Professor Jonathan Friedman Receives Grant for Research of Quantum Mechanical Effects in Single-Molecule Nanomagnets and Superconducting Devices

June 2010

In the Merrill Science Center lab of Associate Professor of Physics Jonathan Friedman, you’ll find magnets consisting of only one molecule each and student researchers who custom-build much of their own equipment. With the support of a recent grant from the National Science Foundation, this summer the professor and his students are continuing their cutting-edge research of quantum mechanical effects in single-molecule nanomagnets and superconducting devices.

“From an everyday person’s point-of-view, a single-molecule magnet is very, very small, but compared to atomic-scale magnets, it’s pretty large,” Friedman explains. “These magnets behave quantum mechanically: they can do things that no typical refrigerator bar magnet could do.”

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The teams’ experiments will, in part, investigate whether these tiny magnets’ behavior will allow them to serve as qubits—the processing elements for quantum computers. “They could become part of the quantum hard drive of the future,” says Friedman. Quantum computing, the use of quantum mechanical phenomena to perform operations on data, is a field still in its infancy, but it could someday be used to solve problems much faster and more efficiently than traditional computing can. It has been proposed, Friedman says, that a computer might be developed that could use an array of quantum magnets to store information in holographic form.

This summer, the professor is working with three Amherst students—Andrew Eddins ’11, John Ware ’11 and Rohan Mazumdar ’12—as well as UMass graduate student Yiming Chen on several interrelated research projects. Some of the research will be done on the Amherst campus, and some at the university.

Eddins’ project, his senior thesis, involves studying the interactions between a crystal of single-molecule magnets and a microwave resonant cavity. The magnets and cavity each have particular frequencies at which they absorb and emit photons (particles of light). “If you adjust things just right, you can get photons to be exchanged back and forth between the magnet and the cavity: the magnet absorbs the photon and then re-emits it into the cavity; the cavity holds onto it for a while, and then the magnet reabsorbs it,” Friedman says. “That’s a process we’ve been able to identify spectroscopically, and we’re going to try and now see it in real time.”

The project builds upon research Eddins conducted last summer, during which he made an exciting observation: that an entire crystal of magnets can, under certain circumstances, work as a unit to absorb a single photon, rather than having each magnet absorb its own photon. (They hope to publish this finding soon.) This summer, he and Friedman will also attempt to create a phenomenon known as superradiance, first predicted in the 1950s, in which the magnets, in an excited state, will emit all of their photons together in one burst of microwave radiation. If they succeed, it will be the first time superradiance has ever been observed in objects larger than the atomic scale.

But it wouldn’t be the first time Friedman has made a major breakthrough. He is one of the scientists credited with discovering, in 1996 at the City University of New York, the first unambiguous evidence of a process called “magnetization tunneling,” by which single-molecule magnets can change the direction of their magnetization even though they don’t have enough energy to do so. Last year, the journal Nature declared that discovery to be among the 23 most important “milestones” in more than a century of “spin physics.” (As a pioneering researcher in this area, Friedman, along with Myriam P. Sarachik, was asked to write an article, titled “Single-Molecule Nanomagnets,” for the inaugural issue of the Annual Review of Condensed Matter Physics, to be published this summer in print and online at http://arjournals.annualreviews.org/loi/conmatphys.) Friedman is also well-known for pioneering work on superconducting devices. In 2000, he and his collaborators at Stony Brook University showed that a superconducting loop could be put into a quantum state in which a large electrical current flowed “two ways at the same time.”

Other members of Friedman’s group are working on observing quantum effects on the macroscopic scale in both nanomagnets and superconducting devices. Ware’s research project is to look for “geometric-phase interference,” in which a magnet can follow more than one possible path in its tunneling. In a uniquely quantum mechanical way, these paths can, in effect, cancel one another out, so that there is no tunneling at all. In a similar project, Chen and Mazumdar are searching for an interference effect in a superconducting device using two Josephson junctions—very thin insulating layers separating two pieces of superconductor. At high temperatures, Friedman says, “that insulating layer would be like a resistor—it would impede the flow of electrons. … But when you cool it down [to a few degrees above absolute zero] and the metals become superconducting … you can get electrons tunneling from one superconductor to the other without losing any energy at all.” A junction can also allow magnetic flux to tunnel through it, he says, but because there will be two paths set up for the experiment, “one through each junction, those paths can interfere and, under certain circumstances, they can interfere destructively, meaning that the flux doesn’t tunnel.” The researchers’ goal will be to use interference to suppress the tunneling of magnetic flux.

Friedman’s $285,000 NSF grant will enable him to pay Chen’s salary, hire a postdoctoral research assistant and purchase equipment and supplies, including, he expects, a helium liquefier. “Most of my experiments are done in liquid helium at temperatures a few degrees above absolute zero,” he says. (Helium is one of very few substances that can remain a liquid at such low temperatures.) “Because the price of helium has skyrocketed in the last few years, there’s been a lot of technological progress in helium liquefaction. So I’m planning on buying a small liquefier that will take whatever helium has boiled off from my cryostat and reliquefy it.” Recycling the helium in this way, rather than buying a new supply each month, will save money in the long run.

Of the grant, Friedman says simply, “It allows my group to continue doing the fun and interesting things we’ve been doing.”

 

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