Stockholm: This year’s Nobel Prize in Physics has been awarded to John Clarke, Michel H. Devoret and John M. Martinis for a discovery that brings the bizarre rules of the quantum world to a human scale11. The Royal Swedish Academy of Sciences announced that the three scientists will share the prestigious prize “for the discovery of macroscopic quantum mechanical tunnelling and energy quantisation in an electric circuit”.
Their groundbreaking experiments demonstrated that the strange properties of quantum mechanics, typically thought to be confined to the microscopic scale of single particles, can be observed in a system large enough to be held in the hand.
A Ball That Passes Through a Wall
One of the strangest aspects of quantum mechanics is “tunneling”. In the everyday world, if you throw a ball at a wall it will always bounce back. But in the microscopic quantum world, a particle can sometimes pass straight through a barrier and appear on the other side, even if it classically doesn’t have enough energy to do so.
In a series of experiments conducted in 1984 and 1985 at the University of California, Berkeley, Clarke, Devoret, and Martinis demonstrated that this tunneling effect can happen on a macroscopic scale, involving billions of particles working in unison.
Building an Artificial ‘Macro-Atom’
The scientists constructed a special electrical circuit containing two superconductors materials that conduct current with zero resistance separated by a thin insulating layer known as a “Josephson junction”. Within this circuit, pairs of electrons called “Cooper pairs” synchronized to behave as if they were a single giant particle filling the entire circuit.
The team showed that this collective, particle-like system could “tunnel” out of a zero-voltage state in which it was trapped, and suddenly generate a voltage much like a particle passing through a wall.
Furthermore, they also confirmed that the system’s energy is “quantized”. This means it can only absorb or emit energy in specific, discrete amounts, just as quantum mechanics predict. They demonstrated this by introducing microwaves into the circuit and observing that only certain wavelengths were absorbed, causing the system to jump to a higher energy level.
The Foundation of Quantum Computing
This research has had a profound impact on the understanding of quantum mechanics. It shows that quantum effects are not just limited to the microscopic world but can also exist in large systems, reminiscent of Erwin Schrödinger’s famous “cat” thought experiment.
The work has also opened the door to practical applications, particularly in the field of quantum computing. Their circuit can be regarded as a large-scale “artificial atom” and Martinis later used these quantized energy states to create quantum bits, or “qubits,” which are the building blocks of quantum computers. Superconducting circuits remain one of the most leading technologies for building quantum computers today.
