CC BY 4.0Luo, ZhenZhenLuoMayer, ThomasThomasMayerZahn, DanielaDanielaZahnMorán Guizán, CarlaCarlaMorán GuizánWeber, JohannesJohannesWeberLang, SimonSimonLangBender, HannesHannesBenderSchwarzenbach, LuisLuisSchwarzenbachNebrich, LarsLarsNebrichPereira, RuiRuiPereiraHagelauer, AmelieAmelieHagelauer2025-09-222025-09-222025-06https://publica.fraunhofer.de/handle/publica/496130https://doi.org/10.24406/publica-551310.1109/LMWT.2025.355922110.24406/publica-5513Flip-chip platforms are among the most promising approaches for scaling up superconducting qubits in quantum computing. This work presents the design, modeling, and analysis of floating transmon qubits implemented on a 3-D flip-chip platform. A major focus during qubit design is addressing coherence challenges caused by two-level system (TLS) losses, particularly from surface interfaces. TLS losses at the metal-air (MA) interface are mitigated by introducing apertures beneath the qubit pads. The aperture size is optimized to balance two key factors: minimizing total TLS loss and ensuring sufficient coupling strength between the transmon and resonator. Subsequently, a packaged 3-D chip comprising 24 floating transmons is demonstrated. The transmission spectrum is measured, with all readout resonators successfully located at their designated positions on the spectrum. Two-tone spectroscopy is demonstrated, enabling precise measurement of the qubit frequency.enQubitFrequency measurementFlip-chip integrationquantum measurementqubit modelingsuperconducting qubittwo-level system (TLS) lossestransmonCouplingsResonant frequencyThree-dimensional displaysAperturesCapacitanceFlip-chip devicesSemiconductor device measurementSuperconducting microwave devicesjournal article