Xiaoyang Zhu and his team at Columbia University just pulled off something physicists have long struggled to achieve: using nothing but light, they flipped quantum materials from metals into insulators in the blink of an eye. The discovery, published in Physical Review Letters, rewrites what's possible with two-dimensional quantum matter—and hints at a entirely new generation of ultrafast devices.
For years, scientists have been able to use laser light to trigger quantum materials to transform from insulators to metals, a feat that has earned significant attention. But the reverse process—pushing metallic materials into an insulating state through photoexcitation alone—remained frustratingly out of reach. Until now.
Zhu's breakthrough came partly by accident. The team was working with moiré heterostructures, exotic quantum materials made by stacking ultra-thin layers of tungsten disulfide and tungsten diselenide on top of each other with a slight misalignment. These twisted layers create a characteristic periodic pattern that gives moiré materials their unique properties. The researchers included graphite electrodes to help control the electrical charge flowing through the devices, and as Zhu explained to Phys.org, "most excitation energy is absorbed by the graphite electrodes. The current work is a result of such a surprising function."
When they fired short laser pulses at the metallic moiré devices, something unexpected happened. The photoexcited holes—missing electrons created by the light—were ultrafast injected from the graphite electrode directly into the moiré structure. This injection of charge disrupted the delicate quantum correlations holding the material in its metallic state, and the entire device snapped into an insulating state within femtoseconds. It was the reverse transition everyone had been seeking, achieved through a mechanism no one had fully anticipated.
"In the photoexcitation of charge-doped moiré quantum matter, pump excitation results in the disruption of a correlation," Zhu explained. "At high pump powers, we made the astonishing observation of a spectroscopic signature for the reverse process, i.e., a metal-to-insulator transition." The team conducted careful analyses to confirm the physics driving this transition, ruling out competing explanations and pinpointing the ultrafast hole injection as the key mechanism.
This matters because controlling how electrons move through materials at ultrafast speeds is the holy grail for next-generation quantum technologies. If you can flip quantum phases on and off with laser pulses—at picosecond or femtosecond timescales—you unlock possibilities that were previously theoretical. Ultrafast quantum memories that store and retrieve information at light speed. Quantum processors that could outpace classical computers by orders of magnitude. Optical switches that respond faster than any electronic component ever built.
Zhu and his colleagues are already plotting their next moves. They want to use this discovery not just to toggle between two states, but to actively tune and control multiple quantum phases in moiré devices on ultrafast timescales. They're also hunting for entirely new phases of matter that might be hidden within these materials, accessible only through the right combination of light and electrical charge. "We now want to take advantage of this discovery to control various moiré quantum phases on ultrafast time scales and to explore potentially hidden quantum phases," Zhu said.
The implications extend beyond Columbia's lab. The methodology Zhu's team developed—using pump-probe spectroscopy to watch quantum transitions happen in real time—provides a foundation for how future researchers will study van der Waals structures with ultrafast laser pulses. In other words, they've not only solved a longstanding puzzle, but opened a door that future scientists will walk through for years to come.