Exquisite sensitivities are a prominent advantage of quantum sensors. Ramsey sequences allow precise measurement of direct current fields, while Hahn-echo-like sequences measure alternating current fields. However, the latter are restrained for use with high-frequency fields (above approximately $1$ kHz) due to finite coherence times, leaving less-sensitive noncoherent methods for the low-frequency range. In this paper, we propose to bridge the gap with a fitting-based algorithm with a frequency-independent sensitivity to coherently measure low-frequency fields. As the algorithm benefits from coherence-based measurements, its demonstration with a single nitrogen-vacancy center gives a sensitivity of $9.4$ nT ${\mathrm{Hz}}^{-0.5}$ for frequencies below about $0.6$ kHz down to near-constant fields. To inspect the potential in various scenarios, we apply the algorithm at a background field of tens of nTs, and we measure low-frequency signals via synchronization.

Flux-tunable qubits are a useful resource for superconducting quantum processors. They can be used to perform cphase gates, facilitate fast reset protocols, avoid qubit-frequency collisions in large processors, and enable certain fast readout schemes. However, flux-tunable qubits suffer from a trade-off between their tunability range and sensitivity to flux noise. Optimizing this trade-off is particularly relevant for enabling fast, high-fidelity, all-microwave cross-resonance gates in large, high-coherence processors. This is mainly because cross-resonance gates set stringent conditions on the frequency landscape of neighboring qubits, which are difficult to satisfy with nontunable transmons due to their relatively large fabrication imprecision. To solve this problem, we realize a coherent, flux-tunable, transmonlike qubit, which exhibits a frequency tunability range as small as $43\mathrm{MHz}$, and whose frequency, anharmonicity and tunability range are set by a few experimentally achievable design parameters. Such a weakly tunable qubit may be used to avoid frequency collisions in a large lattice while exhibiting minimal susceptibility to flux noise.

Long and thin scanning force cantilevers are sensitive to small forces, but also vulnerable to detrimental noncontact interactions. Here we present an experiment with a cantilever whose spring constant and static deflection are dominated by the interaction between the tip and the surface, a regime that we refer to as “overcoupled.” The interactions are an obstacle for ultrasensitive measurements such as nanoscale magnetic resonance imaging (nano-MRI). We discuss several strategies to overcome the challenges presented by overcoupling, and demonstrate proton nano-MRI measurements of individual influenza virus particles.

In this paper, we discuss the dynamic modeling of fluid-filled strawlike elements consisting of serially interconnected elastic frusta with both axisymmetric and antisymmetric degrees of freedom, assuming planar motion. Under appropriate conditions, each substructure has four stable equilibrium states, which gives the system under investigation the ability to remain stable in a large number of complex configurations where each unit cell is in one of its four stable states. This is a vital ability for a myriad of applications, including reconfigurable structures and soft robots. The theoretical model explains the dynamics of a single strawlike element in a discrete manner, considering inertial, damping, and gravitational effects, while taking into account the nonlinear elasticity of the elastic frusta, and assuming hydrostatic behavior of the entrapped fluid. After identifying the geometrical and elastic parameters of the theoretical model based on relatively simple experiments, the model is validated compared to numerical simulations and experiments. The numerical simulations validate the theoretical elasticity of the elastic frusta, whereas the overall dynamic behavior of the system and the influence of unmodeled fluidic effects are examined experimentally. It is demonstrated both theoretically and empirically that strawlike elements cannot be adequately modeled using simple uniaxial deformations. In addition, the experimental validation indicates that the suggested model that allows bending can accurately capture their overall dynamics.

The network of interferometric gravitational-wave observatories has successfully detected tens of astrophysical signals since 2015. In this paper, we experimentally investigate compact sensors that have the potential to improve the sensitivity of gravitational-wave detectors to intermediate-mass black holes. We use only commercial components, such as sensing heads and lasers, to assemble the setup and demonstrate its subpicometer precision. The setup consists of a pair of Michelson interferometers that use deep frequency modulation techniques to obtain a linear, relative displacement readout over multiple interference fringes. We implement a laser-frequency stabilization scheme to achieve a sensitivity of $0.3\mathrm{pm}/\sqrt{\mathrm{Hz}}$ above 0.1 Hz. The device has also the potential to improve other experiments, such as torsion balances and commercial seismometers.

