Quantum entanglement is one of the most cognitively discordant disconnects between the quantum domain and the macroscopic world. When fundamental particles like electrons or photons become entangled, they share a quantum identity and cannot be described as individual entities; they exist in a superposition with no fixed state until at least one is measured. This transcends physical distance, so a quantumly entangled coin toss on Earth would produce an instantaneous and proportional result on Mars. Of course, these results are random and convey no information, crushing hopes of faster-than-light communication or an intricate Martian coin-tossing Earth trap ring. The elementary particles of the Standard Model, which describe our best current understanding of the fundamental particles and forces of the Universe, except gravity. (ScienceAlert) Still, entanglement is fundamental to quantum information technologies; We just need to increase the number of particles we entangle and our ability to measure these systems. To that end, physicists at Kyoto University and Hiroshima University have brought us one or a few steps closer to our imminent quantum future in a landmark study that determined the configuration of entangled states of multiple particles, starting with three photons. In their paper, published in Science Advances in September 2025, the physicists finally reported a new technique for making measurements of so-called W-state entanglement types. Previously, the scientists measured Greenberger-Horne-Zeilinger (GHZ) states, the well-known and fundamental “vanilla ice cream” of multiparticle entanglement systems. “More than 25 years after the initial proposal regarding entangled measurement for GHZ states, we have finally obtained also entangled measurement for the W state, with a true experimental demonstration for 3-photon W states,” explains Shigeki Takeuchi, quantum information researcher. But the ‘chocolate chip cookie dough’ of W state entanglements exhibits greater complexity and an interesting property that differs from the GHZ states. In W states, if one of the entangled particles is lost, the remaining ones retain their useful entangled state. Importantly, particle entanglement is often measured using a technique known as quantum tomography. It works similar to X-ray CT scan that reveals the skeletal secrets of our body. It involves taking many measurements and then putting these “slices” together into a coherent picture. But this can be inefficient and complicated, introducing two practical obstacles. First, scientists must create many identical entangled systems, because measuring (observing) their properties takes them out of their entangled states. Second, the measurements required to reconstruct an entangled state increase exponentially with the number of entangled particles. The researchers managed to avoid these problems with a single measurement that determined the entire interlocking system in a single step. They did this using a discrete Fourier transform (DFT) optical circuit device of their own manufacture, which acted as an advanced interferometer. The experimental setup. (Park et al, Sci. Adv., 2025) In other words, they injected three photons of known polarization into this device, splitting them into different paths, and then crashing them into each other, to see how the peaks and valleys of their wave functions aligned, either combining or canceling out. This allowed the researchers to explore the “cyclic flip symmetry” of the W state, which is a bit like the fingerprint of a system based on the symmetry of its quantum properties. The “cyclic shift” part means that the structural description of the entangled system does not change when its individual photons shift cyclically. Imagine a group of people sitting in a circle around a campfire. Suddenly, each of them stands up and moves on a seat to the left (who knows why, perhaps they are part of an esoteric Borg-like hive mind that has not lost its ancestral fascination with flames). As a result, the cyclically shifted Borg circle looks the same. A simplified illustration of the experimental setup. (KyotoU/Takeuchi Laboratory) And so, the researchers revealed an average measurement discrimination fidelity (MDF) of 0.871 ± 0.039. They correctly identified the W-state condition 87 percent of the time, comfortably exceeding the mathematical threshold of 66.7 percent (two-thirds) to demonstrate that measurement of three-particle entanglement had been achieved, thus demonstrating the effectiveness of their methodology. Why not 100 percent? The team attributes the shortfall to imperfections in the preparation of the photons and in the measurement setup itself. “We believe these results represent an important milestone toward broader application of entangled multi-qubit measurements in photonic quantum computing, quantum communication, and sensing,” the researchers write. Consequently, this work helps open the door (hopefully not a quantum door that is open and closed at the same time, i.e., Schrödinger’s smacked finger) for quantum applications in computing and cryptography. The team now plans to develop photonic quantum circuits on chips for these entangled measurements. Given the enormous technological maw of quantum computing, these advances could come together to improve life-saving drug research or personalized healthcare strategies, for example. They could also make our Pizza Hut rewards points accounts more secure than ever, unless a pepperoni-hungry hacker also has access to quantum computing. The research has been published in Science Advances. This article was fact-checked by Rebecca Dyer and edited by Rebecca Dyer. While we take pride in our process, we are human. If you spot an error, please let us know.
