The requirement at issue is known as indistinguishability. In classical optics, two beams of light can differ in wavelength, timing, polarization or spectral purity and still be perfectly useful. In quantum information processing, however, many of the most powerful protocols rely on two-photon interference, a phenomenon in which two photons arriving simultaneously at a beam splitter become fundamentally impossible to tell apart. When that happens, the photons ‘bunch’ together in a way that classical particles cannot, and this interference effect becomes the raw material for entanglement swapping, quantum teleportation and photonic gate operations. Any difference between the photons, however small, degrades the interference and injects errors into the computation or communication task downstream.

The Paderborn-Basel-Bochum team attacked the problem using semiconductor quantum dots, nanoscale islands of semiconductor material that trap single charge carriers and behave, in the words of researchers in the field, like artificial atoms. A quantum dot can absorb energy and re-emit it as individual photons on demand, one after another. Because the dots are fabricated in semiconductor chips, they are widely regarded as one of the most promising routes to the mass production of single and paired photons, in contrast to sources that rely on delicate bulk optical setups. The challenge has always been quality: photons from quantum dots have historically suffered from temporal correlations and spectral jitter that undermined their indistinguishability.

The specific mechanism the team exploited is called biexciton decay. An exciton is a bound pair consisting of an electron and the electron hole it leaves behind; a biexciton is a molecule-like state made of two such bound pairs. When a quantum dot is doubly excited, the biexciton decays in a cascade: first one photon is emitted as the biexciton collapses to a single exciton, and then a second photon is emitted as the remaining exciton decays to the ground state. ‘The so-called biexciton cascade in a semiconductor quantum dot emits photons at the push of a button, which are of great interest for modern applications,’ explains lead author Timon Baltisberger of the University of Basel. ‘This is a process in which a quantum dot is doubly excited and the excitation then decays. This generates two photons, one after the other.’

On its own, however, the cascade has a weakness. The two photons are emitted at different energies, corresponding to the two transitions, and the decay dynamics leave them temporally smeared in ways that limit how well they can interfere. Left uncontrolled, the indistinguishability of photons from such a cascade hovers around 60 per cent, far below the levels needed for fault-tolerant quantum protocols. The breakthrough of the new study lay in placing the quantum dot inside a specialised optical resonator, a microscopic cavity similar in principle to the mirror arrangement found in a laser. By tuning the cavity to interact strongly with the biexciton transition, the researchers accelerated and controlled the decay process itself, sharpening the emission in time and dramatically improving the quality of the photons that emerged.

‘A quantum dot is often described as an artificial atom within a semiconductor that can generate individual particles of light,’ says Professor Stefan Schumacher, head of the Theory of Functional Photonic Structures research group at the Department of Physics and the Institute for Photonic Quantum Systems at Paderborn University. ‘By integrating it into a specialised optical cavity, similar to that found in a laser, the light emission process was specifically accelerated and controlled in this study.’ The cavity does not merely collect light more efficiently; it modifies the quantum dynamics of the emitter, a regime in which the engineered electromagnetic environment actively shapes how the dot releases its stored energy.

The experimental expertise behind this control was built over years by the group of Professor Richard Warburton at the University of Basel, which has studied the interplay between quantum dots and optical cavities in depth. Applying that accumulated knowledge to the biexciton state proved decisive. With the cavity accelerating the decay in a controlled manner, the emitted photons reached approximately 90 per cent indistinguishability, a striking improvement over the 60 per cent achieved without the effect. ‘The results show excellent agreement with the theoretical prediction and point the way towards generating photons with even higher indistinguishability,’ says Warburton. ‘They demonstrate that biexciton decay can produce very high-quality photons, provided the system is properly controlled using a cavity.’

