The accurate measurement of quantum two-level objects (qubits) is crucial for developing quantum computers. Over the last decade, the measure of choice for benchmarking readout routines for superconducting qubits has been assignment fidelity; however, this method only focuses on the preparation of computational basis states and therefore does not provide a complete characterization of the readout. Here, we expand the focus to the use of detector tomography to fully characterize multiqubit readout of superconducting transmon qubits. The impact of different readout parameters on the rate of information extraction is studied using quantum-state reconstruction infidelity as a proxy. The results are then compared with assignment fidelities, showing good agreement for separable two-qubit states. We therefore propose the rate of infidelity convergence as a validation tool for assignment fidelity and a more comprehensive benchmark for single- and multiqubit readout optimization. To make the best use of experimental resources, we investigate the most efficient distribution of a limited shot budget between detector tomography and state reconstruction within the context of single- and two-qubit experiments. To address the growing interest in three-qubit gates and test the scalability of the validation tool, we perform three-qubit quantum-state tomography that goes beyond conventional readout-error-mitigation methods and find a factor-of-30 reduction in quantum infidelity. Our results demonstrate that qubit readout correlations are not induced by a significantly reduced state distinguishability. Consequently, correlation coefficients can serve as a valuable tool in qubit readout optimization.

Electron spins in semiconductor devices are highly promising building blocks for quantum processors (QPs). Commercial semiconductor foundries can create QPs using the same processes used for conventional chips, once the QP design is suitably specified. There is a vast accessible design space; to identify the most promising options for fabrication, one requires predictive modeling of interacting electrons in real geometries and complex nonideal environments. In this work we explore a modeling method based on real-space grids, an ab initio approach without assumptions relating to device topology and therefore with wide applicability. Given an electrode geometry, we determine the exchange coupling between quantum dot qubits, and model the full evolution of a $\sqrt{\mathrm{SWAP}}$ gate to predict qubit loss and infidelity rates for various voltage profiles. We determine full, three-dimensional solutions and introduce a method that can obtain near-identical predictions using far more efficient two-dimensional computations. Moreover we explore the impact of unwanted charge defects (static and dynamic) in the environment, and test robust pulse sequences. As an example we exhibit a sequence correcting both systematic errors and (unknown) charge defects, observing an order of magnitude boost in fidelity. The technique can thus identify the most promising device designs for fabrication, as well as bespoke control sequences for each such device.

Mechanical systems played a foundational role in computing history, and have regained interest due to their unique properties, such as low damping and the ability to process mechanical signals without transduction. However, recent efforts have focused primarily on elementary computations, implemented in systems based on predefined reservoirs, or in periodic systems such as arrays of buckling beams. Here we numerically demonstrate a passive mechanical system—in the form of a nonlinear mass-spring model—that tackles a real-world benchmark for keyword spotting in speech signals. The model is organized in a hierarchical architecture combining feature extraction and continuous-time convolution, with each individual stage tailored to the physics of the mass-spring systems considered. For each step in the computation, a subsystem is designed by combining a small set of low-order polynomial potentials. These potentials act as fundamental components that interconnect a network of masses. In analogy to electronic circuit design, where complex functional circuits are constructed by combining basic components into hierarchical designs, we refer to this framework as “springtronics.” We introduce springtronic systems with hundreds of degrees of freedom, achieving speech classification accuracy comparable to that of existing submilliwatt electronic systems.

