An ordinary superconducting qubit is like a swing that any gust of wind can set rocking. The slightest change in electric charge nearby, and the quantum state is already spoiled. In the new work, physicists made a swing with a heavy base: the charge on the qubit's island can only change in pairs, as if coins are dropped into a piggy bank strictly two at a time. That's the protection against charge noise.

The circuit is called a $\cos(2\varphi)$-qubit. In it, the tunnel junction only lets through pairs of Cooper pairs — bound quartets of electrons. In the experiment, they observed the splitting of two states with opposite parity by 13.6 megahertz. This is a hundred times smaller than in previous versions, so random charge jolts barely affect the qubit. They even managed to control it coherently and read out its state in a single shot.

But there's a flip side. The low transition frequency makes the qubit vulnerable to magnetic noise in the tunnel element's loop. The lifetime $T_1$ was 70 microseconds, and the coherence time with an echo pulse was 2.5 microseconds. These numbers are still modest compared to the best ordinary qubits, but the potential is enormous: if magnetic noise can be tamed, charge losses could be pushed far beyond 10 milliseconds.

The authors propose two paths: gradiometric designs, where the magnetic field is compensated, or new elements with four-electron tunneling. If this works, quantum computers could become significantly more reliable without complex error correction.