The idea was proposed by David Klyshko in 1988 and verified experimentally in 1995 by Pittman and colleagues, who used pairs of entangled photons to form images of an object that neither photon had individually interacted with in a conventional way. For years, ghost imaging seemed inseparable from quantum entanglement. But researchers soon discovered that classical light sources—thermal lamps, polarized lasers, and laser beams passed through rotating diffusers to create pseudo-thermal speckle—could do the same job. The pivotal moment came in 2008 with computational ghost imaging, which dispensed with the reference arm entirely. Instead of measuring a reference beam, the system projects known patterns onto the object and reconstructs the image from the bucket signal alone, using the patterns as a computational reference. That simplification turned ghost imaging into a practical single-pixel imaging technology.

Mathematically, the reconstruction is straightforward. The image value at each position is obtained by correlating the bucket-detector readings with the known or measured intensity patterns, subtracting the product of the mean signals to isolate genuine fluctuations. Refinements followed quickly. Differential ghost imaging weights the correlation by the total source intensity, suppressing background noise and improving results for highly transparent or reflective objects. Normalized ghost imaging standardizes the reference intensity distribution, making the reconstruction insensitive to absolute brightness fluctuations. Compressive sensing, also introduced in 2008, exploits the fact that most natural images are sparse in some mathematical basis: by acquiring far fewer random measurements than the Nyquist sampling theorem would demand and solving a nonlinear optimization that favors sparse solutions, researchers can reconstruct images from dramatically reduced data. Deep learning entered the field in 2017, when Lyu and colleagues trained a neural network to denoise and refine low-sampling-rate ghost images, opening the door to reconstruction methods that now routinely outperform handcrafted algorithms.

Why go to all this trouble when cameras are everywhere? The answer, the review argues, is that conventional multi-pixel detectors such as CMOS and CCD sensors are optimized for narrow wavelength ranges and degrade badly outside them. In the extreme ultraviolet, at terahertz frequencies, and in much of the mid-infrared, fast high-resolution detector arrays are expensive, scarce, or simply nonexistent. A single-pixel detector, by contrast, can be chosen for peak sensitivity at almost any wavelength, and the simplicity of the setup reduces cost and complexity. Ghost imaging therefore thrives precisely where traditional imaging hardware struggles—decoupling illumination from detection and letting computation do the heavy lifting.

The technique has even crossed from photons to matter. In 2016, Khakimov and colleagues produced correlated pairs of metastable helium atoms by colliding a Bose–Einstein condensate, achieving micron-scale ghost images of a mask with massive particles. Follow-up work showed that higher-order correlations among ultracold atoms can boost image visibility without sacrificing resolution. Neutron ghost imaging, demonstrated on a reactor beamline by Kingston and colleagues, exploits the penetrating power of thermal neutrons, which pass through dense metals yet highlight light elements such as hydrogen—complementing X-rays in nondestructive materials evaluation while reducing dose. Electron ghost imaging, realized by Li and co-workers using patterned electron beams driven by a digital micromirror device, promises to cut acquisition time and specimen damage by orders of magnitude relative to conventional electron microscopy, a boon for radiation-sensitive samples.

X-ray ghost imaging may be the most consequential branch for medicine and materials science. X-rays penetrate soft tissue far more readily than bone or metal, but they also ionize, making radiation dose a critical constraint. In 2016, two groups independently achieved X-ray ghost imaging: Yu and colleagues performed lensless Fourier-transform ghost imaging with pseudo-thermal hard X-rays, while Pelliccia and colleagues split a synchrotron beam in Laue diffraction geometry and exploited natural speckle in the source. By 2018, Zhang and colleagues had demonstrated tabletop X-ray ghost imaging with dramatically reduced radiation, and a follow-up study combined Hadamard-patterned illumination with a multi-level wavelet convolutional neural network to reach roughly 10-micrometer resolution using only 18.75 percent of the Nyquist sampling rate—conditions the authors suggest could suit early-stage cancer detection. Polycapillary optics have since tripled resolution by shrinking the illumination speckle from 166 to 55 micrometers, and ghost tomography has extended the method to three-dimensional volumetric reconstruction. At free-electron lasers, ghost imaging has reached the extreme ultraviolet, enabling nanometer-scale imaging of radiation-sensitive samples.

