Photo credit: www.sciencedaily.com
The realm of quantum states is vast and intricate, representing the myriad ways in which quantum matter can behave, especially when considerable numbers of electrons interact. For many years, various quantum states existed mainly as theoretical concepts, with real-world examples waiting to be uncovered. Many scientists are affectionately referring to this domain as a ‘quantum zoo’, filled with new ‘species’ ripe for discovery and study.
A recent study published on April 3 in Nature adds over a dozen new states to this growing collection of quantum phenomena.
Lead researcher Xiaoyang Zhu, who holds the Howard Family Professorship of Nanoscience at Columbia University, remarked, “Some of these states are completely new to science. We honestly did not anticipate finding such a large number of them.”
Among these newly identified states are configurations that could pave the way for the development of topological quantum computers. This next-generation computational architecture boasts unique quantum characteristics that may reduce the errors that currently plague quantum systems based on superconducting materials. Unlike superconductors, which are affected by magnetic fields, the new states identified by Zhu’s team can be produced without the need for such external magnets, thanks to the remarkable attributes of a material referred to as twisted molybdenum ditelluride.
Looking Back at Quantum History
Some of the newly identified quantum states may relate to the classical Hall effect. Discovered in 1879, the classical Hall effect illustrates how electric currents in metal strips can become concentrated at edges when subjected to magnetic fields; a stronger field increases the voltage disparity across the metal. In a two-dimensional space at ultra-low temperatures, where quantum mechanics is easily observed, this relationship transforms. Instead of a straightforward increase, the voltage change becomes quantized, occurring in steps linked to the charge of an electron, the smallest known charged particle.
These steps can be further divided into smaller segments, resulting in fractional charges such as -½, -â…”, and -â…“. This extraordinary observation won Columbia Emeritus Professor Horst Stormer a share of the Nobel Prize in Physics in 1998. As Stormer explained in his Nobel Lecture, the “fractional quantum Hall effect” reveals a counterintuitive aspect of quantum mechanics, where an aggregation of electrons can generate composite particles with charges smaller than an individual electron’s charge without actually splitting the electrons themselves.
For decades, scientists have actively sought evidence of the fractional quantum Hall effect across various materials. A significant breakthrough emerged in 2023 when physicist Xiaodong Xu from the University of Washington and member of Columbia’s Department of Energy-supported Energy Frontier Research Center on Programmable Quantum Materials (ProQM) identified an anomalous, magnet-free fractional quantum Hall effect in twisted layers of molybdenum ditelluride. His findings were corroborated by experiments conducted at Cornell University and results from Shanghai Jiao Tong University.
Led by doctoral students Jiaqi Cai and Heonjeoon Park, Xu’s research culminated in two published papers in Nature, unveiling two desired fractional quantum anomalous Hall states. Zhu indicated that this was just the beginning.
Unraveling the Moiré Mystique
The research team has focused on Moiré materials, which consist of ultra-thin layers of different elements that are slightly misaligned. This twisting creates a honeycomb structure with attributes unattainable in single-layer forms or bulk crystals from which these layers are derived.
When the layers of molybdenum ditelluride are twisted, they gain topological properties, arranging electrons in ways that encourage collective behavior capable of forming fractional quantum Hall charges. This twisting also produces internal magnetic fields, removing the necessity for external magnets.
Last summer, Yiping Wang, a Max-Planck NYC Center Postdoctoral Fellow at Columbia and lead author of the current Nature publication, tested a material sample from Xu’s laboratory. While Zhu was away, Wang employed a specialized pump-probe spectroscopy method devised by co-author Eric Arsenault. She detected numerous fractional charges, including fractions theoretically predicted as components vital for constructing a topological quantum computer, notably non-Abelian anyons.
This innovative pump-probe technique entails one laser pulse disrupting the material’s quantum states, followed by a second pulse that measures changes in the dielectric constant—essentially gauging the strength of electrical interactions—as the quantum states re-establish themselves. According to Zhu, “This study confirms that pump-probe spectroscopy is currently the most sensitive method for detecting quantum states of matter.”
Besides identifying these states in their lowest energy form, the technique captures how they evolve. Wang commented, “It’s as if we have stepped into a new dimension of time, allowing us to explore correlations and topology in ground states. Every experiment continues to surprise us, especially when we push these states out of equilibrium.”
Moving forward, researchers will work to delineate the exact nature and potential applications of these newly discovered states. Zhu expressed optimism, stating, “There are so many possibilities. We anticipate our findings and methodologies will inspire further exploration in the field.” Indeed, the quantum zoo continues to expand.
Source
www.sciencedaily.com