A Qubit That Isn't Afraid of Noise express
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.
Cooper pairs are electrons that, at extremely low temperatures, pair up and flow without resistance. In this qubit, they tunnel only in fours, like carts coupled in twos.
The idea of protecting quantum information from noise echoes the 'topological insulation' in Greg Egan's novel 'Schild's Ladder', where data is stored in stable nodes of space.