Connect the Quantum Dots for a New Kind of Fuel

It’s easy to forget how far we’ve come. Just a century ago, only half of American homes were hooked up to an electricity supply. Now we can turn lights on with our voices, while smart devices have consigned phone booths and fax machines to the dustbin of history. This is all thanks to our mastery of electrons and the way we can make them surge along wires in dizzying numbers like an invisible, rampaging river. One ampere of electrical current equates to the flow of six million trillion electrons per second. "That’s about a billion times more than the total number of people on the planet," says Klaus Ensslin, a physicist at ETH Zurich, in Switzerland.

Ensslin is part of a team that’s been slowing that river of electrical current right down—using nanometer-sized devices called "quantum dots" to study what happens when you prune the number of electrons back. "We can cut it down to one electron per second, even one electron per day," he says.

Along with Peter Samuelsson and Ville Maisi of Lund University, in Sweden, and Christopher Jarzynski of the University of Maryland, in College Park, Ensslin has been awarded just over US$1 million by FQXi to find out what quantum dots can tell us about the foundations of thermodynamics—the science of heat and energy—in the quantum nanorealm. Manipulating individual electrons could also help reduce energy wastage in electronic devices.

Quantum dots are nanoparticles of semiconductors that were first theorised to exist in the 1970s. The idea was that if you made the particles small enough, quantum effects, which govern tiny objects, would come into play, potentially unleashing new capabilities. The first dots were created in the 1980s, and now physicists and engineers hope to use these mini crystals for a wide range of applications, from biological probes to optoelectronics.

Artificial Atom

"A quantum dot is an artificial atom," says Ensslin; it exhibits a range of quantum mechanical effects that in some ways mimic those of an ordinary atom. The electrons in an ordinary atom can only exist in a number of discrete energy states and can be excited between them. "In a quantum dot, electrons are equally constrained in all three dimensions," Ensslin says. It is possible to manipulate quantum dots, adding and removing charges or exciting them by applying a voltage.

At the heart of the team’s research is a quest to understand quantum modifications to thermodynamics. The laws of thermodynamics were devised in the 18th century by considering large scale engines, but things may become more complicated as devices shrink down. "When we take things that we understand well in our daily world, like energy, work and information, and move them into the quantum world everything becomes fuzzy," Samuelsson says. "We don’t really know what we’re dealing with."

Of particular interest is the famed Second Law of Thermodynamics, which says the disorder in a closed system always increases over time. "When you think about how it applies to very small systems there’s a lot of new physics that you just don’t see at the macroscale," says Jarzynski. "Quantum dots offer a very nice setting for probing some of these ideas."

The team’s approach combines both experiment and theoretical analysis. "The experiments tell us what nature is doing and the theory allows us to understand and describe it," says Maisi, one of the team’s experimentalists.

The lab set-up involves a micrometer-long semi-conducting wire suspended between two electrical contacts. (For comparison, a micrometer is less than a third the width of a strand of spider’s silk.) The team is able to control how individual electrons hop in and out of quantum dots within the wire. Other quantum dots are used to monitor where the electrons are going. (You can take a virtual tour of the Lund lab and investigate the team’s experimental tools here.)

The experiment allows an electron to be in a superposition between two different quantum dots—a feat that impresses quantum physicist and FQXi member Franco Nori, from RIKEN in Japan, who is not part of the team, but is also investigating ways to convert information to fuel. "By turning the electrical knobs of the device they can measure and control the superposition states and explore how to extract work from these states, which do not exist classically," Nori says.

Reducing Waste

Nori also points to "unavoidable challenges" with such systems, including measurement errors and dealing with noise. Conquering these and other obstacles, and monitoring and manipulating individual electrons could also have big practical upsides, such as for reducing energy wastage. Many of us will have felt our smartphones warm up if they’re in near constant use. Indeed, estimates suggest that two thirds of the energy consumed in the USA every year is lost to waste heat. One possible solution to this will be using the precise control that quantum dots allow you to cherry-pick stray electrons and divert their energy to a meaningful task. Ensslin gives an everyday analogy. "There are energy fluctuations in the air molecules all around us," he says. "If you can pick the right ones then it will heat up your living room and you can dump the cold ones outside in your garden." In a very real sense, that’s using information as fuel. "More likely than not, it’s going to pay off someday," adds Jarzynski.

Using information to convert heat into work would be a great achievement, says quantum scientist Christian Glattli, from the Saclay Nuclear Research Centre, in France. "It might lead to a better understanding of the fundamental concepts of information in quantum thermodynamics through a simple, but efficient, toy model," he says. However, he cautions that he doesn’t see any practical applications any time soon. "It is very dangerous to look through a crystal ball," he says.

The team is already working on other ways to upgrade their experiment’s sensitivity. The physicists realised they would be able to glean even more information about the electrons if the wire in their experiment was replaced by a net made of graphene—the much-lauded material made of a layer of carbon just one atom thick. "It’s much thinner than anything you can do in a semi-conductor," says Ensslin, "so you can get the quantum dots one hundred times closer to the detector." That means you know where the electrons are going one hundred times more accurately.

The trouble is that currently the team can’t control the individual electrons in graphene as well as they can in the semi-conducting wire. There’s no fundamental reason stopping them, though. "It’s not money and it’s not equipment," Ensslin says. Instead he thinks that smart students and post-docs with good ideas are the surest routes to unblocking the impasse. Then we can really grapple with the foundational questions.

"Quantum mechanics is one of the most fascinating theories we have, but we still don’t understand it," Ensslin says. "By exploring information theory in this way we hope to learn something fundamental about it."