For the first time, scientists have measured electrons tunnelling their way out of atoms. Each escape happened amazingly quickly, in less than a billionth of a millionth of a second.

Although the feat is largely of academic interest, the team say it could speed up the quest for compact X-ray lasers, which could lead to improved early-cancer diagnosis.

Electrons have a negative charge and are glued into atoms by the attractive force of its positively charged nucleus. In classical physics, an electron could not escape from an atom unless it received enough energy to overcome this force by ascending the nucleus's "potential barrier".

But quantum mechanics allows another way – the electron can "tunnel" straight through the barrier with a certain probability. Quantum tunnelling is commonplace in the microscopic world. But until now, it has proved impossible to time the process because it happens much faster than any clock could possibly measure.

Now Ferenc Krausz at Max Planck Institute for Quantum Optics in Garching, Germany, and colleagues, have achieved the feat using a cunning trick that did away with the need for clocks. They gave electrons in a cloud of neon atoms three fleeting time "windows" in which they could burrow out, then counted how many took up the offer of escape.

Escape tunnel

The team zapped a cloud of the neon atoms with two carefully synchronised brief laser pulses – an ultraviolet one (pulse 1) and an infrared one (pulse 2). Pulse 1 primed tightly bound electrons for escape from neon atoms by raising their energy levels so they could escape as far as the periphery of the atom.

Then, three peaks of the oscillating pulse 2 provided a strong enough electric field to suppress the potential barrier from the nucleus, giving the pre-prepped electrons three windows of opportunity to escape.

By blasting pulse 1 at different times during the course of pulse 2, then measuring the numbers of electrons liberated, Krausz's team could reconstruct their escape strategy. Around 30%, 40% and 30% of the electrons emerged in the three main bursts, in line with a quantum tunnelling theory dating back to the 1960s.

The results proved that a single electron could escape during a half-wave of the infrared laser, fleeing in less than 400 attoseconds – an unimaginably short time. If you slowed down time so that an attosecond lasted for 1 second, that second would last for 30 billion years – more than twice the age of the universe.

Stadium laser

"This measurement is a very important step towards the goal of really understanding how tunnelling happens," Krausz told New Scientist. "Physicists are trying to understand what's going on in the microscopic world, and even without immediate applications, it's just very satisfying to understand another mysterious phenomenon."

But he adds that a better understanding of tunnelling could contribute to the development of compact X-ray lasers, in which electron tunnelling looks set to play a key role. Current powerful X-ray lasers are cumbersome machines housed in buildings the size of football stadiums. Compact ones could be used in hospitals to reveal tumours when they're very small and readily treatable.

"One of our dreams is to be able to build very compact X-ray lasers," says Krausz: "They would allow cancer diagnosis at a very early stage, and it would definitely revolutionise this field."

Journal reference: Nature (vol 446, p 627)