As implemented in commercialized device modeling software, the four-state nonradiative multiphonon model has attracted intensive attention in the past decade for describing the physics in negative bias temperature instability (NBTI) and other reliability issues of ${\mathrm{Si}/\mathrm{Si}\mathrm{O}}_2$ metal-oxide-semiconductor field effect transistor (MOSFET) devices. It was proposed initially based on the assumption that the oxygen vacancy defects (${\mathrm{V}}_{\mathrm{O}}$) in ${\mathrm{Si}\mathrm{O}}_2$ dielectric layer are bistable in the Si-dimer and back-projected structures during carrier capture and emission. Through high-throughput first-principles structural search, we found ${\mathrm{V}}_{\mathrm{O}}$ on nonequivalent O sites in amorphous ${\mathrm{Si}\mathrm{O}}_2$ can take four types of structural configurations in neutral state and seven types of configurations in +1 charged state after capturing holes, which produce a wide range of charge-state transition levels for trapping holes. The finding contrasts the structural-bistability assumption and makes the four-state model invalid for most O sites. To describe the reliability physics accurately, we propose an all-state model to consider all these structural configurations as well as all the carrier capture/emission transitions and thermal transitions between them. With the all-state model, we show that the ${\mathrm{V}}_{\mathrm{O}}$ defects play important roles in causing NBTI, which challenges the recent studies that discarded ${\mathrm{V}}_{\mathrm{O}}$ as a possible hole trap in NBTI. Our systematical calculations on the diversified ${\mathrm{V}}_{\mathrm{O}}$ properties and the all-state model provide the microscopic foundation for describing the reliability physics of MOSFETs and other transistors accurately.

We report laser cooling and trapping of ytterbium atoms in a two-color magneto-optical trap (MOT). Benefited from both the broad singlet transition (${}^1{S}_0\to {}^1{P}_1$) and the narrow intercombination transition (${}^1{S}_0\to {}^3{P}_1$) of ytterbium atoms, the two-color MOT enables rapid loading and efficient cooling. We systematically investigate the shielding effect of the intercombination transition by examining the atom loading and loss rates of single-color and two-color MOTs. Our findings are general and can be extended to other alkaline-earth(-like) atoms.

Reservoir computing (RC), a neural network designed for temporal data, enables efficient computation with low-cost training and direct physical implementation. Recently, quantum RC has opened possibilities for conventional RC and introduced other ideas to tackle open problems in quantum physics and advance quantum technologies. Despite its promise, it faces challenges, including physical realization, output readout, and measurement-induced backaction. Here, we propose to implement quantum RC through quantum transport in mesoscopic electronic systems. Our approach possesses several advantages: compatibility with existing device-fabrication techniques, ease of output measurement, and robustness against measurement backaction. Leveraging universal conductance fluctuations, we numerically demonstrate two benchmark tasks, spoken-digit recognition and time-series forecasting, to validate our proposal. This work establishes another pathway for implementing on-chip quantum RC via quantum transport and expands the mesoscopic physics applications.

We give a general design method for finding the passive, reciprocal surface impedance tensor required to enact any wave transformation. We do this through characterizing the surface in terms of a tensorial surface impedance, showing that a large family of impedance distributions can be found that perform an identical wave transformation. Even when the conditions of reciprocity and passivity are imposed, there still remain many solutions to the design problem. We exploit this as a design method for metasurfaces, giving two examples where the metasurface rotates the input polarization and reshapes the output field, showing we can parameterize the set of equivalent reciprocal metasurfaces in terms of a single complex parameter. In addition, through allowing dissipation and gain within the response, the surface can have many different functionalities in the orthogonal polarization, opening up a new route for the design of multiplexed metasurfaces.

Controlling thermal waves, transient temperature fluctuations propagating through materials, is crucial for thermal management in aerospace, mechanical engineering, and electrical systems with pulsed heat sources and periodic temperature variations. However, their rapid attenuation due to thermal relaxation makes precise control inherently difficult, and even more so in metadevices designed to perform complex functions. Existing effective medium theory, proposed for steady-state heat transfer, is limited in addressing the transient nature of thermal-wave propagation. Here, we propose a diffusive pseudoconformal mapping theory for thermal waves to design metadevices with isotropic materials. As a specific application, we simultaneously design thermal-wave insulating and zero-index carpet cloaks within a unified spatial transformation framework. Furthermore, we uncover a geometric duality between these functionally distinct devices. Numerical simulations show that these cloaks with a carpetlike surface achieve precise invisibility in thermal-wave environments. Meanwhile, objects covered by the zero-index cloak can also sense background temperature variations, similar to sensors detecting environmental changes. We further integrate common materials with a segmented outer-layer structure and propose an experimental scheme to validate the feasibility of cloaks. Our research builds a broader pseudoconformal framework and advances the design of thermal-wave metadevices.

