Tyler O'Neal, Staff Editor PHYSICS

Brown physics profs show a new strategy for detecting non-conformist particles called anyons

By observing how strange particles called anyons dissipate heat, researchers have shown that they can probe the properties of these particles in systems that could be relevant for topological quantum supercomputing.

A team of Brown University researchers has shown a new method of probing the properties of anyons, strange quasiparticles that could be useful in future quantum supercomputers. 436076102 21e90

In research published in the journal Physical Review Letters, the team describes a means of probing anyons by measuring subtle properties of how they conduct heat. Whereas other methods probe these particles using electrical charge, this new method enables researchers to probe anyons even in non-conducting materials. That’s critical, the researchers say, because non-conducting systems have far less stringent temperature requirements, making them a more practical option for quantum computing.

“We have beautiful ways of probing anyons using charge, but the question has been how do you detect them in the insulating systems that would be useful in what’s known as topological quantum computing,” said Dima Feldman, a physics professor at Brown and study co-author. “We show that it can be done using heat conductance. Essentially, this is a universal test for anyons that works in any state of matter.”

Anyons are of interest because they don’t follow the same rules as particles in the everyday, three-dimensional world. In three dimensions, there are only two broad kinds of particles: bosons and fermions. Bosons follow what’s known as Bose-Einstein statistics, while fermions follow Fermi-Dirac statistics. Generally speaking, those different sets of statistical rules mean that if one boson orbits around another in a quantum system, the particle’s wave function — the equation that fully describes its quantum state — does not change. On the other hand, if a fermion orbits around another fermion, the phase value of its wave function flips from a positive integer to a negative integer. If it orbits again, the wave function returns to its original state.

Anyons, which emerge only in systems that are confined to two dimensions, don’t follow either rule. When one anyon orbits another, its wave function changes by some fraction of an integer. And another orbit does not necessarily restore the original value of the wave function. Instead, it has a new value — almost as if the particle maintains a “memory” of its interactions with the other particle even though it ended up back where it started.

That memory of past interactions can be used to encode information in a robust way, which is why the particles are interesting tools for quantum computing. Quantum computers promise to perform certain types of calculations that are virtually impossible for today’s computers. A quantum computer using anyons — known as a topological quantum supercomputer — has the potential to operate without elaborate error correction, which is a major stumbling block in the quest for usable quantum supercomputers.

But using anyons for computing requires first being able to identify these particles by probing their quantum statistics. Last year, researchers did that for the first time using a technique known as charge interferometry. Essentially, anyons are spun around each other, causing their wave functions to interfere with each other occasionally. The pattern of interference reveals the particles’ quantum statistics. That technique of probing anyons using charge works beautifully in systems that conduct electricity, the researchers say, but it can’t be used to probe anyons in non-conducting systems. And non-conducting systems have the potential to be useful at higher temperatures than conducting systems, which need to be near absolute zero. That makes them a more practical option of topological quantum supercomputing.

For this new research, Feldman,  who in 2017 was part of a team that measured the heat conductance of anyons for the first time, collaborated with Brown graduate student Zezhu Wei and Vesna Mitrovic, a Brown physics professor and experimentalist. Wei, Feldman, and Mitrovic showed that comparing properties of heat conductance in two-dimensional solids etched in very specific geometries could reveal the statistics of the anyons in those systems.

“Any difference in the heat conductance in the two geometries would be smoking gun evidence of fractional statistics,” Mitrovic said. “What this study does is show exactly how people should set up experiments in their labs to test for these strange statistics.”

Ultimately, the researchers hope the study is a step toward understanding whether the strange behavior of anyons can indeed be harnessed for topological quantum supercomputing.

Cornell project aims to improve accuracy of climate change models

Tyler O'Neal, Staff Editor PHYSICS

There’s broad scientific consensus that, because of climate change, the western U.S. will have less water and the northeastern U.S. will have more. But how much less and how much more is deeply uncertain, presenting a critical challenge for the scientists, policymakers, and public servants tasked with ensuring the nation’s water supply.

Flavio Lehner, assistant professor of earth and atmospheric sciences at Cornell University, is working to reduce that uncertainty, by improving the climate models on which future water projections are based. Lehner won a three-year, $500,000 grant from the National Oceanic and Atmospheric Administration (NOAA) to do that work, beginning this fall.

Dan Barrie, a program manager in NOAA’s Climate Program Office, said Lehner’s work will improve NOAA’s climate models and enable the agency to make better short-term predictions of floods and droughts and better long-term projections of how surface water systems will evolve in the 21st century.

“The United States is experiencing profound changes in its regional water resources,” Barrie said. “It is more urgent than ever to have the best modeling tools to provide a vision of these future changes so that we can take cost-effective measures now to mitigate and adapt to them.”

Lehner’s research, which will improve climate modeling globally, was based on similar research he began in the Colorado River. Current estimates predict that for every degree Celsius of global warming, the Colorado River will lose between 3 to 15% of its streamflow.

Lehner compared the differences in climate models to the disparity in human reactions to COVID-19 – assessing whether an individual has COVID-19 is relatively simple, but predicting how sick the virus will make each person is much more difficult. A similar principle is at play in climate modeling, he said.