In the visible and near-infrared, where cameras are cheap and excellent, ghost imaging finds its niche in hostile environments. Underwater, blue and violet light between 400 and 500 nanometers penetrates best, and compressive computational ghost imaging with Hadamard patterns and wavelet-domain enhancement has recovered images through turbid water; temporal ghost imaging has even carried error-free optical data across meter-scale underwater channels. Structured beams such as pseudo-Bessel rings and Lorentz beams, which self-heal after obstruction, improve contrast at depth. Deep learning has transformed the visible band: self-supervised dual networks now extract task-relevant features from raw speckle without labeled training data, block-wise networks iteratively refine multi-scale sub-images using only the bucket signal as supervision, and multi-input mutual-supervision frameworks co-train random-pattern and bucket-signal branches to correct artifacts. Multi-polarization fusion networks capture bucket signals under several polarization states and exploit their statistical differences to reconstruct scenes through dynamic scattering media.

The infrared story is equally striking. Because infrared light rides atmospheric windows, it sees through fog, smoke and dust while avoiding photo-damage—ideal for surveillance, remote sensing and diagnostics. Radwell and colleagues replaced the focal-plane array with a single indium-gallium-arsenide detector in a dual-band microscope, then pushed to 10-hertz real-time video through smoke and tinted glass, and later captured video-rate images of methane gas leaks at the 1.65-micrometer absorption line. Near-infrared ghost-imaging LiDAR has produced centimeter-resolution depth maps from a moving aircraft, and broadband single-pixel hyperspectral cameras classify concealed chemicals across 900 to 1700 nanometers at milliwatt illumination. In the mid-infrared, where digital micromirror devices fail and fast detectors are rare, Wu and colleagues used difference-frequency generation to transfer random patterns from a near-infrared signal to a mid-infrared idler between 3.2 and 4.3 micrometers, enabling ultrafast temporal ghost imaging without high-speed electronics. Graphene metasurface modulators now steer centimeter-scale beams at gigahertz rates, and passive thermal ghost imaging reconstructs hot objects in complete darkness from their self-emitted radiation.

Terahertz ghost imaging completes the spectrum. Terahertz waves pass through textiles, polymers and paper while carrying molecular-fingerprint contrast, making them attractive for security screening, pharmaceutical inspection and cultural heritage diagnostics. Chan and colleagues inaugurated the field by replacing pixelated arrays with a single photoconductive antenna and random printed masks; spinning disks, optically controlled silicon modulators, electrically tunable metamaterials and spintronic emitter arrays have since pushed acquisition toward video rate and achieved deep-sub-wavelength near-field resolution. Nonlinear ghost imaging schemes now fuse terahertz generation with time-resolved field sampling, targeting hyperspectral micro-volumetry of semitransparent samples. Water-vapor absorption, scarce modulators and heavy computational loads remain hurdles, but reconfigurable metamaterials, low-noise detectors, wavefront shaping and physics-informed neural networks are converging toward compact, real-time systems. Taken together, the review concludes, multi-wavelength ghost imaging has become a powerful low-dose alternative wherever conventional focal-plane arrays fall short—imaging with light, and with matter, in places no camera can go.

