Current noise levels in physical realizations of qubits and quantum operations limit the applicability of conventional methods to characterize entanglement. In this adverse scenario, we follow a quantum variational approach to estimate the geometric measure of entanglement of multiqubit pure states. The algorithm requires only single-qubit gates and measurements, so it is well suited for noisy intermediate-scale quantum devices. This is demonstrated by successfully implementing the method on IBM Quantum devices for Greenberger-Horne-Zeilinger states of three, four, and five qubits. Numerical simulations with random states show the robustness and accuracy of the method. The scalability of the protocol is numerically demonstrated via matrix-product-state techniques up to $25$ qubits.

We propose here a high-frequency spin-Hall nano-oscillator based on a simple magnetic texture, such as a domain wall, located in an antiferromagnet with an easy-axis anisotropy type. We show that the spin current, polarized along the anisotropy axis, excites a conical precession of the Néel vector in such a domain wall, which allows obtaining a robust ac output signal—contrary to the planar precession in a uniform uniaxial antiferromagnet, where ac output is hard to achieve. The frequency of the auto-oscillations is easily tunable by the applied current up to the terahertz range, and the threshold current vanishes for a pure uniaxial antiferromagnet. By micromagnetic simulations, we demonstrate that the pinning of the domain wall is crucial for the oscillator design, which can be achieved in the nanoconstriction layout of the free layer.

Single-mode high-index-contrast waveguides have been ubiquitously exploited in optical, microwave, and phononic structures for achieving enhanced wave-matter interactions. Although microscale optomechanical and electro-optical devices have been widely studied, optomagnonic devices remain a grand challenge at the microscale. Here, we introduce a planar optomagnonic waveguide platform based on a ferrimagnetic insulator that simultaneously supports single transverse mode of spin waves (magnons) and highly confined optical modes. The colocalization of spin and light waves gives rise to an enhanced inverse Faraday effect, and as a result, magnons are excited by an effective magnetic field generated by interacting optical photons. Moreover, the strongly enhanced optomagnonic interaction allows us to observe such an effect using low-power (milliwatt level) light signals in the continuous-wave form, as opposed to high-intensity (megawatt peak power) light pulses that are typically required in magnetic bulk materials or thin films. The optically driven magnons are detected electrically with preserved phase coherence, showing the feasibility for launching spin waves with low-power continuous optical fields.

The combination of local heating and biasing at the tip-surface junction in temperature-assisted piezoresponse force microscopy (TPFM) opens a pathway for probing local temperature-induced phase transitions in ferroics, exploring the temperature dependence of polarization dynamics in ferroelectrics and potentially discovering coupled phenomena driven by strong temperature and electric field gradients. Here, we analyze the signal-formation mechanism in TPFM and explore the interplay between thermal- and bias-induced switching in model ferroelectric materials. We further explore the contributions of the flexoelectric and thermopolarization effects to the local electromechanical response and demonstrate that the latter can be significant for “soft” ferroelectrics. These results establish a framework for the quantitative interpretation of TPFM observations, predict the emergence of nontrivial switching and relaxation phenomena driven by nonlocal thermal-gradient-induced polarization switching, and open a pathway for exploring the physics of thermopolarization effects in various noncentrosymmetric and centrosymmetric materials.

Single color centers in solid have emerged as promising physical platforms for quantum information science. Creating these centers with excellent quantum properties is a key foundation for further technological developments. In particular, the microscopic understanding of the spin-bath environments is the key to engineer color centers for quantum control. In this work, we propose and demonstrate a distinct high-temperature annealing (HTA) approach for creating high-quality nitrogen vacancy (N-V) centers in implantation-free diamonds. Simultaneously using the created N-V centers as probes for their local environment we verify that no damage is microscopically induced by the HTA. Nearly all single N-V centers created in ultralow-nitrogen-concentration membranes possess stable and Fourier-transform-limited optical spectra. Furthermore, HTA strongly reduces noise sources naturally grown in ensemble samples, and leads to more than threefold improvements of decoherence time and sensitivity. We also verify that the vacancy activation and defect reformation, especially H3 and P1 centers, can explain the reconfiguration between spin baths and color centers. This distinct approach will become a powerful tool in vacancy-based quantum technology.

