Paving the way for the next generation of quantum computers Scientists film electron motion at attosecond speeds for the first time

On October 12, the journal Nature published the latest research by German and American scientists, "Attosecond Timing of the Correlation between Bloch (Bloch) Electrons" [1], in which the team achieved the fastest speed to date (300 attoseconds, 1 attosecond = 10-18 seconds) to photograph the motion of electrons.

Professor Mackillo Kira of the University of Michigan's Department of Electrical Engineering and Computer Science, who led the research, said, "Current computers have processors that run at gigahertz, which is a billionth of a second per run. But for quantum computing, this is very slow because the electrons inside a quantum computer chip collide trillions of times per second, terminating the computation with each collision." [2]

Kira added, "To drive (quantum computer) performance forward, what we need is a billion times faster (one attosecond is one billionth of a billionth) snapshots of electron motion. Now we have it."

01New high-precision attosecond timing

Generally speaking, it takes longer to drive downtown than it does to cover the same distance on country roads. After all, drivers encounter many red lights, roadworks and traffic jams in city centers; instead, to find out how busy a road is, simply measure the time it takes for a car to travel a certain distance: this is exactly how modern navigation systems identify traffic obstacles. This logic can be applied to the microscopic world: as electrons (the smallest charge carriers) move through a solid, they can interact with other electrons and change their dynamics; due to the small mass of electrons, the processes involved happen unimaginably fast, and they follow the laws of quantum physics rather than classical mechanics.

This time, a team led by Professor Rupert Huber of the Institute of Experimental and Applied Physics at the University of Regensburg, Germany, and Professor Mackillo Kira of the Department of Electrical Engineering and Computer Science at the University of Michigan, USA, has succeeded for the first time in tracking the ultrafast motion of free electrons (WSe2) in solids with incredible precision: just a few hundred attoseconds. This resolution is sufficient to study the smallest changes in the quantum dynamics of electrons caused by the attraction of other charge carriers or by complex many-body correlations.

Left: Rupert Huber; Right: Mackillo Kira

An attosecond is equivalent to one billionth of a billionth of a billionth of a second, and it is related to one second in the same way that one second is related to twice the age of the universe. Even light can travel a distance of only one atomic diameter in an attosecond. To measure the movement of electrons on such a short time scale, researchers have developed a new type of attosecond clock.

The "pendulum" of this clock is provided by the oscillating carrier wave of light, the fastest alternating field that humans can control. The optical field actually places charge carriers on a test track through a solid. It first accelerates the electrons in the semiconductor sample in one direction and, after reversing the direction of the field, re-collides them with the gaps (known as holes) from which they were removed. In this process, high-energy photons are emitted. The likelihood of collisions occurring is not always the same, but depends on the point in time when the electrons start moving.

Schematic diagram of the experimental setup

The researchers timed this collision path more precisely than a hundredth of a light oscillation period and were thus able to show how the different strengths of attraction between the charge carriers changed their dynamics [3]. Josef Freudenstein of the Institute for Experimental and Applied Physics at the University of Regensburg, who is the first author, explains, "Just as in busy traffic it is better to leave early in order to reach your destination on time, if there are many strong encounters between electrons in a crystal, then the electrons must start their collision path early. " The resulting off-domain electron attosecond timing could revolutionize the understanding of future quantum dynamics phenomena in the unexpected phase transitions and emergence of electron, photoelectron and quantum information technologies.

02Double pulse sequences: observing the quantum state of electrons

To observe electron motion in two-dimensional quantum materials, researchers typically use short bursts of focused extreme ultraviolet (XUV) light. These bursts can reveal the activity of electrons attached to atomic nuclei, but the large amount of energy carried in these bursts prevents a clear view of electrons passing through semiconductors: as in current computers and materials being explored for quantum computers.

University of Michigan engineers and partners use two pulses of light with energy scales that match those of moveable semiconductor electrons. The first, an infrared light pulse, places the electrons in a state that allows them to pass through the material; then, the second, a lower-energy terahertz pulse, forces those electrons into a controlled head-on collision trajectory. The collisions produce bursts of light whose precise timing reveals the interactions behind quantum information and exotic quantum materials, among other things.

"We use two pulses - one that matches the electron state energetically and then another that causes a state change," said Mackillo Kira, a professor of electrical engineering and computer science at the University of Michigan. "We can basically photograph how these two pulses change the quantum state of the electron and then express that as a function of time."

The double-pulse sequence allows time measurements to be made with better than one-hundredth the accuracy of the terahertz radiation oscillation period of an accelerating electron.

Identification of monolayers and blocks WSe2

Timing quasiparticle collisions in lumpy and monolayer WSe2

"This is really unique and has taken us many years to develop," said Rupert Huber, professor of physics at the University of Regensburg in Germany and corresponding author of the study, "The individual oscillation periods of light are so ridiculously short that this high-precision measurement is even very unexpected: moreover, the we have a time resolution that is a hundred times faster."

03A game changer in many-body physics

Huber says the results of the study could have potential implications in the field of many-body physics beyond their computational impact. "Many-body interactions are the microscopic driving force behind the most coveted properties of solids: from optics and electronics to interesting phase transitions. But they are notoriously difficult to obtain. Our solid-state atomic clocks could be real game-changers, allowing us to design new quantum materials with more precise, tailored properties and develop new materials platforms for future quantum information technologies."

"Until now, no one has been able to build a scalable and fault-tolerant quantum computer, and we don't even know what it will look like," said Markus Borsch, co-first author of the study and a doctoral student in electrical and computer engineering at the University of Michigan, "but fundamental research that such as studying how electron motion in solids works at the most fundamental level, may give us an idea to steer us in the right direction."

Reference links：

[1]https://www.nature.com/articles/s41586-022-05190-2

[2]https://phys.org/news/2022-10-crystalline-attoclock-ultrafast-motion-free.html?deviceType=desktop

[3]https://techxplore.com/news/2022-10-electron-movement-fastest-next-level-quantum.html