The collaboration with Dr Arne Ludwig of Ruhr University Bochum added a deeper understanding of the physics that still limits performance. The team found that the purity of the generated photons can itself be optimised through the resonator, but that the ultimate ceiling is set by phonons, the quantised vibrations of the semiconductor crystal lattice. These vibrations couple to the quantum dot and subtly randomise the energy of the emitted light, a process the researchers identify as ‘cavity feeding’. ‘The purity of the photons generated can also be optimised using the resonator and is limited only by vibrations in the semiconductor’s crystal lattice,’ explains Professor Klaus Jöns, head of the Hybrid Quantum Photonic Devices group at Paderborn. ‘This phenomenon must be taken into account in future designs and can then be systematically minimised even further.’ In other words, the remaining imperfection is not a fundamental barrier but an engineering target, one that cleverer cavity geometries and materials could push back further.

The practical significance of the result lies in the connection between photon quality and error rates. In quantum communication and photonic quantum computing, every imperfection in indistinguishability or purity translates directly into errors in the logical operations performed with the light. The more identical and purer the photons a source delivers, the lower the error rate in data processing, and the shorter the overhead needed for error correction. A source that reliably emits pairs of nearly identical photons on demand, from a chip-based platform, addresses one of the central bottlenecks in scaling photonic quantum technologies from laboratory demonstrations to deployable systems.

The study, titled ‘Indistinguishable Photons from a Two-Photon Cascade’, was published in Physical Review Letters in August 2026, with the associated DOI 10.1103/t8sk-b2w4. In a companion paper in Physical Review Applied, published as an Editors’ Suggestion, the collaboration reported initial detailed findings on the single-photon source application of the same platform, with the DOI 10.1103/78c9-j817. Together, the two papers sketch a roadmap: chip-based quantum dots, embedded in engineered optical cavities, emitting streams of photons whose indistinguishability approaches the thresholds demanded by quantum repeaters, entanglement distribution and photonic logic. If the phonon-limited purity can be systematically suppressed as the researchers suggest, the humble biexciton cascade, a decay process first studied as a curiosity of semiconductor spectroscopy, may become one of the standard engines of the quantum internet.

Subject of Research: Indistinguishable photon generation from cavity-controlled biexciton decay in semiconductor quantum dots

Article Title: Quantum communication: New method generates photons that are virtually indistinguishable

Article References: Quantum communication: New method generates photons that are virtually indistinguishable. (n.d.). Original publication

Image Credits: AI Generated

DOI: Not provided

Keywords: quantum communication, quantum dots, biexciton decay, indistinguishable photons, optical cavity, photon pairs, quantum entanglement, phonons, cavity feeding, semiconductor nanostructures, Physical Review Letters, quantum networks

The research, carried out by Rasha Mushtaq Hashim and Saeed Naif Turki Al-Rashid of the Department of Physics, College of Education for Pure Science, tackles a long-standing challenge in nanoscience: predicting how optical properties change when a material is confined to fewer and fewer dimensions, without resorting to computationally expensive first-principles simulations. Instead, the team built their model around a quantity that is often overlooked in optical studies: cohesive energy, the energy that binds a solid’s atoms together.

The central insight of the work is that as a semiconductor structure shrinks, the fraction of its atoms sitting at the surface grows dramatically, weakening the average bonding in the crystal. This bond-order deficiency, a concept developed in earlier size-dependence literature, feeds directly into a cohesive-energy-based description of how the electronic band gap evolves with size. When the atoms at a surface have fewer neighbors to bond with, the electronic states near the band edges shift, and the band gap widens. The effect is the well-known phenomenon of quantum confinement, in which charge carriers—electrons and holes—are squeezed into a space comparable to or smaller than their natural extent in the bulk crystal, quantizing their allowed energies and pushing the lowest excitation energy upward.

