Physicists generate electrical currents from noise

Two quantum dots have been used to generate an electrical current from voltage noise. The device was created by physicists in Germany, who say that it could lead to the development of systems that convert waste heat into useful energy.

Electronic devices generate large amounts of excess heat that must be dissipated. Instead of simply discarding this energy, using it to do useful work could revolutionize the electronics industry, and make it possible to create more efficient devices. Indeed, for more than a decade, physicists have been thinking up ways to convert this heat into electrical currents that can do work, such as power electronic devices.

Now, Lukas Worschech and colleagues at the University of Würzburg in Germany have verified experimentally that random voltage fluctuations can be rectified to drive a direct current. The experiment uses voltage noise to mimic the hot and cold spots of waste heat, and is therefore not a direct demonstration of waste heat being converted into work. However, team member Fabian Hartmannexplains that it shows that small voltage fluctuations can drive a current: "A device derived from our sample might be able to provide the necessary power to drive autonomous and self-powered systems."

Coupled quantum dots

The experiment comprises two quantum dots, which are discs of semiconductor about 300 nm in diameter. The quantum dots are separated by 150 nm to ensure that they exchange energy via Coulomb coupling – the electrostatic force that charged particles feel at a distance – as opposed to electrons physically jumping between the two subsystems. The researchers then connected three conducting leads to the system: two to the upper quantum dot to allow current to flow across the dot and one linking the lower quantum dot to a voltage source (several volts) with superimposed noise in the millivolt range.

While increasing the intensity of the noise applied to the lower quantum dot, the researchers measured larger currents across the upper quantum dot. The data followed a quadratic trend: doubling the voltage noise quadrupled the maximum current, at least for millivolt noise levels and currents measured in nanoamperes. "Certainly there is an upper bound that limits the device's current. So far we have not tested these limits," says Hartmann. The researchers also showed that they could reverse the direction of the current by changing the voltages of the leads attached to the upper quantum dot. This makes it energetically favourable for electrons to flow in one direction or another, depending on the voltage.

Getting warmer

One limitation to the team's energy-harvesting scheme is that the quantum dots were immersed in liquid helium, which is clearly impractical for consumer electronics. Therefore, it will be critical to demonstrate that currents can still be driven at room temperature. "The biggest challenge is certainly to build a device that operates at room temperature," notes Björn Sothmann, a researcher at the University of Geneva, who was not involved in the study.

In addition, further work is needed to show that currents can be driven by thermal fluctuations rather than voltage noise. "They do not create a true thermal gradient but mimic the effect by introducing noisy gate voltages," says Rafael Sánchez of the Institute of Materials Science in Madrid, who published a theoretical study in 2012 that suggested that thermal fluctuations can drive a directed current. "However, the two effects [voltage fluctuations and thermal gradients] are closely related, so this study serves as a proof of principle for the rectification mechanism," Sánchez concludes.