Subject of Research: Multi-wavelength ghost imaging using single-pixel correlation-based reconstruction across the electromagnetic spectrum and matter waves

Article Title: Multi-wavelength ghost imaging: a review

Article References: Tian, T., Oh, S., & Spielmann, C. (2025). Multi-wavelength ghost imaging: a review. Vicinagearth, 2(1), Article 4. https://doi.org/10.1007/s44336-025-00013-0

Image Credits: AI Generated

DOI: 10.1007/s44336-025-00013-0

Keywords: ghost imaging, single-pixel imaging, computational ghost imaging, X-ray imaging, terahertz imaging, infrared imaging, compressive sensing, deep learning, quantum imaging, matter waves, low-dose imaging, correlation imaging

Writing in Nature Photonics, Jiaqi Liu and colleagues, working under the supervision of Guangda Niu of the Wuhan National Laboratory for Optoelectronics at Huazhong University of Science and Technology and Mengling Xia of the Wuhan University of Technology, describe a design strategy built on so-called zero-dimensional structured materials. In such crystals, the functional building blocks are isolated molecular units that do not share bonding directions with their neighbours in any crystallographic orientation. Because each unit is electronically self-contained, cutting or growing the crystal in any direction never leaves behind broken chemical bonds at the surface. The result is a semiconductor that is, in the researchers’ terminology, dangling-bond-free along all crystallographic directions.

The significance of this property becomes clear when one considers how defects arise in ordinary semiconductors. A conventional three-dimensional crystal, such as silicon, shares covalent bonds in every direction, so any grain boundary or surface necessarily terminates those bonds and creates dangling bonds, chemically unsatisfied sites that trap charge carriers and dissipate energy as heat rather than light. Two-dimensional materials such as transition-metal dichalcogenides avoid this problem within their atomically thin planes, but their edges remain problematic. One-dimensional wires solve the edge problem along their axis yet still expose discontinuities at their tips and side surfaces. Even the celebrated halide perovskites, which have revolutionized solar cells and radiation detectors with their remarkable defect tolerance, rely on passivation strategies to tame their interfacial defects rather than eliminating them structurally.

Zero-dimensional materials sidestep this entire problem, but until now they carried a penalty of their own. Because the functional units are electronically isolated, charge and energy transport within them tends to be isotropic, meaning that a crystal grows with similar rates in all directions and adopts compact, blocky morphologies. That isotropy is excellent for electronic performance but awkward for imaging, where the geometry of the scintillator film matters enormously. In X-ray scintillation imaging, light generated deep inside a thick film scatters as it travels to the detector, and lateral spreading degrades spatial resolution. Columnar structures, in which parallel rods act like optical fibres channeling light toward the detector, are the standard remedy, as demonstrated by the structured caesium iodide scintillators used commercially since the late 1990s. Achieving such columnar growth without sacrificing the defect-free electronic character of the material seemed impossible, because the two requirements pull in opposite directions.

The Chinese team’s insight was that the geometry of a crystal and the electronic dimensionality of its structure can be decoupled. To do so, they exploited a subtle chemical lever built into the uranyl ion, a linear O=U=O unit in which a uranium atom is flanked by two strongly bonded oxygen atoms. In the model compound caesium uranyl chloride, Cs2UO2Cl4, the uranium–oxygen double bonds are dramatically stronger and more directional than the uranium–chlorine bonds that link neighbouring uranyl groups. This bonding disparity means the crystal’s growth kinetics differ from one crystallographic direction to another, even though the uranyl units themselves remain electronically isolated. By tuning the balance between the fast-growing and slow-growing facets, the researchers drove the crystals to elongate into one-dimensional rods while preserving the zero-dimensional, dangling-bond-free electronic framework intact.

The concept draws on classical crystal-growth theory that dates back more than a century. In 1901, Georg Wulff proposed that the equilibrium shape of a crystal is determined by the relative surface energies of its facets, and Carl Herring’s theorems of 1951 formalized how surface free energy governs morphology. The team applied this thermodynamic framework to the chemically anisotropic uranyl system, using it to predict which facets would dominate the equilibrium rod shape. They then validated the prediction at the atomic scale with transmission electron microscopy, characterizing the crystal boundaries to confirm that the expected surfaces form and that no problematic bonding discontinuities appear. The work was a collaboration among the Wuhan University of Technology, Huazhong University of Science and Technology and Beijing University of Technology, with Wei Wang and Yue Lu contributing the atomic-scale electron microscopy characterization.

