University of Maryland Launches Quantum Technology Center

Date: 
Thursday, August 22, 2019

Newfound Superconductor Material Could Be the ‘Silicon of Quantum Computers’

 We have already found lots of superconductors, but this whimsical illustration shows why one superconductor's newfound properties may make it especially useful. Most known superconductors are spin singlets, found on the island to the left. Uranium ditelluride, however, is a rare spin triplet, found on the island to the right, and also exists at the top of a mountain representing its unusually high resistance to magnetic fields. These properties may make it a good material for making qubits, which could maintain coherence in a quantum computer despite interference from the surrounding environment. Credit: N. Hanacek/NIST We have already found lots of superconductors, but this whimsical illustration shows why one superconductor's newfound properties may make it especially useful. Most known superconductors are spin singlets, found on the island to the left. Uranium ditelluride, however, is a rare spin triplet, found on the island to the right, and also exists at the top of a mountain representing its unusually high resistance to magnetic fields. These properties may make it a good material for making qubits, which could maintain coherence in a quantum computer despite interference from the surrounding environment. Credit: N. Hanacek/NIST

 A collaboration of the NIST Center for Neutron Research, the UMD's Center for Nanophysics and Advanced Materials and the Ames Laboratory has yielded a new superconductor with properties highly advantageous for the development of quantum computers. Uranium ditelluride, or UTe2, described in Science magazine, resists magnetism and could maintain coherence in qubits.  Read more at NIST.gov. 

 

Corkscrew Photons May Leave Behind a Spontaneous Twist

A new prediction argues that some materials might experience a torque when they are hotter than their surroundings. (Credit: E. Edwards/JQI)

 

Everything radiates. Whether it's a car door, a pair of shoes or the cover of a book, anything hotter than absolute zero (i.e., pretty much everything) is constantly shedding radiation in the form of photons, the quantum particles of light.

A twin process—absorption—is usually also present. As photons carry away energy, passers-by from the environment can be absorbed to replenish it. When absorption and emission occur at the same rate, scientists say that an object is in equilibrium with its environment. This often means that object and environment share the same temperature.

Far away from equilibrium, new behaviors can emerge. In a paper published August 1, 2019 as an Editors’ Suggestion in the journal Physical Review Letters, scientists at JQI and Michigan State University suggest that certain materials may experience a spontaneous twisting force if they are hotter than their surroundings.

"The fact that a material might feel a torque due to a temperature difference with the environment is very unusual," says lead author Mohammad Maghrebi, a former JQI postdoctoral researcher who is now an assistant professor at Michigan State University.

The effect, which hasn't yet been observed in an experiment, is predicted to arise in a thin ribbon of a material called a topological insulator (TI)—something that allows electrical current to flow on its surface but not through its innards.

In this case, the researchers made two additional assumptions about the TI. One is that it is hotter than its environment. And another is that the TI has some magnetic impurities that affect the behavior of electrons on its surface.

These magnetic impurities interact with a quantum property of the electrons called spin. Spin is part of the basic character of an electron, much like electric charge, and it describes the particle’s intrinsic angular momentum—the tendency of an object to continue rotating. Photons, too, can carry angular momentum.

Although electrons don’t physically rotate, they can still gain and lose angular momentum, albeit only in discrete chunks. Each electron has two spin values—up and down—and the magnetic impurities ensure that one value sits at a higher energy than the other. In the presence of these impurities, electrons can flip their spin from up to down and vice versa by emitting or absorbing a photon that carries the right amount of energy and angular momentum.

Maghrebi and two colleagues, JQI Fellows Jay Deep Sau and Alexey Gorshkov, showed that radiation emanating from this kind of TI carries angular momentum skewed in one rotational direction, like a corkscrew that twists clockwise. The material gets left with a deficit of angular momentum, causing it to feel a torque in the opposite direction (in this example, counterclockwise).

The authors say that TIs are ideal for spotting this effect because they play host to the right kind of interaction between electrons and light. TIs already link electron spin with the momentum of their motion, and it's through this motion that electrons in the material ordinarily absorb and emit light.

If an electron on the surface of this particular kind of TI starts with its spin pointing up, it can shed energy and angular momentum by changing its spin from up to down and emitting a photon. Since the TI is hotter than its environment, electrons will flip from up to down more often than the reverse. That’s because the environment has a lower temperature and lacks the energy to replace the radiation coming from the TI. The result of this imbalance is a torque on the thin TI sample, driven by the random emission of radiation.

Future experiments might observe the effect in one of two ways, the authors say. The most likely method is indirect, requiring experimenters to heat up a TI by running a current through it and collecting the emitted light. By measuring the average angular momentum of the radiation, an experiment might detect the asymmetry and confirm one consequence of the new prediction.

A more direct—and likely more difficult—observation would involve actually measuring the torque on the thin film by looking for tiny rotations. Maghrebi says that he's brought up the idea to several experimentalists. "They were not horrified by having to measure something like a torque, but, at the same time, I think it really depends on the setup," he says. "It certainly didn't sound like it was impossible."

Story by Chris Cesare: https://jqi.umd.edu/news/corkscrew-photons-may-leave-behind-spontaneous-twist

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Davoudi, Manucharyan Receive DOE Early Career Research Funding

Zohreh Davoudi and Vladimir Manucharyan are among the 73 scientists selected by the Department of Energy for Early Career funding. Davoudi’s proposal, Analog and Digital Quantum Simulations of Strongly Interacting Theories for Applications in Nuclear Physics was chosen by the Office of Nuclear Physics. Manucharyan’s proposal, Realization of a Quantum Slide Rule for 1+1 Dimensional Quantum Field Theories Using Josephson Superconducting Circuits was selected for funding by the Office of Advanced Scientific Computing Research.

Davoudi and Manucharyan will each receive $750,000 over five years. The list of awardees and their abstracts can be seen here.