“For example, for the Colorado River, all of the numbers point in the same direction – in a warmer climate, there will be less water. But the big uncertainty is how much less,” Lehner said.

To test the sensitivity of climate models, Lehner’s group is studying 70 years of data on precipitation, temperature, and streamflow, to assess how well current models would have predicted what happened.

“The most important question to us is: How sensitive are these models to changing environmental factors, such as changes in temperature and atmospheric greenhouse gases? And is their sensitivity consistent with what we see in reality?” he said.

The models Lehner and colleagues are using are more complicated and ultimately more useful because they take into account multiple interacting systems. Rather than just measuring rainwater or groundwater, Lehner is examining how atmospheric, terrestrial, and hydrologic systems interplay, in the presence of increasing temperatures and atmospheric greenhouse gases. For example, there is now 40% more carbon dioxide in the air today than there was 100 years ago, and the earth is 1 degree Celsius warmer.  Even if precipitation remained neutral, those changes would cause plants to alter their behavior – consuming more groundwater to prevent parching, and thus leaving less to become stream runoff available to humans. But with added complexity comes added uncertainty.

“We already have a sizable uncertainty because we don’t know how much precipitation is going to change, but if you go one step further and say, how does runoff or streamflow change? The uncertainty becomes even larger,” Lehner said.

Modern climate modeling expanded dramatically in the 1980s and has provided useful and accurate information to help scientists and policymakers plan and adapt, Barrie said. Since 1980, the U.S. has experienced an average of 7.1 major weather or climate disasters per year, each causing losses of more than $1 billion. But in the past five years, the annual average of major disasters has jumped to 16.2, according to NOAA.

“Improving climate models is one step to ensuring that equitable adaptation efforts can be implemented to minimize net negative impacts on people and the economy. The cost of investments like Dr. Lehner's research project pales in comparison to the magnitude of the potential benefits,” Barrie said.

MD simulations show a rare quaternary compound FeO2H2He stabilized under high pressure

Tyler O'Neal, Staff Editor PHYSICS

This study is led by Prof. Hanyu Liu, Prof. Yanming Ma at the College of Physics, Jilin University located in Changchun, China, and Prof. Changfeng, Chen at the Department of Physics and Astronomy, University of Nevada. Their supercomputer simulations present evidence of direct and prevalent chemical association of hydrogen and helium facilitated by their reaction with recently discovered iron peroxide FeO2 in forming rare quaternary compound FeO2H2He, which has been predicted to be viable in a large region of the pressure-temperature phase diagram. Most interestingly, in a wide swath of the phase space corresponding to Earth's lowest-mantle regions, this quaternary compound stays in a superionic state hosting liquid-like hydrogen inside the FeO2He sublattice that remains intact in crystalline form. This exotic solid-liquid mixture state of matter makes an unusually conducive environment promoting close coalescence of hydrogen and helium. “These results highlight a compelling case of a hydrogen-helium chemical association, which may be harbored in deep-Earth regions, providing crucial guidance for exploring the novel solid and superionic phases of FeO2H2He in laboratory experiments and also for modeling interiors of giant solar and extrasolar planets,” Liu says. The phase boundary is marked by the thick black line and different structures within each phase are determined based on the molecular dynamics results, where triangles, stars, and circles represent, respectively, the solid, superionic (with diffusive H atoms), and liquid structures. Also presented are the phase coexistence probabilities, as indicated by the colored scale bar shown in the inset, in the context of the Boltzmann distribution for the FeO2H2He structures relative to the dehydrogenation products FeO2H, H2 and He at selected points along the geotherm in the pressure range of 100-140 GPa. The yellow region presents the geotherm of the Earth's core. CREDIT ©Science China Press

Hydrogen and helium are the most abundant elements in the universe and play crucial roles in geological and astrophysical environments, but they are known to be inert toward each other across wide pressure-temperature and concentration ranges and remain largely immiscible up to multi-megabar pressures and 3,000-4,000 K temperatures. Given their prominent presence and influence on the formation and evolution of celestial bodies, it is of great interest and significance to explore and decipher the nature of interactions between hydrogen and helium, the especially possible chemical association that would have considerable impacts in many scientific fields, from chemistry, physics, geoscience to astrophysics.

By employing an advanced crystal structure search method called CALYPSO, they identified a quaternary compound FeO2H2He, which could be stabilized in a wide range of pressure and temperature conditions. Further molecular dynamics simulations indicate a novel superionic state of FeO2H2H, which hosted liquid-like diffusive hydrogen in the FeO2He sublattice. Meanwhile, a conducive environment for hydrogen-helium chemical association was identified, where the pressure and temperature conditions correspond to the Earth's lowest mantle regions. “These results suggested the surprising chemically facilitated coalescence of otherwise immiscible molecular species highlights a promising avenue for exploring this long-sought but [a] hitherto unattainable state of matter”, Liu says, “this finding raises strong prospects for exotic H-He mixtures inside Earth, as well as providing implications for other astronomical bodies.