On the computational side, the researchers employed density functional theory calculations with the DFT-1/2 correction method, a technique developed to overcome the systematic underestimation of band gaps in conventional approximations to density functional theory. This allowed them to model the electronic structure of the material accurately enough to confirm that the anisotropic crystal growth does not compromise the isotropic, defect-free electronic properties that make zero-dimensional semiconductors attractive in the first place. The combination of thermodynamic growth modelling, atomic-resolution microscopy and corrected band-structure calculations gave the team confidence that their design principle is physically robust rather than a lucky accident of one particular compound.

The payoff came when the columnar Cs2UO2Cl4 crystals were tested as an X-ray scintillator. Uranyl compounds have attracted growing interest as scintillator materials since 2018, when researchers first highlighted uranium as a distinct metal centre for building intrinsic X-ray scintillators, with the heavy uranium nucleus providing strong X-ray absorption and the uranyl luminescence providing efficient light emission. Subsequent work explored uranium-organic frameworks and uranyl-cluster compounds for flexible and thermally adaptive scintillators, but spatial resolution remained limited by the film structures those materials formed. The new columnar geometry changes that picture decisively: the team’s Cs2UO2Cl4 scintillator achieved a spatial resolution of 34 line pairs per millimetre, a figure that places it among the highest-resolution scintillation imaging media reported and corresponds to the ability to distinguish features finer than fifteen micrometres across.

The practical implications extend across medical diagnostics, industrial inspection and scientific instrumentation. In clinical X-ray imaging and computed tomography, higher spatial resolution translates directly into the ability to visualize finer anatomical structures and subtler lesions at lower radiation doses, because fewer photons are wasted on scattered, blurred light. In industrial and security screening, fine-resolution scintillators enable detection of small defects or threats. The researchers also emphasize the generalizability of their approach: the underlying principle, that anisotropic chemical bonding can be introduced within a zero-dimensional framework to sculpt crystal geometry without disturbing electronic isotropy, is not limited to uranyl halides. Any material family containing chemically inequivalent bonds between electronically isolated units could, in principle, be engineered the same way, opening a route to defect-tolerant semiconductors with customizable shapes for light-emitting diodes, photodetectors and other optoelectronic devices where interfacial defects have long been the performance bottleneck.

The study, published on 25 September 2026 in Nature Photonics, arrives amid intense global competition to improve radiation detection materials, with halide perovskites and their derivatives dominating recent advances. What distinguishes this work is its conceptual reframing: rather than passivating defects after they form, the team designed them out of the material at the structural level, then recovered the anisotropic geometry needed for high-resolution imaging through chemistry rather than through growth tricks that would reintroduce broken bonds. The authors declare no competing interests, and the work was supported by the National Natural Science Foundation of China, the National Key Research and Development Program of China, the Beijing Natural Science Foundation, the Shenzhen Science and Technology Program and Huazhong University of Science and Technology. If the zero-dimensional design philosophy proves as transferable as the authors anticipate, the humble dangling bond, long the bane of semiconductor engineering, may finally lose its grip on the next generation of imaging technology.

Subject of Research: Dangling-bond-free zero-dimensional Cs2UO2Cl4 scintillators for ultrahigh-resolution X-ray imaging

Article Title: Dangling-bond-free, chemically anisotropic semiconductors for ultrahigh-resolution X-ray imaging

Article References: Liu, J., Liao, M., Liu, X., Ma, N., Li, H., Wang, W., Lu, Y., Xu, Y., Luo, G., Tang, J., Xia, M., & Niu, G. (2026). Dangling-bond-free, chemically anisotropic semiconductors for ultrahigh-resolution X-ray imaging. Nature Photonics. https://doi.org/10.1038/s41566-026-02013-y