Hashim and Al-Rashid applied this framework across three structural regimes. For zero-dimensional (0D) quantum dots, the relevant parameter is particle radius: smaller dots confine carriers more tightly, widening the gap. For one-dimensional (1D) nanowires and nanotubes, it is the diameter that controls the confinement energy. For two-dimensional (2D) layered nanosheets, the number of layers plays the analogous role—single layers behave very differently from multilayer stacks, and as layers accumulate the material gradually recovers its bulk character. In every case, the model predicts the same qualitative trend: the band gap decreases as the structure grows larger in its confined dimensions, converging toward the bulk value.

Crucially, the study does not stop at the band gap. Using the calculated size-dependent gap as an input, the researchers invoked the empirical Moss relation, a decades-old rule of thumb that links a semiconductor’s refractive index inversely to its band-gap energy, and then the Fresnel equations to compute reflectivity from the refractive index. The result is an elegant inversion of the band-gap trend: while the gap widens as structures shrink, the refractive index and reflectivity fall. Conversely, larger nanostructures—with weaker confinement—exhibit higher refractive indices and reflect more light. The physical logic is straightforward: a narrower band gap means the material’s electrons respond more readily to an oscillating electromagnetic field, increasing optical polarizability and hence the refractive index.

The two materials studied occupy fascinating and contrasting corners of the semiconductor landscape. InSb is a classic narrow-gap III–V semiconductor with a bulk band gap of only about 0.17 electronvolts, placing it squarely in the mid-infrared region of the spectrum. It has long been prized for infrared detectors, and its high-mobility charge carriers have made it a candidate material for next-generation electronics. PdSe₂, by contrast, belongs to the newer family of two-dimensional transition-metal dichalcogenides. Its puckered pentagonal layered structure sets it apart from the hexagonal geometry of graphene or molybdenum disulfide, and its air stability and widely tunable band gap—from around 1.3 electronvolts in monolayer form to a much smaller value in the bulk—have made it a magnet for research into photodetectors and layered electronics.

The model’s predictions capture these differences clearly. PdSe₂ generally exhibits larger band-gap values than InSb across the size ranges examined, consistent with its wider bulk gap and its strongly layer-dependent electronic structure. InSb, meanwhile, shows relatively higher refractive-index and reflectivity values in larger structures—a direct consequence of its narrow bulk band gap, which permits a large optical dielectric response. In practical terms, this means an InSb nanowire or thin film reflects and bends light more strongly than an equivalent PdSe₂ structure, a distinction that matters when these materials are integrated into photonic circuits, antireflection coatings, or infrared emitters.

The authors validated their approach against available experimental measurements and theoretical calculations from the literature, including photoluminescence studies of PdSe₂ quantum dots, layer-dependent optical measurements on chemical-vapor-deposition-grown PdSe₂ films, and experimental work on size-tuned InSb nanowires. The agreement was described as reasonable, particularly for band-gap trends across all three dimensionality regimes. Deviations in the predicted refractive indices and reflectivities were attributed to the limitations of the empirical Moss and Fresnel relations themselves, and to the model’s deliberate omission of wavelength dependence, crystal anisotropy, and surface-related effects—complications that are especially significant for PdSe₂, whose puckered structure is intrinsically anisotropic.

What makes the study notable is not the precision of individual numbers but the generality of the framework. Density functional theory calculations, while accurate, can require enormous computational resources when applied across many sizes, diameters, and layer counts. A cohesive-energy model, by contrast, needs only a handful of bulk material parameters and scales effortlessly across structural regimes. The authors had previously applied the same philosophy to predict size-dependent melting temperatures and Debye temperatures of noble-metal nanoparticles and the optical and thermal properties of CdSe and ZnSe nanoparticles, and the new work extends that program to a direct comparison of a narrow-gap III–V material and a layered dichalcogenide.

