Resonant laser spectroscopy is essential for the characterization, operation, and manipulation of single-quantum systems such as semiconductor quantum dots. The separation of the weak resonance fluorescence from the excitation laser is key for high-quality single- and entangled-photon sources. This is often achieved by cross-polarization laser extinction, which is limited by the quality of the optical elements. Recently, it was discovered that Fresnel-reflection birefringence in combination with single-mode filtering counteracting spin-orbit-coupling effects enables a three-order-of-magnitude improvement of polarization extinction [Benelajla et al., Phys. Rev. X 11, 021007 (2021)]. Here, we demonstrate this method for cross-polarization-extinction enhancement in cryogenic confocal microscopy of a resonantly excited semiconductor quantum dot in a birefringent optical microcavity, and observe a $10\times$ improvement of the single-photon contrast.

To successfully execute large-scale algorithms, a quantum computer will need to perform its elementary operations near perfectly. This is a fundamental challenge since all physical qubits suffer a considerable level of noise. Moreover, real systems are likely to have a finite yield, i.e., some nonzero proportion of the components in a complex device may be irredeemably broken at the fabrication stage. We present a threshold theorem showing that an arbitrarily large quantum computation can be completed with a vanishing probability of failure using a two-dimensional array of noisy qubits with a finite density of fabrication defects. To complete our proof we introduce a robust protocol to measure high-weight stabilizers to compensate for large regions of inactive qubits. We obtain our result using a surface-code architecture. Our approach is therefore readily compatible with ongoing experimental efforts to build a large-scale quantum computer.

Chalcogenide-based semiconductors have recently emerged as promising optoelectronic materials. In particular, bismuth-based sulfides have demonstrated great potential for device applications, such as in photovoltaics and photodetection. However, most of the devices reported have been based on silver-bismuth-sulfide-based solar cells. Work on bismuth sulfide based on precursor methods has been barely reported, and studies of its optoelectronic properties are lagging far behind. In this paper, we first report solution processing of ${\mathrm{Bi}}_2{\mathrm{S}}_3$ thin films, describe the systematic optimization of the precursors, and investigate the influence of composition and annealing on the properties of the films, including their morphology, crystallinity, charge-carrier dynamics, charge transport, and photoconductive gain. We also demonstrate excellent field-effect transistors based on ${\mathrm{Bi}}_2{\mathrm{S}}_3$ thin films, and introduce the use of these ${\mathrm{Bi}}_2{\mathrm{S}}_3$ phototransistors for weak-light detection. The optimized devices exhibit extremely high responsivity and low dark current, paving the way for high-performance photodetection.

Despite the usefulness of the anomalous Nernst conductivity (${\alpha }_{xy}^A$) for studying electronic band structures and exploring magnetic materials with large transverse thermopower, there has not been a straightforward way to obtain ${\alpha }_{xy}^A$ in experiments. Here we propose a simple and versatile method enabling direct electrical probing of ${\alpha }_{xy}^A$, which is realized by creating a closed circuit consisting of a target magnetic material and a nonmagnetic conductor. This method is experimentally demonstrated on a thin film of the magnetic Weyl semimetal ${\mathrm{Co}}_2\mathrm{Mn}\mathrm{Ga}$, where the closed circuit is formed simply by our connecting both ends of the ${\mathrm{Co}}_2\mathrm{Mn}\mathrm{Ga}$ film with a $\mathrm{Au}$ wire. A good approximation of ${\alpha }_{xy}^A$ is obtained in a wide temperature range, validating the proposed method and showing its potential for aiding the further development of topological-materials science and transverse thermoelectrics.

We present the parametric excitation of spin-wave modes in yttrium iron garnet (YIG) microdisks via parallel pumping. Their spectroscopy is performed using magnetic resonance force microscopy (MRFM), while their spatial profiles are determined by microfocus Brillouin light scattering (BLS). We observe that almost all the fundamental eigenmodes of an in-plane magnetized YIG microdisk, calculated using a micromagnetic eigenmode solver, can be excited using the parallel pumping scheme, as opposed to the transverse one. A comparison between the MRFM and BLS data on the one hand, and the simulations on the other hand, provides the complete spectroscopic labeling of over 40 parametrically excited modes. Our findings could be promising for spin-wave-based computation schemes, in which the amplitudes of a large number of spin-wave modes have to be controlled.

