What is Macroscopic Quantum Tunneling : Nobel Prize for Physics 2025

The 2025 Nobel Prizes in Physics have been announced. John Clarke and Michael Deboret, as well as John Martinis, who developed ‘macroscopic quantum tunneling’, shared the Nobel Prize this time. They have received the Nobel Prize in 2025 for the experiments conducted by these three together in the 1980s. What is that one experiment worthy of a Nobel Prize? What is Macroscopic Quantum Tunneling? That is what we are going to discuss today. 

You can watch video explanation in malayalam language here

If you are reading this article for academic purposes then you can skip this article. Because many of the things that are going to be said will have to be presented in a simplified way that anyone can understand. If you want to study this subject academically or in a more deep and technical way, you can read a very detailed description of it on the Nobel Prize website : Link 

What is quantum mechanics?

We have made many laws with the knowledge we have acquired about our universe over time. All the things we saw and understood back then were things that could be explained by these laws. But after a phase we started getting some different results from some experiments. There was an observation that the particles in our universe like molecules, atoms, electrons, protons and neutrons do not obey the laws of physics that we have made up. Thus, quantum mechanics is a field of physics that we have created to explain the properties of some small particles. 

A fundamental theory of quantum mechanics is wave-particle duality. What is a particle? A particle is an object that needs a definite size and shape or a specific place to exist. What is a wave? A wave exhibits the opposite properties of a particle. No exact shape, no size. That means they are something you don't want to confine to a particular location. This will continue to spread.

The basic law of quantum mechanics states that every object in our universe has both of these properties. That is, if I am standing before you now, I have a particle nature within me. When we say particle nature, we have a definite form. I need a specific place in which to exist. And there is wave nature. So I'm behaving like a wave, I have a specific wavelength and frequency and I want it to spread out. It is really not common sense. Because even if we see me or you or many objects around us, we don't feel it as a wave but rather as an object made of particles. When we take an electron or an atom or a molecule we are observing things on a quantum scale. If I look at this atom, I can actually observe the atom as a wave, whereas I can observe it as a particle. This is basically a feature of quantum mechanics. What makes it different from normal physical laws is that both of these types of behavior are learned and proceeded in the same way. In many of the observations we see, it is possible to see some conflicts like particle behavior or wave behavior in many places. 

Quantum tunneling

Now let's see what quantum tunneling is. I am telling a case. As you walk down a path, you see a wall in front of you. It is a fairly high wall. There are no other gaps or doors on the sides. Only after crossing this wall you can go to the other side of the road. But it is impossible for you to jump over a wall of such height. Think of it as a wall higher than a ladder can climb. Your friend is on the other side. Your friend is also unable to cross this wall. I am talking to my friend in the same context. He can hear me talking to my friends. Even though he is standing on the other side of the wall, he can hear me and I can hear him back. 

Sound is a wave. I am a particle. A particle has a physical form, shape, and weight. So for me it is impossible to cross it. But the wave I emitted was able to cross this one barrier and go beyond. I create an energy barrier on a particle scale like the wall before me. That is, something like a wall that can only be crossed if there is a lot of energy, so I create a barrier and then I bring an electron to one side of this barrier and then I try to get this electron to cross this barrier. For a small particle to cross a barrier, it needs more energy than the energy gap for that barrier. We have learned how to knock out electrons from an atom. That is, if I want to knock out an electron that is rotating at a certain energy in an atom, I can only give more energy to that electron. But even if the electron does not have as much energy as the barrier, we see that the electron has crossed this barrier and reached the other side. It is not common sense. Because what have we learned? To overcome an energy barrier we need more energy than that energy barrier. But in quantum mechanics, or going down to the quantum scale, the exact opposite happens. That is, even if there is a big wall in front of you, without jumping over that wall, you can run a short run on that wall and reach the other side through that run. Similarly, we can only see such observations at the quantum scale. 

The ability of microscopic particles to cross such an energy barrier is what we call quantum tunneling, the word tunnel meaning pathway across a barrier. 

Even though we understand the electron to be a particle, quantum mechanics says that it has a wave nature. A characteristic of a wave is that it cannot remain localized at any one place. Then the wave that cannot stay localized penetrates our barrier and a small portion of this wave reaches beyond our barrier. Now if the barrier is too big then we will never be able to do this even through tunnelling. Generally we use very small size barriers for tunneling. So basically quantum mechanical tunneling allows us to meet our needs by crossing an energy barrier with tiny particles. We use it in many ways. The most common use of quantum tunneling is in flash memory. In other words, things like pendrives work by trapping electrons through quantum tunneling. One such thing that has many applications is called quantum mechanical tunneling. 

