One circuit that behaves like a single quantum particle

Some particles are so small they can do something that looks impossible. An electron can slip straight through a wall that should stop it, a move called tunnelling. Until now, tunnelling had only ever shown up in single, invisibly small particles.

The prize winners built a small electric circuit and cooled it colder than deep space. Inside, trillions of electrons paired up and moved in perfect step, like a whole crowd swaying as one. Because they moved together, the entire circuit began to act like a single tiny particle, even though it was big enough to hold.

That was the shock. Quantum rules people thought belonged only to the invisibly small were caught working in something large. The same trick now sits at the heart of superconducting quantum computers.

To find quantum behaviour in something large, start with superconductivity. When certain metals are cooled below a critical temperature, their electrons pair up into Cooper pairs, and all those pairs fall into a single shared quantum state. The whole sea of electrons now moves in lockstep and can be described by one wavefunction with a single phase.

Now take two superconductors and separate them with an insulating barrier thin enough for Cooper pairs to cross. This is a Josephson junction. Feed a small current through it and, up to a point, that current flows with no voltage at all. Physicists call this the zero-voltage state, and you can picture the circuit as a ball resting at the bottom of a valley, trapped behind a hill.

They went further and showed the trapped state has quantised energy levels, like the rungs of a ladder. Shining microwaves on the junction, they found it absorbed energy only at certain frequencies, hopping to a higher rung, exactly as an atom absorbs light only at its own special colours. A circuit big enough to hold was behaving like a single artificial atom.

Tunnelling is normally a single-particle story. A lone electron, described by one wavefunction, has a small but real chance of passing through a barrier that classical physics forbids. The laureates asked whether an entire electric circuit could be coaxed into doing the same thing. The key is superconductivity. Below its critical temperature a superconductor's conduction electrons bind into Cooper pairs, each carrying charge 2e, and all the pairs condense into a single macroscopic wavefunction with one shared phase. The circuit's quantum state is then set not by countless individual electrons but by one collective coordinate.

In that tilted-washboard picture the zero-voltage state is the phase trapped in one local well: current flows with no voltage across the junction. Classically the only way out is to be kicked over the barrier by thermal energy, so the escape rate should fall off as exp(minus barrier over kT) and vanish as the circuit is cooled. Clarke, Devoret and Martinis cooled their junction below about 50 millikelvin and found the opposite. The escape rate flattened out and stayed constant, independent of temperature. The phase was not climbing the barrier. The whole collective coordinate was tunnelling straight through it, and each escape announced itself as a sudden voltage across the junction.

What made this macroscopic was the number of particles moving as one. Billions of electrons, paired and condensed, acted through a single quantum variable that could be prepared, tunnelled and measured. This answered a question pressed by Anthony Leggett: how large can a system be and still obey quantum mechanics? The answer turned out to be large enough to hold in your hand.