Although two-qubit entangling gates are necessary for universal quantum computing, they are notoriously difficult to implement with high fidelity. Recently, tunable couplers have become a key component for realizing high-fidelity two-qubit gates in superconducting quantum computers. However, it is still difficult to achieve tunable coupling free of unwanted residual coupling—in particular, for highly detuned qubits, which are desirable for mitigating qubit-frequency crowding or errors due to crosstalk between qubits. We thus propose a kind of tunable coupler that we call a double-transmon coupler, because it is composed of two transmon qubits coupled through a common loop with an additional Josephson junction. By controlling the magnetic flux in the loop, we can achieve not only fast high-fidelity two-qubit gates but also no residual coupling during the idle time, where computational qubits are highly detuned fixed-frequency transmons. The proposed coupler is expected to offer an alternative approach to higher-performance superconducting quantum computers.

The demonstration of universal quantum logic operations near the fault-tolerance threshold has established ion-implanted near-surface donor atoms as a plausible platform for scalable quantum computing in silicon. The next technological step forward requires a deterministic fabrication method to create large-scale arrays of donors, featuring a few-hundred-nanometer interdonor spacing. Here, we explore the feasibility of this approach by implanting low-energy ions into silicon devices featuring a $60\times 60\mu {\mathrm{m}}^2$ sensitive area and an ultrathin 3.2-nm gate oxide—capable of hosting large-scale donor arrays. We employ a characterization system consisting of a modified focused-ion-beam machine and ultralow-noise ion-detection electronics to demonstrate a method for evaluating the device response characteristics to shallowly implanted 12-keV ${}^1$$\mathrm{H}$${}_2^{+}$ ions. Despite the weak internal electric field, near-unity charge-collection efficiency is obtained from the entire sensitive area. This can be explained by the critical role that the thermal gate oxide plays in the ion-detection response, allowing an initial rapid diffusion of ion-induced charge away from the implant site. Next, we adapt our approach to perform deterministic implantation of a few thousand 24-keV ${}^{40}$$\mathrm{Ar}$${}^{2+}$ ions into a predefined microvolume, without any additional collimation. Despite the reduced ionization from the heavier ion species, a fluence-independent detection confidence of $\ge 99.99$% is obtained. Our system thus represents not only a method for mapping the near-surface electrical landscape of electronic devices but also a framework toward mask-free prototyping of large-scale donor arrays in silicon.

Qubit measurement and control in circuit quantum electrodynamics (QED) rely on microwave drives, with higher drive amplitudes ideally leading to faster processes. However, degradation in qubit coherence time and readout fidelity has been observed even under moderate drive amplitudes corresponding to a few photons populating the measurement resonator. Here, we numerically explore the dynamics of a driven transmon-resonator system under strong and nearly resonant measurement drives and find clear signatures of transmon ionization where the qubit escapes out of its cosine potential. Using a semiclassical model, we interpret this ionization as resulting from resonances occurring at specific resonator-photon populations. We find that the photon populations at which these spurious transitions occur are strongly parameter dependent and that they can occur at low resonator-photon population, something that may explain the experimentally observed degradation in measurement fidelity.

Harnessing the quantum interference between generation processes of visible-infrared photon pairs, infrared quantum absorption spectroscopy (IRQAS) enables the estimation of the optical properties of a medium in the infrared region from interferograms obtained by detecting visible photons. Since IRQAS enables infrared spectroscopy without using a light source or detector in the infrared region, infrared spectrometers can be made more compact and less invasive, which will find many alternative applications. A broader spectroscopy bandwidth is key, however, the spectral bandwidths of the reported IRQAS system have been limited to less than $1\mu \mathrm{m}$ so far. Here, we report a wavelength-tunable IRQAS system covering the spectral range of $1.9–5.2\mu \mathrm{m}$ with a bandwidth of $3.3\mu \mathrm{m}$. In this system, the wavelengths of spontaneous parametric down-conversion photon pairs are tuned by rotating a nonlinear crystal with respect to the pump beam. Rapid spectral measurements (coarse scan) over a broad bandwidth ($1.9–5.2\mu \mathrm{m}$) is successfully demonstrated using silica glass as a sample, and the obtained spectrum clearly shows $\mathrm{OH}$-stretching absorption bands, which agreed well with the result measured by a conventional infrared spectrometer. Furthermore, quantum Fourier-transform infrared spectroscopy measurements are demonstrated for these absorption bands with a high resolution (fine scan). The reported concept of the ultrabroadband IRQAS system paves the way for the use of IRQAS in real-world applications.

In this paper, we present a direct loss-tangent measurement of a high-resistivity intrinsic (100) silicon wafer in a temperature range from approximately $70\mathrm{mK}$ to 1 K, approaching the quantum regime. The measurement is performed using a technique that takes advantage of a high-quality-factor superconducting niobium resonator and allows us to directly measure the loss tangent of insulating materials with a high level of accuracy and precision. We report silicon-loss-tangent values at the lowest temperature and for electric field amplitudes comparable to those found in planar transmon devices, and these are one order of magnitude larger than what was previously estimated. In addition, we discover a nonmonotonic trend in the loss tangent as a function of temperature, which we describe by means of a phenomenological model based on variable-range hopping conduction between localized states around the Fermi energy. We also observe an increasing dependence of the dissipation on the electric field, which can be qualitatively described by the variable-range hopping-conduction mechanism as well. These findings are important for the optimization of quantum devices and for advancing the understanding of their decoherence channels.

