For nearly a century, the peculiar phenomenon of quantum tunneling has captivated physicists and challenged our understanding of the subatomic world. In the realm of quantum mechanics, particles can seemingly pass through impenetrable barriers, defying the laws of classical physics. But how long does this mysterious process take? A landmark study by researchers Patrik Schach and Enno Giese from the Technical University of Darmstadt in Germany from the Technical University of Darmstadt in Germany sheds new light on this age-old question, using the precision of atomic clocks to probe the elusive nature of tunneling time. The exciting findings have been published in the journal Science Advances.
Quantum tunneling is a bizarre consequence of the wave-particle duality that lies at the heart of quantum mechanics. In the everyday world, a ball thrown at a solid wall will simply bounce back. However, at the atomic scale, particles like electrons behave more like waves, and these waves have a small but finite probability of passing through seemingly insurmountable barriers. This puzzling effect is crucial to many technologies we rely on, from scanning tunneling microscopes to the flash memory in our smartphones.
But the question of how long a particle spends inside the barrier during the tunneling process has been a subject of intense debate among physicists. Some experiments have suggested that tunneling is instantaneous, while others have hinted at a finite duration. The problem is that the act of measuring the tunneling time can itself interfere with the delicate quantum process, making it difficult to obtain a clear answer.
This is where Schach and Giese’s ingenious approach comes in. Instead of trying to directly measure the tunneling time of a single particle, researchers propose using a quantum clock — essentially an atom with two internal energy states that can act as a highly precise timekeeper. By preparing the clock in a special quantum state and letting it tunnel through a barrier, scientists show that the time spent inside the barrier can be inferred from the final state of the clock, without disrupting the tunneling process itself.
The key to this method lies in a classic technique from atomic physics known as Ramsey interferometry. Named after Nobel laureate Norman Ramsey, this technique involves putting an atom into a superposition of two energy states, allowing it to evolve for a precise time interval, and then measuring the final state to determine how much the two states have shifted relative to each other. By comparing the phase shift of a tunneling clock to that of a free-moving reference clock, Schach and Giese demonstrate that the tunneling time can be isolated and measured with astonishing precision.
One of the most striking findings of their study is that the tunneling time is not fixed, but depends on the properties of the barrier and the energy of the incident particle. For opaque barriers, the maximum tunneling time occurs when the particle’s energy is just slightly above the barrier height. As the particle’s energy increases further, the tunneling time actually decreases, eventually vanishing for particles that have enough energy to simply pass over the barrier.
Researchers also explore how imperfections in the experimental setup, such as slight differences in the initial velocities or masses of the two clock states, can affect the measured tunneling time. By carefully analyzing these effects and proposing ways to mitigate them, they lay the groundwork for future experiments that could measure tunneling times with unprecedented accuracy.
Perhaps most intriguingly, Schach and Giese show that the sensitivity of their method can be greatly enhanced by using optical atomic clocks, which operate at frequencies hundreds of trillions of times higher than microwave clocks.
“We are currently discussing this idea with experimental colleagues and are in contact with our project partners,” says Giese.
By exploiting the extreme precision of these cutting-edge timepieces, it may one day be possible to measure tunneling times as short as a few billionths of a trillionth of a second — a timescale so brief that light itself travels only the width of an atom.
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