Beijing time on August 3, according tomedia reports, the so-called “quantum tunneling” refers to a particle can pass through a “tunnel”, through a seemingly insurmountable obstacle. While the quantum tunneling effect won’t take you through the brick walls of the nine-and-three-quarters of the platforms and board the Hogwarts Express, it’s always a confusing, seemingly counterintuitive phenomenon. However, some experimental physicists in Toronto have recently measured for the first time how long the niobium atom spends crossing the barrier, and the findings are published July 22 in the journal Nature.
Studies have shown that, contrary to recent news reports, quantum tunneling is not an instantaneous phenomenon. “It’s a beautiful experiment.” Igor Litvinjak of Griffith University in Australia points out. He also studies quantum tunneling, but was not involved in the study, “just doing this experiment is a heroic move.” “
To understand how weird quantum tunneling is, imagine a ball that rolls on flat ground. The ball rolled and suddenly came across a round hill. What happens next depends on how fast the ball is rolling. It will either roll up to the top of the mountain and slide down from the other side, or it will roll halfway through the low energy and have to roll down the road.
However, particles in the quantum world do not encounter this. Even if one particle has enough energy to reach the top of the mountain, it can sometimes reach the foot of the other side. “It’s like the particles digging a tunnel under the mountain and then getting out of the other side.” The study’s co-author, Efrem Steinberg of the University of Toronto, noted.
To understand this weird phenomenon, it’s best to look at particles from a wave function perspective. Wave functions are mathematical expressions of quantum states of particles that evolve and expand. Using the amplitude of the wave function at any point in time and at any point in space, we can calculate the probability of finding the particle at that point in time and at the point of space. By its definition, this probability can appear at multiple locations at the same time with non-zero values.
If a particle encounters an energy barrier, the way the particle wave function expands changes and begins to show exponential decay inside the barrier. Nevertheless, some wave functions seep past, and their amplitude does not decay to zero on the other side of the barrier. This makes it possible to detect the particle on the other side of the barrier, although the probability is low.
Physicists have known about quantum tunneling since the late 1920s. Today, the phenomenon has become the core of devices such as tunnel diodes, scanning tunnel microscopes, and superconducting qubits used in quantum computing.
Since discovering this effect, experimentalists have been trying to figure out what happened during quantum tunneling. In 1993, for example, Steinberg, Paul Cuyatt and Raymond Zio, who were at the University of California, Berkeley, detected photons that passed through a light barrier. The barrier is made of a special piece of glass that reflects 99% of the incident photons, and 1% of the photons penetrate the past. The average time to reach the past photons through the barrier is earlier than the photons that pass through the same distance but are not blocked on the road. In other words, the lightchildren worn by the tunnel appear to move faster than the speed of light.
Detailed analysis shows that, from a mathematical point of view, the peak of the wave function of the tunnel through photons (i.e. where the particles are most likely to be found) do do hyper-light-speed motion. However, the time at the front end of the wave function of freely transmitted photons and tunneling photons reaches the detector the same time, and therefore does not contradict Einstein’s theory of relativity. “The wave function’s peak can move faster than the speed of light, without causing information or energy to travel faster than the speed of light.” Steinberg points out.
The results of a study published last year by Mr Litvinenko and his colleagues showthat that when electrons in hydrogen atoms are subject to an external electric field, the equivalent of a barrier, they occasionally escape through the electric field. As the strength of the outer electric field oscillates, the number of electrons passing through the past increases or decreases, in line with theoretical predictions. The team demonstrated that the delay between the lowest barrier strength and the highest number of tunnel-through electrons is 1.8 Aseconds (i.e. 1.8 x 10 to 18 seconds). Within 1 a second, even light can travel only three hundred millionths of a meter, equivalent to the diameter of an atom. “This delay may simply be zero, or it may be calculated in fractions (10-21 seconds). Mr. Litvinenko pointed out.
Some media reports say the experiment, conducted by Griffith University, shows that tunneling occurs in a flash. 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 does not mean that electrons travel within the barrier for a time of zero. Mr Litvinenko and his colleagues have yet to study this aspect of quantum tunneling.
Steinberg’s new experiment begins with this. His team measured the average length of time the gamma atoms spent inside the barrier before they crossed the barrier. And measured time of up to milliseconds, must not be used to describe “in a momentary”.
Steinberg and his colleagues first cooled the niobium atoms to about 1 Nakelin, and then lasered them to move them slowly in one direction. They then blocked the path of the thorium atom with another laser, creating an optical barrier about 1.3 microns thick. The key is to measure how long a particle has stayed inside the barrier before it crosses it.
To do this, the team created a so-called Larmor clock that uses a complex series of lasers and magnetic fields to manipulate atomic transitions. In theory, it should happen if a particle is originally spinning in a fixed direction, just like a watch pointer. Then, the particle suddenly meets a barrier with a magnetic field that causes the “pointer” to start turning. The longer the particles stay inside the barrier, the longer they interact with the magnetic field, and the greater the rotation of the “pointer”. By measuring the magnitude of the “pointer” rotation, you can get how long the particles move inside the barrier.
However, if the magnetic field interacting with the particles is strong enough to allow scientists to accurately calculate how long the particles will take inside the barrier, the quantum state will collapse, disrupting the particle’s tunneling process.
As a result, Steinberg’s team used a technique called “weak measurement”: having a set of identically-occurring niobium atoms reach the barrier at the same time, and when they enter the barrier, they interact faintly with a weak magnetic field. This interaction does not interfere with the tunneling of atoms, but causes the “pointers” of each atom to rotate at unpredictable magnitudes. Once these atoms leave the barrier, the rotation of their “pointer” can be measured. Take the average of the rotation of all atomic “pointers” and interpret them as representative values for a single atom. Based on this “weak measurement” approach, the researchers found that the atoms in the experiment consumed about 0.61 milliseconds inside the barrier.
They also validated another strange prediction of quantum mechanics: the lower the energy of the tunneling particles, or the slower the movement, the shorter the time spent inside the barrier. This conclusion seems to contradict intuition, because, according to our understanding of everyday life, the slower the particles should move within the barrier for longer.
In this study, Litvinenko was shocked by the way the particle “pointer” rotates. “I haven’t seen any loopholes for the time being.” But he remains cautious, “but the link between this and the length of the particle tunnel ingon needs to be further interpreted.” “
Irfan Sadic, a quantum physicist at the University of California, Berkeley, was shocked by the precision of the technique. “We are witnessing a remarkable achievement. Now we finally have the right tools to validate the philosophical thinking of the last century. “
Satya Senada Nderti, co-author of the Litvinenko study, agrees: “The Larmor clock is undoubtedly the right way to answer the question of tunneling time. The design of the experiment was very clever. “
Steinberg also acknowledged that his team’s interpretation of the results is bound to be questioned by some quantum physicists, especially those who are skeptical of the “weak measurement” approach. Still, he believes the experiment clearly reveals some of the truth about the length of the tunnel. “If you use the right definition, there is a key difference between quantum tunneling that is not happening in an instant, but at very fast speeds.” (Leaf)