11 November 2016
by Rebecca Boyle
It’s like catching light in action. For the first time, physicists have measured changes in an atom to the level of zeptoseconds, or trillionths of a billionth of a second – the smallest division of time yet observed.
In this case, the speed demon was an electron escaping the bonds of its parent atom. When light strikes electrons, they get excited and can break free from their atoms. The photon’s energy is either entirely consumed by one electron or divided among several. This electron ejection is known as the photoelectric effect, and was described by Albert Einstein in 1905.
Previous experiments studying this effect could only measure what happened after the electron was kicked out of an atom, says Martin Schultze at the Max Planck Institute of Quantum Optics in Garching, Germany.
Now, he and his colleagues have seen the other end of the process. They measured the entire ejection of electrons from a helium atom from start to finish with zeptosecond precision (10-21 seconds), marking the smallest time slot ever measured.
In a series of experiments, the team fired an unspeakably brief, extremely ultraviolet laser pulse at a helium atom to start exciting its pair of electrons. This pulse lasted just 100 to 200 attoseconds, or 10-18 seconds. But by making many readings and calculating their statistical spread, they were able to measure events at a rate of 850 zeptoseconds.
They also fired a near-infrared laser pulse, lasting just four femtoseconds (1 femtosecond is 10-15 seconds). This pulse was able to detect an escaping electron as soon as it was freed from the helium atom. Depending on the electromagnetic field of the laser pulse, the electron either accelerated or decelerated.
“Using this information, we can measure the time it takes the electron to change its quantum state from the very constricted, bound state around the atom to the free state,” says Marcus Ossiander at the Max Planck Institute.
The ejections took between 7 and 20 attoseconds, according to Schultze, depending on how the electron interacted with the nucleus and the other electron.
“We only need to know the time centre of the pulses very precisely, and our technique allows us to measure this to very high precision,” Ossiander says. “An even shorter pulse would give better time resolution, but the resolution can far exceed the pulse duration. We can confirm this by doing statistics for a lot of measurements and calculate the statistical standard error of the mean, which in our case is 850 zeptoseconds.”
The researchers were also able to measure how the electrons divided up the laser’s energy, taking an even or uneven share. In some cases, one of the two electrons grabbed all of the energy. Several factors influenced this energy split, from the quantum correlation between the electrons to the electromagnetic state of the laser field, Schultze says.
One reason for choosing helium is because it has just two electrons, allowing direct measurement of their quantum mechanical behaviour. For an atom with more electrons, some assumptions would be needed about how the energy was divided and the time taken for ejection.
The results are an important window into the quantum behaviour of atoms, especially how their electrons work, Schultze says. Understanding that could lead to insights into phenomena like superconductivity or quantum computing.
“There is always more than one electron. They always interact. They will always feel each other, even at great distances,” he says. “Many things are rooted in the interactions of individual electrons, but we handle them as a collective thing. If you really want to develop a microscopic understanding of atoms, on the most basic level, you need to understand how electrons deal with each other.”