Macroscopic quantum tunneling

Now let's talk about what ‘macroscopic quantum tunneling’ is. What is microscopic? When we talk about things on a small scale, what we talk about in nanometers and micrometers is what we call microscopic things. What do you mean macroscopic? Objects that can be seen with our eyes or some observations that are of a size that can be seen with our eyes. This is what we refer to as macroscopic. What happens if the microscopic scale phenomena described in quantum mechanics, such as quantum tunneling, are magnified to the macroscopic scale, to the large scale that we can see with our eyes? That's where macroscopic quantum tunneling comes into play. Note that pen drives do not work by macroscopic quantum tunneling. All the basic processes in pendrive take place in each electron. It is the sum total of all the specific activity of each electron that we call pendrive as a big object works. It is microscopic quantum tunneling. 

Before talking about macroscopic quantum tunneling and John Clarke's experiment, we must first understand what superconductivity is. We know what conductivity is. Superconductivity is the ability of a material to allow current to flow very fast without any other resistance. Even if it is a material that conducts current very well, for example silver. It is a material that conducts current the fastest and best. Even that silver has a definite resistance value. That means it opposes the current. But silver is a material that conducts better than other materials around it. But superconductivity says that it is possible to bring some materials to a state where there is absolutely no resistance. We have already done videos on superconductivity in detail on our channel. You can see it. Basically, superconductivity works by cooling a material, completely cutting off the vibrations of the atoms inside, and allowing electrons to move very fast inside the material without thermal energy. Electrons also have a property that enables superconductivity. Electrons are changing into a different form. That is called a Cooper pair. Electrons are called fermions and fermions have definite energy levels. Two electrons cannot be in same state or energy state. 

When a material becomes a superconductor, the electrons within it become pairs. That means the two electron chains work as a single unit.  So the current must be generated by the flow of electrons but how is the current generated in a superconductor? Two electrons flow as a single unit. A similar superconducting circuit was made by John Clark and Michael Devoret as well as John Martinis.

 

This circuit consisted of a silicon chip. It is a chip about a centimeter in size. They have created many barriers in this chip. That is, they created barriers to stop the flowing electrons. Because it is a superconducting circuit, electrons flow in the form of Cooper pairs. So these Cooper pairs started to cross the barriers inside this silicon chip. Quantum tunneling, of course. But what's special here is that we could observe in this one circuit that was about one centimeter in size, all these Cooper pairs acted as a unit. Or quantum tunneling has occurred here in such a way that the entire circuit shows the behavior of a single electron at the time of completing that circuit. This is something we do not observe in normal quantum tunneling. What happened here? All of these electrons are about a centimeter in size, which is the size that we can observe with the naked eye. An electron can never have the same wave function as another electron, but what has happened here is that in an area that is about a centimeter in size or on a macroscopic scale, we have been able to bring many electrons into the same wave function. It is something that has never happened before. This is also something that no one had thought of until then. 

How did they verify this? Normally, all processes at the quantum scale involve the transfer of energy in small, discrete pieces. That is, energies are always transmitted in the form of one unit of energy and two units of energy, whereas continuous energy measurements such as 1.5 units or 1.001 units are not possible for us at the quantum scale. We use the word 'quanta' to refer to the mechanism by which energy is transferred in precise units. So an energy quanta is a packet of energy. All processes at the normal quantum level are carried out by transferring discrete, non-continuous, small packets of energy. So when they measured energy transfer in the circuit that John Clarke's group made, they realized that energy is quantized. This means that all energy transfer takes place on a quanta scale, just as it does on a quantum scale. Even if it happened on a macroscopic scale, all its properties show the same properties on the quantum scale, that is, in the old belief, when we take things to a bigger object, we lose the quantum properties, that's why we had to take another ladder to jump over that wall.  If not, we can create a wave nature like that of an electron and easily cross a wall. At the macroscopic scale it is impossible. But what did this experiment prove? Observations at the quantum scale can be brought down to the macroscopic scale to recreate the same observations. So this is a very ground breaking discovery. 

For those who still do not understand the significance of this observation, I will give one more example. Quantum chips. 

Quantum chips are one of the most exciting technologies we see today. How does a quantum computer work? A quantum computer works with basic units called qubits. This can only happen if all the qubits work in sync. Quantum processors are built by making many qubits work in a synchronized manner on a macroscopic-scale object called a quantum chip. Quantum computing is one area where many advancements have been made in the last year or so. We are yet to see its great benefits in the future. 

Another interesting thing about this is that a person named Michael Deboret is the current Chief Scientist of Google's Quantum AI. Another Nobel laureate, John Martinis, was the chief scientist there before Deboret.