The concept of quantum entanglement is emblematic of the gap between classical and quantum physics. Referring to a situation in which it is impossible to describe the physics of each photon separately, this key characteristic of quantum mechanics defies the classical expectation that each particle should have a reality of its own, which gravely concerned Einstein. Understanding the potential of this concept is essential for the realization of powerful new quantum technologies.
Developing such technologies will require the ability to freely generate a multi-photon quantum entangled state, and then to efficiently identify what kind of entangled state is present. However, when performing conventional quantum tomography, a method commonly used for state estimation, the number of measurements required grows exponentially with the number of photons, posing a significant data collection problem.
If available, an entangled measurement can identify the entangled state with a one-shot approach. Such a measurement for the Greenberger-Horne-Zeilinger -- GHZ -- entangled quantum state has been realized, but for the W state, the other representative entangled multi-photon state, it has been neither proposed nor discovered experimentally.
This motivated a team of researchers at Kyoto University and Hiroshima University to take on this challenge, ultimately succeeding in developing a new method of entangled measurement to identify the W 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," says corresponding author Shigeki Takeuchi.
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