Krishna Prasad Maity, Assistant Professor Department of Physics, SRM University AP
The Nobel Prize is one of the most prestigious awards in the world, conferred upon individuals who have made outstanding contributions to the advancement of human life across diverse fields—from literature to science and technology.
In the field of physics, several groundbreaking discoveries have been recognized with this honor in the past years. For example, in 2014, three Japanese scientists—Isamu Akasaki, Hiroshi Amano, and Shuji Nakamura—received the Nobel Prize in Physics for inventing the blue light-emitting diode (LED). Their innovation revolutionised lighting technology, enabling the use of low-power bulbs (5 W, 10 W, or 12 W) in place of traditional high-power filament bulbs (60 W or 100 W) in our home. This has significantly reduced energy consumption and improved our daily life.
India has also made significant contributions to physics. In 1930, C. V. Raman received the Nobel Prize for his discovery of light scattering, known as the Raman Effect. However, since then, our country has lagged behind in achieving further Nobel Prizes in science.
This year, the Nobel Prize in Physics has been awarded to John Clarke, Michel H. Devoret, and John M. Martinis for the discovery of Macroscopic Quantum Tunneling (MQT) in superconducting circuits. Their discovery paves the way for quantum computers capable of ultra-fast computation and efficient quantum communication, transforming technology and everyday life in the near future. To understand their achievement, it is essential to grasp the concept of quantum tunneling. In the classical world, if an object’s energy is less than the height of an energy barrier, it cannot cross that barrier. However, for extremely small particles (~ 10-10 meters, while the thickness of human hair is about 10-5 meters), quantum mechanics allows them to “tunnel” through the barrier—even when their energy is lower than the barrier height.
For example, if you throw a cricket ball at a wall, it will not pass through unless it has enough energy to overcome the wall’s potential barrier, related to height. But if that ball were a tiny quantum particle, it could penetrate the wall with much less energy. This happens because, in the quantum realm, particles also behave like waves (as described by De Broglie’s hypothesis). Just as part of a light wave is transmitted through a material, a particle’s wave can also partially transmit through a potential barrier—this is quantum tunneling.
Theoretical predictions of tunneling date back to 1926, and it has been observed in many microscopic phenomena, such as alpha decay in radioactivity—where an alpha particle escapes the nucleus of an atom by tunneling through an electrostatic barrier. However, until the 1980s, tunneling was observed only at the microscopic scale, involving single particles. No one had demonstrated macroscopic quantum tunneling, in which a collective quantum state involving many particles tunnels through a barrier.
To explore macroscopic quantum tunneling (MQT), John Clarke began experiments in the early 1970s at the University of California, Berkeley, using superconducting circuits. Later, John M. Martinis and Michel H. Devoret joined his research group as a Ph.D. student and a postdoctoral researcher, respectively. Superconductors are the materials that conduct electricity without any resistance and also floats near a magnet (below a critical temperature, e.g., 9.2K/-264°C for Niobium metal). Remember the material ‘unobtainium’ in the Avatar movie – a superconductor at room temperature! In this state, electrons form pairs known as Cooper pairs, which condense into a coherent quantum phase and behave as a single macroscopic quantum entity. These Cooper pairs move without scattering, resulting in extremely high conductivity.
When two superconductors are separated by a thin insulating layer, called a Josephson junction, Cooper pairs can tunnel through the insulator, generating a supercurrent without an applied voltage. The current depends solely on the quantum phase difference between the two superconductors.
When a steady current is applied to the Josephson junction, it controls the voltage that appears across it. At first, as long as the current stays below a certain critical value, no voltage is observed— the particles remain quietly trapped in their potential well. But once the current increases and some particles manage to tunnel through the energy barrier, a small but measurable voltage suddenly appears.
By carefully adjusting the current, the Nobel laureates watched this transition from the silent, no- voltage state to the active, voltage-producing state. What amazed them was that the tunneling continued even at temperatures as low as fifteen millikelvin—far too cold for thermal energy to play any role. They confirm a macroscopic quantum state was tunneling through the insulating barrier, something that is distinct from microscopic one.
They also noticed that not all escapes were equal—different excitations had different chances to tunnel. This revealed something even deeper: the trapped particles did not have continuous energies, but discrete, quantized energy levels, just as quantum theory predicted.
This ground breaking work is the stepping stone to make qubit, a two level quantum system. These qubits are the building block of quantum computers recently developed across the world. These computers will have capability to solve calculations in a seconds which our modern super computers even takes several years to complete.
Acknowledgement: Author thank Dr. Siddhartha Ghosh for reading the article and valuable comments.