Recently, there has been growing interest in developing superresolution lenses to focus light beyond the conventional diffraction limit in the far field. Achromatic superresolution is particularly attractive for many practical applications. However, it remains a great challenge to realize an achromatic superresolution focusing lens with both a high numerical aperture and a small depth of focus, which is critical for superresolution imaging with high longitudinal resolution. Here, a method for preparing a multiwavelength achromatic superresolution lens with a high numerical aperture is proposed to control the wave front through a binary-phase zone plate for generating a transverse superresolution focal spot with a small depth of focus. The approach is experimentally verified with an achromatic superresolution lens with a high numerical aperture of 0.95 at six wavelengths in the visible range between 405 and 684 nm. The proposed method is practical for superresolution-related applications, including imaging, microscopy, and data storage.

We investigate quantum repeater protocols that rely on three-qubit repetition codes using nitrogen-vacancy (N-$V$) centers in diamond as quantum memories. N-$V$ centers offer a two-qubit register, corresponding to their electron and nuclear spins, which makes it possible to perform deterministic two-qubit operations within one N-$V$ center. For quantum repeater applications, however, we need to do joint operations on two separate N-$V$ centers. Here, we study two N-$V$-center-based repeater structures that enable such deterministic joint operations. One structure offers less consumption of classical communication, at the cost of more computation overhead, whereas the other relies on a smaller number of physical resources and operations. We assess and compare their performance for the task of secret-key generation under the influence of noise and decoherence with current and near-term experimental parameters. We quantify the regimes of operation where one structure outperforms the other, and find the regions where encoded quantum repeaters offer practical advantages over their nonencoded counterparts.

Quantum sensing is considered to be one of the most promising subfields of quantum information to deliver practical quantum advantages in real-world applications. However, its impressive capabilities, including high sensitivity, are often hindered by the limited quantum resources available. Here, we incorporate the expected information gain (EIG) and techniques such as accelerated computation into Bayesian experimental design (BED) in order to use quantum resources more efficiently. A simulated nitrogen-vacancy center in diamond is used to demonstrate real-time operation of the BED. Instead of heuristics, the EIG is used to choose optimal control parameters in real time. Moreover, combining the BED with accelerated computation and asynchronous operations, we find that up to a tenfold speedup in absolute time cost can be achieved in sensing multiple surrounding ${}^{13}\mathrm{C}$ nuclear spins. Our work explores the possibilities of applying the EIG to BED-based quantum-sensing tasks and provides techniques useful to integrate BED into more generalized quantum sensing systems.

Exciton-polaritons are hybrid light-matter states that arise from strong coupling between an exciton resonance and a photonic cavity mode. As bosonic excitations, they can undergo a phase transition to a condensed state that can emit coherent light without a population inversion. This aspect makes them good candidates for thresholdless lasers, yet short exciton-polariton lifetime has made it difficult to achieve condensation at very low power densities. In this sense, long-lived symmetry-protected states are excellent candidates to overcome the limitations that arise from the finite mirror reflectivity of monolithic microcavities. In this work we use a photonic symmetry-protected bound state in the continuum coupled to an excitonic resonance to achieve state-of-the-art polariton condensation threshold in a $\mathrm{Ga}\mathrm{As}$/($\mathrm{Al},\mathrm{Ga}$)$\mathrm{As}$ waveguide. Most important, we show the influence of fabrication control and how surface passivation via atomic layer deposition provides a way to reduce exciton quenching at the grating sidewalls.

The edge states in a one-dimensional Su-Schrieffer-Heeger (SSH) chain have been extensively studied for their topologically protected properties when there are onsite dissipations induced by independent environments. We here show that common environments shared by nearest neighboring sites in the SSH chain can result in dissipative couplings between sites, and thus change the topologically trivial phase to a nontrivial one. In contrast to usual hybridization of two edge states in the SSH chain with finite size, we find that the dissipative couplings result in oscillations of these hybridized edge states, which are similar to those of Majorana modes in the semiconductor-superconductor nanowire. The parameter for controlling the topological phase of the SSH chain plays the role of the magnetic field in a nanowire for controlling Majorana oscillation. A measurement method for oscillations is proposed. Our study provides a useful way to manipulate and ascertain edge states, and is experimentally feasible within current technology of superconducting quantum circuits.

