Quantum tunneling allows particles to bypass energy barriers. A new method for measuring the time it takes for particles to tunnel has been proposed, which could challenge previous claims about superluminal tunneling rates. This method involves using atoms as clocks to detect subtle time differences. Credit: SciTechDaily.com
In an amazing quantum physics phenomenon known as tunneling, particles appear to move faster than the speed of light. However, the Darmstadt physicists believe that the time required for the particles to tunnel has been measured incorrectly until now. They propose a new method to stop the speed of quantum particles.
In classical physics there are strict laws that cannot be circumvented. For example, if a rolling ball lacks sufficient energy, it will not make it over the hill; instead, it goes back down before reaching the top. In quantum physics, this principle is not so strict. Here, a particle can pass through a barrier even if it does not have enough energy to overcome it. It behaves as if it were slipping through a tunnel, which is why this phenomenon is also known as “quantum tunneling”. This phenomenon is far from mere theoretical magic and has practical uses, for example in the operation of flash memory units.
Quantum tunneling and the theory of relativity
Experiments in which particles tunnel faster than light have attracted some attention in the past. After all, Einstein’s theory of relativity forbids speeds faster than light. So the question is whether the time needed to drive the tunnel was correctly “stopped” during these experiments. Physicists Patrik Schach and Enno Giese from TU Darmstadt are pursuing a new approach to defining “time” for a tunneling particle. Now they have proposed a new way of measuring this time. In their experiment, they measure it in a way that they say is more appropriate for the quantum nature of tunneling. They published the design of their experiment in a renowned journal Scientific advances.
Wave-particle duality and quantum tunneling
According to quantum physics, small particles such as atoms or light particles have a dual nature.
Depending on the experiment, they behave like particles or like waves. Quantum tunneling highlights the wave nature of particles. A “packet of wool”, comparable to a rush of water, rolls towards the barrier. The height of the wave indicates the probability with which the particle would materialize at that location if its position were measured. If the wave packet hits an energy barrier, part of it will be reflected. However, a small fraction penetrates the barrier and there is little chance that the particle will appear on the other side of the barrier.
Rethinking tunneling speed
Previous experiments observed that a light particle traveled a longer distance after tunneling than one that had a free path. It would therefore travel faster than light. However, scientists had to define the location of the particle after its passage. They chose the highest point of his wave deck.
“But the particle does not follow a path in the classical sense,” argues Enno Giese. It is impossible to say exactly where a particle is at a particular time. This makes it difficult to state the time required to travel from point A to point B.
A new approach to tunneling time measurement
Schach and Giese, on the other hand, follow Albert Einstein’s quote: “Time is what you read off the clock. They propose to use the tunneling particle itself as a clock. The second particle, which does not tunnel, serves as a reference. By comparing these two natural clocks, it is possible to determine whether time passes more slowly, faster, or at the same rate during quantum tunneling.
The wave nature of particles facilitates this approach. The oscillation of waves is similar to the oscillation of a clock. Specifically, Schach and Giese propose using atoms as clocks. Energy levels of atoms oscillate at certain frequencies. After addressing an atom by a laser pulse, its levels initially oscillate in sync – the atomic clock is started. When tunneling, however, the rhythm shifts slightly. The second laser pulse causes the two internal waves of the atom to interfere. Interference detection makes it possible to measure how far apart two waves of energy levels are, which in turn is an accurate measure of elapsed time.
The other non-tunneling atom serves as a reference for measuring the time difference between tunneling and non-tunneling. Calculations by the two physicists suggest that the tunneling particle will exhibit a slightly delayed time. “Tunneled clocks are a bit older than the others,” says Patrik Schach. This appears to contradict experiments that attributed superluminal speeds to tunneling.
A challenge in conducting an experiment
In principle, the test can be done with today’s technology, Schach says, but it’s a big challenge for experimenters. This is because the time difference to be measured is only around 10-26 seconds – an extremely short time. It helps to use clouds of atoms as clocks instead of individual atoms, explains the physicist. The effect can also be strengthened, for example by artificially increasing clock frequencies.
“We are currently discussing this idea with experimental colleagues and are in contact with our project partners,” adds Giese. It is quite possible that the team will soon decide to conduct this exciting experiment.
Reference: “A unified theory of tunnel times supported by Ramsey clocks” by Patrik Schach and Enn Giese, 19 Apr 2024, Scientific advances.
DOI: 10.1126/sciadv.adl6078