Tunable two-qubit couplers offer an avenue to mitigate errors in multiqubit superconducting quantum processors. However, most couplers achieve this by fine tuning circuit parameters and often target specific couplings, such as the spurious $ZZ$ interaction. We introduce a superconducting coupler that alleviates these limitations by suppressing all two-qubit interactions with an exponentially large on:off ratio and without the need for fine tuning. Our approach is based on a bus mode supplemented by an ancillary nonlinear resonator mode. Driving the ancillary mode leads to a coupler-state-dependent field displacement in the resonator that, in turn, results in an exponential suppression of real and virtual two-qubit interactions with respect to the drive power. A superconducting circuit implementation supporting the proposed mechanism is presented.

We address an outstanding problem that represents a critical roadblock in the development of the Majorana-based topological qubit using semiconductor-superconductor hybrid structures: the quantitative characterization of disorder effects generated by the unintentional presence of charge impurities within the hybrid device. Given that disorder can have far-reaching consequences for the Majorana physics but is intrinsically difficult to probe experimentally in a hybrid structure, providing a quantitative theoretical description of disorder effects becomes essential. To accomplish this task, we develop a microscopic theory that (i) provides a quantitative characterization of the effective potential generated by a charge impurity embedded inside a semiconductor wire proximity coupled to a superconductor layer by solving self-consistently the associated three-dimensional Schrödinger-Poisson problem, (ii) describes the low-energy physics of the hybrid structure in the presence of $s$-wave superconductivity, spin-orbit coupling, Zeeman splitting, and disorder arising from multiple charge impurities by using the results of (i) within a standard free-fermion approach, and (iii) links the microscopic results to experimentally observable features by generating tunneling differential-conductance maps as a function of the control parameters (e.g., Zeeman field and chemical potential). We find that charge impurities lead to serious complications regarding the realization and observation of Majorana zero modes, which have direct implications for the development of Majorana-based qubits. More importantly, our work provides a clear direction regarding what needs to be done for progress in the field, including specific materials-quality and semiconductor-purity targets that must be achieved to create a topological qubit.

The genuine concurrence is a standard quantifier of multipartite entanglement, detection, and quantification of which still remains a difficult problem from both the theoretical and experimental points of view. Although many efforts have been devoted to the detection of multipartite entanglement (e.g., using entanglement witnesses), measuring the degree of multipartite entanglement, in general, requires some knowledge about the exact shape of a density matrix of the quantum state. An experimental reconstruction of such a density matrix can be done by full state tomography, which amounts to having the distant parties share a common reference frame and well-calibrated devices. Although this assumption is typically made implicitly in theoretical works, establishing a common reference frame, as well as aligning and calibrating measurement devices in experimental situations, are never trivial tasks. It is therefore an interesting and important question whether the requirements of having a shared reference frame and calibrated devices can be relaxed. In this work, we study both theoretically and experimentally the genuine concurrence for the generalized Greenberger-Horne-Zeilinger states under randomly chosen measurements on individual qubits without a shared frame of reference and calibrated devices. We present the relation between genuine concurrence and the so-called nonlocal volume, a recently introduced indicator of nonlocality.

An understanding of nano- and microscale crystal strain in chemical-vapor-deposition diamond is crucial to the advancement of diamond quantum technologies. In particular, the presence of such strain and its characterization presents a challenge to diamond-based quantum sensing and information applications—as well as for future dark-matter detectors, where the directional information about incoming particles is encoded in crystal strain. Here, we exploit nanofocused scanning x-ray diffraction microscopy to quantitatively measure crystal deformation from defects in diamond with high spatial and strain resolution. The combination of information from multiple Bragg angles allows stereoscopic three-dimensional modeling of strain-feature geometry; the diffraction results are validated via comparison to optical measurements of the strain tensor based on spin-state-dependent spectroscopy of ensembles of nitrogen-vacancy centers in the diamond. Our results demonstrate both strain and spatial resolution sufficient for directional detection of dark matter via x-ray measurement of crystal strain and provide a promising tool for diamond growth analysis and improvement of defect-based sensing.

Quantum technology is playing an increasingly important role due to the intrinsic parallel processing capabilities enabled by quantum superposition, exceeding the upper limits of classical performances in diverse fields. An integrated photonic chip offers an elegant way to construct large-scale quantum systems in a physically scalable fashion; however, nonuniformity of quantum sources prevents all the elements from being connected coherently for exponentially increasing Hilbert space. Here, we experimentally demonstrate 128 identical quantum sources integrated on a single silica chip. By actively controlling the light-matter interaction in femtosecond laser direct writing, we are able to unify the properties of waveguides comprehensively and therefore the spontaneous four-wave mixing process for quantum sources. We verify the indistinguishability of the on-chip sources by a series of heralded two-source Hong-Ou-Mandel interferences, with all the dip visibilities above 90%. In addition, the brightness of the sources is found easily reaching the megahertz level and being applicable to both discrete-variable and continuous-variable platforms, showing either clear antibunching feature or large squeezing parameter under different pumping regimes. The demonstrated scalability and uniformity of the quantum sources, together with integrated photonic network and detection, will enable large-scale all-on-chip quantum processors for real-life applications.

