Page 12 - EngineerIT Nov-Dec 2025

QUANTUM PHYSICS

Experiments on a chip reveal

quantum physics in action

By Hans van de Groenendaal, Science and technology writer*

he Royal Swedish Academy of In 1984 and 1985, John Clarke, Michel H Devoret and John M Martinis
Sciences awarded the Nobel conducted a series of experiments with an electronic circuit built of
TPrize in Physics 2025 jointly to superconductors, components that can conduct a current with no electrical
Prof John Clarke of the University of resistance. In the circuit, the superconducting components were separated
California, Berkeley, Prof Michel H by a thin layer of non-conductive material, a setup known as a Josephson
Devoret of Yale University and the junction. By refining and measuring all the various properties of their circuit,
University of California, Santa Barbara, they were able to control and explore the phenomena that arose when they
and Prof John M Martinis of the passed a current through it.
University of California, Santa Barbara,
and Qolab, Los Angeles, USA, for the This macroscopic particle-like system is initially in a state in which current flows
discovery of macroscopic quantum without any voltage. The system is trapped in this state, as if behind a barrier
mechanical tunnelling and energy that it cannot cross. In the experiment the system shows its quantum character
quantisation in an electric circuit. by managing to escape the zero-voltage state through tunnelling. The system’s
changed state is detected through the appearance of a voltage.
A significant question in physics is the
maximum size of a system that can
demonstrate quantum mechanical
effects. This year’s Nobel Prize
laureates conducted experiments
with an electrical circuit in which
they demonstrated both quantum
mechanical tunnelling and quantised
energy levels in a system big enough to
be held in the hand.

Quantum mechanics allows a particle
to move straight through a barrier
using a process called tunnelling. As
soon as large numbers of particles
are involved, quantum mechanical
effects usually become insignificant.
The laureates’ experiments
demonstrated that quantum-
mechanical properties can be
realised on a macroscopic scale.

12 | EngineerIT November/December 2025