Image Credits: AI Generated

DOI: 10.1038/s41566-026-02013-y

Keywords: X-ray imaging, scintillators, zero-dimensional materials, dangling bonds, Cs2UO2Cl4, uranyl halide, crystal growth, spatial resolution, defect tolerance, semiconductors, Nature Photonics, optoelectronics

STIX is not a conventional telescope. It is an indirect imaging spectrometer, a device that never forms a direct picture of the Sun. Instead, it observes the bremsstrahlung X-rays emitted by hot flare loops and accelerated electrons in the energy range from 4 to 150 kiloelectronvolts, using pixelated cadmium telluride detectors with an energy resolution of about 1 kiloelectronvolt. The imaging trick relies on 30 pairs of grids, each a stack of tungsten foils etched with fine slits, placed in front of the detectors. As X-rays stream through a grid pair, they cast a shifting Moiré pattern of light and shadow onto the detector behind it. By measuring the contrast and phase of that pattern, each sub-collimator, as a detector-grid unit is called, records one Fourier component of the X-ray source, much as a radio interferometer records a single visibility. Combine enough of these components and an image of the flare emerges from the mathematics.

The trouble is that every Fourier component depends on knowing precisely how transparent each grid pair really is. Before Solar Orbiter launched, engineers measured the grids optically, characterizing the pitch, slit width, and orientation of each one, and those measurements have underpinned the data analysis software ever since. But optical measurements have a fundamental blind spot: they can only see the top and bottom layers of a stack. The STIX grids for the 24 sub-collimators with the coarsest angular resolution are built from tungsten foils 33 or 50 micrometers thick, stacked to reach a total thickness of 400 micrometers, with the pattern etched into each layer. If the layers are not perfectly aligned, or if the etching leaves slits that vary slightly from layer to layer, the effective opening of the grid narrows in ways no optical inspection of the outer surfaces can reveal. Unlike its predecessors on Yohkoh, RHESSI, and the Chinese ASO-S mission, STIX also lacked a full pre-launch X-ray calibration at the small incident angles relevant to solar flare observations, a constraint of time and budget.

The new work turns that limitation into an opportunity. The key insight is that STIX carries a built-in flux monitor it was not entirely designed to be one: the Coarse Flare Locator, or CFL. This sub-collimator uses an H-shaped front grid and an empty rear grid, and for certain flare locations on the solar disk, the shadow cast by the front grid leaves up to three of the CFL’s large detector pixels fully illuminated. A fully illuminated pixel, by definition, sees the total incident X-ray flux, unmodulated by any grid. By comparing the flux recorded by the fully illuminated CFL pixels with the flux transmitted through each imaging sub-collimator, the team could compute the true effective transmission of every grid pair directly from flight data. The transmission is simply the ratio of the transmitted flux to the total flux, a beautifully direct measurement that sidesteps the uncertainties of the ground-based optical characterization.

Building the calibration dataset required patient mining of the STIX archive. From roughly 25,000 flares recorded between January 2021 and February 2025, the team selected events with at least one fully illuminated large CFL pixel, accurate flare locations, and enough counts to keep statistical errors at or below about 3 percent. The winnowing left 91 flares, a small fraction of the total, because only flares at particular solar latitudes illuminate the CFL pixels completely. The team focused on the 10 to 15 kiloelectronvolt band, where the transmission of the Solar Black coating on the spacecraft heatshield is high enough that any inhomogeneities introduce only second-order errors, and where photon statistics remain strong. Careful corrections for background counts, detector live time, and onboard energy channel binning were applied, and a self-consistency check of the CFL pixels themselves showed the total flux measurements are accurate to about 2.3 percent.

