Techniques based on classical and quantum correlations in light beams, such as ghost imaging, allow us to overcome many limitations of conventional imaging and sensing protocols. Despite their advantages, applications of such techniques are often limited in practical scenarios where the position and the longitudinal extension of the target object are unknown. In this work, we propose and experimentally demonstrate an imaging technique, named light-field ghost imaging, that exploits light correlations and light-field imaging principles to enable going beyond the limitations of ghost imaging in a wide range of applications. Notably, our technique removes the requirement to have prior knowledge of the object distance, allowing the possibility of refocusing in postprocessing, as well as performing three-dimensional imaging while retaining all the benefits of ghost imaging protocols.

In the design and investigation of superconducting qubits and related devices, a lumped-element circuit model is the standard theoretical approach. However, many important physical questions lie beyond its scope, such as the behavior of circuits with strong Josephson junctions carrying substantial currents and the properties of very small superconducting devices. By performing gauge transformations on self-consistent solutions of the Bogoliubov–de Gennes equations, we develop here a formalism that treats Josephson couplings nonperturbatively. We apply the formalism to (a) show that Fermi sea effects can contribute to the effective capacitance of small charge qubits; (b) demonstrate an asymmetry in clockwise and counterclockwise current states in small rf superconducting quantum interference device qubits; and (c) provide a microscopic wave function of superconducting Schrödinger cats suitable for computing the number of entangled electrons.

Refractive-index sensors have been used in a wide range of applications, including biological diagnosis and therapy, material analysis and shaping, (bio)chemical sensing, environmental monitoring, and many more. A good sensor should have sufficiently high sensitivity and figures of merit to detect the slightest changes in the environment. Here, we study the performance of high-$Q$ Mie modes of dielectric cylinders with ultrahigh permittivity for refractive-index sensing. First, we experimentally demonstrate the emergence of high-$Q$ Mie modes in the scattering spectrum of a single dielectric cylinder with ultrahigh permittivity ($\epsilon \approx 40–100$). We also investigate the sensitivity of the high-$Q$ Mie modes to the background refractive index of the surrounding environment. The simulation results demonstrate that the high-$Q$ Mie modes are sensitive to changes in the surrounding refractive index. This results in high sensitivities and remarkable figures of merit, indicating their potential for refractive-index sensing applications. However, the material losses limit the observation of high-$Q$ modes and their sensing capability.

The Stoner-Wohlfarth model (SW model) is used to explain the relationship between the external magnetic field and the magnetization reversal of a single magnetic domain. However, it is inappropriate for inhomogeneous magnetic films or bulk materials, such as granular and defective films, because the magnetization reversal of the films is affected by magnetic particles or grains, which could have a different anisotropy for uniaxial magnetic anisotropy (UMA) of the films. In this paper, we propose a model based on the SW model to understand the magnetization switching behavior of magnetic films with misoriented grains. Our model considers the distribution effect of different anisotropies originating from magnetic defects or domains against the UMA of a magnetic thin film, and a Gaussian function is employed for the probability distribution of tilted anisotropy. Finally, we experimentally demonstrate that our model describes the magnetic switching behavior of a granular magnetic film ($\mathrm{Fe}\mathrm{Pt}$-$\mathrm{C}$) with perpendicular magnetic anisotropy. This study provides an alternative analytical method for the magnetization reversal in magnetic devices.

Characterizing the time over which quantum coherence survives is critical for any implementation of quantum bits, memories, and sensors. The usual method for determining a quantum system’s decoherence rate involves a suite of experiments probing the entire expected range of this parameter, and extracting the resulting estimation in postprocessing. Here we present an adaptive multiparameter Bayesian approach, based on a simple analytical update rule, to estimate the key decoherence timescales ($T_1$, $T_2^{\ast }$, and $T_2$) and the corresponding decay exponent of a quantum system in real time, using information gained in preceding experiments. This approach reduces the time required to reach a given uncertainty by a factor up to an order of magnitude, depending on the specific experiment, compared to the standard protocol of curve fitting. A further speedup of a factor approximately $2$ can be realized by performing our optimization with respect to sensitivity as opposed to variance.

Skyrmions are topologically protected quasiparticles, originally studied in condensed-matter systems and recently in photonics, with great potential in high-density information storage. Despite the recent attention, most optical solutions require complex systems yet produce limited topologies. Here we demonstrate an extended family of quasiparticles beyond normal skyrmions that are controlled in compact photonic gradient-index media, extending to higher-order members such as multiskyrmions and multimerons, with increasingly complex topologies. We introduce multiple topological numbers (centrality, radiality, vorticity, and polarity) in addition to the skyrmion number to describe these photonic quasiparticles. Our compact creation system lends itself to integrated and programmable solutions of complex particle textures, with potential impacts on both photonic and condensed-matter systems for revolutionizing topological informatics and logic devices.

