In 2023, physicist Xiaodong Xu at the University of Washington —working with researchers from Cornell and Shanghai Jiao Tong University —found that twisting atom-thin layers of molybdenum ditelluride into a special pattern called a moiré lattice could produce the fractional quantum anomalous Hall effect without magnets. This was a huge leap, because magnets can disrupt superconducting materials used in quantum technology.

Xu’s team discovered two such magnet-free fractional states. That alone was remarkable. But Zhu and his colleagues suspected there were more waiting to be found. The secret lies in the moiré pattern. When the layers are slightly rotated relative to each other, they form a honeycomb-like grid at the atomic scale.

This structure changes the way electrons move, encouraging them to team up in unusual ways that create fractional charges. In other words, the twist turns the material into a playground for exotic quantum phases.

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.