Reef-building corals are inherently sessile organisms. However, motion is an important behavioral trait of coral polyps, which plays an essential role in feeding, competition, defense, reproduction, and thus, survival and fitness. Notwithstanding the importance of inherent temporal and spatial multiscale features of polyps, their quantitative properties and modeling still remain challenging and unexplored. Here, we observe Pocillopora acuta in vivo under different light and temperature conditions using a fluidic platform that allows the direct microscopic study of small live coral fragments, where the stochastic dynamics of the in-plane waving motion of polyps is uncovered. The relationship between polyps on nubbins is described by motion-correlation analysis. Additionally, the fractional Brownian motions of polyps under certain light conditions and temperatures are revealed by the Hurst index via power spectral analysis. Finally, the motion of polyps is modeled by Langevin dynamics, numerically obtained by data-driven parameterization. This combination of experimental observations, numerical analysis, and theoretical modeling opens an avenue to boost our understanding of the biological and physical behaviors of corals in relation to changing environmental conditions.

The electronic and optical properties of graphene can be precisely tuned by generating deterministic arrangements of strain features. In this paper, we report the formation of widespread and controlled buckling delamination of monolayer graphene deposited on hexagonal boron-nitride promoted by a significant squeezing of the graphene flake and induced by polymeric microactuators. The flexibility of this method offers a promising technique to create arbitrary buckling geometries and arrays of wrinkles which could also be subjected to iterative folding-unfolding cycles. Further development of this method could pave the way to tune the properties of several kinds of other two-dimensional materials, such as transition metal dichalcogenides, by tailoring their surface topography.

Currently available superconducting quantum processors with interconnected transmon qubits are noisy and prone to various errors. The errors can be attributed to sources such as open quantum system effects and spurious interqubit couplings (crosstalk). Static $ZZ$ coupling between qubits in transmon architectures is always present and contributes to both coherent and incoherent crosstalk errors. Its suppression is therefore a key step toward enhancing the fidelity of quantum computation using transmons. Here, we propose the use of dynamical decoupling to suppress the crosstalk and we demonstrate the success of this scheme through experiments performed on several IBM quantum cloud processors. In particular, we demonstrate improvements in quantum memory as well as the performance of single-qubit and two-qubit gate operations. We perform open quantum system simulations of the multiqubit processors and find good agreement with the experimental results. We analyze the performance of the protocol based on a simple analytical model and elucidate the importance of the qubit drive frequency in interpreting the results. In particular, we demonstrate that the XY4 dynamical decoupling sequence loses its universality if the drive frequency is not much larger than the system-bath coupling strength. Our work demonstrates that dynamical decoupling is an effective, practical, and scalable way to suppress crosstalk and open-system effects, thus paving the way toward higher-fidelity logic gates in transmon-based quantum computers.

Quantum key distribution (QKD) is a crucial component for truly secure communication, which enables the leakage of information due to eavesdropper attacks to be analyzed. Although impressive progress was made in the field of long-distance implementations, user-oriented applications involving short-distance links have mostly remained overlooked. Recent technological advances in integrated photonics now enable developments towards QKD also for existing hand-held communication platforms. In this work we report on the design and evaluation of a hand-held free-space QKD system including a micro-optics-based sender unit. This system implements the BB84 protocol employing polarization-encoded faint laser pulses at a rate of $100\mathrm{MHz}$. Unidirectional beam tracking and live reference-frame alignment systems at the receiver side enable a stable operation over tens of seconds when aiming the portable transmitter to the receiver input by hand from a distance of about half a meter. The user-friendliness of our system was confirmed by successful key exchanges performed by different untrained users with an average link efficiency of about $20\mathrm{\%}$ relative to the case of the transmitter being stationarily mounted and aligned. In these tests we achieve an average quantum bit error ratio (QBER) of $2.4\mathrm{\%}$ and asymptotic secret key rates ranging from $4.0\mathrm{kbps}$ to $15.3\mathrm{kbps}$. Given its compactness, the versatile sender optics is also well suited for integration into other free-space communication systems enabling QKD over any distance.

