The concept of quantum entanglement is a fundamental aspect that highlights the distinction between classical and quantum physics. It describes a scenario where it becomes impossible to separately describe the physics of individual photons. This critical characteristic of quantum mechanics challenges the classical notion that each particle exists independently, a concept that greatly troubled physicist Albert Einstein. Gaining a comprehensive understanding of this phenomenon is vital for the advancement of innovative and powerful new quantum technologies.
To develop these groundbreaking technologies, researchers must be able to generate multi-photon quantum entangled states freely and efficiently identify the type of entangled state present. However, conventional quantum tomography—a common method for state estimation—presents a significant challenge: the number of measurements required increases exponentially with the number of photons involved. This creates a substantial data collection problem for researchers.
When an entangled measurement is available, it can identify the entangled state using a one-shot approach. Such a measurement for the Greenberger-Horne-Zeilinger (GHZ) entangled quantum state has been successfully realized. However, for the W state, another critical representation of multi-photon entanglement, no experimental proposals or discoveries had been made until now.
This gap motivated a dedicated team of researchers from Kyoto University and Hiroshima University to tackle the challenge of developing a method for entangled measurement of the W state. Their groundbreaking findings were published in the journal Science Advances. "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," stated corresponding author Shigeki Takeuchi.
The research team concentrated on the unique characteristics of the W state's cyclic shift symmetry. They theoretically proposed a method to create an entangled measurement using a photonic quantum circuit designed to perform quantum Fourier transformations for the W state across any number of photons. To demonstrate their proposed method, they developed a high-stability optical quantum circuit that could operate without active control for extended periods.
By inserting three single photons into the device in the appropriate polarization states, the researchers were able to demonstrate the capability to distinguish between different types of three-photon W states. Each type corresponds to a specific non-classical correlation among the three input photons. The team evaluated the fidelity of the entangled measurement, which indicates the probability of obtaining the correct result for a pure W-state input.
This significant achievement paves the way for advancements in quantum teleportation—the transfer of quantum information—as well as the development of new quantum communication protocols. It could also facilitate the transfer of multi-photon quantum entangled states and introduce novel methods for measurement-based quantum computing. "To accelerate the research and development of quantum technologies, it is crucial to deepen our understanding of basic concepts to come up with innovative ideas," emphasizes Takeuchi.
Looking ahead, the research team plans to apply their method to larger-scale, more general multi-photon quantum entangled states. Additionally, they are working on developing on-chip photonic quantum circuits for entangled measurements, which could further enhance the capabilities and applications of quantum technologies in the future.
