One of the shortest signals ever created by humans was created by physicists at the University of Konstanz. They used paired laser pulses to compress a series of electron pulses to a period of only 0.000000000000000005 seconds.
The compressor from the experimental set-up. Image Credit: Johannes Thurner
Attoseconds and femtoseconds are the units of time used to describe various natural processes that take place in molecules or solids. Even more quickly are nuclear reactions.
The University of Konstanz’s Maxim Tsarev, Johannes Thurner, and Peter Baum is now developing a new experimental setup to produce signals with an attosecond (billionth of a nanosecond) duration, which opens up new avenues for studying ultrafast processes.
Even light waves cannot achieve such a time resolution since it takes too long for a single oscillation. Here, electrons offer a solution since they permit noticeably higher time resolution.
The Konstanz researchers create their incredibly brief electron pulses in a free-space beam using pairs of femtosecond light flashes from a laser in their experimental setup. The journal Nature Physics reported the findings.
How Did the Scientists Go About It?
Light waves can also superimpose to form stationary or moving wave crests and troughs, just like water waves can. The co-propagating electrons, which travel through a vacuum at half the speed of light, overlap with optical wave crests and troughs that travel at exactly the same speed according to the physicists’ choice of incidence angles and frequencies.
The electrons are then propelled into the next wave trough by what is known as ponderomotive force. Thus, following a brief interaction, a succession of extremely brief electron pulses is produced, particularly in the center of the pulse train where the electric fields are strongest.
The temporal duration of the electron pulses is only a few attoseconds long. The researchers take measurements of the electrons’ remaining velocity distribution following compression to comprehend that mechanism.
Instead of a very uniform velocity of the output pulses, you see a very broad distribution that results from the strong deceleration or acceleration of some electrons in the course of compression. But not only that: The distribution is not smooth. Instead, it consists of thousands of velocity steps, since only a whole number of light particle pairs can interact with electrons at a time.
Johannes Thurner, Physicist, University of Konstanz
Significance for Research
According to the scientist, this is a temporal superposition (interference) of the electrons with themselves as described by quantum mechanics, after they underwent the same acceleration at several periods. For experiments involving quantum mechanics, such as those involving the interaction of electrons and light, this effect is important.
The fact that heavy electrons and a light particle (photon) with a zero-rest mass cannot have their total energy and momentum conserved makes it extraordinary that plane electromagnetic waves like a light beam can alter the velocity of electrons in a vacuum permanently.
The Kapitza-Dirac phenomenon, which allows two photons to exist concurrently in a wave moving slower than the speed of light, resolves this issue.
These findings are still clearly basic research, according to University of Konstanz professor of physics Peter Baum, leader of the Light and Matter Group, but he highlights the enormous potential for further study.
If a material is hit by two of our short pulses at a variable time interval, the first pulse can trigger a change and the second pulse can be used for observation—similar to the flash of a camera.
Peter Baum, Professor, Physics, University of Konstanz
According to Peter Baum, the main advantage is that no material is used in the experimental principle, and everything takes place in free space. In theory, lasers of any power could be utilized in the future to achieve even more compression.
Baum concluded, “Our new two-photon compression allows us to move into new dimensions of time and perhaps even film nuclear reactions.”
Journal Reference
Tsarev, M., et al. (2023) Nonlinear-optical quantum control of free-electron matter waves. Nature Physics. doi:10.1038/s41567-023-02092-6.