Every electronic device around us operates through the flow of electrons. From smartphones to massive data centers, there has been only one fundamental principle for moving electrons through a circuit: applying an external voltage (electric field) to push the electrons along. However, a new device developed by a research team centered at the University of Michigan and the University of Toronto shakes up this convention, which has remained unchanged for more than half a century.
What they published in the journal Physical Review Letters is a technology that can aim electrons in a specific direction using nothing but light—without applying any external electric field. The research team likens this to a rotating beam of light sweeping across the sea, calling it an "electron lighthouse."
Electrons That Scatter Versus Electrons That Fly Straight
Inside electronic circuits, when a voltage is applied, electrons begin moving as they are pushed by the electric field. However, electrons don't travel in a straight line. They repeatedly collide—like a pinball—with impurities and defects present in the material, as well as with thermal vibrations of atoms, bouncing back and forth while gradually flowing forward overall. This is the "diffusion current" that underlies conventional electronic devices.
What physicist Steven T. Cundiff of the University of Michigan and his colleagues achieved is a fundamentally different kind of electron behavior. Inside their device, electrons are generated as a "ballistic current" that flies straight like a bullet, without being scattered by impurities. Moreover, the direction in which this current is launched can be freely altered solely through the manipulation of light from outside.
The driving force behind this phenomenon is "quantum interference." In the quantum mechanical world, particles possess wave-like properties. Just as two waves on a water surface can reinforce or cancel each other when they collide, electron waves also undergo interference.
Theories for controlling electric current using quantum interference have existed since the 1990s, studied as interference between one-photon and two-photon absorption (1+2 QuIC). The development of optical frequency comb technology—the precise "ruler" of light also related to the 2005 Nobel Prize in Physics—opened a path to precisely controlling this interference. However, with the combination of one-photon and two-photon processes, it was difficult to sharply focus the electron momentum into a single specific direction.
Theoretical physicist John Sipe of the University of Toronto and colleagues then predicted that using a higher-order interference—interference between two-photon and three-photon absorption (2+3 QuIC)—would allow the electron momentum to be concentrated into an extremely narrow range, creating a beam-like current. The Michigan team brilliantly demonstrated this theoretical prediction through experiment. This demonstration is not merely an extension of existing technology; it marks the arrival of an entirely new stage in controlling carrier distribution in momentum space using light.
Trajectories Drawn by Two-Photon and Three-Photon Interference
The research team simultaneously irradiated a semiconductor material—aluminum gallium arsenide ()—with two laser pulses of different wavelengths. One had a wavelength of 1040 nm, and the other 1560 nm. The pulse duration was an extremely short approximately 85 femtoseconds (one femtosecond is one-quadrillionth of a second).
For an electron to jump to a specific high-energy state, it must absorb either two photons at 1040 nm or three photons at 1560 nm. The crux of the matter is that these two entirely different pathways—two-photon absorption and three-photon absorption—arrive at exactly the same final energy state.
When multiple pathways lead to the same outcome, quantum interference occurs. When the research team precisely adjusted the phase (wave timing) and polarization (direction of wave oscillation) of the two laser beams, electron waves heading in a particular direction reinforced each other, while waves heading in other directions completely canceled out. As a result, electrons were not pushed along by a voltage but instead flew out in a bundle toward a specific direction, guided by the light waves.
The aluminum gallium arsenide chosen for the experiment allows the band gap to be precisely engineered by adjusting the aluminum concentration. Through this band-gap engineering, the research team succeeded in selectively triggering the targeted two-photon and three-photon absorption processes while suppressing other competing processes that would introduce noise.
Invisible Fluctuations of Light Alter the Current
What is remarkably unique about this light-based current control is an extraordinary sensitivity to light. Because the phase difference between the two lasers directly governs the generation of current, even minute fluctuations in the air within the laboratory, or changes in optical path length of merely tens of nanometers (one nanometer is one-billionth of a meter), clearly manifested as changes in the output current.
Initially, the research team struggled with noise caused by this hypersensitivity. They used an acousto-optic modulator (AOM) to thoroughly suppress frequency offset noise in the lasers. As a result of reducing the linewidth from several hundred kHz down to about 1 Hz, a beautiful sinusoidal waveform—evidence of quantum interference—emerged from beneath the noise.
When the polarization of the light was rotated by 90 degrees, current was detected at a separate, orthogonally arranged pair of electrodes, and the direction of current generation also rotated precisely by 90 degrees. This is decisive proof that it was not merely the magnitude of the current that changed, but that the polarization vector of the light was directly converted into the current vector of the electrons. Two pairs of mutually orthogonal electrodes were fabricated into the device for measurement, allowing the team to simultaneously capture how the current rotated within the plane of the sample as a two-dimensional vector.
Ohmic Contacts That Captured the Ballistic Current
Behind the success of this experiment lies advanced device fabrication technology. To purely measure only the ballistic current injected by light, a special electrode structure was needed that would not generate unintended electric fields within the material.
While existing research often used Schottky-junction-type electrodes, lead author Yiming Gong and colleagues, working at the University of Michigan's Lurie Nanofabrication Facility (LNF), devised their own approach. Through trial and error—adjusting the composition and thickness of metal layers, fine-tuning annealing temperatures, and removing surface oxide films via plasma etching—they achieved low-resistance ohmic contacts. This was the key that, for the first time, allowed them to read out the trajectory of electrons launched by light as an electrical signal, without interference from internal electric fields.
New Perspectives for Spin and Topological Materials
Optical phase information—a signal that oscillates at an ultrafast rate of hundreds of terahertz and thus cannot normally be directly read by conventional electronic circuits—can be directly converted into a low-frequency current signal that is easy to handle. This could serve as the foundation for next-generation measurement infrastructure supporting ultra-precise optical clocks and high-capacity optical communication systems. In particular, the ability to directly convert elusive, minute optical phase fluctuations—such as the carrier-envelope offset frequency of a frequency comb—into a direct electrical signal for use in feedback control is an advantage of immeasurable value.
This technique for generating current by targeting specific electron momenta offers a new lens for materials science. Whether it can be applied not only to aluminum gallium arsenide but also to other quantum materials such as transition metal dichalcogenides and topological materials will be a key focus going forward.
If it becomes possible to directly target the spin of electrons within a material, or its unique valley dynamics, using light and read them out as current, a groundbreaking analytical tool will be born—one capable of unraveling the behavior of unknown quantum geometry, such as Berry curvature. In the history of electronics, which has long depended on external electric fields, an era is now beginning in which light directly draws the trajectories of electrons.