What the flares revealed was unambiguous. The measured transmission of the sub-collimators is systematically lower than the values predicted from the optical grid characterization, and the internal shadowing effect, the progressive narrowing of the effective slit width as photons arrive at increasing angles, is far weaker than a model of perfect grids would predict. Simple geometric simulations explain why. When the team modeled grids with random etching and stacking imperfections of a few micrometers, drawn from Gaussian distributions with a standard deviation of about 2 micrometers, the simulated on-axis transmission dropped and the shadowing effect flattened out, exactly matching the pattern in the flight data. For sub-collimator 5, with slits nominally 83 micrometers wide, imperfections of this scale reduce the effective slit width by roughly 6 micrometers. In some sub-collimators, such as 6b, a slight tilt of the stacked layers, plausibly caused by tilted alignment pins during assembly, produces a sloped transmission profile that the simulations reproduce as well.

The practical payoff is substantial. When the team applied the new transmission calibration to 25 independent flares observed between March and December 2025, the total flux estimates from the different sub-collimators agreed within about 2 percent, whereas the old calibration produced systematic biases ranging from 3 to 39 percent for the coarse-resolution sub-collimators and up to 175 percent for the finest ones. A second test on the powerful X2.2-class flare of December 8, 2024 drove the point home: photon spectra derived independently from the individual sub-collimators now agree within roughly 2 percent across the 8 to 20 kiloelectronvolt range, a dramatic improvement over the 6.9 percent scatter of the old calibration. The new calibration also means that derived photon fluxes are about 13 percent higher than previously computed, and fitted emission measures, a measure of the amount of hot plasma in the flare, increase by a similar 13 percent. Fitted flare temperatures, reassuringly, do not change, and the team emphasizes that conclusions from previously published STIX spectral analyses remain valid.

There are honest caveats. The calibration is valid only at low energies, below about 20 kiloelectronvolts, and only for offset angles between roughly minus 0.5 and plus 0.5 degrees, because at higher energies the tungsten grids become partially transparent and the CFL statistics grow too thin. Below 8 kiloelectronvolts, inhomogeneities in the Solar Black coating introduce discrepancies that the team hopes to self-calibrate in a future study using the dependence of transmission on flare location. Three sub-collimators, 3a, 5a, and 5b, showed energy-dependent discrepancies likely tied to detector rather than grid calibration and are recommended for exclusion from spectral analyses for now. The high-energy grid calibration, crucial for studying whether flare X-ray emission is anisotropic, will follow in a companion paper, and the team notes that cross-calibration better than 10 percent between Solar Orbiter and Earth-orbiting observatories is needed to settle that question.

Perhaps the most enduring lesson is a design philosophy. The authors strongly recommend that future indirect X-ray imagers include a dedicated total flux monitor, so that every recorded flare can contribute to self-calibration, as the Hard X-ray Imager aboard ASO-S already does. A flux monitor with sufficient effective area would even allow the same technique to be extended to high energies. For now, the new transmission model will be released in the upcoming version of the STIX analysis software, quietly improving every flare image and spectrum the mission produces. It is a striking demonstration that a spacecraft instrument, far from any laboratory, can diagnose its own imperfections using nothing more than the raw fury of the Sun, and that a few micrometers of tungsten misalignment, invisible to any optical microscope on Earth, can be measured from 100 million kilometers away by letting solar flares do the talking.

Subject of Research: In-flight self-calibration of the tungsten grid transmission of the STIX hard X-ray imaging spectrometer on board Solar Orbiter using solar flare observations

Article Title: In-Flight Self-Calibration of the STIX Grid Transmission

Article References: Massa, P., Volpara, A., Stiefel, M. Z., Bilgili, E., Degen, M., Limousin, O., Hurford, G. J., & Krucker, S. (2026). In-Flight Self-Calibration of the STIX Grid Transmission. Solar Physics, 301(9), Article 138. https://doi.org/10.1007/s11207-026-02739-6

Image Credits: AI Generated

DOI: 10.1007/s11207-026-02739-6

Keywords: Solar Orbiter, STIX, solar flares, X-ray imaging, instrument calibration, tungsten grids, Solar Physics, space instrumentation, hard X-rays, spectroscopy, ESA, helio physics