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.
Original abstract
as written by the authors arXiv:2603.13114
Superconducting circuits with embedded symmetries are good candidates to robustly protect quantum information from dominant error channels. The $$\cos(2\varphi)$$ qubit, consisting of an island shunted to ground through a tunneling element that selectively transmits pairs of Cooper pairs, leverages charge-parity symmetry to protect from charge-induced errors. In this experiment, we observe a doublet of states of opposite Cooper-pair parity split by $$13.6~\mathrm{MHz}$$. Operating in a soft-transmon regime, this splitting is two orders of magnitude smaller than in previous implementations, pushing charge-induced losses well beyond the measured coherence times. Despite the low transition frequency, we demonstrate coherent qubit control, single-shot readout, and resolve quantum jumps. Charge protection of the qubit is evidenced by a $$100-$$fold suppression of the island charge matrix element compared to the unprotected plasmon transition, placing dielectric loss limits above $$10~\mathrm{ms}$$. The measured $$T_1 = 70~\mu\mathrm{s}$$ and $$T_2^\mathrm{echo}= 2.5~\mu\mathrm{s}$$ are instead limited by $$1/f$$ flux noise in the tunnelling element's loop. This experiment shows that pushing Cooper-pair pairing in the transmon regime sets high limits on charge-induced losses while preserving coherent control and single-shot readout of the low-frequency qubit. We identify flux noise as the dominant remaining limitation, calling for gradiometric designs or novel $$4e$$-tunneling elements.
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.