Experimental realization of a $\cos(2φ)$ transmon qubit
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arXiv
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| Natura: | Preprint |
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2026
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| author | Roverc'h, Erwan Borgognoni, Alvise Villiers, Marius Gerashchenko, Kyrylo Smith, W. Clarke Wilson, Christopher Douçot, Benoit Petrescu, Alexandru Campagne-Ibarcq, Philippe Leghtas, Zaki |
| author_facet | Roverc'h, Erwan Borgognoni, Alvise Villiers, Marius Gerashchenko, Kyrylo Smith, W. Clarke Wilson, Christopher Douçot, Benoit Petrescu, Alexandru Campagne-Ibarcq, Philippe Leghtas, Zaki |
| contents | Superconducting circuits with embedded symmetries are good candidates to robustly protect quantum information from dominant error channels. The $\cos(2φ)$ 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 $T_1 = 70~μ\mathrm{s}$ and $T_2^\mathrm{echo}= 2.5~μ\mathrm{s}$ 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. |
| format | Preprint |
| id |
arxiv_https___arxiv_org_abs_2603_13114 |
| institution | arXiv |
| publishDate | 2026 |
| record_format | arxiv |
| spellingShingle | Experimental realization of a $\cos(2φ)$ transmon qubit Roverc'h, Erwan Borgognoni, Alvise Villiers, Marius Gerashchenko, Kyrylo Smith, W. Clarke Wilson, Christopher Douçot, Benoit Petrescu, Alexandru Campagne-Ibarcq, Philippe Leghtas, Zaki Quantum Physics Superconductivity Superconducting circuits with embedded symmetries are good candidates to robustly protect quantum information from dominant error channels. The $\cos(2φ)$ 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 $T_1 = 70~μ\mathrm{s}$ and $T_2^\mathrm{echo}= 2.5~μ\mathrm{s}$ 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. |
| title | Experimental realization of a $\cos(2φ)$ transmon qubit |
| topic | Quantum Physics Superconductivity |
| url | https://arxiv.org/abs/2603.13114 |