Particle physics is challenging because of the tiny dimensions of atoms, and studying changes in atoms is equally difficult owing to the short amount of time in which these changes take place.

But now for the first time ever, German and Austrian physicists have observed a photon ejecting an electron from a helium atom with a precision of 850 trillionths of a billionth of a second, or 850 zeptoseconds (10^-21 seconds). This is the highest ever recorded accuracy of an event. The research is published in Nature Physics.

The team from the Universities of Munich and Vienna chose a complex scenario to push the envelope on hyperquick observations. The photoemission of a helium electron usually takes between 5 and 15 attoseconds (10^-18 seconds). The precision achieved is enough to allow the researchers to now perfectly describe the system using quantum mechanics.

“Our understanding of these processes within the helium atom provides us with a tremendously reliable basis for future experiments,” explained team leader Martin Schultze, a specialist in laser physics at Munich's Chair of Experimental Physics, in a statement.

“We can now derive the complete wave mechanical description of the entangled system of electron and ionized helium parent atom from our measurements.”

The researchers directed an attosecond-long ultraviolet laser pulse at a helium atom to excite an electron. At the same time an infrared laser pulse, thousands of times slower, was shot at the escaping electron. The second laser measured the change in speed of the electron, which allowed the researchers to reach the record-breaking precision.

This photoemission is a particular case of the photoelectric effect that was discovered by Albert Einstein at the beginning of the last century; he won the Nobel prize for this discovery in 1921. The energy of the photon is the crucial discriminant in this process, as it needs to be equal or greater than the energy that binds the electron to the atom.

Einstein worked out that you could shoot an infinite amount of low-energy photons without having an electron emission, but a single photon that is energetic enough can suddenly liberate the particle. Thanks to this study, researchers now understand better how the energy of the photon actually breaks the electron free.