Physicists Achieve Groundbreaking Quantum Measurement With W States

Physicists in Japan have made a significant breakthrough in quantum measurement by successfully performing a collective measurement on a W state composed of three entangled photons. This achievement, reported in the journal Science Advances, represents a crucial advancement in the control of quantum states, which has implications for a range of technologies including quantum cryptography and quantum computing.

Quantum entanglement allows two particles to be interconnected in such a way that measuring one instantly reveals information about the other, regardless of the distance separating them. Traditional methods for measuring entangled particles often involve a technique known as quantum tomography. This process requires creating many identical copies of a particle and measuring each copy at different angles to reconstruct its full quantum state.

The recent experiment, conducted by researchers from Kyoto University and Hiroshima University, moves beyond this traditional approach. Instead of measuring individual particles sequentially, the team achieved simultaneous measurement of all three photons in a W state, which is more complex than the previously studied Greenberger–Horne–Zeilinger (GHZ) states. In a GHZ state, measuring one qubit collapses the entire superposition, while in a W state, entanglement persists even after one particle is measured.

Shigeki Takeuchi, the corresponding author of the study, highlighted the advantages of W states, stating, “In a W state, even if you measure one particle, entanglement still remains.” This robustness makes W states particularly promising for future quantum technologies.

To distinguish between nearly identical W states, the researchers utilized a method based on a discrete Fourier transform (DFT) circuit. This approach allowed them to decode tiny phase shifts that act as hidden labels distinguishing different states. The DFT method exploits inherent symmetries of W states, making it adaptable for systems with varying numbers of photons.

In their experiment, the physicists prepared photons in controlled polarization states and passed them through the DFT to identify each state’s phase label. The photons were then separated into vertically and horizontally polarized groups using polarizing beam splitters. By counting the number of photons in each category and combining this with DFT data, the team successfully identified the W state with an accuracy of approximately 87%. This success rate significantly exceeds the usual 15% achieved through traditional tomography-based measurements.

Maintaining this performance presented challenges, as fluctuations in optical paths or photon loss can disrupt the delicate interference patterns necessary for accurate measurement. Nevertheless, the team’s ability to sustain stable performance long enough to collect statistically reliable data marks an important technical milestone in the field. Takeuchi emphasized the efficiency of their method, stating, “Our device is not just a single-shot measurement: it works with 100% efficiency.”

While the current results demonstrate the technique with three photons, the researchers believe the method can be scaled up for larger systems. The key insight is the ability of the DFT to detect symmetries, which can theoretically apply to any number of photons.

The implications of this research are particularly relevant for quantum communication. Takeuchi noted, “Because our device is highly efficient, our protocol could be used for robust communication between quantum computer chips.” The next steps involve developing this technology on a compact photonic chip to minimize errors and photon loss, paving the way for practical applications in quantum computing and communication networks.

This groundbreaking work not only enhances our understanding of quantum systems but also positions Japan at the forefront of quantum technology research, potentially leading to significant advancements in the field in the coming years.