Maximizing the computational utility of near-term quantum processors requires predictive noise models that inform robust, noise-aware compilation and error mitigation. Conventional models often fail to capture the complex error dynamics of real hardware or require prohibitive characterization overhead. We introduce a data-efficient framework that first constructs a physically motivated, parameterized noise model, and subsequently employs machine-learning-driven Bayesian optimization to identify its parameters. Our approach circumvents costly characterization protocols by estimating algorithm- and hardware-specific error parameters directly from readily available experimental data derived from existing application and benchmark circuit executions. The generality and robustness of the framework are demonstrated across diverse algorithms and superconducting devices, yielding high-fidelity predictions by estimating an independent parameter set tailored to each specific algorithm-hardware context. Crucially, we show that a model calibrated exclusively on small-scale circuits accurately predicts the behavior of larger validation circuits. Our data-efficient approach achieves up to a 65% improvement in model fidelity quantified by the Hellinger distance between predicted and experimental circuit output distributions, compared to standard noise models derived from device properties. This work establishes a practical paradigm for application-aware noise characterization, enabling compilation and error-mitigation strategies tailored to the specific interplay between quantum algorithms and device-specific noise dynamics.

Spin shuttling offers a promising approach for developing scalable silicon-based quantum processors by addressing the connectivity limitations of quantum dots. In this work, we demonstrate high-fidelity bucket-brigade spin shuttling in a silicon MOS device, utilizing Pauli-spin-blockade readout. We achieve an average shuttling fidelity of 99.8%. The residual shuttling error is highly sensitive to the ratio between interdot tunnel coupling and Zeeman splitting, with tuning of these parameters enabling up to a 20-fold variation in error rate. An appropriate four-level Hamiltonian model supports our findings. These results provide valuable insights for optimizing high-performance spin-shuttling systems in future quantum architectures.

Silicon nitride microresonators driven by strong pump pulses can generate squeezed light in a dominant spectral-temporal mode, a central resource for continuous-variable quantum computation. In the high parametric gain regime, several effects, including self- and cross-phase modulation as well as time-ordering corrections, become significant and can degrade source performance. In this work, we comprehensively investigate the generation of squeezed light from a silicon nitride resonator under pulsed pumping, spanning from low to high parametric gain up to approximately $16$ photons/pulse. We experimentally study how the average photon number and the first- and second-order correlations of the squeezed marginal modes evolve with increasing pulse energy, across various frequency detunings and pulse durations. Furthermore, we analyze the errors introduced by multipair emissions in estimating the joint temporal intensity via time-resolved coincidence measurements. We propose and demonstrate an error-correction strategy based on the marginal distributions of time-resolved multiphoton events. Our results provide a practical strategy for optimizing the gain and the temporal mode structure of pulsed squeezed light sources in microresonators, elucidating the physical mechanisms and limitations that govern source performance in the high-gain regime.

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.

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.

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.

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.

$\mathrm{Si}$/$\mathrm{Si}\mathrm{Ge}$ 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.

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.

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.

We introduce a new method, rooted in estimation theory, to detect individual atoms in site-resolved images of microtrap arrays, such as optical lattices or optical tweezer arrays. Using labeled test images, we demonstrate drastic improvement of the detection accuracy, compared with the popular method based on Wiener deconvolution, when the intersite distance is comparable with the radius of the point spread function. The runtime of our method scales approximately linearly with the number of sites, and remains well below 100 ms for an array of $100\times 100$ sites on a desktop computer. It is therefore fully compatible with a real-time usage. Finally, we propose a rigorous definition for the signal-to-noise ratio of the problem and show that it can be used as a predictor for the detection error rate. Our work opens the prospect for future experiments with increased array sizes or reduced intersite distances.

We report the fabrication and optical characterization of an air-suspended photonic crystal nanobeam cavity in yttrium iron garnet (YIG) realized by focused-ion-beam milling. YIG’s combination of low optical loss and ferrimagnetism makes it highly attractive for quantum technologies, yet prior work has largely been focused on millimeter-scale spheres and simple microstructures, hindering true on-chip integration. Demonstrating nanometer-scale patterning in a suspended geometry therefore represents an important advance. Finite-element simulations predict that the same structure supports a flapping-type mechanical mode at ${\mathrm{\Omega }}_m/2\pi \approx 1.52\mathrm{GHz}$ and a backward-volume spin-wave mode at ${\omega }_{\mathrm{mag}}/2\pi =11.59\mathrm{GHz}$ under an in-plane bias field. Although we measured only the photonic resonance (intrinsic $Q\approx 2\times {10}^3$) in this study, the device lays the groundwork for future exploration of coupled photon-phonon-magnon dynamics once higher optical quality factors are achieved.

