Researchers Control of Quantum Bits with Ultrafast Light Pulses to Process Data a Thousand Times Faster

Image

Researchers from the Madrid Institute of Materials Science (ICMM-CSIC), part of the Spanish National Research Council (CSIC), participate in an international study, in collaboration with the Politecnico di Milano and the Max-Born-Institut in Berlin, which has successfully encoded and coherently manipulated valley quantum bits using ultrafast light pulses. Valley quantum bits are a way of storing information in certain materials formed by a single atomic layer (ultrathin materials), where electrons can reside in two different states called valleys, which function as "two quantum drawers" for storing data.

The advance, which falls within valleytronics—an area that precisely explores this mode of storing and controlling information—uses light flashes controlled with extreme precision to move electrons between those two "drawers." This type of optical control would allow processing information at speeds far superior to those of current electronic devices. The study has just been published in the journal Nature Photonics.

Achieving quantum computing involves having ever smaller and faster electronic devices, but following this path will soon lead to a dead end. As components become too tiny (ultrasmall), signal leakage, overheating, and operational failures caused by unwanted interactions between electrons appear.

Faced with this bottleneck, a promising alternative is so-called ultrafast physics and, within it, valleytronics. These areas study how to manipulate quantum properties of electrons using extremely brief light pulses, controlled with such precision that they can last even attoseconds, that is, one trillionth of a second. This approach allows moving electrons without the need for wires or electrical currents, thus bypassing many limitations of traditional electronics.

"Manipulating electrons in these valleys [the quantum 'drawers' for storing information] is very difficult due to their speed [on the order of femtoseconds, i.e., a millionth of a millionth of a second], but at the same time, it is essential to manipulate them for use in quantum computing," explains Álvaro Jiménez Galán, a researcher at ICMM-CSIC and participant in the study.

In this work, the researchers have used "a sequence of light pulses to excite and manipulate the valley pseudo-spin in a tungsten disulfide monolayer and we achieved, for the first time, coherent control at room temperature," adds Rui E. F. Silva, also from ICMM-CSIC and a participant in the research. These ultrafast light pulses allow moving electrons between the two quantum "drawers" of the material—the valleys—and directing their behavior, even though it is a material formed by a single atomic layer. "Pseudo-spin" is the technical way of describing which of those two valleys the electron leans towards.

The results of this research open a new route for faster information processing, with control using a type of ultrafast light pulse that is already available: "This has been possible now because we have managed to apply a pioneering ultrafast control technology in materials, based on attosecond physics, developed by researchers in Giulio Cerullo's group at the Politecnico di Milano," adds Eduardo Bernal Molinero, former researcher at ICMM-CSIC, now at Indra. Attosecond physics allows controlling light in extremely brief times, a domain where until a few years ago, work was only done with gases, not solid materials.

"Our work overcomes two critical obstacles for the implementation of valleytronic devices at room temperature: the fully optical control of the valley degree of freedom and the execution of cascaded logic operations at a very high speed, exceeding terahertz," adds Pablo San-José, also a researcher at ICMM-CSIC participating in the study. The optical control of the "valley degrees of freedom" refers to the ability to direct electrons towards one or the other of the two quantum "drawers" (valleys) of the material using only light pulses, without the need to apply electrical currents. Cascaded logic operations are sequences of steps—like those performed by a processor when comparing, adding, or storing data—that this study paves the way to execute at terahertz speeds, that is, trillions of operations per second.

Working on this terahertz scale represents an enormous leap compared to current electronic processors, which operate only in gigahertz, or in other words, billions of operations per second. The technique demonstrated in this study would therefore operate a thousand times faster than today's most advanced chips.