Flux Activated Controlled-Z Gate with a Cryogenically Integrated Digital Flux Controller

Dynamic flux control of superconducting qubits is commonly implemented by applying voltage pulses synthesized by an AWG at room temperature. These analog pulses are susceptible to distortions that form as they traverse the electrical lines connecting the room temperature control hardware to the qubits at cryogenic temperatures [1]. The pulse distortions limit qubit gate fidelities. Furthermore, the analog flux control is not scalable when one considers the wiring and instrumentation overhead required to operate a quantum processor. In this talk, we present cryogenically integrated digital flux control of tunable transmon qubits using Energy-Efficient Rapid Single Flux Quantum (ERSFQ) technology [2] at 10 mK. Here, the tunable transmon is inductively coupled to a current loop, where fluxons are injected or drained by triggering an on-chip dc-to-SFQ converter. With this novel flux control method, we demonstrate a digital flux activated controlled-Z gate between a fixed- and tunable- frequency transmon, without the need for complex pulse-shaping. The gate fidelity is mainly limited by the qubit coherence and finite flux resolution of the SFQ-based flux controller. Finally, we present simulations with optimized parameters, showing that a CZ fidelity greater than 0.9999 is achievable with this architecture.

 

[1] M.A. Rol et al. Appl. Phys. Lett. 116, 054001 (2020)

[2] D.E. Kirichenko et al. IEEE Trans. Appl. Supercond. 21, 776 (2011)