The practical implications reach into several hot areas of technology. Infrared photodetection, mid-infrared photonics, and quantum information science all demand semiconductors whose optical response is precisely engineered. InSb nanowires are actively explored as infrared sensors and as platforms for devices that exploit strong spin-orbit coupling. PdSe₂ nanosheets, with their tunable, layer-number-controlled gaps, are candidates for photodetectors spanning the visible to the infrared, for tunable optical coatings, and for van der Waals heterostructures in which different two-dimensional materials are stacked like atomic-scale LEGO bricks. A fast, inexpensive model that predicts how band gap, refractive index, and reflectivity evolve with dimensionality gives device designers a comparative shortcut before committing to synthesis or heavy simulation.

The study also serves as a reminder of a deeper principle in nanoscience: in the quantum world, geometry is destiny. The same chemical compound can behave as a wide-gap quantum emitter as a tiny dot, a moderate-gap wire, and a narrow-gap bulk absorber, with its reflectivity shifting in parallel. By capturing this geometry–optics relationship in a single cohesive-energy framework, Hashim and Al-Rashid have offered the nanoscience community a compact map of territory that would otherwise demand expedition after expedition with supercomputers.

The work, which received no external funding, was designed and simulated by Hashim, who developed the theoretical model and MATLAB implementation, with Al-Rashid supervising the theoretical framework and verifying the findings. The authors report no competing interests. As low-dimensional materials continue their march from laboratory curiosity to commercial device components, tools of this kind—simple, transparent, and physically interpretable—are likely to become fixtures in the optoelectronics design toolkit, guiding engineers toward the right material, in the right geometry, at the right size, for the optical job at hand.

The relentless drive to miniaturize and accelerate information processing systems is pushing electronics to their physical limits. Photonics, leveraging photons instead of electrons for information transmission and processing, promises a faster, more efficient alternative. However, photonics faces a fundamental obstacle: light behaves as a wave with a wavelength ranging from hundreds of nanometers (visible light) to several micrometers (infrared). Conventional photonic devices require components on the order of these wavelengths, limiting how small and dense integrated photonic circuits can become. The crux of advancing photonics thus hinges on manipulating light at scales smaller than its wavelength.

Subwavelength gratings, structures composed of periodic arrays of materials spaced at intervals smaller than the wavelength of the incident light, offer a powerful tool for light manipulation. Acting similarly to prisms, these gratings diffract and control light. When designed with periods shorter than the wavelength, such gratings can function as nearly perfect mirrors while simultaneously confining light within their minuscule volumes. Yet, until now, subwavelength gratings have required thicknesses on the order of hundreds of nanometers to effectively trap light, thus limiting their miniaturization.

The Research Team’s innovation lies in employing molybdenum diselenide (MoSe₂), a layered van der Waals semiconductor with a uniquely high refractive index, to fabricate ultrathin subwavelength gratings capable of confining infrared light in merely a 40-nanometer-thick film. The refractive index of MoSe₂ is approximately 4.5, considerably larger than that of traditional photonic materials like silicon or gallium arsenide, which reduces the effective wavelength of light inside the material. This property facilitates a drastic reduction in grating thickness without impairing light confinement, enabling structures thousands of times thinner than a human hair to manipulate infrared photons with exceptional efficiency.

Beyond its high refractive index, molybdenum diselenide also exhibits intriguing nonlinear optical phenomena. Chief among these is third harmonic generation, a nonlinear process whereby photons interacting within the material combine their energies to produce new photons at triple the original frequency. This means infrared light entering the MoSe₂ grating can emerge transformed into blue light, signifying a frequency tripling effect. Importantly, the intense localization of infrared light within the nanostructured grating enhances this nonlinear response by over 1,500-fold compared to simple layers of MoSe₂ without patterning, showcasing the immense potential for integrated nonlinear photonic applications.

The fabrication method developed by the researchers represents a critical advance that addresses previous challenges in producing high-quality, scalable MoSe₂ films. Traditionally, such layers have been created through mechanical exfoliation — peeling thin sheets from bulk crystals using adhesive tape — a technique well-known from graphene research but limited by randomness, small size, and poor reproducibility. To overcome these problems, the team adopted molecular beam epitaxy (MBE), a sophisticated, controllable thin-film fabrication technique prominently used in semiconductor industry but never before applied to materials like MoSe₂. MBE facilitates the growth of large-area, uniform films with precisely controlled thickness, crucial for industrial-scale photonic device integration.