The cooperative effects of coherent and dissipative couplings have demonstrated significant potential for developing nonreciprocal photonic devices. However, beyond photon transmission, directional phonon lasing and coherent amplification of mechanical oscillations remain largely unexamined in the context of these cooperative effects. Here the potential for generating nonreciprocal phonon lasing within an open-cavity magnomechanical system is investigated. It is found that the interplay of coherent and dissipative couplings can lead to robust nonreciprocal mechanical gain, facilitated by the asymmetric dissipation of hybrid magnon-photon supermodes in opposite driving directions. Consequently, the phonon lasing can be selectively induced by driving the system from one direction, while remaining inactive with driving from the other direction. The study presents an experimentally viable approach to explore one-way phonon lasing through dissipation engineering, with potential applications in sound sensing, imaging, and chiral acoustics.

Quantum random-number generators (QRNGs) produce true random numbers, which are guaranteed by the fundamental principles of quantum physics. Miniaturization of QRNGs is crucial for a wide range of communication and cryptography applications. Here, we first report a fully functional QRNG chip based on vacuum-state fluctuations, with dimensions of $16.6\times 7.8{\mathrm{mm}}^2$. The quantum entropy source, which is achieved via hybrid photonic integration with an ${\mathrm{Si}\mathrm{O}}_2$ waveguide, generates raw quantum random numbers. The hybrid photonic and electrical components are assembled into a compact ceramic package using system-in-package technology. A microcontroller unit acquires the raw data and outputs the processed quantum random numbers via a serial peripheral interface. According to the characterization results, the QRNG chip achieves a constant real-time output rate of 5.2 Mbps across the industrial temperature range of $-40$ to $85{}^{\circ }\mathrm{C}$, making it suitable for practical applications.

The ability to amplify ultrashort pulses has revolutionized modern laser science, driving advances in various fields such as ultrafast optics and spectroscopy. A pivotal development in this field is chirped pulse amplification (CPA), which stretches, amplifies, and recompresses ultrashort optical pulses using dispersive elements to overcome amplification limits. However, CPA faces limitations due to gain narrowing, restricting the final pulse duration. Here we propose Doppler pulse amplification (DoPA), an approach for amplifying ultrashort pulses. While DoPA shares similarities with CPA in that it also stretches, amplifies, and recompresses pulses, it differs in how it achieves this temporal compansion (expansion and compression). Unlike CPA, DoPA exploits Doppler shifts induced by space-time-modulated interfaces through a space-time wedge implementation without chirping. We show that DoPA dynamically shifts the pulse spectrum, effectively mitigating the gain narrowing issue of CPA. Additionally, we show that DoPA enables more compact amplification systems via a space-time Fresnel implementation. This approach may pave the way for more efficient, high-intensity laser systems and expand the potential for applications in both laboratory research and practical environments.

Quantum nonlinear optics using Rydberg polaritons can enable single-photon transistor and switch, single-photon source, and deterministic quantum information processing. A major hindrance in this study is the fast motional decoherence. Here, we devise a scheme to significantly enhance the coherence of Rydberg polaritons by letting the atoms remember their velocities or, alternatively, by changing the phase of a Rydberg polariton according to its storage time. After the Rydberg polariton is prepared with a Rydberg state $|r_1⟩$, i.e., during the storage time, two laser fields induce a transition between $|r_1⟩$ and a nearby Rydberg state $|r_2⟩$ via a low-lying intermediate state $|f⟩$ which is largely detuned. In particular, we find that either a $2\pi \mathbb{N}$ protocol, a $\pi$-wait-$\pi$ protocol, or a wait-$\pi$ protocol, along with an appropriate choice of $|f⟩$ can lead to a phase-coherent Rydberg polariton upon its retrieval. Importantly, the coherent transition between $|r_1⟩$ and $|r_2⟩$ ensures that the Rydberg polariton can block the Rydberg excitation of nearby atoms as in usual applications of Rydberg polaritons. Numerics show that the theory can nearly completely eliminate the motional dephasing, leaving Rydberg-state decay as the only fundamental channel of decoherence. This sheds light on a broad application of Rydberg-mediated quantum nonlinear optics.