Fundamental aspects of spin-orbit interaction in commercially relevant $\mathrm{Ga}\mathrm{N}$/${\mathrm{Al}}_x{\mathrm{Ga}}_{1-x}\mathrm{N}$ heterostructures hosting two-dimensional electron systems are extensively studied by electron spin resonance (ESR). This unprecedentedly accurate experimental technique allows access to the fine details of coupling between the spin degree of freedom and the quantized orbital motion of an electron in the quantum Hall regime through the precise measurement of the electron $g$-factor. Filling-factor-dependent changes of the $g$-factor value in strong magnetic fields allow extraction of the Rashba spin-orbit interaction constant $\alpha$ in various $\mathrm{Ga}\mathrm{N}$/${\mathrm{Al}}_x{\mathrm{Ga}}_{1-x}\mathrm{N}$ heterojunctions with electron sheet densities in the range $0.8–5.2\times {10}^{12}{\mathrm{cm}}^{-2}$. Despite three significantly different approaches used to tune the electron density, the extracted value of $\alpha$ is on the order of $5.3\pm 0.4$ meV $\text{Å}$ for all experimental realizations. This striking finding can be explained by assuming that the spin-orbit interaction is of bulk origin. Theoretical calculations confirm this observation, as the bulk cubic in-plane wave-vector term of the spin-orbit interaction compensates the rising contribution due to the change in the quantum well shape. Finally, the value of $\alpha$ is cross-checked and confirmed by weak antilocalization measurements in the longitudinal magnetoresistance at substantially lower magnetic field values compared to the ESR approach. The presented experimental findings provide knowledge for estimating the true scales of spin-orbit interaction in GaN-based spintronic devices.

We demonstrate the storage and retrieval of an on-demand single photon generated by a collective Rydberg excitation in a low-noise Raman quantum memory located in a different cold atomic ensemble. We generate single photons on demand by exciting a cold cloud of rubidium atoms off resonantly to a Rydberg state, with a generation probability up to 15% per trial. We then show that the single photons can be stored and retrieved with a storage-and-retrieval efficiency of 21% and a low noise floor in the Raman quantum memory. This leads to a signal-to-noise ratio ranging from 11 to 26 for the retrieved single photon, depending on the input-photon-generation probability, which allows us to observe significant antibunching. We also evaluate the performance of the Raman memory as a built-in unbalanced temporal beam splitter, tunable by varying the write-in control-pulse intensity. In addition, we demonstrate that the Raman memory can be used to control the single-photon wave shape. These results are a step forward in the implementation of efficient quantum repeater links using single-photon sources.

We design a pair of resonators to realize multiple spoof plasmonically induced transparency (PIT) with high-Q group delays, which show potential in sensing applications. To reveal the mechanism of the multiple spoof PIT scheme, we deduce mathematical equations of a simplified model composed of multiple harmonic oscillators, and we further propose an equivalent-circuit model to analyze the metamaterials in an intuitive way. A prototype consisting of a broadband stub resonator and a spoof localized surface-plasmonic resonator is further fabricated to detect the complex permittivity of high-lossy liquids and solid samples in microwave bands. Experimental results show that the group-delay-based sensor can work well even when high-order modes are weakly excited, and the trade-off between sensitivity and properties of samples is discussed. The proposed double-resonator scheme with multiple spoof PIT is universal, and we design a metasurface working in terahertz bands to show that the concept can be widely used in various surface-sensing applications.

We achieve a compact optical frequency standard with an extended cavity diode laser locked to the 459-nm $6S_{1/2}$-$7P_{1/2}$ transition of thermal ${}^{133}\mathrm{Cs}$ atoms in a $\varphi 10\mathrm{mm}\times 50\mathrm{mm}$ glass cell, using modulation transfer spectroscopy (MTS). The self-estimated frequency stability of this laser is $1.4\times {10}^{-14}/\sqrt{\tau }$. With heterodyne measurement, we verify the linewidth-narrowing effect of MTS locking and measure the frequency stability of the locked laser. The linewidth of each laser is reduced from the free-running 69.6 to 10.3 kHz after MTS stabilization, by a factor of 6.75. The Allan deviation measured via beat detection is $2.1\times {10}^{-13}/\sqrt{\tau }$ for each MTS-stabilized laser. In addition, we measure the hyperfine structure of the $7P_{1/2}$ energy level based on the heterodyne measurements, and calculate the magnetic dipole constant A of the Cs $7P_{1/2}$ level to be 94.38(6) MHz, which agrees well with previous measurements. This compact optical frequency standard can also be used in other applications that require high-stability lasers, such as laser interferometry, laser cooling, geodesy, and so on.