Water oxidation at the oxygen-evolving complex (OEC) of the photosystem II is catalyzed by the core cluster ${\mathrm{Ca}\mathrm{Mn}}_4{\mathrm{O}}_5$, which has been projected to experience five intermediate states ${\mathrm{S}}_i$ in the Kok’s cycle since the 1970s. However, the detailed dynamics of state evolutions still remains unclear, albeit with the general fact that the process is initiated by the transfer of photoelectrons with the steady electron donors of the water molecules. Based on density-functional simulations, we find that the spin flips of manganese atoms between the consecutive states in the electric polarization field can be used as a marker to uncover the intricate dynamic evolutions and the underlying dynamics. The dynamic electron and proton transfers and water insertion and dissociation are traced to reveal the evolution pathways of ${\mathrm{S}}_0–{\mathrm{S}}_3$ with commensurate spin flips towards the exact spin configuration of the next state. In particular, the various water insertions and dissociations at coordination sites of the ${\mathrm{S}}_2$ open and closed cubane isomers are predicted with constraints on the necessary spin flips. Our study paves the way to uncover the animated OEC evolutions with the spin flips that await more experimental verifications and lays a solid ground for revealing the mechanism of dioxygen generation via the pending ${\mathrm{S}}_4$ state.

The increasing complexity of recent photonic experiments challenges the development of efficient multichannel coincidence counting systems with high-level functionality. Here, we report a coincidence unit able to count detection events ranging from single to 16-fold coincidences with full channel-number resolution. The device operates within sub-100-ps coincidence time windows, with a maximum input frequency of 1.5 GHz and an overall jitter of less than 10 ps. The unit high-level timing performance renders it suitable for quantum photonic experiments employing low-timing-jitter single-photon detectors. Additionally, the unit can be used in complex photonic systems to drive feed-forward loops. We demonstrate the developed coincidence counting unit in photon-number-resolving detection to directly quantify the statistical properties of light, specifically coherent and thermal states, with a fidelity exceeding 0.999 up to 60 photons.

We propose and analyze a protocol for stabilizing a maximally entangled state of two noninteracting qubits using active state-dependent feedback from a continuous two-qubit half-parity measurement in coordination with a concurrent, noncommuting dynamical decoupling drive. We demonstrate that such a drive can be simultaneous with the measurement and feedback, while also playing a key part in the feedback protocol itself. We show that robust stabilization with near-unit fidelity can be achieved even in the presence of realistic nonidealities, such as time delay in the feedback loop, imperfect state-tracking, inefficient measurements, dephasing from $1/f$-distributed qubit-frequency noise, and relaxation. We mitigate feedback-delay error by introducing a forward-state-estimation strategy in the feedback controller that tracks the effects of control signals already in transit. More generally, the steady state is globally attractive without the need for ancillas, regardless of the error state, in contrast to most known feedback and error-correction schemes.

Oxide thin films and interfaces with strong spin-orbit coupling have recently shown exceptionally high charge-to-spin conversion, making them potential spin-source materials for spintronics. Epitaxial strain engineering using oxide substrates with different lattice constants and symmetries has emerged as a mean to further enhance charge-to-spin conversion. However, high relative permittivity and dielectric loss of commonly used oxide substrates, such as ${\mathrm{Sr}\mathrm{Ti}\mathrm{O}}_3$, can cause significant current shunting in substrates at high frequency, which may strongly affect spin-torque measurement and potentially result in an inaccurate estimation of charge-to-spin conversion efficiency. In this study, we systematically evaluate the influence of various oxide substrates for the widely used spin-torque ferromagnetic resonance (ST-FMR) measurement. Surprisingly, we observed substantial spin-torque signals in samples comprising only ferromagnetic metal on oxide substrates with high relative permittivity (e.g., ${\mathrm{Sr}\mathrm{Ti}\mathrm{O}}_3$ and ${\mathrm{KTaO}}_3$), where negligible signal should be initially expected. Notably, this unexpected signal shows a strong correlation with the capacitive reactance of oxide substrates and the leakage rf current within the substrate. By revising the conventional ST-FMR analysis model, we attribute this phenomenon to a 90° phase difference between the rf current flowing in the metal layer and in the substrate. We suggest that extra attention should be paid during the ST-FMR measurements, as this artifact could dominate over the real spin-orbit torque signal from high-resistivity spin-source materials grown on substrate with high relative permittivity.

Quantum microwave photonics (QMWP) technology, which harnesses energy-time-entangled biphotons as optical carriers and employs time-correlated single-photon detection for high-speed rf-signal recovery, has demonstrated remarkable potential in achieving nonlocal rf-signal coding and efficient rf-signal recovery from significant dispersion interference. In this paper, we expound on the principle of maximizing the recovered rf signals in QMWP systems. Through theoretical analysis and experimental validation, we establish a precise relationship between the recovered rf signal’s magnitude, the input rf signal’s frequency, involved dispersions, and the width of the coincidence selection window. This enables us to determine the optimal width for the coincidence selection window and the corresponding magnitude of the maximized rf signal given the dispersion. These findings offer valuable guidance for improving QMWP systems, thus expanding their potential applications.

