two different heterostructures and apply the process to realize a spin-qubit shuttling device on a membrane for future valley mapping experiments. The membranes have a thickness in the micrometer range and can be metallized to form a back-gate contact for extended control over the electric field. To probe their elastic properties, the membranes are stressed by loading with a profilometer stylus at room temperature. We distinguish between linear elastic and buckling modes, each offering mechanisms through which strain can be coupled to spin qubits.

Portable quantum technologies require robust lightweight apparatus with superior performance. For techniques dependent upon high-vacuum environments, such as atom interferometers and atomic clocks, three-dimensional (3D) printing enables additional avenues to tailor in-vacuum gas propagation dynamics. We demonstrate intricate fine-scale surface patterning of 3D-printed vacuum components to increase the rate at which gas particles collide with the surface. By applying a nonevaporable getter coating for use as a surface pump, we show that the patterned surface pumps gas particles 3.8 times faster than equivalent flat areas. These patterns can be directly integrated into additively manufactured components, enabling application in close proximity to key experimental regions and contributing to overall mass reduction. We develop numerical simulations that show good agreement with this result and predict up to a tenfold increase in the pumping rate, for realistic surface structures. Our work has direct applications in enabling passively pumped portable quantum technologies but also establishes 3D printing as a powerful technique for the creation of optimized surface patterning to provide enhanced control over high-vacuum gas dynamics for a broad range of applications.

Two-level systems (TLSs) are known to be a dominant source of dissipation and decoherence in superconducting qubits. Superconducting resonators provide a convenient way to study TLS-induced loss due to easier design and fabrication in comparison with devices that include nonlinear elements such as Josephson junctions. However, precisely measuring TLS-induced loss in a resonator in the quantum regime is challenging due to low signal-to-noise ratio (SNR) and the temporal fluctuations of the TLS, leading to uncertainties of 20% or more in the loss measurement. To address these limitations, we develop a multiwavelength resonator device that extends the resonator length from a standard quarter wavelength $\lambda /4$ to $N\lambda /4$, where $N=37$, at 6 GHz. This design provides two key advantages: the TLS-induced fluctuations are reduced by a factor of $\sqrt{N}$ due to spatial averaging over an increased number of independent TLSs, and the measurement SNR for a given intraresonator energy density increases by a factor of $\sqrt{N}$. The multiwavelength resonator also has fundamental and harmonic resonances spanning a wide frequency range, allowing one to study the frequency dependence of TLS-induced loss. In this work we fabricate both multiwavelength and quarter-wave coplanar waveguide resonators formed from thin-film aluminum on a silicon substrate, and characterize their TLS properties at 10 and 200 mK. Our results show that the power-dependent TLS-induced loss measured from the $N\lambda /4$ resonator agrees well with that measured from the $\lambda /4$ resonator, while achieving a fivefold reduction in measurement uncertainty due to TLS fluctuations, down to $5\mathrm{\%}$. The $N\lambda /4$ resonator also provides a measure of the fully unsaturated TLS-induced loss due to the increased measurement SNR at low intraresonator energy densities. Finally, measurements across seven harmonic resonances of the $N\lambda /4$ resonator between 4 and 6.5 GHz reveal no frequency dependence in the TLS-induced loss over this range. These results highlight the benefits of the multiwavelength resonator approach for material development in superconducting quantum circuits.

Microwave drives play a central role in the control of superconducting quantum circuits, enabling qubit gates, readout, and parametric interactions. As the drive frequencies are typically an order of magnitude smaller than (twice) the superconducting gap, it is generally assumed that such drives do not disturb the BCS ground state. However, sufficiently strong drives can activate multiphoton pair-breaking processes that generate quasiparticles (QPs) and result in qubit errors. In this work, we present a theoretical framework for calculating the rates of multiphoton-assisted pair-breaking transitions induced by charge- or flux-coupled microwave drives. Through illustrative examples, we show that photon-assisted QP generation may affect novel high-frequency dispersive readout architectures, as well as Floquet-engineered superconducting circuits operating under strong driving.

This paper is a contribution to the Physical Review Applied collection titled Physics-Inspired Computing.

We demonstrate that thermally stable perpendicular magnetic tunnel junctions, widely used in spin-transfer torque magnetic random-access memory, can be actuated with nanosecond pulses to exhibit tunable stochastic behavior. This actuated-stochastic tunnel junction (A-sMTJ) concept produces random telegraph noise, with control over fluctuation rate and probability bias. The device response is shown to be consistent with a Poisson process, with fluctuation rates tunable over two orders of magnitude, with average state dwell times ranging from approximately 30 ns to $2\text{μ}\mathrm{s}$. These results establish A-sMTJ as a versatile platform for integrating deterministic, stochastic, and in-memory functionality on a single chip—advancing the development of probabilistic, neuromorphic, and unconventional computing systems.

Hole-spin qubits in $\mathrm{Ge}$/$\mathrm{Ge}\mathrm{Si}$ heterostructures benefit from the clean environment of epitaxial interfaces and from the intrinsic spin-orbit coupling that enables efficient electrical control, which makes them promising candidates for quantum computation. However, spin-orbit coupling also enhances the sensitivity to electrical disorder, potentially increasing variability. In this work, we perform numerical simulations in a realistic device geometry to quantify the variability of the charge and spin properties of $\mathrm{Ge}$ qubits induced by charge traps at the $\mathrm{Si}\mathrm{Ge}$/oxide interfaces. We show that while the variability of charge properties remains moderate, spin properties ($g$ factors and Rabi frequencies) show significant dispersion. We explore the implications of this variability for large-scale architectures, and provide guidelines to minimize variability both in terms of interface quality requirements and optimal operation strategies.

The development of deterministic single-photon sources emitting in the telecommunication bands is a key challenge for quantum communication and photonic quantum computing. Here, we investigate the optical properties and single-photon emission of molecular beam epitaxy (MBE) grown semiconductor quantum dots (QDs) emitting in the telecom O- and C-bands. The QDs are embedded in an $\mathrm{In}\mathrm{Ga}\mathrm{As}$ matrix with fixed indium content grown on top of a compositionally graded $\mathrm{In}\mathrm{Ga}\mathrm{As}$ buffer. This structure allows for the future implementation of electrically contacted nanocavities to enable high-quality and bright QD emission. In detailed optical characterizations we observe linewidths as low as $50\text{μ}\mathrm{eV}$, close to the spectrometer resolution limit, low fine structure splittings close to $10\text{μ}\mathrm{eV}$, and $g^{(2)}(0)$ values as low as $0.08$. These results advance the current performance metrics for MBE-grown QDs on $\mathrm{Ga}\mathrm{As}$ substrates emitting in the telecom bands and showcase the potential of the heterostructures presented for further integration into photonic devices.