Breaking the Speed ​​of Light: The Quantum Tunneling Enigma

Quantum tunnel concept

Quantum tunneling allows particles to bypass energy barriers. A new method has been proposed to measure the time it takes for particles to tunnel, which could challenge previous claims about superluminal tunneling speeds. This method involves using atoms as clocks to detect subtle time differences. Credit:

In an astonishing phenomenon in quantum physics known as tunneling, particles appear to move faster than the speed of light. However, physicists from Darmstadt believe that the time it takes for 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 hard rules that cannot be circumvented. For example, if a rolling ball does not have enough energy, it will not get over a hill, but will turn around before reaching the top and change direction. In quantum physics, this principle is not so strict: a particle can pass a barrier even if it does not have enough energy to cross it. It behaves as if it were sliding through a tunnel. That is why the phenomenon is also called ‘quantum tunneling’. What sounds magical has tangible technical applications, for example in flash memory drives.

Quantum tunneling and relativity

In the past, experiments in which particles tunneled faster than light attracted some attention. After all, Einstein’s theory of relativity prohibits speeds faster than light. The question is therefore whether the time required for tunneling was properly ‘stopped’ in these experiments. Physicists Patrik Schach and Enno Giese from TU Darmstadt are taking a new approach to defining ‘time’ for a tunneling particle. They have now proposed a new measurement method. In their experiment, they measure it in a way that they believe better fits the quantum nature of tunneling. They published the design of their experiment in the renowned journal Scientific progress.

Wave-particle duality and quantum tunneling

According to quantum physics, small particles such as atoms or light particles have a dual character.

Depending on the experiment, they behave like particles or waves. Quantum tunneling emphasizes the wave nature of particles. A ‘wave packet’ rolls towards the barrier, comparable to a flood of water. The height of the wave indicates the probability with which the particle would materialize at this location if its position were measured. When the wave packet hits an energy barrier, part of it is reflected. However, a small part penetrates the barrier and there is a small chance that the particle appears on the other side of the barrier.

Reevaluate tunnel speed

Previous experiments have shown that a light particle has traveled a longer distance after tunneling than a light particle that had a free path. It would therefore have traveled faster than light. However, the researchers had to determine the location of the particle after its passage. They chose the highest point of the wave package.

“But the particle does not follow a path in the classical sense of the word,” Enno Giese objects. It is impossible to say exactly where the particle is at any given time. This makes it difficult to make statements about the time it takes to get from A to B.

A new approach to measuring tunnel time

Schach and Giese, on the other hand, are guided by a quote from Albert Einstein: “Time is what you read from a clock.” They propose using the tunneling particle itself as a clock. A second particle that does not tunnel serves as a reference. By comparing these two natural clocks, it is possible to determine whether time passes slower, faster, or equally fast 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. The energy levels of atoms oscillate at certain frequencies. After addressing a atom with a laser pulse the levels initially oscillate in synchronization – the atomic clock is started. However, during tunneling the rhythm shifts slightly. A second laser pulse causes the two internal waves of the atom to interfere. Detecting the interference makes it possible to measure how far apart the two waves of energy levels are, which in turn provides an accurate measure of elapsed time.

A second atom, which is not tunneling, serves as a reference to measure the time difference between tunneling and non-tunneling. Calculations by the two physicists suggest that the tunneling particle will exhibit a slightly delayed time. “The clock in a tunnel is a little older than the others,” says Patrik Schach. This seems to contradict experiments that attribute superluminal speed to tunneling.

The challenge of implementing the experiment

In principle, the test can be performed with today’s technology, says Schach, but it is a major challenge for experimentalists. This is because the time difference to be measured is only approximately 10-26 seconds – an extremely short time. It helps to use clouds of atoms as clocks instead of individual atoms, the physicist explains. It is also possible to enhance the effect, for example by artificially increasing the clock frequencies.

“We are currently discussing this idea with experimental colleagues and are in contact with our project partners,” Giese adds. It is quite possible that a team will soon decide to carry out this exciting experiment.

Reference: “A unified theory of tunnel times promoted by Ramsey clocks” by Patrik Schach and Enno Giese, April 19, 2024, Scientific progress.
DOI: 10.1126/sciadv.adl6078

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