Working at Ludwig Maximilians Universitat Munchen (LMU Munich) and the Max Planck Institute of Quantum Optics (MPQ), laser physicists have measured the time taken between a photon striking a helium atom and an electron being ejected with zeptosecond (a trillionth of a billionth of a second) precision. This marks the first time the timescale for this kind of atomic-level event has been absolutely determined with such accuracy.

This phenomenon, known as photoionization, whereby an ion is formed through the interaction of a photon with an atom, is specifically related to the quantum-mechanical redistribution or absorption of energy within an atom – namely, the photon's energy is either distributed between the two electrons in the helium atom, or completely absorbed by one of them. Whichever way things go, one of the electrons is ejected from the atom and being able to measure the time interval between photon strike and electron release is therefore very important in verifying an aspect of quantum theory.


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"Our understanding of these processes within the helium atom provides us with a tremendously reliable basis for future experiments," said Martin Schultze, LMU's Chair of Experimental Physics. "We can now derive the complete wave mechanical description of the entangled system of electron and ionized helium parent atom from our measurements".

The ejection of an electron after photionization is known as photoemission, or the photoelectric effect – a phenomena Albert Einstein detailed in his 1905 paper that described light energy being carried in discrete packets and which eventually led to quantum theory itself. To see this effect occurring requires both high-energy lasers and a camera with a stupendously fast shutter speed.

This is because excitation levels of this magnitude require energy inputs in the mega-electronvolt (MeV) range, and the timeframe of the entire event from when the photon strikes the electrons to the moment when one of the electrons exits the atom, is somewhere around 5 to 15 attoseconds (an attosecond is one quintillionth of a second or 1018 seconds).

No ordinary camera could possibly hope to capture this moment. Even a superfast cameras like the Japanese STAMP system is way too slow, despite being able to snap images in femtoseconds (one quadrillionth or 1015 seconds). In fact, the new system used by the Munich physicists operates at orders of magnitude faster, accurately capturing events down to 850 zeptoseconds (a zeptosecond is 1021 seconds).

To achieve such blistering capture speeds, the researchers fired an attosecond-long, extremely ultraviolet (XUV) light pulse onto a helium atom whilst simultaneously shooting a four femtosecond infrared laser pulse at the same area. When the electron was ejected, it was immediately detected by the infrared laser pulse. Depending on the state of the oscillating electromagnetic field of the laser pulse at the time of capture, the released electron was either accelerated or decelerated by the photon interaction and measurements of this speed change enabled researchers to record the length of the photoemission event down to zeptosecond accuracy.

The practical upshot from a quantum perspective to this experiment is that the physicists were also able to ascertain how the energy of the photon fired in to the helium atom is quantum mechanically distributed between the two electrons contained in that atom in the very few attoseconds prior to photoemission taking place.

As the relatively simple properties of the helium atom allow its behavior to be calculated solely from the application of quantum theory, this new zeptosecond accuracy experiment means that it is now possible to definitively reconcile theory and experiment.

The results of this research were recently published in the journal Nature Physics.

Source: LMU Munich