A ferromagnetic-antiferromagnetic (FM-AFM) thin-film heterostructure is proposed to be a potential system that can induce perpendicular magnetic anisotropy (PMA) in a ferromagnet, although there are few material combinations available and the underlying mechanism is not sufficiently understood. Here, we demonstrate that the AFM phase of an $\mathrm{Fe}\text{−}\mathrm{Rh}$ ordered alloy induces PMA in an adjacent $\mathrm{Fe}$ layer in an $\mathrm{Fe}/\mathrm{Fe}\text{−}\mathrm{Rh}$ heterostructure, which manifests itself as an additional mode of ferromagnetic resonance. The induced interfacial PMA disappears following a magnetic phase transition of $\mathrm{Fe}\text{−}\mathrm{Rh}$ from the AFM to the FM state, suggesting that the AFM order is crucial for the stabilization of PMA. The absence of the additional resonance mode in an $\mathrm{Fe}/\mathrm{Rh}/\mathrm{Fe}\text{−}\mathrm{Rh}$ control sample suggests that the PMA originates from a magnetic exchange coupling at the $\mathrm{Fe}/\mathrm{Fe}\text{−}\mathrm{Rh}$ interface. The results are promising for the development of high-density spintronic devices, in which PMA is controllable through the phase transition of $\mathrm{Fe}\text{−}\mathrm{Rh}$.

Harnessing the properties of phonons on a chip, phononic integrated circuits are playing an increasingly important role in optoelectronic integration. To date, most of such phononic waveguides are realized by etching a piezoelectric layer and forming a wire, which not only involves complicated fabrication processes but also limits integration with electronic and photonic elements on the same chip. To overcome these difficulties, here we propose and demonstrate a type of overlay slot phononic waveguide obtained by patterning a thin silicon cladding layer on an unetched gallium-nitride-on-sapphire substrate. We experimentally demonstrate the guiding and power splitting of gigahertz surface acoustic waves in such waveguides, with a measured propagation loss of about 1.80 dB/mm for the Rayleigh mode. We also theoretically predict that the Love mode in such waveguides can turn into a phononic bound state in the continuum under special conditions. Such waveguides introduce an alternative paradigm for phononic integrated circuitry and will enable applications in information processing, environmental monitoring, and noninvasive manipulation of biomolecules on a chip.

The metrological characteristics of source masses are essential in the determination of the Newtonian gravitational constant $G$. This paper presents an air-bearing method to check the density uniformity of spherical source masses by measuring the mass center offset from the geometric center. With detailed error analysis, we find that the eccentricity measurement accuracy using this method has the potential to be better than $0.1\text{μ}\text{m}$, and the uncertainty of actual experimental measurement is mainly limited by the nonsphericity of spheres. Two kinds of spherical source masses that are directly used in measurements of $G$ are measured. The eccentricity of the 57-mm-diameter spheres used in the time-of-swing method is determined to be less than $0.3\text{μ}\text{m}$ and that of the 127-mm-diameter spheres used in the angular acceleration feedback method is less than $1.3\text{μ}\text{m}$. Two density distribution models are discussed to estimate the uncertainty in the value of $G$ in terms of the density uniformity of source masses.

Nitrogen-vacancy (N-V) centers are diamond lattice defects that may be manipulated and controlled by visible light and microwave irradiation. They are considered a promising solid-state platform for a broad range of quantum technologies, such as magnetic field sensing. A major limitation in realizing such applications is the weak optical signal attained from the NVs, making the readout inefficient and prone to noise. Here, we report the increased brightness and shortened lifetime of N-V centers coupled to hyperbolic metamaterial photonic cavities with optimized dispersion characteristics. As a result, we demonstrate the enhancement of magnetic field sensitivity and measurement SNR. These results introduce a broadly applicable, robust, and technically accessible platform, promising improved performance relevant for a multitude of solid-state defects and their applications.

Measurement of the Landé $g$ factor (LGF) normally involves determining the energy splitting of spectral features under a dc magnetic field of several teslas. We show that circular-polarization-resolved magnetomodulation spectroscopy, with a small ac field of the order of 50 mT rms, can be used to measure the LGF of excitons in semiconductors. The technique is easy to implement with a microscope arrangement, enabling measurements with high spatial resolution at cryogenic temperatures. We validate the technique and determine its limits through measurements on excitons in a ${\text{GaAs/Al}}_{0.3}{\mathrm{Ga}}_{0.7}\mathrm{As}$ quantum well. We then apply it to microscopic flakes of bulk $2H$-${\mathrm{Mo}\mathrm{S}}_2$ and determine the LGF of the ground-state $A_{1s}$ exciton and the next-higher-energy exciton labeled $IL$, at energies around the direct gap of $2H$-${\mathrm{Mo}\mathrm{S}}_2$. The LGF of $IL$ is found to be much larger in magnitude and of opposite sign compared to that of $A_{1s}$. This suggests that it cannot be an excited state of the $A$ exciton as was previously assigned. Using an approximate model, the LGF value of the $IL$ exciton is shown to be consistent with an interlayer exciton that was earlier identified in few-layer $2H$-${\mathrm{Mo}\mathrm{S}}_2$.

