20 September 2012

Experiment in a UF laboratory tests important Quantum Theory

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This week’s issue of Nature contains groundbreaking work in two very different approaches to physics. The first is work from the MagLab’s High B/T user program. High B/T uses a technique that combines ultra-cold temperatures and high magnetic fields to slow down the ultrafast particles enough to observe quantum systems, whose behavior underlies the mysteries of many of the technologies we use today.

The second collaboration, also published in Nature this week, has resulted in substantial improvements to EPR spectrometer technology. EPR spectrometers are used to blast electrons — each a tiny magnet — with electromagnetic radiation, exciting them enough to tease out information about the molecular structure of the material they’re in. Learn more about the latter collaboration here.

Postdoctoral scholar Chao Haun works in a magnet bay at the MagLab’s High B/T Facility at the University of Florida.Postdoctoral scholar Chao Haun works in a magnet bay at the MagLab’s High B/T Facility at the University of Florida.GAINESVILLE, Fla. — An international team of scientists is re-writing a page from the quantum physics rulebook with help from the National High Magnetic Field Laboratory’s High B/T Facility at UF, funded by the National Science Foundation. The Facility provides access to users from around the world who need to conduct experiments at high magnetic fields (B) and at low temperatures (T). It is located in UF’s Microkelvin Laboratory.

Much of what we know about quantum mechanics is theoretical and tested via computer modeling because quantum systems, like electrons whizzing around the nucleus of an atom, are difficult to pin down for observation. One can, however, catch matter in the quantum act if it is subjected to extremely low temperatures. New research, published in the Sept. 20 edition edition of Nature, describes how this freeze frame approach was recently used to overturn an accepted rule of thumb in quantum theory.

The experiments specifically test the leading theory of what happens to a quantum system when it is subject to disorder that can break the system into pieces that are no longer in phase with each other. This phenomenon is very important in superconductivity, Bose-Einstein condensation (BEC), superfluids, electrical conduction, etc.

We introduced disorder an XY quantum magnet (dichloro-tetrakis-thiourea-nickel, or DTN) that if pure undergoes BEC at low temperatures. With sufficient disorder, the XY magnetic order of the BEC state will break apart into independent pieces each with a different phase, leading to new Bose Glass (BG) state.

The phase diagram can be traversed with magnetic field at very low temperatures with a BEC occurring for intermediate fields and BG for low and high fields. Mathew Fischer and collaborators predicted in 1989 that the critical fields for these field-induced quantum phase transitions scales as |H-Hc| ~ Tα with α less than 0.5, However, we observe a near to 1.0 for both experiments and simulations.

“We are in the age of quantum mechanics,” said Neil Sullivan, a UF physics professor and director of the Microkelvin Laboratory. “If you’ve had an MRI, you have made use of a quantum technology.”

The magnet that powers an MRI scanner is a superconducting coil transformed into a quantum state by very cold liquid helium. Inside the coil, electric current flows friction free.

Quantum magnets and other strange, almost otherworldly occurrences in quantum mechanics could provide inspiration for the next big breakthroughs in computing, alternative energy and transportation technologies like magnetic levitating trains. But innovation cannot proceed without a proper set of guidelines to help engineers navigate the quantum road.

Schematic representation of BEC and BG states for an XY magnet.Schematic representation of BEC and BG states for an XY magnet. The arrows represent local magnetic ordering with phase coherence for the BEC state.That’s where the Microkelvin lab at UF comes in. It is one of the few facilities in the world equipped to deliver the extremely cold temperatures needed to slow the “higgledy-piggledy” world of hot systems to a manageable pace where it can be observed and manipulated, said Sullivan.

“Room temperature is approximately 300 kelvin,” he said. “Liquid hydrogen pumped into a rocket at the Kennedy Space Center is at 20 kelvin. Physicists need to cool things down another three factors of ten to bring matter into this completely different realm where new properties can be exlpored.”

One fundamental state of quantum mechanics that scientists are keen to understand more fully is a fragile, ephemeral phase of matter called a Bose-Einstein Condensate. In this state, individual particles that make up a material begin to act as a single coherent unit. It’s a tricky condition to induce in a laboratory setting, but one that researchers need to explore if technology is ever to fully exploit the properties of the quantum world. Two theorists,Tommaso Roscilde (University of Lyon, France) and Rong Yu (Rice University) developed the underlying ideas for the experimental test and the sample was made by Armando Paduan-Filho (University of Sao Paulo, Brazil).

“Our measurements definitively tested an important prediction because we could measure a very large number of bosons down to 1 millikelvin,” said Vivien Zapf, staff scientist at the National High Magnetic Field Laboratory at Los Alamos and a driving force behind the international collaboration.

The experiment monitored the atomic spin of subatomic particles called bosons in a crystalline sample of Chlorine and Bromine to detect signs that the condensate phase had been achieved. The extreme temperatures were needed to take the sample all the way through the Bose-Einstein Condensate phase to another quantum state where the condensate properties decay.

“We wanted to test how impurities in a sample would affect its ability to maintain a Bose-Einstein Condensate phase,” said Zapf. “It’s nice to know what happens in pure samples, but the real world, unlike the lab, is messy. We need to know what the quantum rules are in those situations. In fact, you can get interesting new and useful behavior by making the samples messy.”

Having performed a series of simulations in advance, they knew that the experiment would require them to generate temperatures down to 1 millikelvin, one thousandth of a kelvin above absolute zero.

“You have to go to the Microkelvin Laboratory at UF for that,” she said. Other laboratories can get to the extreme temperature required, but none of them could sustain it long enough to collect all of the data needed for the experiment.

“It took six months to get the readings,” said Liang Yin, an assistant scientist at UF who operated the equipment in the Microkelvin lab. “Because the magnetic field we used to control the wave intensity in the sample also heats it up. You have to adjust it very slowly.”

Their findings literally re-wrote the rule for predicting the conditions under which the transition would occur between the two quantum states.

“All the world should be watching what happens as we uncover properties of systems at these extremely low temperatures,” said Sullivan. “A superconducting wire is superconducting because of this Bose-Einstein Condensation concept. If we are ever to capitalize on it for quantum computing or magnetic levitation for trains, we have to thoroughly understand it.”

The above release has been reprinted with permission from the University of Florida.