The flow of electrons powers our modern lives.
From household conveniences like the indispensable air fryer to the variously sized screens we rely on for work and meme-browsing leisure, modern tech functions through the directed flow of electrons.
Yet electrons play by the weird rules of the quantum universe, displaying strange behaviors unseen in the macroscopic world, like the superposition that allows them to exist in many states until they're observed, à la Schrödinger's theoretically undead cat.
Sometimes, however, their odd activities do display real-world analogs that help us non-quantum-physicists wrap a fraction of our minds around unseeable processes.
In a breakthrough study, physicists at the University of Michigan (U-M) have illuminated new physics by developing an electron lighthouse device that emits a beam of electrons that can be controlled using different 'colors' of light without requiring any electrical power source.
"This isn't the way things normally work. When you think about electrons moving through a material, they're moving because you've applied an electrical field and they actually bounce around and drift across the materials," explains Steven Cundiff, an experimental physicist at U-M, as well as the study's senior author.
"Here, using light, you can actually sort of squirt the electrons in a specific direction without applying an electric field."
With support from the US National Science Foundation, the researchers built a device that uses two laser beams of different 'colors' (in the invisible infrared region of the electromagnetic spectrum) to control the flow of electrons through a semiconductor.
"Probably the most surprising thing is that the currents produced by this process are even detectable using the device we made," Cundiff told ScienceAlert.
"The generated current is a 'ballistic' current, meaning the electrons travel along a trajectory determined by their initial 'launch' velocity. Usually, electrical currents are not ballistic, but rather are diffusive because the electrons are constantly scattering off impurities and defects in the material."
The researchers built their device at U-M's Lurie Nanofabrication Facility (LNF), testing different methods to establish an experimental setup that didn't introduce external electric fields that could skew their results.
"That was the biggest puzzle to solve for me, because there isn't a standard way to do that," says Yiming Gong, a machine learning scientist now at Grainger and the study's first author.
"So I worked with the LNF staff to play around with different recipes and temperatures to come up with a manufacturing process."
Previous theoretical and experimental work has shown that light alone can supply the energy needed to get electrons flowing.
"Actually, this project is kind of interesting, because I started working on [its predecessor] many years ago as a way to make a phase-sensitive detector for use in stabilizing frequency combs," Cundiff told ScienceAlert.
Such frequency combs are central tools for allowing precision control of electromagnetic waves, creating more accurate clocks, computers, and communications systems.
This research then builds on previous works, demonstrating that light can be used to entice electrons to flow in a beam-like configuration, which the authors liken to a lighthouse.
"The light no longer merely switches the current on; it also aims it," Cundiff explains.
Yet aiming this electron beam is more complicated than simply swiveling a lamp. Instead, the researchers rotate their electron beam by rotating the polarization – or the direction of the oscillations – of the light.
This work also represents the fruition of a prediction made by a previous collaborator, John Sipe, a theoretical physicist at the University of Toronto.
Sipe and colleagues predicted that this experimental setup would produce the observed 'electron lighthouse' effect, generating a beam of directional electrons in the absence of an applied voltage.
The underlying mechanism is known as quantum interference, a "hallmark" quantum process.
Unlike classical particles with clearly defined paths, in the wildly counterintuitive subatomic realm, where particles act like indistinct waves, quantum interference describes how the wave functions of these particles can align constructively or destructively.
In the future, this research could help elucidate the quantum geometry of materials that dictate their fundamental behaviors. As a result, it may help improve multiple technologies, including the quantum sensing techniques that aim to describe physical quantities with ever-greater precision.
Related: The First Room-Temperature Quantum Material of Its Kind Is Spun From Atoms of Gold
Quantum interference advances could also enhance imaging and telecommunications, improving how signals are transmitted between devices and increasing the amount of data they carry.
Similarly, quantum interference can lead to significant speedups in computation by amplifying the probability of receiving desired computing solutions and suppressing the probability of receiving undesired ones.
The future is quantum, and this lighthouse may guide us in navigating the still-murky, unseen sea of subatomic particles and fields that govern the tangible world.
This research was published in Physical Review Letters.
This article was fact-checked by Rebecca Dyer and edited by Peter Dockrill. While we pride ourselves on our process, we are only human. If you spot a mistake, please let us know.