University of Minnesota
School of Physics & Astronomy

Spotlight

Exotic Phases of Matter

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Fiona Burnell
Annie Bartels
                                                       

Fiona Burnell is a condensed matter theorist who studies exotic phases of matter. These are materials which do not display long-ranged order at low temperatures (which is typical of low-temperature behaviour in many materials, such as magnets), but are also not ordinary metals. One of the most bizarre examples of an exotic phase is a fractionalized system, where there appear to be particles that carry a fraction of the charge of the electron. “This seems very surprising because we know that you can’t subdivide an electron,” Burnell explains.

Fractionalization is an example of a collective phenomenon where the whole can behave differently than the sum of its parts. Burnell explains that if you want to understand waves in the ocean, the best approach is not to study the motion of the individual water molecules, but to consider the behaviour of the fluid as a whole. Similarly when you zoom in closely on a fractionalized system, you’ll see only the electrons from which it is made. As you zoom out, though, you’ll see the collective behavior that makes it look like you have particles carrying (say) a third of a charge of the electron.

One potential application of fractionalized phases is quantum computing. In a quantum computer, information is stored and manipulated in a quantum system. For example, imagine using the presence or absence of an electron in a nano-scale "box” to represent one of the bits in a hard drive (either a one or a zero). Quantum mechanics tells us that the electron can be simultaneously in the box and not in the box. This strange fact allows information to be manipulated in ways that are not possible on conventional hard drives, making certain problems that are difficult using conventional computers much easier. One big challenge with quantum computing is ensuring that the information captured in these quantum bits is not lost over time: a single electron interacts with the outside world, so that typically your quantum bit will quickly change its state randomly between 0 and 1. One of the exciting things about the fractionalized systems that Burnell studies is that they encode information in a non-local way, meaning that it is stored in the pattern of correlations of all of the constituent particles of the system, rather than in a particular particle at a fixed location in space. This opens the possibility of more robust ways to store and process quantum information.

One device that Burnell has studied which could be promising for quantum information involves tiny “topological superconducting” wires. In these systems the state of the bit --whether it is counted as a one or a zero--is distributed between the two ends of the wire. The hope is that physicists will be able to change the bit by using a series of manipulations involving other wires. “We are a long way from that right now,” she says. “The current challenge is to determine whether the systems that are being built in the lab are actually storing electrons in the way that theorists have predicted.” Burnell’s proposal suggests a way to test such devices to see if they behave in a way that is useful for computation.