Temporally varying electromagnetic materials have recently attracted substantial attention as a platform for building tunable devices, being directly controlled by the developed waves. Such a concept can be expanded to metasurfaces, where the time modulation involves only the imposed boundary conditions without requiring the solving of a wave equation with time-dependent coefficients. In this framework, a generic class of two coupled sheets with temporally modulated surface conductivities is investigated under unilateral excitation. Various reflected and transmitted waves at different Floquet frequencies are determined by rigorously formulating the temporal dynamics and are found to give multiharmonic resonances. This spectral redistribution of power is adjustable via the time change of metasurface admittances, and thus can be used in a broad range of applications from secure transmission to dispersionless propagation.

Quantum simulation on noisy intermediate-scale quantum devices is severely limited by short qubit coherence times. A variational pulse-shaping algorithm known as ctrl-VQE was recently proposed to address this issue by eliminating the need for parameterized quantum circuits, which lead to long state preparation times. Here, we find the fastest possible pulses that prepare target molecular wave functions for a given device Hamiltonian describing coupled transmon qubits using simulations. We find that the resulting time-optimal pulses develop a bang-bang form consistent with Pontryagin’s maximum principle. We further investigate how the minimal state preparation time is impacted by the number of energy levels active in the transmon simulations. We find that leakage outside the computational subspace (something that is usually considered problematic) speeds up the state preparation, further reducing device coherence-time demands. Our analysis reveals that this speedup is due to both an enlarged solution space of target wave functions and the appearance of additional channels connecting initial and target states.

We propose a quantum charging scheme fueled by measurements on ancillary qubits serving as disposable chargers. A stream of identical qubits are sequentially coupled to a quantum battery of $N+1$ levels and measured by projective operations after joint unitary evolutions of optimized intervals. If charger qubits are prepared in excited state and measured on ground state, then their excitations (energy) can be near perfectly transferred to battery by iteratively updating the optimized measurement intervals. Starting from its ground state, the battery could be constantly charged to an even higher energy level. Starting from a thermal state, the battery could also achieve a near-unit ratio of ergotropy and energy through less than $N$ measurements, when a population inversion is realized by measurements. If charger qubits are prepared in ground state and measured on excited state, useful work extracted by measurements alone could transform the battery from a thermal state to a high-ergotropy state before the success probability vanishes. Our operations in charging are more efficient than those without measurements and do not invoke the initial coherence in both battery and chargers. Particularly, our finding features quantum measurement in shaping nonequilibrium systems.

We report experimental studies of a driven spin-mechanical system, in which a nitrogen-vacancy ($\mathrm{N}\text{−}$V) center couples to out-of-plane vibrations of a diamond cantilever through the excited-state deformation potential. Photoluminescence excitation studies show that in the unresolved sideband regime and under strong resonant mechanical driving, the excitation spectra of a $\mathrm{N}\text{−}$V optical transition feature two spectrally sharp peaks, corresponding to the two turning points of the oscillating cantilever. In the limit that the strain-induced frequency separation between the two peaks far exceeds the $\mathrm{N}\text{−}$V zero-phonon linewidth, the spectral position of the individual peak becomes sensitive to minute detuning between the mechanical resonance and the external driving force. For a fixed optical excitation frequency near the $\mathrm{N}\text{−}$V transition, $\mathrm{N}\text{−}$V fluorescence as a function of mechanical detuning features resonances with a linewidth that can be orders of magnitude smaller than the intrinsic linewidth of the mechanical mode. This enhanced sensitivity to mechanical detuning can potentially provide an effective mechanism for mechanical sensing, for example, mass sensing via measurements of induced changes in the mechanical oscillator frequency.

Predictive design and optimization methods for controlled quantum systems depend on the accuracy of the system model. Any distortion of the input fields in an experimental platform alters the model accuracy and eventually disturbs the predicted dynamics. These distortions can be nonlinear with a strong frequency dependence so that the field interacting with the microscopic quantum system has limited resemblance to the input signal. We present an effective method for estimating these distortions, which is suitable for nonlinear transfer functions of arbitrary lengths and magnitudes provided the available training data has enough spectral components. Using a quadratic estimation, we have successfully tested our approach for a numerical example of a single Rydberg atom system. The transfer function estimated from the presented method is incorporated into an open-loop control optimization algorithm allowing for high-fidelity operations in quantum experiments.

Fluctuations of the qubit frequencies are one of the major problems to overcome on the way to scalable quantum computers. Of particular importance are fluctuations with a correlation time that exceeds the decoherence time due to decay and dephasing by fast processes. The statistics of the fluctuations can be characterized by measuring the correlators of the outcomes of periodically repeated Ramsey measurements. We describe qubit dynamics during repeated measurements and evaluate the two-time correlator for noise from two-level systems and two- and three-time correlators for Gaussian noise. The explicit expressions for the correlators are compared with simulations. We find that, even though the three-time correlators for noise from two-level systems and for Gaussian noise are generally significantly different, already ten two-level systems can mimic Gaussian noise provided they are symmetric. This is not the case for asymmetric systems.