Accurately controlling the quantum coherence of photons is pivotal for their applications in quantum sensing and quantum imaging. Here, we propose the utilization of quantum entanglement and local phase manipulation techniques to control the higher-order quantum coherence of photons. By engineering the spatially varying phases in the transverse plane, we can precisely manipulate the spatial structure of the second-order coherence function of entangled photon pairs without changing the photon intensity distribution of each photon. Our approach can readily be extended to higher-order quantum coherence control. These results could potentially stimulate experimental research and applications of optical quantum coherence.

As quantum gates improve, it becomes increasingly difficult to characterize the remaining errors. Here we describe a class of coherent non-Markovian errors—excitations due to an off-resonant drive—that occur naturally in quantum devices that use time-dependent fields to generate gate operations. We show how these errors are mischaracterized using standard quantum computer verification and validation techniques that rely on Markovianity and are therefore often overlooked or assumed to be incoherent. We first demonstrate off-resonant errors within a simple toy model of $Z$ gates created by the ac Stark effect, then show how off-resonant errors manifest in all gates driven on a fixed-frequency transmon architecture, a prominent example being incidental cross-resonance interaction driven during single-qubit gates. Furthermore, the same methodology can access the errors caused by two-level systems, showing evidence of coherent, off-resonant interactions with subsystems that are not intentional qubits. While we explore these results and their impact on gate error for fixed-frequency devices, we note that off-resonant excitations potentially limit any architectures that use frequency selectivity.

In this work we experimentally demonstrate the implementation of a recently proposed robust and state-independent heat-bath algorithmic cooling (HBAC) method [Sadegh Raeisi, Mária Kieferová, and Michele Mosca, Phys. Rev. Lett. 122, 220501 (2019). doi:10.1103/PhysRevLett.122.220501] on an NMR quantum processor. While HBAC methods improve the purity of a quantum system via iterative unitary entropy compression, they are difficult to implement experimentally since they use sort operations that are different for each iteration. The new robust HBAC method proved that optimal HBAC is possible without prior state information and using a single fixed operation. We modified the protocol to experimentally perform efficient cooling of ${}^{13}\mathrm{C}$ and ${}^{15}\mathrm{N}$ spins and provide an optimal decomposition of this modified protocol in terms of quantum gates. We examined the relaxation dynamics of these algorithmically cooled spins, in order to ascertain the effect of decoherence on the cooled states.

High-fidelity universal quantum gates are widely acknowledged as essential for scalable quantum computation. However, in solid-state quantum systems, which hold promise as physical implementation platforms for quantum computation, the inevitable $ZZ$ crosstalk resulting from interqubit interactions significantly impairs quantum operation performance. Here we propose a scalable protocol to achieve $ZZ$-crosstalk mitigation in universal quantum gates. This method converts the noisy Hamiltonian with $ZZ$ crosstalk into a framework that efficiently suppresses all $ZZ$-crosstalk effects, leading to ideal target quantum operations. Specifically, we first analytically derive the $ZZ$-crosstalk mitigation conditions and then apply them to enhance the performance of target universal quantum gates. Moreover, numerical simulations validate the effectiveness of $ZZ$-crosstalk mitigation when multiple qubit gates operate concurrently. As a result, our protocol presents a promising approach for implementing practical parallel quantum gates in large-scale quantum computation scenarios.

The strong anharmonicity and high coherence times inherent to fluxonium superconducting circuits are beneficial for quantum information processing. In addition to requiring high-quality physical qubits, a quantum processor needs to be assembled in a manner that minimizes crosstalk and decoherence. In this paper, we report on fluxonium qubits packaged in a flip-chip architecture, where a classical control and readout chip is bump bonded to the quantum chip, forming a multichip module. The modular approach allows for improved connectivity between the qubits and control and readout elements, and separate fabrication processes. We characterize the coherence properties of the individual fluxonium qubits, demonstrate high-fidelity single-qubit gates with 6-ns microwave pulses (without DRAG), and identify the main decoherence mechanisms to improve on the reported results.

Coherent high-quality laser sources are important for a variety of photonic devices and applications, yet they are especially challenging for high-power semiconductor lasers, because high-power and single-mode lasing present conflicting requirements; high power drives nonlinear processes causing multimode lasing and instabilities. Here, we present a distributed feedback (DFB) microring laser coupled to a uniform ring, which displays single-mode lasing up to 10 times the threshold value. This is the first microring DFB laser in gallium nitride ($\mathrm{Ga}\mathrm{N}$) and the first monolithic coupled microring $\mathrm{Ga}\mathrm{N}$ lasers. Our work paves the way for future designs of high-quality high-power coupled lasers for applications, such as topological lasers, high-power monolithic frequency combs, optical clocks, gyros, sensing, and quantum systems.