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| Main Authors: | , , , , , |
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| Format: | Preprint |
| Published: |
2026
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| Subjects: | |
| Online Access: | https://arxiv.org/abs/2603.26374 |
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| _version_ | 1866910078398889984 |
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| author | Jakobsen, Leo Uhre Shagalov, Ksenia Feldstein-Bofill, David Kjaergaard, Morten Flensberg, Karsten Krøjer, Svend |
| author_facet | Jakobsen, Leo Uhre Shagalov, Ksenia Feldstein-Bofill, David Kjaergaard, Morten Flensberg, Karsten Krøjer, Svend |
| contents | The superconductor-insulator-superconductor Josephson junction is the fundamental nonlinear element of superconducting circuits. Connecting two junctions in series gives rise to higher-harmonic content in the total energy-phase relation, enabling new design opportunities in multimode circuits. However, the double-junction element hosts an internal mode whose spectrum is set by the finite capacitances of the individual junctions. Using a Born-Oppenheimer approximation that treats the additional mode as fast compared to the qubit mode, we analyze the double-junction circuit element shunted by a large capacitor. Here, we derive an effective single-mode model of the qubit containing a correction term owing to the presence of the internal mode. We explore experimentally relevant parameter regimes and find that our model accurately describes the low-energy spectrum of the qubit. We further discuss how eliminating the internal degree of freedom affects the system's periodic boundary conditions and how this leads to non-uniqueness in performing the Born-Oppenheimer approximation. Finally, we analyze the harmonic content of the double-junction element and discuss its sensitivity to charge noise. |
| format | Preprint |
| id |
arxiv_https___arxiv_org_abs_2603_26374 |
| institution | arXiv |
| publishDate | 2026 |
| record_format | arxiv |
| spellingShingle | Low-energy spectrum of double-junction superconducting circuits in the Born-Oppenheimer approximation Jakobsen, Leo Uhre Shagalov, Ksenia Feldstein-Bofill, David Kjaergaard, Morten Flensberg, Karsten Krøjer, Svend Quantum Physics The superconductor-insulator-superconductor Josephson junction is the fundamental nonlinear element of superconducting circuits. Connecting two junctions in series gives rise to higher-harmonic content in the total energy-phase relation, enabling new design opportunities in multimode circuits. However, the double-junction element hosts an internal mode whose spectrum is set by the finite capacitances of the individual junctions. Using a Born-Oppenheimer approximation that treats the additional mode as fast compared to the qubit mode, we analyze the double-junction circuit element shunted by a large capacitor. Here, we derive an effective single-mode model of the qubit containing a correction term owing to the presence of the internal mode. We explore experimentally relevant parameter regimes and find that our model accurately describes the low-energy spectrum of the qubit. We further discuss how eliminating the internal degree of freedom affects the system's periodic boundary conditions and how this leads to non-uniqueness in performing the Born-Oppenheimer approximation. Finally, we analyze the harmonic content of the double-junction element and discuss its sensitivity to charge noise. |
| title | Low-energy spectrum of double-junction superconducting circuits in the Born-Oppenheimer approximation |
| topic | Quantum Physics |
| url | https://arxiv.org/abs/2603.26374 |