Nobel Prize in Physics, 2025
Mundur V N Murthy

This year we are celebrating one hundred years of the discovery of quantum mechanics. This year's Nobel Prize in physics celebrates this occasion by awarding the prize to a specific discovery in quantum mechanics known as "quantum tunnelling".

BOX The Physics, in Brief
The Nobel Prize in Physics for 2025 was awarded to John Clarke (University of California at Berkeley, USA), Michel H. Devoret (Yale and University of California at Santa Barbara, USA) and John M. Martinis (also at Santa Barbara and Qolab, USA), who used a series of experiments to demonstrate that the strange properties of the quantum world can be seen in a system big enough to be held in the hand. Read all about the details in the text.
END OF BOX

What is Quantum Mechanics?
You will learn about the Quantum World in Class 11 or 12. Our entire Universe is believed to be governed by the laws of quantum mechanics. However, the so-called "macroscopic" or large-scale world that we see around us is easily described by Newton's laws of motion. This is called the classical world.

When you observe phenomena at very small scales -- atomic scales -- smaller even than a single cell, the usual classical behaviour is not seen because they are governed by quantum laws. The key element of quantum mechanics is that energy, momentum, angular momentum, and such properties come in discrete units called "quanta". "Discrete" means that they cannot take continuous values.

Also, certain properties are inter-linked: for instance, you cannot measure the position and momentum of a particle simultaneously because measurement of one alters the other. This is called the uncertainty principle. Finally, a quantum system can exist in multiple states at the same time. This is called superposition. Only upon measurement will the system "collapse" to a single one of these available states. These properties allow for what is called quantum tunnelling.

Quantum tunnelling
We said that quantum mechanics is relevant for single particles. A ball, on the other hand, is built up of an astronomical number of molecules and displays no quantum mechanical effects. We know that the ball will bounce back every time it is thrown at a wall. A single particle, however, will sometimes pass straight through an equivalent barrier in its microscopic world and appear on the other side! This quantum mechanical phenomenon is called tunnelling.

Quantum mechanics of atoms and nuclei
The protons and neutrons in a nucleus are together called nucleons. These are bound together by the attractive nuclear force inside the nucleus. Energy is required to "break" these bonds and remove either a neutron or a proton from the nucleus (like removing an electron from an atom). It is like there is an energy barrier they need to overcome before they can come out of the nucleus. So it appears that they can only come out of the nucleus when it is hit by another particle which supplies enough energy to overcome the attraction. But this is not always so, as alpha decay shows.

Alpha decay and quantum tunnelling
A helium nucleus (with two protons and two neutrons) is called an alpha particle. George Gamow in 1927 formulated the theory of the so-called `alpha decay' of heavy nuclei (nuclei with mass numbers exceeding 100 typically) where the nucleus of such heavy atoms (like Uranium) emits an alpha particle. This decay occurs
without any external intervention.

The interesting thing here is that classically the alpha particle is a prisoner for ever inside the nucleus due to the energy barrier created by the attractive nuclear potential. Gamow showed that the laws of quantum mechanics allow for the alpha particle to tunnel through the potential barrier. That is, there is not enough energy for the particle to go over the barrier, but there is a chance it can simply tunnel through it, like a mole burrowing in the earth.

The physics of the small
This seems amazing to our classical brains! But it has been shown to occur in many nuclei whose size is of the order of 10-13 cm. Actually, the laws of quantum mechanics operate at all scales, not just the smallest scale. However, when the system grows bigger in physical size and number, it so happens that the quantum effects get washed out, due to a phenomenon now called decoherence, leaving us with the simple classical laws as we use and experience every day. Not quite, though, there are some quantum phenomena visible even at larger scales.

The physics of the large
There are some cases where the effect of quantum mechanics is seen on macroscopic (large) scales and this is the content of this year's Nobel prize winning work. The examples are rather advanced and difficult to explain at the high school level. One example of importance for this year's Nobel prize is the phenomenon of superconductivity.

Superconductivity
The experiments conducted in 1984 by Clarke, Devoret and Martinis involved superconducting circuits. Superconductivity, as the name indicates, is the flow of current in circuits where the resistance is near zero. Normal conductors have finite resistance at any given voltage. Using Ohm's law we can determine the current (I) at any voltage (V) for a given resistance (R) as I = V/R.

The resistance is the property of the material. Bad conductors (plastics) have high resistance while good conductors (metals like copper, aluminium) which are commonly used in transmission, have low resistance. It is expected that the resistance of all these metals smoothly goes to zero as the temperature goes to absolute zero (see the Answers to `Do You Know' for more details on Absolute zero).

Rarely, in some materials, the resistance already becomes almost zero below when the temperature is still non-zero. This temperature is called the critical temperature. Above the critical temperatures these materials may be just normal conductors or insulators. Below this critical temperature, there is no energy required to keep the current flowing since there is no resistance that would heat up the sample. Since there is no loss, due to zero resistance, the current once induced can remain circulating for ever! This is called superconductivity and can be explained by quantum mechanics.

While the explanation is complicated, the key point is that, below the critical temperature, the electrons in the atoms pair up to form so-called “Cooper pairs” which results in the superconducting behaviour. This has huge technological applications: every day medical applications involving MRI machines as well as sophisticated particle accelerators use this technology. We are now ready to understand the Nobel prize-winning work.

The Nobel experiment
The Nobel Laureates set up an electronic circuit which is superconducting with billions of electrons combining to form Cooper pairs. The charged particles flowing through the system behave as a collective single particle. The system is confined in this zero voltage state from which it cannot escape as it does not have enough energy. They built two superconducting electrical circuits separated by a thin layer of material which does not conduct any current. This in-between layer acts as a barrier separating the two superconducting components.

It was found that the system generated an electrical voltage on the other side of the barrier! This should remind you of the barrier tunnelling problem we discussed earlier; that is, the electron Cooper pairs were able to tunnel through the barrier to the other side. This is a purely quantum phenomenon, but on a very large scale, involving billions of electrons. They also made other more complicated experiments of a similar nature.

In alpha particle decay, we observe particles cross the barrier individually; also, this is seen on a very small scale of the size of the nucleus. Here we have macroscopic tunnelling involving billions of particles acting in unison.

These experiments proved that quantum mechanics operates on very large scales as well, if we are clever enough to observe it. In addition, the experiment has many technological applications, especially in quantum computers. All these factors make this year's prize worthwhile and appropriate to celebrate the centenary of the birth of quantum mechanics.

Sources: Several, including www.nobelprize.org