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الملخص الأصلي

كما كتبه المؤلفون، بالإنجليزية 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.