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In a strange quantum mechanics phenomenon called “tunneling”, particles appear to be able to travel faster than light – contradicting Einstein’s theory of relativity. However, the researchers suggest in the new study that previous measurements showing this phenomenon are incorrect and that, instead, there is no such thing as superluminal energy or “instantaneous tunneling.”
Quantum tunneling is a phenomenon in which a particle can pass through an energy barrier that it cannot otherwise cross, according to the laws of classical physics. The latter is particularly regulated by strict laws.
In contrast, quantum mechanics is not so limited. Even if its energy is lower than the minimum required to cross the barrier, the particle can pass through it, as if sliding through a tunnel, hence the name “tunnel effect”. First described in 1928, this effect explains many previously mysterious phenomena, such as radioactive decay, as well as how the Sun’s hydrogen nuclei can overcome mutual repulsion and fuse together to produce energy. Flash memory storage media work on the same principle.
However, the speed at which particles pass through quantum tunnels is a matter of debate. The researchers suggested that for quantum particles, the barriers act as shortcuts. If the particles were to “dig” tunnels there, their movement would take less time than if there was no barrier, which seems contradictory.
Furthermore, the thickness of the barriers does not appear to increase the time required for particles to pass through them. In other words, particles “tunnel” faster than light traveling the same distance in empty space. However, Einstein’s relativity forbids any faster-than-light motion. This led to the questioning of some fundamental aspects of physics, including the very definition of time.
On the other hand, researchers at the Technical University of Darmstadt (in Germany) suggest that the quantum tunneling time may not have been measured correctly in previous experiments. As part of their new study, recently published in the journal Scientific progresspropose a new measurement protocol that, according to them, is more adapted to the nature of tunneling.
Previous experiments based on wave-particle duality
Previous tunneling time measurements were generally based on
wave-particle duality, a phenomenon in which particles can behave both as waves and as particles. The tunneling effect would particularly highlight the wave nature of particles, when moving towards a barrier such as a water wave as they gradually transform into a wave packet (a property that allows a massive particle to represent several frequencies and combined wavelengths).
If the wave packet comes into contact with the energy barrier, part of it is reflected, while the other part passes through it. The height of the wave (or waves) indicates the probability that the particle will materialize at a particular location on the barrier after tunneling. In order to locate the point of materialization of the particle, the researchers were based on the highest height reached by the wave packet.
However, “the particle does not follow a path in the classical sense of the word”, explains ua communicated co-author of the new study, Enno Giese, from the Technical University of Darmstadt. As a result, “it is impossible to say exactly where a particle is at any given time. Therefore, it is difficult to comment on the time it takes to get from point A to point B,” he says.
An approach based on Einstein’s temporal model
A new protocol by Giese and his colleague aims to overcome this obstacle by relying on Einstein’s temporal model, according to which time is simply defined as that measured by a clock. In this vision, they propose using a tunneling particle as a clock, while another, which does not tunnel, serves as a reference. By comparing two clocks, it would be possible to determine how fast time passes when tunnelling.
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Diagram summarizing the experimental approach of the study. (HAVE) The first laser pulse initializes the clock by creating an equal superposition of internal states ∣ Mr in / e in〉two-layer system. (B) During tunneling, each internal state receives a state-dependent phase shift encoded in complex transmission amplitudes t Mr / e
. After the diffusion process, a second laser pulse reads out the accumulated phase which includes laboratory time contributions
t from time dilation δ t and the tunneling time τ (see (A)). (vs) is obtained characterized by the contrast ∣〈e T ∣ Mr T〉∣/ NOT T
with the total number of transferred atoms NOT T and average transmission coefficient (D) for a rectangular barrier. This transfer coefficient shows different characteristics for different kinetic energies at different levels. © Patrick Schach et al.
The realization of this approach also partly relies on the wave nature of the particles. Their oscillations as waves would be comparable to those characteristic of a clock. The energy levels of atoms (used as clocks) oscillate at specific frequencies. Therefore, by exposing them to a laser beam, these levels would oscillate in a synchronized manner, thus inducing atomic clock-type operation.
However, the tunneling effect slightly disturbs this synchronization, which can be adjusted using a second laser pulse that causes the two internal waves of the atom to interfere. Detecting this disturbance then allows obtaining precise measurements of the elapsed time during tunneling.
By applying the process to another non-tunneling atom, it is possible, according to the researchers, to measure the difference between tunneling time and non-tunneling time. Surprisingly, the experts’ calculations revealed that the tunneled particle showed a slightly delayed time compared to the non-tunneled particle, contradicting previous experiments that attributed it to a speed greater than the speed of light.
However, conducting such an experiment faces major challenges. In fact, the time difference to be measured would be on the order of 10-26 second, which is remarkably short, even considering current measurement techniques. To overcome these challenges, experts suggest using clouds of atoms as clocks instead of individual atoms. It would also be possible to amplify the effect of the time difference by manually increasing the frequency of the clocks, which would facilitate the measurements.