We show that the lower levels of a large-spin network with a collective antiferromagnetic interaction and collective couplings to three reservoirs may function as a quantum-absorption refrigerator. In appropriate regimes, the steady-state cooling current of this refrigerator scales quadratically with the size of the working medium, i.e., the number of spins. The same scaling is observed for the noise and the entropy production rate.

Despite quantum networking concepts, designs, and hardware becoming increasingly mature, there is no consensus on the optimal wavelength for free-space systems. We present an in-depth analysis of a daytime free-space quantum channel as a function of wavelength and atmospheric spatial coherence (the Fried coherence length). We choose decoy-state quantum key distribution bit yield as a performance metric in order to reveal the ideal wavelength choice for an actual qubit-based protocol under realistic atmospheric conditions. Our analysis represents a rigorous framework to analyze requirements for spatial, spectral, and temporal filtering. These results will help guide the development of free-space quantum communication and networking systems. In particular, our results suggest that shorter wavelengths in the optical band should be considered for free-space quantum communication systems. Our results are also interpreted in the context of atmospheric compensation by higher-order adaptive optics.

Owing to ever increasing gate fidelities and to a potential transferability to industrial CMOS technology, silicon spin qubits have become a compelling option in the strive for quantum computation. In a scalable architecture, each spin qubit will have to be finely tuned and its operating conditions accurately determined. In view of this, spectroscopic tools compatible with a scalable device layout are of primary importance. Here we report a two-tone spectroscopy technique providing access to the spin-dependent energy-level spectrum of a hole double quantum dot defined in a split-gate silicon device. A first gigahertz-frequency tone drives electric dipole spin resonance enabled by the valence-band spin-orbit coupling. A second lower-frequency tone (approximately $500\mathrm{MHz}$) allows for dispersive readout via rf-gate reflectometry. We compare the measured dispersive response to the linear response calculated in an extended Jaynes-Cummings model and we obtain characteristic parameters such as $g$ factors and tunnel and spin-orbit couplings for both even and odd occupation.

In this work, ultrastrong photon-to-magnon coupling is demonstrated for on-chip multilayered superconductor-ferromagnet-insulator hybrid thin-film structures reaching a coupling strength above 6 GHz, coupling ratio about 0.6, single-spin coupling strength about 350 Hz, and cooperativity about ${10}^4$. Strong characteristics of coupling are achieved owing to a radical suppression of the photon phase velocity in an electromagnetic resonator. With achieved coupling, the spectrum reveals the inapplicability of the Dicke quantum model, and evidences a contribution of the diamagnetic $A^2$ interaction term in the Hamiltonian of the system, which satisfies the Thomas-Reiche-Kuhn sum rule. The contribution of the $A^2$ term denotes the validity of the Hopfield quantum model and manifests observation of a different hybrid polariton quasiparticle, namely the plasmon-magnon polariton.

Quantum machine learning is expected to be one of the first practical applications of near-term quantum devices. Pioneer theoretical works suggest that quantum generative adversarial networks (GANs) may exhibit a potential exponential advantage over classical GANs, thus attracting widespread attention. However, it remains elusive whether quantum GANs implemented on near-term quantum devices can actually solve real-world learning tasks. Here, we devise a flexible quantum GAN scheme to narrow this knowledge gap. In principle, this scheme has the ability to complete image generation with high-dimensional features and could harness quantum superposition to train multiple examples in parallel. We experimentally achieve the learning and generating of real-world handwritten digit images on a superconducting quantum processor. Moreover, we utilize a gray-scale bar dataset to exhibit competitive performance between quantum GANs and the classical GANs based on multilayer perceptron and convolutional neural network architectures, respectively, benchmarked by the Fréchet distance score. Our work provides guidance for developing advanced quantum generative models on near-term quantum devices and opens up an avenue for exploring quantum advantages in various GAN-related learning tasks.

