AVS 72 Session QS1-MoA: Scalable Superconducting Qubits, Materials, and Integration

In this seminar, I will present our team's recent efforts in developing high-performance superconducting qubits. Topics will include improving performance and processing windows of base layer films made from niobium and tantalum on silicon substrates. Particularly, this effort includes a reduction of the substrate temperature well into the back-end-of-the-line process window via use of krypton as a sputtering gas. Transmon qubits with a twenty-micron capacitor gap made from these films exhibit quality factors exceeding fifteen million (coherence times up to 1 millisecond), demonstrating their performance with processing that is compatible with a greater number of tools. Additionally, I will highlight high-performance qubits made with Josephson junctions formed by a resist-free fabrication method and our efforts to make high-performance qubit fabrication more broadly available through developing standard process flows at the Cornell Nanoscale Facility.

This prototype was partially supported by the Microelectronics Commons Program, a DoW initiative, under award number N00164-23-9-G061. Partial funding for shared facilities used in this prototype was provided by the Microelectronics Commons Program, a DoW initiative, under award number N00164-23-9-G061.

The superconducting A15 phase of vanadium silicide (V₃Si) is a promising candidate for quantum device applications. V₃Si-Si based Josephson junctions can be used to fabricate voltage-controlled transmon qubits, also known as gatemons. Apart from higher temperatures of operation due to the higher T_c (~ 17 K) of V₃Si, these superconductor-insulator-superconductor type gatemons are also expected to have longer coherent times. Because V₃Si is almost perfectly lattice-matched to Si, fabrication of these qubits is compatible with existing CMOS technology.

In order to realize V₃Si-based gatemons, it is crucial to understand how the V₃S-Si Schottky barrier height is tuned by the carrier density in silicon. In this talk, I report a method for reproducibly synthesizing superconducting vanadium silicide on Si substrates at various levels of doping and a systemic temperature-dependent transport study to determine the V₃S-Si Schottky barrier height. It is observed that, as compared to undoped Si, doping increases the barrier transparency by reducing both barrier height and width. Interestingly, n-type doping in Si lowers the barrier height more than p-type doping. We also demonstrate a gate-voltage tunable V₃Si-Si-V₃Si based superconducting field effect transistor.

These results evidence that V₃Si is a suitable platform for realizing voltage-based Josephson field effect transistors (JoFETs) and can have a significant broader impact not only in the field of semiconductor-based quantum computing research but also in the development of novel energy-efficient microelectronics.

As quantum computing processors continue to increase in size, there is growing interest in developing cryogenic electronics to address the challenges associated with conventional brute-force approaches to system scaling. Single Flux Quantum (SFQ) technology, implemented using a multilayer niobium (Nb) process and adapted for operation near quantum processors at millikelvin temperatures, offers a promising alternative to remote, bulky, and power-hungry room-temperature electronics. SEEQC's digital quantum management approach places energy-efficient SFQ (ERSFQ) circuits in close proximity to qubits within a multi-chip module (MCM) featuring tightly controlled inter-chip spacing. This architecture enables extremely low power dissipation, compatible with the limited cooling capacity of a typical dilution refrigerator, while maintaining high processing speeds and low error rates. In this presentation, we describe our multilayer fabrication and MCM assembly processes. We demonstrate that, by deploying digital ERSFQ circuitry in close proximity to qubits at millikelvin temperature, all essential functions required to operate a full-stack quantum computer can be realized, including digital qubit charge control, magnetic flux control, and qubit readout.