Optically accessible solid-state-defect spins serve as a primary platform for quantum information processing, where precise control of the electron spin and ancillary nuclear spins is essential for operation. With use of the nitrogen-vacancy color center in diamond as an example, by a combined group theory and density functional theory study we demonstrate that spin-lattice relaxation of the ${}^{14}\mathrm{N}$ nuclear spin is significantly enhanced due to strong entanglement with orbital degrees of freedom in the ${|}^{3}E⟩$ optically excited state of the defect. This mechanism is common to other solid-state-defect nuclear spins with similar optically excited states. Additionally, we propose a straightforward and versatile ab initio scheme for predicting orbital-dependent spin Hamiltonians for trigonal defects exhibiting orbital degeneracy.

In this work, we present a new diagrammatic method for computing the effective Hamiltonian of driven nonlinear oscillators. At the heart of our method is a self-consistent perturbation expansion developed in phase space, which establishes a direct correspondence between the diagram and algebra. Each diagram corresponds to a Hamiltonian term, the prefactor of which, like those in Feynman diagrams, involves a simple counting of topologically equivalent diagrams. Leveraging the algorithmic simplicity of our diagrammatic method, we provide a readily available computer program that generates the effective Hamiltonian to arbitrary order. We show the consistency of our schemes with existing perturbation methods, such as the Schrieffer-Wolff method. Furthermore, we recover the classical harmonic balance scheme from our result in the limit of $\hslash \to 0$. Our method contributes to the understanding of dynamic control within quantum systems and achieves precision essential for advancing future quantum information processors. To demonstrate its value and versatility, we analyze five examples from the field of superconducting circuits. These include an experimental proposal for the Hamiltonian stabilization of a three-legged Schrödinger cat, modeling of energy renormalization phenomena in superconducting circuit experiments, a comprehensive characterization of multiphoton resonances in a driven transmon, a proposal for an inductively shunted transmon circuit, and a characterization of classical ultrasubharmonic bifurcation in driven oscillators. Lastly, we benchmark the performance of our method by comparing it with experimental data and exact Floquet numerical diagonalization.

Theoretically understanding and experimentally characterizing and modifying the underlying Hamiltonian of a quantum system is of utmost importance in achieving high-fidelity quantum gates for quantum computing. In this work, we explore the use of dynamical decoupling (DD) in characterizing and suppressing undesired two-qubit couplings as well as the underlying single-qubit decoherence, both significant hurdles to achieving precise quantum control and realizing quantum computing on many hardware prototypes. Through discrete search of DD sequences, we identify sequences that protect against decoherence and selectively target unwanted two-qubit interactions of general form. On a transmon-qubit-based superconducting quantum device, we identify separate white and $1/f$ noise components underlying the single-qubit decoherence and a static ZZ coupling between pairs of qubits. A family of syncopated DD sequences is found and their efficiency is demonstrated in two-qubit benchmarking experiments. The syncopated decoupling technique significantly boosts performance in a realistic algorithmic quantum circuit.

Type-II fusion is a probabilistic entangling measurement that is essential to measurement-based linear-optical quantum computing and can be used for quantum teleportation more broadly. However, it remains under-explored for high-dimensional qudits. Our main result gives a Type-II fusion protocol with proven success probability approximately $2/d^2$ for qudits of arbitrary dimension $d$. This generalizes a previous method that only applied to even-dimensional qudits. We believe this protocol to be the most efficient known protocol for Type-II fusion, with the $d=5$ case beating the previous record by a factor of approximately $723$. We discuss the construction of the required $(d-2)$-qudit ancillary state using a silicon spin-qudit ancilla coupled to a microwave cavity through time-bin multiplexing. We then introduce a general framework of extra-dimensional corrections, a natural technique in linear optics that can be used to nondeterministically correct nonmaximally entangled projections into Bell measurements. We use this method to analyze and improve several different circuits for high-dimensional Type-II fusion and compare their benefits and drawbacks.