What quantum effect did they demonstrate in a macroscopic electric circuit?

John Clarke, Michel Devoret, and John Martinis demonstrated in the 1980s that a superconducting electrical circuit, built at a scale you could hold in your hand, could display quantum tunnelling and quantized energy levels — behaviors previously thought to only occur at the scale of individual atoms. Their experiments showed that an entire circuit, made of billions of atoms, could act as a single collective quantum object.

Nuclear fission is an entirely different physical process involving splitting atomic nuclei, and superconducting magnetism alone doesn't capture what made this discovery special — the breakthrough was specifically about quantum tunnelling and quantized states appearing at macroscopic scale.

This work laid essential groundwork for today's superconducting quantum computers, which rely on exactly these kinds of engineered macroscopic quantum circuits, sometimes called 'artificial atoms,' as their basic building blocks.