Spins in ${\mathrm{Si}}_x$${\mathrm{Ge}}_{1-x}$ quantum dots are promising candidates for quantum bits but are also challenging due to the valley degeneracy and weak spin-orbital coupling. In this work, we demonstrate that valley states can serve as an asset that enables full two-axis control of a singlet-triplet qubit formed in a double quantum dot. We measure the valley spectrum in each dot using magnetic field spectroscopy of Zeeman split triplet states. We find a finite probability of valley states to flip during interdot transitions, which in turn provides a $g$-factor difference $\mathrm{\Delta }g$ between two dots. This $\mathrm{\Delta }g$ serves as an effective magnetic field gradient and allows for qubit rotations with a rate that increases linearly with an external magnetic field. We measure several interdot transitions and find that this valley-introduced $\mathrm{\Delta }g$ is universal and electrically tunable. This could potentially simplify the scaling up of quantum information processing in the ${\mathrm{Si}}_x$${\mathrm{Ge}}_{1-x}$ platform by removing the requirement for magnetic field gradients, which are difficult to engineer.

Nitrogen-vacancy (N-$V$) centers in diamond have shown promise as inherently localized electric field sensors, capable of detecting individual charges with nanometer resolution. Working with N-$V$ ensembles, we demonstrate that a detailed understanding of the internal electric field environment enables enhanced sensitivity in the detection of external electric fields. We follow this logic along two complementary paths. First, using excitation tuned near the N-$V$’s zero-phonon line, we perform optically detected magnetic resonance (ODMR) spectroscopy at cryogenic temperatures in order to precisely measure the N-$V$ center’s excited-state susceptibility to electric fields. In doing so, we demonstrate that the characteristically observed contrast inversion arises from an interplay between spin-selective optical pumping and the N-$V$ centers’ local charge distribution. Second, motivated by this understanding, we propose and analyze a method for optically enhanced electric field sensing using N-$V$ ensembles; we estimate that our approach should enable order-of-magnitude improvements in the dc electric field sensitivity.

Previous demonstrations of free-space quantum communication in daylight have been touted as significant for the development of global-scale quantum networks. Until now, no one has carefully tuned their atmospheric channel to reproduce the daytime sky radiance and slant-path turbulence conditions as they exist between space and Earth. In this article we report a quantum communication field experiment under conditions representative of daytime downlinks from space. Higher-order adaptive optics increased quantum channel efficiencies far beyond those possible with tip-tilt correction alone while spatial filtering at the diffraction limit rejected optical noise without the need for an ultranarrow spectral filter. High signal-to-noise probabilities and low quantum-bit-error rates are demonstrated over a wide range of channel radiances and turbulence conditions associated with slant-path propagation in daytime. The benefits to satellite-based quantum key distribution are quantified and discussed.

We present a lab-on-chip experiment to accurately measure losses of superconducting microstrip lines at microwave and submillimeter wavelengths. The microstrips are fabricated from $\mathrm{Nb}\text{−}\mathrm{Ti}\text{−}\mathrm{N}$, which is deposited using reactive magnetron sputtering, and amorphous silicon which is deposited using plasma-enhanced chemical vapor deposition (PECVD). Submillimeter wave losses are measured using on-chip Fabry-Perot resonators (FPRs) operating around 350 GHz. Microwave losses are measured using shunted half-wave resonators with an identical geometry and fabricated on the same chip. We measure a loss tangent of the amorphous silicon at single-photon energies of $\tan \delta =3.7\pm 0.5\times {10}^{-5}$ at approximately $6\mathrm{GHz}$ and $\tan \delta =2.1\pm 0.1\times {10}^{-4}$ at 350 GHz. These results represent very low losses for deposited dielectrics, but the submillimeter wave losses are significantly higher than the microwave losses, which cannot be understood using the standard two-level system loss model.

The coherence of state-of-the-art superconducting qubit devices is predominantly limited by two-level-system defects, which are found primarily at amorphous interface layers. Reducing microwave loss from these interfaces by proper surface treatments is key to pushing the device performance forward. Here, we study niobium resonators after removing the native oxides with a hydrofluoric acid etch. We investigate the reappearance of microwave losses introduced by surface oxides that grow after exposure to air. We find that microwave loss in resonators are reduced by approximately an order of magnitude: internal Q factors increase from 1 × ${10}^6$ to 7 × ${10}^6$ in the single-photon regime when oxides are removed and device exposure to air is no longer than 16 min. Furthermore, we observe that ${\mathrm{Nb}}_2{\mathrm{O}}_5$ is the only surface oxide that grows significantly within the first 200 h, following the extended Cabrera-Mott growth model. In this time, microwave losses scale linearly with the ${\mathrm{Nb}}_2{\mathrm{O}}_5$ thickness, with an extracted loss tangent, $\tan {\delta }_{\mathrm{N}{\mathrm{b}}_2{\mathrm{O}}_5}$, of about $1\times {10}^{-2}$. Our findings are of particular interest for devices spanning from superconducting qubits, quantum-limited amplifiers, and microwave kinetic inductance detectors to single-photon detectors.

