A photonic chip no larger than a laboratory component has produced a form of quantum entanglement that could reshape how researchers think about scalable quantum technologies. In a study published in Light: Science & Applications, Chi, Ding, Wang and their colleagues report the generation of qudit W states and Greenberger–Horne–Zeilinger (GHZ) states using a platform based on only two photons. The result combines two of the most important ideas in quantum information—high-dimensional quantum systems and multipartite entanglement—inside an integrated optical device.

At the heart of the work is the qudit, a quantum system that can occupy more than the two states available to a conventional qubit. A qubit may be represented by a photon’s horizontal or vertical polarization, for example, while a qudit can use several distinguishable levels encoded in paths, time bins, orbital angular momentum, frequency, or other optical properties. Increasing the number of available levels can allow a single quantum carrier to store more information and may improve the efficiency of quantum communication, sensing, and computation.

The researchers focus on two distinct patterns of entanglement. In a GHZ state, several quantum systems are linked so strongly that their measured properties show collective correlations across the entire state. A simplified three-qubit GHZ state can be written as a superposition of all systems being in one state and all systems being in another. W states have a different structure: the excitation is distributed among several possibilities, creating a state that remains entangled even if one component is lost. These contrasting forms make W and GHZ states valuable for testing the limits of quantum networks and for developing protocols that rely on different types of nonclassical correlations.

What makes the new demonstration especially striking is that the chip uses two photons while producing states that behave as though several high-dimensional quantum subsystems are participating. In photonic experiments, a single photon can carry multiple degrees of freedom, and carefully engineered optical circuits can arrange these degrees of freedom into effective quantum modes. By controlling how the photons interfere and how their properties are measured, the device can create complex entangled states without requiring a separate physical photon for every logical subsystem.

The chip’s operation relies on the quantum interference of indistinguishable photons. When photons enter an integrated circuit through carefully selected pathways, their probability amplitudes combine rather than behaving like independent classical particles. Waveguides, beam splitters, phase shifters, and other on-chip elements manipulate these amplitudes with high precision. The resulting output is not a single predetermined configuration but a coherent superposition of many possibilities. Measurements then reveal correlations that cannot be explained by assigning fixed classical states to the photons before detection.

Moving this process onto a chip is important because conventional optical experiments often depend on large collections of mirrors, lenses, interferometers, and alignment systems. Even tiny mechanical shifts can change the phase relationships required for quantum interference. Integrated photonics replaces many of these free-space components with structures fabricated directly into a solid substrate. The components become more stable, compact, and potentially easier to reproduce, creating a route toward quantum devices that can leave the laboratory and operate as practical hardware.

The ability to generate high-dimensional entanglement with a small number of photons may also address one of the major bottlenecks in photonic quantum technology: the difficulty of producing and controlling many identical photons. Single-photon sources remain technically demanding, and losses increase rapidly as optical systems grow. If different degrees of freedom can be exploited efficiently, researchers may be able to encode more quantum information without simply multiplying the number of particles. This approach does not eliminate the challenges of photon loss, imperfect detectors, or environmental noise, but it offers a different strategy for increasing the information capacity of quantum circuits.

The reported platform could have implications beyond a single state-generation experiment. High-dimensional entangled states are candidates for more efficient quantum key distribution, in which additional levels can increase the information carried by each detected photon and may provide stronger resistance to certain noise processes. They are also relevant to quantum teleportation, distributed quantum computing, measurement-based computation, and precision sensing. W states are particularly interesting for networks in which the loss of one link must not destroy all useful correlations, while GHZ states are central to coordinated measurements and nonlocality tests.

The work also highlights a broader shift in quantum engineering: the field is moving from demonstrating isolated quantum effects toward designing compact architectures that can generate several classes of states on demand. Producing W and GHZ states on the same two-photon chip suggests that programmable photonic platforms may eventually support a wider library of quantum resources. The next steps will include improving source brightness, reducing fabrication imperfections, increasing detection efficiency, and proving that the generated states can be integrated into complete communication or computation protocols. If those challenges can be overcome, a small optical chip may become a powerful gateway to quantum systems far more complex than its physical size suggests.

Subject of Research: High-dimensional photonic entanglement, qudit W states, GHZ states, and integrated two-photon quantum chips

Article Title: Qudit W and Greenberger–Horne–Zeilinger states on a two-photon chip

Article References: Chi, Y., Ding, H., Wang, F. et al. Qudit W and Greenberger–Horne–Zeilinger states on a two-photon chip. Light Sci Appl 15, 340 (2026). https://doi.org/10.1038/s41377-026-02285-7

Image Credits: AI Generated

DOI: 10.1038/s41377-026-02285-7

Keywords: quantum photonics, qudits, W states, GHZ states, quantum entanglement, integrated photonic chips, two-photon quantum technology, high-dimensional quantum information, quantum communication, quantum computing

Tags: Greenberger–Horne–Zeilinger stateshigh-dimensional quantum systemsintegrated optical quantum devicesmulti-level quantum encodingmultipartite entanglementorbital angular momentum in quantum statesquantum communication and sensingquantum information encodingQuantum photonic chipqudit W statesscalable quantum technologiestwo-photon quantum entanglement