We demonstrate a small-footprint electromechanical phase shifter with a compact active length of $10\text{μ}\mathrm{m}$ fabricated on a suspended $\mathrm{Ga}\mathrm{As}$ membrane, offering versatile integration with quantum-dot single-photon sources. The phase shifter is based on a slot waveguide, whose slot width can be controlled by electrostatic forces, enabling a large effective refractive index change $\mathrm{\Delta }{n}_{\mathrm{eff}}>0.1$. We observe up to $3\pi$ phase modulation with 10.6 V applied bias, and a figure of merit $V_{\pi }L=5.7\times {10}^{-3}\mathrm{V}\mathrm{cm}$. Integration with a Mach-Zehnder interferometer further allows routing of single photons with an extinction ratio of up to 24 dB at cryogenic temperatures. This device enables advanced manipulation of quantum emitters and the realization of reconfigurable quantum photonic integrated circuits.

Noisy voltage sources can be a limiting factor for both fundamental physics experiments and device applications in quantum information, mesoscopic circuits, magnetometry, and other fields. The best commercial dc voltage sources can be programmed to approximately six digits and have intrinsic noise in the microvolt range. On the other hand, the noise levels in metrological Josephson-junction-based voltage standards is subfemtovolt. Although such voltage standards can be considered “noiseless,” they are generally not designed for continuous tuning of the output voltage nor for supplying current to a load at cryogenic temperatures. We propose a Josephson-effect-based voltage source, as opposed to a voltage standard, operating in the $30-160\text{−}\text{μ}\mathrm{V}$ range, which can supply over 100 nA of current to loads at mK temperatures. We describe the operating principle, the sample design, and the calibration procedure to obtain continuous tunability. We show the current-voltage characteristics of the device, demonstrate how the voltage can be adjusted without dc control connections to room-temperature electronics, and showcase an experiment coupling the source to a mesoscopic load: a small Josephson junction. Finally, we characterize the performance of our source by measuring the voltage noise at the load, 50 pV rms, which is attributed to parasitic resistances in the cabling. This work establishes the use of the Josephson effect for voltage biasing of extremely sensitive quantum devices.

Partial tomography, which focuses on reconstructing reduced density matrices (RDMs), has emerged as a promising approach for characterizing complex quantum systems, particularly when full state tomography is impractical. Recently, overlapping tomography has been proposed as an efficient method for determining all $k$-qubit RDMs using logarithmic polynomial measurements, though it has not yet reached the ultimate limit. Here, we introduce a unified framework for optimal quantum overlapping tomography by mapping the problem to the clique cover model. This framework provides the most efficient and experimentally feasible measurement schemes to date, significantly reducing the measurement costs. Our approach is also applicable to determining RDMs with different topological structures. Moreover, we experimentally validate the feasibility of our schemes on a practical nuclear-spin processor using average measurements, and further apply our method to noisy data from superconducting quantum processors employing projection measurements. The results validate the utility of overlapping tomography, paving the way for the characterization of advanced quantum systems and learning about state properties in the future.

To realize long-distance quantum communication, it is crucial to design quantum repeater architectures that can deal with transmission losses and operational errors. Code concatenation of photonic graph codes is a promising way to achieve this; however, existing concatenated codes that can correct both transmission losses and operational errors are extremely hardware demanding. We propose a one-way quantum repeater architecture based on concatenated ring graph codes and linear optical Bell-state measurements. We construct a scheme to generate the concatenated ring graph codes using quantum emitters, where the number of matter qubits scales linearly with concatenation depth. Furthermore, we devise a measurement strategy at each repeater station with a simple experimental setup where photons are measured in the order that they are created and show that entanglement swapping is fault tolerant to both transmission losses and operational errors. This allows for long-distance quantum communication ($>{10}^4$ km) at a kHz rate even in the presence of single-qubit error rates $ϵ>{10}^{-3}$.

We present a Josephson traveling-wave parametric amplifier (JTWPA) based on a low-loss lumped-element coplanar waveguide architecture. By employing open-stub capacitors and Manhattan-pattern junctions, our device achieves an insertion loss below 1 dB up to 12 GHz. We introduce windowed sinusoidal modulation for phase matching, demonstrating that a smooth transition in the impedance-modulation strength effectively suppresses intrinsic gain ripples. Using Tukey-windowed modulation with 8% impedance variation, we achieve 20–23-dB gain over 5-GHz bandwidth under ideal matching conditions. In a more practical circuit having impedance mismatches, the device maintains 17–20-dB gain over 4.8-GHz bandwidth with an added noise of 0.18 quanta above standard quantum limit at 20-dB gain and $-99$-dBm saturation power, while featuring zero to negative backward gain below the band-gap frequency.