Physicists from Kyoto University and Hiroshima University have developed a new technique to measure W-state quantum entanglement in three photons. Published in September 2025, the method solves a 25-year-old challenge by using a one-shot measurement approach, potentially accelerating the development of quantum teleportation, secure communication, and photonic quantum computing.
Quantum entanglement links particles so deeply that they share a single identity, regardless of the distance between them. When fundamental particles such as electrons or photons are entangled, they exist in a superposition with no fixed state until at least one is measured. For decades, this phenomenon has been a cornerstone of quantum information technology, but measuring complex multi-particle systems has remained a significant bottleneck. While researchers previously mastered the measurement of Greenberger-Horne-Zeilinger (GHZ) states, the more complex W state remained elusive.
The 25-Year Gap Between GHZ and W States
The distinction between entanglement types is more than academic. GHZ states are often described as the fundamental or vanilla ice cream
of multi-particle systems. In contrast, W states—the chocolate chip cookie dough
of the group—possess a unique resilience: if one entangled particle is lost, the remaining particles retain their useful entangled state.
More than 25 years after the initial proposal concerning the entangled measurement for GHZ states, we have finally obtained the entangled measurement for the W state as well, with genuine experimental demonstration for 3-photon W states,Shigeki Takeuchi, quantum information researcher
Until now, the primary tool for these measurements was quantum tomography. This process acts like a medical X-ray, taking numerous “slices” of measurements to reconstruct a coherent image of the state. However, this method is inefficient because observing a system snaps it out of entanglement, requiring scientists to create many identical systems.
Cyclic Shift Symmetry Bypasses the Tomography Bottleneck
To bypass the tomography bottleneck, the researchers utilized a mathematical property of W states known as cyclic shift symmetry
. This symmetry means the structural description of the entangled system remains unchanged even when individual photons are shifted in a cyclical fashion, similar to a ring of lights where shifting the position of each bulb preserves the overall light pattern.
The team implemented this via a custom-built discrete Fourier transform (DFT) optical circuit. This device functions as an advanced interferometer using high-stability optical quantum circuits that could operate for long periods without active control. By analyzing how the wave functions combine or cancel out, the device can identify the W state in a single step.
The experimental results demonstrated a high level of reliability. The team recorded an averaged measurement discrimination fidelity (MDF) of 0.871 ± 0.039, meaning they correctly identified the W state 87 percent of the time. This result comfortably exceeds the 66.7 percent mathematical threshold required to prove the efficacy of the methodology and demonstrate that three-particle entanglement measurement has been achieved.
Impact on Quantum Teleportation and Sensing
This one-shot measurement capability has direct implications for quantum teleportation—the process of transferring quantum information between locations without moving physical matter. Instead, entanglement acts as a bridge to copy the quantum information from one spot to another. By making multi-photon states easier to identify and verify, the technique clears a hurdle for secure data transfer protocols and measurement-based quantum computing.
- Quantum Communication: Improved protocols for transferring multi-photon entangled states.
- Sensing: Enhanced precision in multi-qubit entangled measurements.
- Cryptography: Potential for more secure accounts and data encryption, including Pizza Hut rewards points accounts, unless a hacker also possesses quantum computing access.
In order to accelerate the research and development of quantum technologies, it is crucial to deepen our [understanding],Researchers, via Science Advances
Scaling Toward On-Chip Photonic Circuits
While the current demonstration used three photons, the mathematical shortcut is theoretically applicable to W states containing any number of photons. The researchers attributed the gap between their 87 percent accuracy and a perfect 100 percent to imperfections in the measurement setup and photon preparation.
The next phase of development focuses on miniaturization. The team intends to develop on-chip photonic quantum circuits. Moving these optical circuits onto microchips would make the technology more compact, affordable, and easier to integrate into the broader quantum networks of the future.
Shigeki Takeuchi, corresponding author of the study, believes that these advancements will eventually enable the practical implementation of high-fidelity quantum communication systems across larger and more complex networks.
