HMNTL Master Calendar

Electrical and Computer Engineering Faculty Candidate Seminar

Dr. Faranak Bahrami

Postdoctoral Research Associate, Princeton University

Tuesday, May 5, 2026, 10:00-11:00 am

1000 HMNTL or Online via Zoom

Title: Superconducting Devices: Materials and New Opportunities 

Abstract: Superconducting qubits are one of the most promising platforms for building large-scale quantum processors. Despite recent progress in mid-scale processors and quantum error correction demonstrations, the performance of superconducting qubit processors is still largely limited by single qubit performance. In this context, materials improvements and fabrication optimization provide powerful pathways to improve single qubit performance, thereby bending the scaling curve for quantum error correction.

Recent advances using tantalum as a base layer in transmon qubits have led to significant improvements in coherence times [1]. However, detailed investigations of the material stack reveal that the remaining losses are dominated by two-level systems (TLS), with comparable contributions from both surface and bulk dielectrics [2]. This indicates that both sources must be addressed to achieve significant improvements beyond the current state of the art.

In this talk, I will show that replacing the substrate with high-purity silicon effectively eliminates bulk dielectric loss, enabling 2D transmons with average quality factors (Q) around 10 million. Our best device achieves a maximum Q of 25 million, corresponding to lifetime  (T₁) of up to 1.68 ms. This low-loss regime also enables the observation of decoherence channels associated with the Josephson junction. By implementing a low-contamination junction deposition process, we achieve Hahn echo coherence times (T_2E) exceeding T₁, enabling single-qubit gate fidelities of 99.994% [3].

The remaining loss in these millisecond-scale coherence qubits is dominated by dielectric loss in the surface oxide of tantalum. Further improvements thus require elimination or mitigation of the tantalum surface oxide. I will describe new fabrication improvements that reduce loss by at least a factor of two compared to current state-of-the-art devices. Finally, I will conclude with a roadmap toward understanding the microscopic origins of decoherence and outline strategies to systematically improve qubit performance through materials and device engineering.

Faranak Bahrami is a postdoctoral research associate at Princeton University, working with Prof. Nathalie de Leon and Prof. Andrew Houck on advancing superconducting qubits and resonators through materials and device optimization. Her work focuses on identifying microscopic sources of decoherence and developing materials-driven strategies to improve coherence in superconducting devices. She earned her Ph.D. from Boston College under Prof. Fazel Tafti, where she investigated Kitaev spin liquid candidates in honeycomb-layered Mott insulators, systems known for their exotic quantum properties and potential to host fractionalized excitations.