In quantum mechanics, entanglement allows two or more particles separated in space to have the same physical properties, which are correlated. This means that if a measurement is performed on one particle, it will prove to be the same on the entangled particle. Quantum entanglement is important, but difficult to study, both theoretically and experimentally. It has been possible to entangle small groups of particles, but scaling up these experiments has proven difficult until now.
Researchers from Shanghai’s University of Science and Technology of China were successfully able to entangle eight photons, which beats the previous record of six. The photos were equally likely to be polarized in a specific orientation, which is known colloquially as a “Schrödinger’s cat” state. Their research was published in Nature Photonics, and the authors describe a new technique using ultra-bright photon sources to control some of the problems that had plagued earlier experiments.
Xing-Can Yao et al. excited beta-barium borate (BBO) with an ultraviolet laser. The resulting photos produced a particular transition inside the crystal that produced new photons with polarizations that are complimentary to each other. Their quantum states were entangled. The photons are in an undetermined state before measurement, and according to the standard interpretation of quantum mechanics, they are seen as possessing both polarization states with equal probability.
Out of the 256 possible polarization combinations from eight photons, only one of them is consistent with a fully entangled state. Their setup is more complex, so it would be surprising if they’re able to scale it further. However, the system is also powerful enough that it is a step forward in terms of optical quantum computation, suggesting that the setup may enable quantum simulations to tackle more complicated problems in condensed matter physics.