New quantum states found in crystal layers could boost computing

Quantum experiments keep stirring up surprises. The latest involves an unusual property inside thin layers of molybdenum ditelluride crystals, twisted slightly to form a patterned lattice. Scientists have discovered more than a dozen new quantum states in these twisted layers.

The research was led by Xiaoyang Zhu at Columbia University, whose recent experiments hint at possible paths toward sturdier quantum computers.

Discovery of new quantum states

The Hall effect was first observed in 1879, when Edwin Hall noticed how electrons drift to one side of a metal strip under a magnetic field. It seemed straightforward until researchers spotted a related behavior at frigid temperatures, known as the fractional quantum Hall effect. 

“It implies that many electrons, acting in concert, can create new particles having a charge smaller than the charge of any individual electron,” explained Horst Stormer, Columbia Professor Emeritus at the end of the 20th century.

Those fractional charges baffled many scientists. Before long, entire families of quantum states were predicted. Over the past few decades, teams around the world searched for them in different materials, adding to what some call a growing zoo of possibilities.

New quantum behavior in crystal layers

Scientists found that twisting ultrathin layers of certain crystals rearranges their electrons. This pattern is called a moiré configuration, and it sets the stage for unusual effects. One result is a hidden internal magnet-like field, which emerges even when no external magnet is used.

“Some of these states have never been seen before,” said Xiaoyang Zhu. The researchers hope some of these new discoveries will bolster topological quantum computing, an approach that could avoid many of the errors seen in present devices.

Strange particles carry electron pieces

Another set of curious particles found in this material might be non-Abelian anyons, which carry fractional charges.

Experts have suggested these unusual entities could store information better than standard bits, thanks to topological properties. They have been difficult to verify in actual experiments, so spotting fresh hints of them fires up new discussions.

Many prior attempts relied on strong magnets to prod electrons. The trouble was that superconducting materials, often used in quantum computing, get thrown off by magnetic fields. Twisted molybdenum ditelluride appears to solve that conflict by generating internal fields on its own.

Laser pulses reveal quantum states

Zhu’s team used a pump-probe method. One powerful laser pulse “melts” the electronic arrangement, while a second pulse measures subtle changes in the dielectric constant, which indicates how charges interact.

This approach is sensitive enough to catch a long list of fractional energy states that remain hidden under static measurements.

“This discovery also establishes pump-probe spectroscopy as hitherto the most sensitive technique in detecting quantum states of matter,” said Zhu. The quick laser flashes reveal short-lived details that are otherwise missed. 

“They just keep surprising us, especially when we push them out of equilibrium,” said Yiping Wang, a postdoctoral researcher involved in the experiments.

Connections to quantum computers

Researchers have long chased designs for topological quantum computers that might avoid many forms of noise.

The idea is that braided groups of non-Abelian anyons could lock data into robust states. Finding ways to generate those states without magnets is a big deal, because it clears away a major engineering hurdle.

It’s still unknown which of these newly observed states will prove the most useful. But there is hope that a fraction of them might become the missing pieces needed to assemble next-level quantum machines.

How quantum states react to rapid changes

Pump-probe methods also show how these states respond to small disturbances over billionths of a second. Researchers observed that some states scramble quickly, while others respond more slowly.

These separate timescales may tell us something about how electrons and crystal vibrations pass information back and forth.

Extended data from these results will keep theorists busy. Shining rapid lasers on twisted bilayers adds a time dimension to the usual mix of charge and spin. Lab tests that focus on these sub-picosecond signals give a deeper look at electron interactions.

More quantum states may emerge

What happens when other moiré materials are twisted at different angles? Scientists anticipate more quantum discoveries as they fine-tune the spacing between atoms. The group that studied molybdenum ditelluride thinks similar approaches could reveal additional hidden states in other layered systems.

This research also opens the door to exotic states on both the electron-doped side and the hole-doped side of these structures. If patterns continue, there may be more undiscovered phases lurking in corners of the moiré lattice. The quest is still unfolding.

A growing variety of quantum states

Quantum science is a rapidly expanding field. Teams worldwide are racing to characterize new materials and see if these phases lead to better electronics or advanced sensors. A single crystal can support many unique charges that lock together in unexpected ways.

There is a sense that this exploration is nowhere near complete. New tools, including ultrafast lasers, keep shining light on subtle effects that traditional instruments can’t detect. The results might be far greater than what we can predict.

The study is published in the journal Nature.

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