The subwavelength gratings crafted from these epitaxial MoSe₂ films measure several inches across yet maintain uniform thickness down to 40 nanometers. Such an extraordinary aspect ratio—width and length dimensions millions of times greater than thickness—mimics a factor unattainable in conventional photonic materials and foreshadows a new era of ultrathin, scalable photonic designs. To put it in perspective, the thickness-to-size ratio of these films surpasses that of standard A4 paper by a factor of about 500, underscoring the unprecedented level of thinness combined with structural integrity.

This research heralds a new paradigm in controlling light at the nanoscale. By demonstrating that infrared photons can be confined and nonlinear optical effects amplified within films just a few tens of nanometers thick, the team has overcome a key technological barrier to the miniaturization and performance enhancement of photonic devices. The capacity to fabricate such nanostructured van der Waals materials reliably and at scale elevates their suitability for applications ranging from highly compact optical circuits to advanced sensors, frequency converters, and quantum photonic systems.

The study published in the high-impact journal ACS Nano describes not only the experimental realization of these optical bound states in the continuum — resonant modes trapped within the grating despite continuous radiation bands — but also elucidates their fundamental physical mechanisms through detailed theoretical modeling. The findings pave the way for integrating layered dichalcogenide materials into next-generation photonic platforms, delivering performance and form factors previously constrained by material limitations and fabrication hurdles.

In essence, molybdenum diselenide subwavelength gratings redefine how light can be controlled at scales far below its natural wavelength. These ultrathin nanostructures constitute versatile building blocks for future photonic devices that combine speed, miniaturization, and nonlinear optical functionality. With molecular beam epitaxy facilitating reproducible large-area production, their technological translation from laboratory breakthroughs to real-world applications appears promisingly within reach.

This collaboration between academic and technological institutions in Poland exemplifies the fusion of fundamental physics and applied engineering. It highlights how interdisciplinary approaches and novel material systems can disrupt long-held limitations in optics, photonics, and beyond. As industries increasingly seek to harness photonics for computing, communications, and sensing, such scientific achievements stand to impact the evolving landscape of information technology and optical science globally.

The funding for this cutting-edge research was provided by national and international agencies, including the National Science Centre of Poland, the European Union’s ERC-ADVANCED grant, and the Foundation for Polish Science. The Faculty of Physics at the University of Warsaw, centrally involved in this project, continues to push frontiers in modern physics through collaborative endeavors that merge quantum science, materials engineering, and photonics.

In summary, the advent of ultrathin subwavelength gratings made of epitaxially grown MoSe₂ marks a significant milestone towards the ultimate goal of manipulating light on the nanoscale with unparalleled precision and efficiency. This achievement promises to accelerate the integration of photonics into everyday technology, from ultrafast optical communication devices to compact nonlinear optical converters, heralding a future enriched by light-driven innovation.

Subject of Research: Ultrafast manipulation and confinement of infrared light in ultrathin subwavelength gratings fabricated from epitaxial molybdenum diselenide (MoSe₂).

Article Title: Optical Bound States in the Continuum in Subwavelength Gratings Made of an Epitaxial van der Waals Material

News Publication Date: 26-February-2026

References: Emilia Pruszyńska-Karbownik et al., ACS Nano 2026, 20 (9), 7426-7437. DOI: 10.1021/acsnano.5c12870

Image Credits: E. Pruszyńska-Karbownik, Faculty of Physics, University of Warsaw

Keywords

nanophotonics, molybdenum diselenide, subwavelength grating, infrared light confinement, nonlinear optics, third harmonic generation, molecular beam epitaxy, van der Waals materials, ultrathin semiconductor films, photonic integration, optical bound states, frequency tripling