Superconducting resonators enable fast characterization and readout of mesoscopic quantum devices. Finding ways to perform measurements of interest on such devices using resonators only is therefore of great practical relevance. We report an experimental investigation of an $\mathrm{In}\mathrm{As}$ nanowire multiquantum dot device by probing gigahertz resonators connected to the device. First, we demonstrate accurate extraction of the dc conductance from measurements of the high-frequency admittance. Because our technique does not rely on dc calibration, it could potentially obviate the need for dc measurements in semiconductor qubit devices. Second, we demonstrate multiplexed gate sensing and the detection of charge tunneling on microsecond timescales. The gigahertz detection of dispersive resonator shifts allows rapid acquisition of charge stability diagrams, as well as resolving charge tunneling in the device with a signal-to-noise ratio of up to 15 in $1\mu \mathrm{s}$. Our measurements show that gigahertz-frequency resonators may serve as a universal tool for fast tuneup and high-fidelity readout of semiconductor qubits.

Group-IV color centers in diamond have attracted significant attention as solid-state spin qubits because of their excellent optical and spin properties. Among these color centers, the tin-vacancy ($\mathrm{Sn}$-$V^{-}$) center is of particular interest because its large ground-state splitting enables long spin coherence times at temperatures above 1 K. However, color centers typically suffer from inhomogeneous broadening, which can be exacerbated by nanofabrication-induced strain, hindering the implementation of quantum nodes emitting indistinguishable photons. Although strain and Raman tuning have been investigated as promising tuning techniques to overcome the spectral mismatch between distinct group-IV color centers, other approaches need to be explored to find methods that can offer more localized control without sacrificing emission intensity. Here, we study the electrical tuning of $\mathrm{Sn}$-$V^{-}$ centers in diamond via the direct-current Stark effect. We demonstrate a tuning range beyond 1.7 GHz. We observe both quadratic and linear dependence on the applied electric field. We also confirm that the tuning effect we observe is a result of the applied electric field and is distinct from thermal tuning due to Joule heating. Stark tuning is a promising avenue toward overcoming detunings between emitters and enabling the realization of multiple identical quantum nodes.

We explore a two-qubit system defined on valley isospins of two electrons confined in a gate-defined double quantum dot created within a ${\mathrm{Mo}\mathrm{S}}_2$ monolayer flake. We show how to initialize, control, interact, and read out such valley qubits by only electrical means using voltages applied to the local planar gates, which are layered on the top of the flake. By demonstrating the two-qubit exchange or readout via the Pauli blockade, we prove that valley qubits in the transition-metal-dichalcogenide semiconductor family fulfill the universality criteria and represent a scalable quantum-computing platform. Our numerical experiments are based on the tight-binding model for a ${\mathrm{Mo}\mathrm{S}}_2$ monolayer, which gives single-electron eigenstates that are then used to construct a basis of Slater determinants for the two-electron configuration space. We express screened electron-electron interactions in this basis by calculating the Coulomb matrix elements using localized Slater-type orbitals. Then we solve the time-dependent Schrödinger equation and obtain an exact time evolution of the two-electron system. During the evolution we simultaneously solve the Poison equation, finding the confinement potential controlled via voltages applied to the gates.

We present a hybrid semiconductor-based superconducting qubit device that remains coherent at magnetic fields up to 1 T. The qubit transition frequency exhibits periodic oscillations with the magnetic field, consistent with interference effects due to the magnetic flux threading the cross section of the proximitized semiconductor nanowire junction. As the induced superconductivity revives, additional coherent modes emerge at high magnetic fields, which we attribute to the interaction of the qubit and low-energy Andreev states.

An important requirement for spin-based quantum information processing is reliable and fast readout of electron spin states, allowing for feedback and error correction. However, common readout techniques often require additional gate structures, hindering device scaling, or impose stringent constraints on the tuning configuration of the sensed quantum dots. Here, we operate an in-line charge sensor within a triple quantum dot, where one of the dots is coupled to a microwave cavity and used to readout the charge states of the other two dots. Owing to the proximity of the charge sensor, we observe a near-digital sensor response with a power signal-to-noise ratio greater than $450$ at an integration time of $t_{\mathrm{int}}=1\mu \mathrm{s}$. Despite small singlet-triplet splittings of approximately 40 $\mu \mathrm{eV}$, we further utilize the sensor to measure the spin relaxation time of a singlet-triplet qubit, achieving an average single-shot spin readout fidelity greater than 99%. Our approach enables high-fidelity spin readout, combining minimal device overhead with flexible qubit operation in semiconductor quantum devices.