Engineering desired Hamiltonians in quantum many-body systems is essential for applications such as quantum simulation, computation, and sensing. Many widely used quantum Hamiltonian engineering sequences are designed using human intuition based on perturbation theory, which may not describe the optimal solution and is unable to accommodate complex experimental imperfections. Here we numerically search for Hamiltonian engineering sequences using deep reinforcement learning (DRL) techniques and experimentally demonstrate that they outperform celebrated decoupling sequences on a solid-state nuclear magnetic resonance quantum simulator. As an example, we aim at decoupling strongly interacting spin-1/2 systems. We train DRL agents in the presence of different experimental imperfections and verify robustness of the output sequences both in simulations and experiments. Surprisingly, many of the learned sequences exhibit a common pattern that had not been discovered before, to our knowledge, but has a meaningful analytical description. We can thus restrict the searching space based on this control pattern, allowing us to search for longer sequences, ultimately leading to sequences that are robust against dominant imperfections in our experiments. Our results not only demonstrate a general method for quantum Hamiltonian engineering, but also highlight the importance of combining black-box artificial intelligence with an understanding of the physical system in order to realize experimentally feasible applications.

We demonstrate the use of radio-frequency (rf) resonators to measure the capacitance of nanoscale semiconducting devices in field-effect transistor configurations. The rf resonator is attached to the gate or the lead of the device. Consequently, tuning the carrier density in the conducting channel of the device affects the resonance frequency, quantitatively reflecting its capacitance. We test the measurement method on InSb and InAs nanowires at dilution-refrigerator temperatures. The measured capacitances are consistent with those inferred from the periodicity of the Coulomb blockade of quantum dots realized in the same devices. In an implementation of the resonator using an off-chip superconducting spiral inductor we find the measurement sensitivity values reaching down to $75\mathrm{zF}/\sqrt{\mathrm{Hz}}$ at 1 kHz measurement bandwidth, and noise down to 0.45 aF at 1 Hz bandwidth. We estimate the sensitivity of the method for a number of other implementations. In particular, we predict a typical sensitivity of about $40\mathrm{zF}/\sqrt{\mathrm{Hz}}$ at room temperature with a resonator composed of off-the-shelf components. Of several proposed applications, we demonstrate two: the capacitance measurement of several identical 80-nm-wide gates with a single resonator, and the field-effect mobility measurement of an individual nanowire with the gate capacitance measured in situ.

Speckle illumination is extensively applied in various imaging scenarios due to its advantages in enhancing the performance of imaging. Generally, the existing speckle-illumination methods are mainly based on external spatial modulation of light through a rotating diffuser or spatial light modulator. Both the lighting efficiency and the system’s compactness are limited for these conventional external modulation methods, due to their free-space light coupling and insertion or diffraction loss. Here, a multimode fiber laser with self-reconfigurable speckle illumination is proposed and realized, benefiting from the temporal-spectral randomness of a homemade coherent random-fiber laser and the spectral-sensitive output pattern of an out-coupling multimode fiber. Output characteristics in the spectral and spatial domain of the proposed laser are analyzed and show dynamically varying speckles within a moderate bandwidth. These features pave efficient ways for superresolution imaging through an opaque scattering medium as well as ghost imaging, which is realized and demonstrated based on the lighting of the proposed laser.

Magnetic skyrmions are topologically protected particlelike spin textures initially discovered in chiral magnetic materials. When driven by an electric current, magnetic skyrmions move at a certain angle (i.e., skyrmion Hall angle) with respect to the driving current due to the Magnus force, giving rise to the so-called skyrmion Hall effect (SHE). The SHE often leads to skyrmion annihilation at the sample edge, which is detrimental for practical applications. In this work, we study, experimentally and through micromagnetic simulations, the current-driven dynamics of skyrmion bubbles (SBs) in $\mathrm{Pt}/\mathrm{Co}/\mathrm{Ru}$ multilayer stacks. It is observed that, under certain circumstances, a long stripe domain will form and align along the sample edge, which separates the skyrmion bubbles from the edge and avoids their annihilation at the edge. Furthermore, the velocity of the skyrmion bubbles can be increased by the stripe domain wall (DW) through the DW-SB interaction, as predicted in our early theory. Our results deepen our understanding of skyrmion dynamics and pave the way to the realization of highly efficient information processing and computing devices based on ultrafast skyrmion motion.