Category Archives: Quantum
Brown University physicist Vesna Mitrovic and colleagues have discovered magnetic waves that fluctuate when exposed to certain conditions in a superconducting material. The find may help scientists understand more fully the relationship between magnetism and superconductivity. (Credit: Lauren Brennan/Brown University)
At the quantum level, the forces of magnetism and superconductivity exist in an uneasy relationship. Superconducting materials repel a magnetic field, so to create a superconducting current, the magnetic forces must be strong enough to overcome the natural repulsion and penetrate the body of the superconductor. But there’s a limit: Apply too much magnetic force, and the superconductor’s capability is destroyed.
This relationship is pretty well known. But why it is so remains mysterious. Now physicists at Brown University have documented for the first time a quantum-level phenomenon that occurs to electrons subjected to magnetism in a superconducting material. In a paper published in Physical Review Letters, Vesna Mitrovic, joined by other researchers at Brown and in France, report that at under certain conditions, electrons in a superconducting material form odd, fluctuating magnetic waves. Apply a little more magnetic force, and those fluctuations cease: The electronic magnets form repeated wave-like patterns promoted by superconductivity.
The discovery may help scientists understand more fully the relationship between magnetism and superconductivity at the quantum level. The insight also may help advance research into superconducting magnets, which are used in magnetic resonance imaging (MRI) and a host of other applications. “If you don’t understand [what is happening at] the quantum [level], how can you design a more powerful magnet?” asked Mitrovic, assistant professor of physics.
When a magnetic field is applied to a superconducting material, vortices measured in nanometers (1 billionth of a meter) pop up. These vortices, like super-miniature tornadoes, are areas where the magnetic field has overpowered the superconducting field state, essentially suppressing it. Crank up the magnetic field and more vortices appear. At some point, the vortices are so widespread the material loses its superconducting ability altogether.
At an even more basic level, sets of electrons called Cooper pairs (named for Brown physicist Leon Cooper, who shared a Nobel Prize for the discovery) form superconductivity. But scientists believe there also are other electrons that are magnetically oriented and spin on their own axes like little globes; these electrons are tilted at various angles on their imaginary axes and move in a repeating, linear pattern that resembles waves, Mitrovic and her colleagues have observed.
“These funny waves most likely appear because of superconductivity, but the reason why is still unsettled,” Mitrovic said.
Adding to the mystery, Mitrovic and fellow researchers, including Brown graduate student Georgios Koutroulakis and former Brown postdoctoral associate Michael Stewart, saw that the waves fluctuated under certain conditions. After nearly three years of experiments at Brown and at the national magnetic field laboratory in Grenoble, France, Mitrovic’s team was able to produce the odd waves consistently when testing a superconducting material — cerium-cobalt-indium5 (CeCoIn5) — at temperatures close to absolute zero and at about 10 Tesla of magnetic force.
The waves appeared to be sliding, Mitrovic said. “It’s as if people are yanking on the wave,” she added. Mitrovic and her colleagues also observed that when more magnetic energy is added, the fluctuations disappear and the waves resume their repeating, linear patterns.
The researchers next want to understand why these fluctuations occur and whether they crop up in other superconducting material.
The research was funded by the National Science Foundation and a European Community grant, as well as the Alfred P. Sloan Foundation.
Adapted from materials provided by Brown University.
Quantum Mechanics at Work in Photosynthesis: Algae Familiar With These Processes for Nearly Two Billion Years
A team of University of Toronto chemists have made a major contribution to the emerging field of quantum biology, observing quantum mechanics at work in photosynthesis in marine algae.
“There’s been a lot of excitement and speculation that nature may be using quantum mechanical practices,” says chemistry professor Greg Scholes, lead author of a new study published in Nature. “Our latest experiments show that normally functioning biological systems have the capacity to use quantum mechanics in order to optimize a process as essential to their survival as photosynthesis.”
Special proteins called light-harvesting complexes are used in photosynthesis to capture sunlight and funnel its energy to nature’s solar cells — other proteins known as reaction centres. Scholes and his colleagues isolated light-harvesting complexes from two different species of marine algae and studied their function under natural temperature conditions using a sophisticated laser experiment known as two-dimensional electronic spectroscopy.
“We stimulated the proteins with femtosecond laser pulses to mimic the absorption of sunlight,” explains Scholes. “This enabled us to monitor the subsequent processes, including the movement of energy between special molecules bound in the protein, against a stop-clock. We were astonished to find clear evidence of long-lived quantum mechanical states involved in moving the energy. Our result suggests that the energy of absorbed light resides in two places at once — a quantum superposition state, or coherence — and such a state lies at the heart of quantum mechanical theory.”
“This and other recent discoveries have captured the attention of researchers for several reasons,” says Scholes. “First, it means that quantum mechanical probability laws can prevail over the classical laws of kinetics in this complex biological system, even at normal temperatures. The energy can thereby flow efficiently by — counter intuitively — traversing several alternative paths through the antenna proteins simultaneously. It also raises some other potentially fascinating questions, such as, have these organisms developed quantum-mechanical strategies for light-harvesting to gain an evolutionary advantage? It suggests that algae knew about quantum mechanics nearly two billion years before humans,” says Scholes.
Scholes’ colleagues in the research at the University of Toronto include Elisabetta Collini, Cathy Y. Wong, and Paul Brumer. Other team members include Paul Curmi and Krystyna Wilk of the University of New South Wales. The research was funded with support from the Natural Sciences and Engineering Research Council of Canada, in part by a Steacie Fellowship awarded to Scholes.
Adapted from materials provided by University of Toronto. Original article written by Sean Bettam.
- Elisabetta Collini, Cathy Y. Wong, Krystyna E. Wilk, Paul M. G. Curmi, Paul Brumer & Gregory D. Scholes. Coherently wired light-harvesting in photosynthetic marine algae at ambient temperature. Nature, 2010; 463 (7281): 644 DOI: 10.1038/nature08811