The manipulation of antiferromagnetic order by means of spin-orbit torques opens opportunities to exploit the dynamics of antiferromagnets in spintronic devices. In this work, we investigate the current-induced switching of the magnetic octupole vector in the Weyl antiferromagnet ${\mathrm{Mn}}_3\mathrm{Sn}$ as a function of pulse shape, magnetic field, temperature, and time. We find that the switching behavior can be either bistable or tristable depending on the temporal structure of the current pulses. Time-resolved Hall effect measurements performed during the current pulsing reveal that ${\mathrm{Mn}}_3\mathrm{Sn}$ switching proceeds via a two-step demagnetization-remagnetization process caused by self-heating over a timescale of tens of nanoseconds followed by cooling in the presence of spin-orbit torques. Single-shot switching measurements with 50-ps temporal resolution indicate that chiral spin rotation is either damped or incoherent in polycrystalline ${\mathrm{Mn}}_3\mathrm{Sn}$. Our results shed light on the switching dynamics of ${\mathrm{Mn}}_3\mathrm{Sn}$ and prove the existence of extrinsic limits on its switching speed.

Entangled microwave photons form a fundamental resource for quantum information processing and sensing with continuous variables. We use a low-loss Josephson metamaterial comprising superconducting, nonlinear, asymmetric inductive elements to generate frequency-entangled photons from vacuum fluctuations at a rate of 2 giga entangled bits per second spanning over the 4-GHz bandwidth. The device is operated as a traveling-wave parametric amplifier under Kerr-relieving biasing conditions. Furthermore, we demonstrate single-mode squeezing in such devices—$3.1\pm 0.7\mathrm{dB}$ below the zero-point level at half of modulation frequency.

Quantum networks facilitate the secure transmission of information between different users. Establishing communication links among multiple users in a scalable and efficient way is important for realizing a large-scale quantum network. Here we develop an energy-time entanglement-based dense wavelength division multiplexed network based on an integrated silicon nitride microring resonator, which offers a wide frequency span (covering at least the entire C-band) and narrow bandwidth modes ($\sim 650\mathrm{MHz}$). Six pairs of photons are selected to form a fully and simultaneously connected four-user quantum network. The observed quantum interference visibilities are well above the classical limits among all users. Each pair of users perform the BBM92 protocol for quantum key distribution. Our results pave the way for realizing large-scale quantum networks with integrated photonic architecture.

We propose and demonstrate a scheme for optical nanoparticle characterization, optofluidic force induction (OF2i), which achieves real-time optical counting with single-particle sensitivity, high throughput, and for particle sizes ranging from tens of nanometers to several micrometers. The particles to be analyzed flow through a microfluidic channel alongside a weakly focused laser vortex beam, which accomplishes two-dimensional trapping of the particles in the transverse directions and size-dependent velocity changes due to the optical forces in the longitudinal direction. Upon monitoring the trajectories and velocity changes of each individually tracked particle, we obtain detailed information about the number-based particle size distribution. A parameter-free model based on Maxwell’s equations and Mie theory is shown to provide very good agreement with the experimental results for standardized particles of spherical shape. Our results prove that OF2i can provide a flexible work bench for numerous pharmaceutical and technological applications, as well as for medical diagnostics.

Clusters of contaminations have been identified within rehearsing choirs during the COVID-19 pandemic. In particular, singing and playing wind instruments are known to generate enhanced release of respiratory droplets, which are then transported by the expiratory flows. By tracking the air exhaled by professional opera singers and musicians from the MET Orchestra in New York City, we measure the spatial extent of the various air flows in opera. While loud singing is often associated with fast flows, professional opera singers and musicians are usually exhaling air flows slower than the air jets exhaled by a person breathing at rest. However, we identify a few situations leading to the release of rapid air jets that are able to enhance the transport of pathogenic droplets within an orchestra. Finally, we show how singing with a facemask and covering the bell of a wind instrument provide a strong reduction of the transport of respiratory droplets, in addition to the filtration features of a mask.

The peak power of a femtosecond laser is limited by the size and damage threshold of its solid-state optical components, with the final grating of a chirped pulse amplification compressor posing a challenging bottleneck on the path to higher-power systems. Practical hundred-petawatt to exawatt lasers will require optics that draw on the higher damage tolerance of plasma to manipulate high-intensity light, but plasma is a difficult medium to control and sets stringent limits on optical performance. Here we describe the design of a compact high-power laser system that uses plasma transmission gratings—with currently achievable parameters—for chirped pulse amplification. A double compression architecture compensates for the low angular dispersion of the plasma gratings. We explore the design constraints set by available plasma parameters and use particle-in-cell simulations to examine performance at high light intensity. These simulations suggest that the meter-scale final grating for a 10-PW laser could be replaced with a 1.5-mm-diameter plasma grating, allowing compression to, for example, 22 fs with 90% efficiency and providing a path towards compact multipetawatt laser systems.