Physicists proved that quantum tunneling is not an instantaneous phenomenon

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  ”quantum tunneling” (quantum Tunneling) means that a particle can pass through a”tunnel” and pass through a seemingly insurmountable obstacle. Although the quantum tunneling effect will not take you through the brick walls of platforms nine and three-quarters and board the Hogwarts Express, it is always a puzzling and seemingly counterintuitive phenomenon. However, some experimental physicists in Toronto recently measured for the first time the time spent by rubidium atoms in traversing the barrier, and their findings were published in the July 22 issue of the journal Nature.

   research shows that, contrary to some recent news reports, quantum tunneling is not an instantaneous phenomenon.”This is a very beautiful experiment.” Igor Litvinyake of Griffith University in Australia pointed out. He also studies the phenomenon of quantum tunneling, but did not participate in this research.”It is already a heroic measure to carry out this experiment.”

  To understand how weird quantum tunneling is, imagine a ball rolling on flat ground. The ball rolled, and suddenly encountered a round small hill. What happens next depends on how fast the ball rolls. Either it will roll up to the top of the mountain and then slide down from the other side; or because of insufficient energy, it will not roll halfway through, so it will have to roll down the same path.

   However, particles in the quantum world do not encounter this situation. Even if a particle has insufficient energy to climb to the top of the mountain, it can sometimes reach the foot of the other side of the mountain.”It’s as if the particles dug a tunnel under a mountain and then got out of the other side.” The co-author of the study, Everem Steinberg of the University of Toronto, pointed out.

   To understand this weird phenomenon, it is best to look at particles from the perspective of wave function. The wave function is a mathematical expression of the quantum state of a particle, and it will continue to evolve and expand. Using the amplitude of the wave function at any point in time and space, we can calculate the probability of finding the particle at that point in time and space. According to its definition, this probability can have a non-zero value in multiple locations at the same time.

  If a particle encounters an energy barrier, the expansion method of the particle’s wave function will change and begin to show an exponential decay inside the barrier. Nevertheless, part of the wave function will still leak, and its amplitude will not decay to zero on the other side of the barrier. In this way, although the probability is low, it is still possible to detect the particle on the other side of the barrier.

   Since the late 1920s, physicists have been aware of the existence of quantum tunneling. Today, this phenomenon has become the core of devices such as tunnel diodes, scanning tunneling microscopes, and superconducting qubits used in quantum computing.

   Since the discovery of this effect, experimentalists have been trying to figure out what happens in the quantum tunneling process. For example, in 1993, Steinberg, Paul Quiat, and Raymond Zio, who were also at the University of California, Berkeley, detected photons passing through a light barrier. This barrier is made of a special piece of glass that can reflect 99%of the incident photons, and 1%of the photons penetrate the past. Compared with photons that have traveled the same distance but are not blocked on the road, the photons tunneled through the barrier arrive earlier on average. In other words, the speed of the tunneled photon seems to exceed the speed of light.

Detailed analysis of    shows that from a mathematical point of view, the peak of the wave function of the tunneling photon (that is, the place where the particle is most likely to be found) does indeed move faster than light. However, the free-propagating photon and the tunneling photon’s wave function’s foremost end of the wave function arrive at the detector at the same time, so it does not violate Einstein’s theory of relativity.”The movement speed of the peak of the wave function can exceed the speed of light, and will not cause the problem of information or energy propagation speed exceeding the speed of light.” Steinberg pointed out.

  Litvinyake and colleagues published last year’s research results showed that when electrons in hydrogen atoms are constrained by an external electric field (equivalent to a barrier), they Occasionally can escape through the electric field. As the strength of the external electric field oscillates, the number of electrons passing through the past will increase or decrease, which is consistent with theoretical predictions. The research team proved that the time delay between the minimum barrier strength and the maximum number of tunneling electrons is at most 1.8 attoseconds (1.8 x 10–18 seconds). Within 1 attosecond, even light can only travel three hundred millionths of a meter, which is equivalent to the diameter of an atom.”This time delay may be zero altogether, or it can be calculated in units of tens of seconds (10-21 seconds).” Litvinyak pointed out.

