The quantum world is already full of surprises, but a new discovery has added an entire wing to what scientists call the \u201cquantum zoo.\u201d <\/p>\n
Using a new kind of laser<\/a> technique, researchers have uncovered nearly 20 previously hidden states of matter\u2014some of which may be the building blocks for a new generation of quantum computers. These states exist in a material called twisted molybdenum ditelluride, or tMoTe\u2082, and they don\u2019t require an external magnet to appear.<\/p>\n The discovery, published in Nature<\/a>, builds on decades of searching for exotic forms of quantum matter. In the past, scientists predicted these states through theory and computer models, but they hadn\u2019t been able to find them all in real materials. <\/p>\n Now, thanks to a collaboration led by Columbia University\u2019s<\/a> Xiaoyang Zhu, these elusive states have finally stepped into the light. \u201cSome of these states have never been seen before,\u201d said Zhu, the Howard Family Professor of Nanoscience. \u201cAnd we didn\u2019t expect to see so many either.\u201d<\/p>\n Device Images. a-b. 50X microscope image of device D1 and D2, the scale bar is 20 \u00b5m. (CREDIT: Xiaoyang Zhu, et al.) From Hallways of History to the Fractional Quantum Anomalous Hall Effect<\/p>\n At the heart of this breakthrough is a phenomenon called the fractional quantum anomalous Hall effect. To understand it, you need to know about its ancestor\u2014the classical Hall effect. Discovered in 1879, the Hall effect describes how electrons moving through a strip of metal shift toward the edges when exposed to a magnetic field, creating a measurable voltage difference.<\/p>\n When this experiment is done in two dimensions at extremely low temperatures, something stranger happens: the voltage jumps in fixed steps, a phenomenon called the quantum Hall effect. Sometimes, those steps break into smaller ones\u2014fractions of an electron\u2019s charge\u2014giving rise to the fractional quantum Hall effect. This bizarre behavior was so significant that Columbia\u2019s Horst Stormer shared the 1998 Nobel Prize in Physics for helping uncover it.<\/p>\n The \u201canomalous\u201d twist comes when these effects appear without any external magnet. That\u2019s exactly what happens in twisted molybdenum ditelluride, where the arrangement of atoms creates its own internal magnetic field<\/a>.<\/p>\n Related StoriesThe Magnet-Free Twist in Quantum Discovery<\/p>\n In 2023, physicist Xiaodong Xu at the University of Washington<\/a>\u2014working with researchers from Cornell<\/a> and Shanghai Jiao Tong University<\/a>\u2014found that twisting atom-thin layers of molybdenum ditelluride into a special pattern called a moir\u00e9 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.<\/p>\n Xu\u2019s 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 moir\u00e9 pattern. When the layers are slightly rotated relative to each other, they form a honeycomb-like grid at the atomic scale. <\/p>\n This structure changes the way electrons<\/a> 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.<\/p>\n Pump\u2013probe spectroscopy detects hidden states at fractional fillings in tMoTe2. (CREDIT: Xiaoyang Zhu, et al.) Pump-Probe Spectroscopy Opens the Gate<\/p>\n Last summer, Yiping Wang, a postdoctoral fellow at Columbia, got a sample of twisted molybdenum ditelluride from Xu\u2019s lab. While Zhu was traveling, she decided to try a special laser method known as pump-probe spectroscopy, developed by fellow researcher Eric Arsenault.<\/p>\n This technique uses two laser pulses. The first pulse, the \u201cpump,\u201d briefly excites electrons and \u201cmelts\u201d the quantum states in the material. The second, the \u201cprobe,\u201d measures how those states recover over time. By tracking these changes in the material\u2019s dielectric constant, the researchers could spot extremely subtle energy levels\u2014many invisible to standard experiments.<\/p>\n What Wang saw lit up her screen. Peaks appeared at fractions that scientists had only theorized before. Among them were fractional fillings such as -4\/3, -3\/2, -5\/3, -7\/3, -5\/2, and -8\/3. These are strong candidates for predicted exotic phases, including fractional topological insulators and non-Abelian anyons\u2014exotic particles thought to be crucial for stable topological quantum computing<\/a>. \u201cThis discovery also establishes pump-probe spectroscopy as the most sensitive technique in detecting quantum states of matter,\u201d Zhu said.<\/p>\n Melting and recovery dynamics of correlated states. (CREDIT: Xiaoyang Zhu, et al.) Hidden States and Exotic Phases<\/p>\n The team didn\u2019t just confirm known states\u2014they revealed nearly 20 new ones. These included fractional fillings between 0 and -1, as well as many on the electron-doped side, where the material has more electrons than usual.<\/p>\n The most intriguing are the newly observed fractional fillings of the Chern bands\u2014energy bands with unique topological properties. These may host non-Abelian states, a rare class of quantum matter where swapping particles changes the system in ways that depend on the order of the swaps. <\/p>\n This property could help create quantum computers that are far more resistant to errors. \u201cWe hope these results and our technique inspire others to explore,\u201d Zhu said. \u201cThere are just so many.\u201d<\/p>\n Why the Fractional Quantum Anomalous Hall Effect Matters<\/p>\n The pump-probe experiments also revealed how these correlated quantum states break apart and recover. The melting happens on two very different timescales. Some states collapse in just 2\u20134 trillionths of a second, a speed linked to pure electronic changes. Others take much longer\u2014about 180\u2013270 trillionths of a second\u2014due to the movement of atomic vibrations, or phonons.<\/p>\n Pump-probe spectroscopy detects hidden states at fractional fillings of the first and second Chern bands in tMOTe2 device 2 (D2) with a twist angle q = 3.1 degrees. (CREDIT: Xiaoyang Zhu, et al.) <\/p>\n The difference between electron-doped and hole-doped states, where electrons are removed instead of added, comes from the distinct shapes of the material\u2019s conduction and valence bands. These differences could be important for tuning the material for future technologies.<\/p>\n The fractional quantum anomalous Hall effect isn\u2019t just another curiosity in the quantum zoo\u2014it\u2019s a potential gateway to practical quantum technology<\/a>. Topological quantum computers, which rely on stable, error-resistant quantum states, have long been a goal for physicists. Until now, creating the right states has required magnets, which interfere with most quantum computing hardware.<\/p>\n The fact that these new fractional states exist without magnets means they could be integrated into devices more easily. The discovery also provides a powerful new way to find and study exotic quantum matter in other materials. \u201cEvery new state is like finding a new species,\u201d said Wang. \u201cYou can study it, understand it, and maybe even use it for something entirely new.\u201d<\/p>\n A Growing Quantum Zoo<\/p>\n