It’s not every day that physicists discover a new type of quasiparticle. And it’s even rarer that they give it a super-cute nickname like “dropleton.”
So today, my fellow physics fans, we are in luck. Not only have scientists announced the discovery of a thing called a “quantum droplet,” but the quasiparticle is making its debut on the cover of this week’s edition of the journal Nature.
A quantum droplet is a collection of electrons and “holes,” which are places where an electron could exist but doesn’t. Most droplets are made up of thousands or even millions of electrons and holes. But a quantum droplet has only about five of each.
In addition to being incredibly small, it’s incredibly fleeting. In the lab where it was discovered at JILA – a joint institute of the University of Colorado, Boulder and the National Institute of Standards and Technology – each quantum droplet lasted for only 25 trillionths of a second. Still, scientists say that’s long enough to give them a chance to study some of their exotic properties.
Before moving on, let’s give Mackillo Kira a chance to offer some background on the work. Kira is a member of the Theoretical Semiconductor Physics Group at Philipp University of Marburg in Germany and a coauthor of the Nature paper.
“In a nutshell, a quasiparticle consists of a stable configuration of particles and its properties are strongly modified by interactions with the surrounding material and/or other particles,” Kira said. “Each found quasiparticle acts [as] a building block of more complicated structures in the same way as elementary particles eventually form molecules, polymers, cells, and so on.”
The collaborators in Germany and Colorado have been looking for new “elements” they can add to the periodic table of quasiparticles, Kira explained. To do this, they create the equivalent of a “Big Bang” inside a semiconductor by pulsing it with laser light.
“Before the light, semiconductors contain no electrons and no holes; it is a void,” Kira said. “Then the light creates the ‘elementary’ particles.”
The JILA team created the dropletons by beaming a seriously high-speed laser at a gallium-arsenide semiconductor. As expected, the laser pulses first prompted the formation of a simple type of quasiparticle called an exciton, which pairs an electron and a hole via electrostatic forces. As the pulses got faster, more excitons were created.
Finally, when the pulses reached a rate of about 100 million per second, the electrons and holes rearranged themselves into the microscopic quantum droplets. These bubble-like structures were held in place ever-so-briefly by the plasma that surrounded them.
“We managed to detect and control a completely new element, the dropleton, which we find extremely exciting,” Kira said.
The quantum droplets had some unusual features. They weren’t content to combine in pairs, like other quasiparticles. Instead, because they were arranged in small groups of droplets, they behaved sort of like liquids, with rings reminiscent of the ripples you would see if you threw a stone in a pond.
“The liquid-like behavior of dropletons was … a surprise,” Kira said.
Steven Cundiff, a JILA physicist who worked on the Nature study, said he was surprised to find that the dropletons became more stable as the laser pulses became more intense. Usually, the greater the laser intensity, the more electrons and holes are injected into a semiconductor – and that weakens the forces that make particles stable, he said.
Both researchers readily acknowledged that the discovery of the quantum droplet won’t cure cancer or enable cold fusion or have any other direct practical benefits.
“Since the dropletons are a completely new concept, it is not yet obvious how to convert such fundamental particles directly to actual applications,” Kira said.
But Conduff said they will allow researchers to test the properties of optoelectronic devices like laser diodes. These are used in DVD players, bar-code scanners, laser printers and fiber optic communications equipment, among others.