The macroscopic world is full of walls, boxes, and barriers that keep things confined. But if you are small enough, where the laws of quantum mechanics rule, barriers don’t affect you. You could say that walls are for suckers and just tunnel your way out.
This is known as quantum tunneling. It is a property that has fascinated – and perplexed – physicists for decades. One thorny question has been how long it takes for particles to go through a “potential barrier,” the walls within which they are confined. Theoretically, the barrier can be crossed faster than light without violating the laws of physics, and it was even suggested that particles could do it instantaneously.
To provide an answer to this question, researchers from the University of Toronto have developed a clever setup. As reported in Nature, they measured how long rubidium atoms in a Bose-Einstein condensate take to cross a 1.3-micrometer-thick optical barrier for the first time. They found it took them 0.6 milliseconds.
“We’ve known about tunneling for nearly a century, and use it in some of the fastest electronics, highest-precision magnetometers, superconducting qubits, etc – it is a disgrace that so much time has gone by without us truly understanding how long the process takes," senior author Professor Aephraim Steinberg told IFLScience. "Knowing this could help us understand many other related processes where a system can end up in more than one final state, which is pretty ubiquitous in quantum theory."
"In general, to me, the context here isn’t even as much about tunneling as about trying to find out how much quantum mechanics allows us to infer about the past,” he added.
A Bose-Einstein condensate is often referred to as the fifth state of matter. Particles, often atoms, are cooled to a temperature very close to absolute zero. The individual atoms begin to behave in ways where quantum mechanical properties become apparent macroscopically. The rubidium atoms can also be imagined as tiny magnetic spinning tops, so an external magnetic field will make them rotate, and this is how the researchers timed how long it took the atoms to cross the barrier.
The team created a localized magnetic field within the barrier. As the atoms went through it, they began to rotate, and then stopped as they came out the other side. The team used the amount of rotation, or spin precision, of the atoms as a stopwatch to measure the time it took them to cross this "forbidden" region, which they revealed as 0.6 miliseconds. This also confirmed a weird result that showed particles that reach the barrier with less energy get through more quickly.
"We’ve... found a finite time, consistent with theory, and have begun to confirm the weird result that particles which reach the barrier with less energy actually get through more quickly than particles with more energy," Professor Steinberg told IFLScience.
"More importantly, the techniques we’ve developed open up the path to probing more detailed questions. Not just how much time the particle spends in the barrier region, but exactly where it spends that time, how transmitted and reflected particles’ behavior differs, and in essence, a kind of 'conditional probability distribution' for what a particle was doing as a function of time if you know where it winds up in the end: not a “trajectory,” since quantum mechanics can’t uniquely specify those, but perhaps the closest we can come."
This is an important milestone in research, but it's far from the end for the team. They are already improving this experimental setup, as well as ways to better probe quantum tunneling, such as understanding in more detail just what goes on in the barrier from the particles' point of view.