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Physicists Collide Quasiparticles for the First Time

Femtosecond lasers offer a peek into a whole new quasiworld.
A femtosecond laser system at Ohio State University. Image: OSU

There are particles in nature that aren't quite particles but are nonetheless real and behave in ways that relate to the that we all know and love. These are the quasiparticles, which we can understand as different sorts of collective behavior performed by multiple particle particles that wind up looking a whole lot like new versions of particles that doesn't actually exist in the universe. Pretty weird, eh.

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As an example, you can imagine a single electron bound to an atomic nucleus within a metal. Something comes along and shifts that electron out of position, and the consequence is a collective shift of the nuclei and other electrons around it. This shift has the neat property of looking an awful lot like one, single really massive electron rather than a collection of particles reacting to the motion of one, single electron with a normal mass.

Something physicists would really love to be able to do is smash quasiparticles together in a collider as if they were photons and protons being smashed in the LHC. This would help us understand them, to put it mildly, which could be very important given the role of quasiparticles in and our understanding of quantum mechanics in general.

As described Wednesday in and accompanying commentary in Nature, physicists at the University of Regensburg, University of California, Santa Barbara, and the University of Marburg have succeeded in colliding pairs of quasiparticles in an insulating material called tungsten diselenide. Courtesy of femtosecond-scale laser pulses, they were able to do this such that the initial distances between the quasiparticles and their initial speeds were well-defined, prerequisites for smashing them together.

It turns out that colliding quasiparticles isn't easy. They don't tend to stick around for long and the collision byproducts that physicists would like to see appear at attosecond scales.

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Just how deeply do quasiparticles imitate their legit particle brethren?

"Like in a conventional particle accelerator it is important to avoid accidental collisions of the quasiparticles somewhere in-between," Rupert Huber, a study co-author and leader of the Ultrafast Quantum Electronics and Photonics group at the University of Regensburg, told me. "Since quasiparticles are embedded in a very dense background of billions over billions of interacting particles such accidental collisions happen on very short time scales set by the femtosecond," he explained. "It is crucial to be fast, therefore. Using the oscillating carrier wave of light as the fastest acceleration field we can control as physicists, we are able to complete the entire life cycle of [quasiparticle] preparation, acceleration, and recollision faster than the unintentional scattering of quasiparticles with the dense environment takes place, which allows us to interpret the outcome of the experiment microscopically."

A commentary accompanying the new study, penned by University of Geneva quantum physicist Dirk van der Marel, unpacks things a bit more. First, pairs of quasiparticles of opposite charges are created within the tungsten diselenide material with a femtosecond laser pulse. These particles are then launched onto a linear track created by a second light pulse, which establishes an electric field. The various properties of this second light pulse are adjusted to direct the quasiparticles into a head-on collision. Note again that all of this is happening in a time span smaller than a single wavelength of light.

"The collision caused mutual annihilation of the quasiparticles and the emission of a photon, which the authors detected," van der Marel explains. "The experiment is therefore similar to studies of electron–positron annihilation in high-energy particle accelerators (positrons are the antiparticles of electrons, which means that they have opposite charge and equal mass to an electron)." Of particular interest to the researchers was the effect of the Coulomb interaction—that is, the force experienced by electrically charged particles—on their quasiparticles. Just how deeply do quasiparticles imitate their legit particle brethren?

"The beauty of Langer and colleagues' experimental toolkit is that it might finally allow quasiparticles and their mutual interactions to be studied in the materials in which they arise," van der Marel notes. "The negative and positive quasiparticles in the authors' experiment are similar to electrons and positrons in a vacuum, but a rich variety of unconventional quasiparticles could also be studied, for which no equivalent elementary particles are known."

"Quasiparticles are not only of academic interest," he continues. "They also determine many of the properties and functionalities of materials, such as electrical resistivity, heat capacity and magnetism. There are thus many reasons to study quasiparticles in the materials in which they are manifested."

Colliding quasiparticles still isn't exactly easy, but its possibility offers the eventual promise of understanding a whole other quasiworld.