  Some media reports claim that this experiment conducted by Griffith University shows that the tunneling phenomenon occurs in an instant. But this statement is not accurate, and may be largely related to scientists’ theoretical definition of tunneling time. The delay measured by the team is indeed close to zero, but it does not mean that the time for electrons to travel inside the barrier is zero. Litvinyak and his colleagues have not studied this aspect of quantum tunneling.

   and Steinberg’s new experiment started from this point. His team measured the average amount of time that rubidium atoms spend inside the barrier before passing through the barrier. The measured time is as long as milliseconds, and it must not be described as”instantly”.

  Steinberg and his colleagues first cooled the rubidium atoms to about 1 nanoKelvin, and then used a laser to move them slowly in one direction. Then, they used another laser to block the path of rubidium atoms, creating an optical barrier about 1.3 microns thick. The key is to measure how long a particle stays inside the barrier before passing through it.

   To this end, the team made a so-called Larmor clock, which uses a series of complex lasers and magnetic fields to manipulate the transition of atomic states. In theory, the following should happen:Suppose a particle is originally rotating in a fixed direction, just like the hands of a clock. Then, the particle suddenly encountered a barrier. There was a magnetic field in the barrier, causing the”pointer” to start to rotate. The longer the particles stay inside the barrier, the longer they interact with the magnetic field and the greater the amplitude of the”pointer” rotation. By measuring the amplitude of the rotation of the”pointer”, the length of time the particles move inside the barrier can be obtained.

   However, if the strength of the magnetic field interacting with the particle is large enough for scientists to accurately calculate the time it takes for the particle to stay inside the barrier, its quantum state will collapse. The tunneling process of particles causes interference.

   Therefore, Steinberg’s team adopted a method called “weak measurement”:let a group of rubidium atoms in exactly the same state reach the barrier at the same time and enter After the barrier, these atoms will interact weakly with a weak magnetic field. This interaction does not interfere with atomic tunneling, but it causes the”pointer” of each atom to rotate at an unpredictable magnitude. Once these atoms leave the barrier, the amplitude of their”pointer” rotation can be measured. Taking the average value of the rotation amplitude of all the atomic”pointers” can be understood as the representative value of a single atom. Based on this”weak measurement” method, the researchers found that the atoms in the experiment spent about 0.61 milliseconds inside the barrier.

   They also verified another strange prediction of quantum mechanics:the lower the energy of the tunneling particle, or the slower the motion, the more time it takes inside the barrier short. This conclusion seems to be counter-intuitive, because according to our perception of daily life, the slower the particle, the longer the movement time inside the barrier should be.

   The method of measuring the rotation amplitude of the particle “pointer” in this study shocked Litvinyak.”I haven’t seen any loopholes for the time being.” But he still maintains a cautious attitude,”However, the relationship between this and particle tunneling duration needs further interpretation.”

   University of California, Berkeley quantum physicist Irfan Sadic was shocked by the precision of the technology used in this experiment.”We are witnessing an amazing achievement. Now we finally have the right tools to verify the philosophical thinking of the last century.”

  利特Satya Senada Enderti, co-author of the Wenyak study, agrees with this point:”The Larmor clock is undoubtedly the correct way to answer the tunneling time problem. The design of this experiment is very clever.”

  Steinberg also admitted that their team’s interpretation of experimental results will inevitably be questioned by some quantum physicists, especially those who are skeptical of the “weak measurement” approach . Nevertheless, he still believes that the experiment clearly revealed some truth about the length of tunneling.”If the correct definition is adopted, quantum tunneling does not happen instantaneously, but it is extremely fast. There is a key difference between the two.” (Leaf)