Seminar: "Tunable superconducting flux qubits with long coherence times" by Prof. Michael Stern, Bar-Ilan University, Israel 

(!!There will be some coffee and snacks before the talk!!)

Abstract: The superconducting flux qubit is a micron-size superconducting aluminium loop intersected by several Josephson junctions, among which one is smaller than others by a factor α. When the flux threading the loop is close to half a flux quantum, this circuit behaves as a two-level system and can exhibit long coherence times. Thus, it is often considered as a strategic building block for the physical realization of superconducting quantum computers. Yet, a good control of the transition energy of the qubit at its optimal working point is required to perform efficient gates on a scalable system. Here we study a series of tunable flux qubits inductively coupled to a coplanar waveguide resonator fabricated on a sapphire substrate [1]. Each qubit includes an asymmetric superconducting quantum interference device which is controlled by the application of an external magnetic field and acts as a tunable Josephson junction. The tunability of the qubits is typically ±3.5 GHz around their central gap frequency. The measured relaxation times are limited by dielectric losses in the substrate and can attain T1 ∼ 8 µs. The echo dephasing times are limited by flux noise even at optimal points and reach T2E ∼ 4 µs, almost an order of magnitude longer than state of the art.

Bio: Michael Stern graduated from Ecole Centrale Paris in 2000 with a specialization in Computer Science and Electrical Engineering. During his studies, he developed a system on a chip (SoC) dedicated for network security and cryptography and founded a start-up company named Everbee. In 2004, he decided to return to academia to undertake a PhD in the fields of semiconductor physics and optical spectroscopy. During his thesis in the group of Prof. Israel Bar-Joseph at Weizmann Institute, Michael investigated the phase diagram of dipolar excitons in coupled quantum wells. He spent two additional postdoctoral years at Weizmann Institute and worked on fractional quantum Hall effect. After this work on semiconductor physics, he opted for a post-doc in mesoscopic physics and spent three postdoctoral years working on superconducting circuits in the Quantronics group at Saclay. In 2015, Michael returned to Israel to start the new Quantum Nanoelectronics Laboratory at BIU which focuses its research on hybrid quantum systems.

[1] T. Chang et al., Phys. Rev. Applied,  18, 064062 (2022).

[2] T. Chang et al., Tunable flux qubits with long coherence times, arXiv: 2207.01460 (2022).